Adhesives, their manufacturing methods and applications

Abstract

The present application discloses an adhesive comprising polyvinylidene fluoride having a weight average molecular weight of 1.5 to 5,000,000, and preferably, the weight average molecular weight of polyvinylidene fluoride is 1.8 to 3.2 million.

Description

[Technical Field]

[0001] The present application relates to the field of secondary batteries, and more particularly to adhesives and their manufacturing methods and applications. [Background technology]

[0002] The statements herein are provided only as background information related to the present application and may not necessarily constitute prior art.

[0003] The positive electrode active material layer of a secondary battery, such as a lithium-ion battery, requires the addition of an adhesive to improve the adhesive strength between the active material layer and the current collector and between the active material particles. Polyvinylidene fluoride is one of the most widely used adhesive types for lithium-ion batteries, offering advantages such as strong adhesiveness, insolubility in the electrolyte, non-reactivity with the electrolyte, resistance to cracking after repeated charging, and no significant deterioration in adhesive strength. However, in conventional technologies, the amount of polyvinylidene fluoride used as an adhesive often accounts for 2.5% or more of the mass of the raw material for the active material layer. If the amount of adhesive used is too high, the proportion of active material in the active material layer is reduced, resulting in brittle electrode sheets with poor toughness and potentially reduced battery cycle performance. Summary of the Invention

[0004] According to various embodiments of the present application, adhesives and methods for their manufacture and applications are provided.

[0005] A first aspect of the present invention provides an adhesive comprising polyvinylidene fluoride having a weight-average molecular weight of 1.5 to 5.0 million, and preferably the weight-average molecular weight of the polyvinylidene fluoride is 1.8 to 3.2 million.

[0006] By selecting polyvinylidene fluoride with a weight-average molecular weight of 1.5-5 million as the adhesive component, when this adhesive is used to manufacture an electrode sheet for a lithium-ion battery, the content of the adhesive in the active material layer can be effectively reduced, avoiding defects such as brittleness of the electrode pieces and damage to battery cycle performance caused by using too much adhesive in the prior art, and not reducing the adhesive strength between the active material layer components and between the active material layer and the current collector.

[0007] In some embodiments, the polyvinylidene fluoride has a polydispersity coefficient of 1.5-2.5, preferably 2-2.3. An appropriate polydispersity coefficient can improve the dispersibility of polyvinylidene fluoride having a weight average molecular weight of 1.5-5 million in a solvent when preparing a positive electrode slurry, effectively increasing the solids content of the slurry and reducing production costs.

[0008] In some embodiments, the number average molecular weight of the polyvinylidene fluoride is 600,000 to 3,000,000, and preferably 1,000,000 to 2,000,000. By setting the number average molecular weight of polyvinylidene fluoride within a certain range, the polydispersity coefficient of polyvinylidene fluoride can be set within an appropriate range.

[0009] In some embodiments, the polyvinylidene fluoride has a Dv50 particle size of 50 μm-150 μm, preferably 60 μm-100 μm. By controlling the Dv50 particle size of polyvinylidene fluoride within an appropriate range, polyvinylidene fluoride having a weight-average molecular weight of 1.5 million-5 million can have appropriate solubility, which not only prevents the dissolution of high-molecular-weight polyvinylidene fluoride from being too slow when producing a positive electrode sheet for a lithium-ion battery, but also speeds up the production of positive electrode slurry. Most importantly, the amount of adhesive used can be controlled to a relatively low level without significantly affecting adhesive performance, effectively improving the damage to electrode sheets and battery performance caused by the high amount of adhesive used in the prior art.

[0010] In some embodiments, the crystallinity of the polyvinylidene fluoride is 38%-48%, and preferably, the crystallinity of the polyvinylidene fluoride is 40%-45%. The crystallinity of polyvinylidene fluoride also affects its solubility, so an appropriate crystallinity range is also an important factor in being able to maintain a low usage amount of the adhesive.

[0011] In some embodiments, the adhesive solution prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone has a viscosity of 3500 mPa·s-5000 mPa·s, and the weight percentage of the adhesive in the adhesive solution is 3%-5%. When preparing a positive electrode slurry, the adhesive solution must have a certain viscosity to prevent settling of the positive electrode active material and auxiliary agents, such as conductive agents, and to ensure stable storage of the slurry. While conventional techniques require at least 7% weight percentage of adhesive to achieve a solution viscosity of 3500 mPa·s-5000 mPa·s, the adhesive of the present application can be controlled to within the range of 3%-5%, providing the possibility of a low content of the adhesive in the subsequent positive electrode active material layer.

[0012] In a second aspect of the present application, there is provided a method for producing the adhesive according to one or more of the previous embodiments.

[0013] Vinylidene fluoride monomer is polymerized in a non-reactive gas atmosphere at a reaction pressure of 6 MPa to 8 MPa and a reaction temperature of 45°C to 60°C for 6 to 10 hours.

[0014] A chain transfer agent is added, the pressure in the reaction system is reduced to 2MPa-2.5MPa, the reaction is stopped, solid-liquid separation is carried out, and the solid phase is retained.

[0015] By controlling the temperature, pressure, and time of the polymerization reaction within appropriate ranges, vinylidene fluoride can be reacted in the form of homopolymerization, and polyvinylidene fluoride with higher regularity can be obtained. Furthermore, by controlling the weight-average molecular weight of polyvinylidene fluoride within a predetermined range, it is possible to achieve a low usage amount of adhesive in the positive electrode slurry.

[0016] In some embodiments, the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.

[0017] In some embodiments, the amount of the chain transfer agent used is 1.5%-3% of the mass of the vinylidene fluoride monomer. By controlling the amount of the chain transfer agent used within an appropriate range, the polymer chain length can be controlled, and a polyvinylidene fluoride product with an appropriate molecular weight range can be obtained.

[0018] In some embodiments, the polymerization reaction comprises the following steps:

[0019] A solvent and dispersant are added to a vessel, the vessel is evacuated and then filled with a non-reactive gas.

[0020] Add initiator and pH adjuster to the vessel, adjust the pH value to 6.5-7, then add vinylidene fluoride monomer, and make the pressure in the vessel to 6MPa-8MPa.

[0021] After stirring for 30-60 minutes, the temperature is raised to 45-60°C to carry out the polymerization reaction.

[0022] Before the temperature is raised to carry out the polymerization reaction, the materials are first mixed uniformly, so that the reaction can be carried out more thoroughly, and the polydispersity coefficient, crystallinity and particle size of the resulting polyvinylidene fluoride are more suitable.

[0023] In some embodiments, the amount of the solvent used is 2-8 times the mass of the vinylidene fluoride monomer.

[0024] In some embodiments, the dispersing agent comprises one or more of a cellulose ether and a polyvinyl alcohol, preferably the cellulose ether comprises one or more of a methyl cellulose ether and a carboxyethyl cellulose ether.

[0025] In some embodiments, the dispersant is used in an amount of 0.1%-0.3% by weight of the vinylidene fluoride monomer.

[0026] In some embodiments, the initiator is an organic peroxide, preferably comprising one or more of t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, and t-butyl peroxypivalate.

[0027] In some embodiments, the initiator is used in an amount of 0.15%-1% by weight of the vinylidene fluoride monomer.

[0028] In some embodiments, the pH adjuster comprises one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and aqueous ammonia.

[0029] In some embodiments, the pH adjuster is used in an amount of 0.05%-0.2% by weight of the vinylidene fluoride monomer.

[0030] In a third aspect of the present application, there is provided an application of polyvinylidene fluoride as defined in one or more of the preceding embodiments in the manufacture of adhesives.

[0031] A fourth aspect of the present application provides a secondary battery comprising a positive electrode sheet, a separator, and a negative electrode sheet, the separator being disposed between the positive electrode sheet and the negative electrode sheet.

[0032] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the adhesive described in the one or more embodiments.

[0033] In some embodiments, the weight percentage of the adhesive in the positive electrode active material layer is 0.6%-1.2%, preferably 0.6%-0.8%. In conventional techniques, the amount of adhesive used in the positive electrode active material layer is typically 2.5% or more, and a usage amount of 2% or less significantly affects the adhesion between particles in the positive electrode active material layer or the adhesion between the active material layer and the current collector. However, by using the adhesive of the present application, the amount of adhesive used can be significantly reduced to 1.2% or less compared to conventional techniques, without significantly affecting adhesive performance. The low amount of adhesive used effectively improves the brittleness and cycle performance of the electrode sheet.

[0034] In a fifth aspect of the present application, there is provided a battery module including the secondary battery according to one or more of the above embodiments.

[0035] In a sixth aspect of the present application, there is provided a battery pack including the battery module.

[0036] In a seventh aspect of the present application, there is provided a power consumption device including one or more of the secondary battery, the battery module, and the battery pack according to the one or more embodiments.

[0037] The details of one or more embodiments of the application are set forth in the drawings and description below. Other features, objects, and advantages of the application will become apparent from the description, drawings, and claims. [Brief explanation of the drawings]

[0038] To better depict and explain these presently disclosed embodiments or examples, reference may be made to one or more drawings, in which additional details or examples for the purposes of illustrating the drawings should not be considered as limiting the scope of either the disclosed application, the presently described embodiments or examples, and the best mode of these applications as currently understood.

[0039] FIG. 1 is a scanning electron micrograph of polyvinylidene fluoride particles according to one embodiment of the present application.

[0040] FIG. 2 shows polyvinylidene fluoride particles according to an embodiment of the present application. of This is a scanning electron microscope photograph.

[0041] FIG. 3 shows polyvinylidene fluoride particles according to an embodiment of the present application. of This is a scanning electron microscope photograph.

[0042] FIG. 4 shows polyvinylidene fluoride particles according to an embodiment of the present application. of This is a scanning electron microscope photograph.

[0043] FIG. 5 is a scanning electron microscope photograph of Comparative Example 1 of the present application.

[0044] FIG. 6 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0045] FIG. 7 is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG.

[0046] FIG. 8 is a schematic diagram of a battery module according to an embodiment of the present application.

[0047] FIG. 9 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0048] FIG. 10 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG.

[0049] FIG. 11 is a schematic diagram of a power consuming device that uses a secondary battery as a power source according to an embodiment of the present application.

[0050] Symbols: 1: battery pack, 2: upper housing, 3: lower housing, 4: battery module, 5: secondary battery, 51: case, 52: electrode assembly, 53: cover plate. DETAILED DESCRIPTION OF THE INVENTION

[0051] In order to clarify the purpose, technical solution and advantages of the present application, the present application will be described in more detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only for the purpose of interpreting the present application, and are not intended to limit the present application.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. The terms used herein are only for describing specific examples and are not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0053] The "ranges" disclosed herein are defined by lower and upper limits. A given range is defined by selecting one lower limit and one upper limit, and the selected lower and upper limits define the boundaries of that particular range. Such defined ranges may or may not include the end values ​​and may be arbitrarily combined, i.e., any lower limit and any upper limit may be combined to form a single 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 also contemplated. Furthermore, if a minimum range value of 1, a minimum range value of 2, a maximum range value of 3, a maximum range value of 4, and a maximum range value of 5 are listed, all ranges of 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5 are contemplated. Unless otherwise specified, the numerical range "ab" herein is a shorthand notation for any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that the text has enumerated all real numbers between "0-5," and "0-5" is merely a shorthand notation for combinations of these numbers. Furthermore, expressing a parameter as an integer ≧2 is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0054] Unless otherwise stated, all embodiments and optional embodiments in the present application can be combined with each other to form a new technical solution.

[0055] Unless otherwise stated, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0056] Unless otherwise stated, all steps in this application may be performed in order or randomly, preferably in order. For example, when a method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, when a method is described as further including step (c), it means that step (c) may be added to the method in any order, e.g., the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0057] Unless otherwise specified, the terms "comprise" and "contain" used in this application indicate an open system and may also be a closed system. For example, "comprise" and "contain" indicate that other components not listed may be further included or may be included, or that only the listed components may be included or may be included.

[0058] Unless otherwise stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any condition in which A is true (or exists) and B is false (or does not exist), or A is false (or does not exist) and B is true (or exists), or both A and B are true (or exist) also satisfies the condition "A or B."

[0059] Generally, adhesives are essential raw materials for the manufacture of positive electrode active material layers. They provide adhesion between each raw material particle of the active material layer and between the active material layer and the current collector layer, ensuring normal operation of secondary batteries through multiple charge / discharge cycles. To ensure sufficient adhesion, conventional techniques typically require the use of adhesives with a mass percentage of 2.5% or more in the positive electrode active material layer. However, if the amount of adhesive used is too high, the finished electrode sheet becomes brittle and prone to cracking during use and processing, affecting the production yield and safety of the battery. Furthermore, excessive adhesive content is also detrimental to the cycle performance of the battery. Therefore, how to reduce the amount of adhesive used in the positive electrode active material layer without adversely affecting its adhesion is a challenge to be solved in the field of secondary batteries.

[0060] Based on this, the inventors of the present application have conducted extensive research and have provided an adhesive containing polyvinylidene fluoride having a weight-average molecular weight of 1.5 to 5.0 million, preferably polyvinylidene fluoride having a weight-average molecular weight of 1.8 to 3.2 million. The weight average molecular weight of polyvinylidene fluoride may be, for example, 1.6 million, 1.7 million, 1.8 million, 1.9 million, 2.0 million, 2.1 million, 2.2 million, 2.3 million, 2.4 million, 2.5 million, 2.6 million, 2.7 million, 2.8 million, 2.9 million, 3.0 million, 3.1 million, 3.2 million, 3.3 million, 3.4 million, 3.5 million, 3.6 million, 3.7 million, 3.8 million, 3.9 million, 4.0 million, 4.1 million, 4.2 million, 4.3 million, 4.4 million, 4.5 million, 4.6 million, 4.7 million, 4.8 million, or 4.9 million, or within a range consisting of any two of these values.

[0061] By selecting polyvinylidene fluoride with a weight-average molecular weight of 1.5 million to 5 million as the adhesive component, when this adhesive is used to manufacture an electrode sheet for a lithium-ion battery, the content of the adhesive in the active material layer can be effectively reduced, avoiding defects such as brittleness of the electrode sheet and damage to battery cycle performance caused by using too much adhesive in the prior art, and not reducing the adhesive strength between the active material layer components and between the active material layer and the current collector.

[0062] In some embodiments, the polyvinylidene fluoride has a polydispersity coefficient of 1.5-2.5, preferably 2-2.3. The polydispersity coefficient of the polyvinylidene fluoride may be, for example, 1.6, 1.7, 1.8, 1.9, 2, 2.1, or 2.2, or within a range consisting of any two of the above values. A suitable polydispersity coefficient can improve the dispersibility of polyvinylidene fluoride having a weight-average molecular weight of 1.5 million to 5 million in a solvent when preparing a positive electrode slurry, effectively increasing the solids content of the slurry and reducing production costs.

[0063] In this application, the polydispersity coefficient can be measured using a method known in the art, for example, using a Waters 2695 Isocratic HPLC-type gel chromatography (differential refractive index detector 2141). A 3.0% mass fraction polystyrene solution sample was used as a reference, and a matching column (oil-based: Styragel HT 5 DMF 7.8*300mm + Styragel HT 4) was selected. A 3.0% adhesive gum solution was prepared using purified N-methylpyrrolidone (NMP) solvent, and the prepared solution was left to stand for one day before use. For measurement, first, tetrahydrofuran was aspirated into the syringe, followed by washing, and this was repeated several times. Then, 5 ml of the experimental solution was aspirated, the air in the syringe was removed, and the needle tip was wiped dry. Finally, the sample solution was slowly injected into the injection port. After the displayed value stabilized, data was acquired. The weight-average molecular weight a and number-average molecular weight b were read, respectively. Polydispersity coefficient = a / b

[0064] In some embodiments, the number average molecular weight of polyvinylidene fluoride is 600,000 to 3,000,000, and preferably 1,000,000 to 2,000,000. The number average molecular weight of polyvinylidene fluoride may be, for example, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, 1,500,000, 1,600,000, 1,700,000, 1,800,000, or 1,900,000, or within a range consisting of any two of these values. By controlling the number average molecular weight of polyvinylidene fluoride within a certain range, the polydispersity coefficient of polyvinylidene fluoride can be adjusted to an appropriate range.

[0065] In some embodiments, the Dv50 particle size of the polyvinylidene fluoride is 50 μm-150 μm, preferably 60 μm-100 μm, and may be, for example, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, or 140 μm, or within a range consisting of any two of these values. By controlling the Dv50 particle size of polyvinylidene fluoride within an appropriate range, polyvinylidene fluoride having a weight average molecular weight of 1.5 million to 5 million can have appropriate solubility, which not only prevents the high molecular weight polyvinylidene fluoride from dissolving too slowly when producing a positive electrode sheet for a lithium ion battery, but also increases the speed at which the positive electrode slurry is prepared. Most importantly, the amount of adhesive used can be controlled to a relatively low level without excessively adversely affecting adhesion, effectively improving the situation in the prior art where high amounts of adhesive are used, resulting in damage to the electrode sheet and battery performance.

[0066] In this application, Dv50 refers to the particle size corresponding to the cumulative particle size distribution number of particles reaching 50% in a particle size distribution curve, and its physical meaning is that particles with a particle size smaller (or larger) than that value account for 50%. For example, Dv50 can be easily measured using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer manufactured by Markov Instruments Ltd., UK, in accordance with GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0067] In some embodiments, the crystallinity of the polyvinylidene fluoride is 38%-48%, and preferably, the crystallinity of the polyvinylidene fluoride is 40%-45%. The crystallinity of the polyvinylidene fluoride may be, for example, 39%, 40%, 41%, 42%, 43%, or 44%, or may be within a range consisting of any two of the above values. Since the crystallinity of polyvinylidene fluoride also affects its solubility, an appropriate crystallinity range is also an important factor in being able to maintain a low adhesive usage level.

[0068] In the present application, the crystallinity can be measured using a method known in the art, for example, by differential scanning calorimetry (DSC). DSC can measure the amount of heat released when a crystalline polymer melts, and this amount of heat is the heat of melting ΔHf of the crystalline portion of the polymer. Since the heat of melting of a polymer is proportional to its crystallinity, the crystallinity θ = heat of melting at crystallization of the polymer portion / heat of melting at crystallization of 100% of the polymer × 100%, and the heat of melting at crystallization of 100% of the polymer is known.

[0069] In some embodiments, the viscosity of an adhesive solution prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone is 3500 mPa·s-5000 mPa·s, and the mass percentage of the adhesive in the adhesive solution is 3%-5%. The viscosity of the adhesive solution may be, for example, 3750 mPa·s, 4000 mPa·s, 4250 mPa·s, 4500 mPa·s, or 4750 mPa·s, or may be within a range consisting of any two of the above values. When preparing a positive electrode slurry, the adhesive solution must have a certain viscosity to prevent settling of the positive electrode active material and auxiliary agents such as conductive agents and to ensure stable storage of the slurry. In the prior art, a mass percentage of at least 7% adhesive is required to achieve a solution viscosity of 3500 mPa·s-5000 mPa·s. However, by using the adhesive of the present invention, the amount of adhesive used can be controlled within the range of 3%-5%, providing the possibility of having a low content of the subsequent adhesive in the positive electrode active material layer.

[0070] In this application, the viscosity of the adhesive solution can be measured using methods known in the art. For example, the viscosity of the adhesive solution can be measured using, for example, an Anton Paar rotational rheometer to measure the dynamic viscosity of the material at a shear rate of 0.1 s -1 -100s -1 is adopted, 31 points are measured and fitted to obtain the consistency coefficient, which can represent the viscosity of the adhesive solution.

[0071] In a second aspect of the present application, there is provided a method for producing the adhesive of one or more of the above embodiments, comprising the steps of:

[0072] Vinylidene fluoride monomer is polymerized in a non-reactive gas atmosphere at a reaction pressure of 6 MPa to 8 MPa and a reaction temperature of 45°C to 60°C for 6 to 10 hours.

[0073] A chain transfer agent is added, the pressure in the reaction system is reduced to 2MPa-2.5MPa, the reaction is stopped, solid-liquid separation is carried out, and the solid phase is retained.

[0074] A non-reactive gas refers to a gas that does not react with the reactants in the reaction system, and typical non-reactive gases can include, for example, noble gases such as argon gas and nitrogen gas.

[0075] In some embodiments, the reaction pressure may be, for example, 6.5 MPa, 7 MPa, or 7.5 MPa.

[0076] In some embodiments, the reaction temperature may be, for example, 50°C or 55°C.

[0077] In some embodiments, the duration of the polymerization reaction may be, for example, 7 hours, 8 hours, or 9 hours.

[0078] By controlling the temperature, pressure, and time of the polymerization reaction within appropriate ranges, vinylidene fluoride can be reacted in the form of homopolymerization, and polyvinylidene fluoride with higher regularity can be obtained. Furthermore, by controlling the weight-average molecular weight of polyvinylidene fluoride within a predetermined range, it is possible to achieve a low usage amount of adhesive in the positive electrode slurry.

[0079] In some embodiments, the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.

[0080] In some embodiments, the amount of chain transfer agent used is 1.5%-3% of the mass of vinylidene fluoride monomer, and the amount of chain transfer agent used may be, for example, 2% or 2.5%. By controlling the amount of chain transfer agent used within an appropriate range, the polymer chain length can be controlled, and a polyvinylidene fluoride product with an appropriate molecular weight range can be obtained.

[0081] In some embodiments, the polymerization reaction comprises the following steps:

[0082] A solvent and dispersant are added to the vessel, the vessel is evacuated, and then filled with a non-reactive gas.

[0083] An initiator and a pH adjuster are added to the vessel, and the pH value is adjusted to 6.5-7. Then, vinylidene fluoride monomer is added, and the pressure in the vessel is set to 6MPa-8MPa.

[0084] After stirring for 30-60 minutes, the temperature is raised to 45-60°C to carry out the polymerization reaction.

[0085] Before the temperature is raised to carry out the polymerization reaction, the materials are first mixed uniformly, so that the reaction can be carried out more thoroughly, and the polydispersity coefficient, crystallinity and particle size of the resulting polyvinylidene fluoride are more suitable.

[0086] In some embodiments, the amount of solvent used is 2-8 times the mass of vinylidene fluoride monomer. The amount of solvent used may be, for example, 3, 4, 5, 6, or 7 times the mass of vinylidene fluoride monomer. Suitable solvents may include, for example, water, preferably deionized water.

[0087] In some embodiments, the dispersing agent comprises one or more of a cellulose ether and a polyvinyl alcohol, preferably the cellulose ether comprises one or more of a methyl cellulose ether and a carboxyethyl cellulose ether.

[0088] In some embodiments, the amount of dispersant used is 0.1%-0.3% by weight of the vinylidene fluoride monomer. The amount of dispersant used may be, for example, 0.2%.

[0089] In some embodiments, the initiator is an organic peroxide, preferably the organic peroxide comprises one or more of t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate, and t-butyl peroxypivalate.

[0090] In some embodiments, the initiator loading is 0.15%-1% of the weight of vinylidene fluoride monomer. The initiator loading may be, for example, 0.2%, 0.4%, 0.6%, or 0.8%.

[0091] In some embodiments, the pH adjuster comprises one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and aqueous ammonia.

[0092] In some embodiments, the amount of pH adjuster used is 0.05%-0.2% by weight of vinylidene fluoride monomer. The amount of pH adjuster used may be, for example, 0.1% or 0.15%.

[0093] In a third aspect of the present application, there is provided an application of polyvinylidene fluoride as defined in one or more of the preceding embodiments in the manufacture of adhesives.

[0094] A fourth aspect of the present application provides a secondary battery comprising a positive electrode sheet, a separator, and a negative electrode sheet, the separator being provided between the positive electrode sheet and the negative electrode sheet.

[0095] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the adhesive of one or more of the above embodiments.

[0096] In some embodiments, the weight percentage of the adhesive in the positive electrode active material layer is 0.6%-1.2%, preferably 0.6%-0.8%. The amount of adhesive used may be, for example, 0.7%, 0.8%, 0.9%, 1%, or 1.1%. In conventional techniques, the amount of adhesive used in the positive electrode active material layer is typically 2.5% or more. If the amount is reduced to 2% or less, the adhesion between particles in the positive electrode active material layer or the adhesion between the active material layer and the current collector is significantly affected. However, by using the adhesive of the present application, the amount of adhesive used can be significantly reduced to 1.2% or less compared to conventional techniques without significantly affecting adhesion. The low amount of adhesive used effectively improves the brittleness and cycle performance of the electrode sheet.

[0097] In a fifth aspect of the present application, there is provided a battery module including the secondary battery of one or more of the above embodiments.

[0098] In a sixth aspect of the present application, there is provided a battery pack including the battery module.

[0099] In a seventh aspect of the present application, there is provided a power consumption device including one or more of the secondary batteries according to the one or more embodiments, the battery module, and the battery pack.

[0100] The secondary battery, battery module, battery pack, and power consuming device of the present application will be described below with appropriate reference to the drawings.

[0101] In one embodiment of the present application, a secondary battery is provided.

[0102] A typical secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charge and discharge process of the battery, active ions are inserted and removed between the positive and negative electrode sheets. The electrolyte serves to conduct ions between the positive and negative electrode sheets. The separator, located between the positive and negative electrode sheets, primarily serves to prevent short-circuiting between the positive and negative electrodes and allows ions to pass through.

[0103] Positive electrode sheet

[0104] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector and including the adhesive according to the first aspect of the present invention.

[0105] As an example, the positive electrode current collector has two surfaces opposing each other in the thickness direction, and the positive electrode active material layer is provided on one or both of the two opposing surfaces of the positive electrode current collector.

[0106] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, an aluminum foil may be used as the metal foil. The composite current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base layer. The composite current collector may be formed by forming a metal material (such as aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, or a silver alloy) on a polymeric material substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0107] In some embodiments, the positive electrode active material may be a positive electrode active material for batteries known in the art. As an example, the positive electrode active material may include at least one of a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and a modified compound thereof. The present application is not limited to these materials, and other conventional materials usable as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include lithium cobalt oxide (e.g., LiCoO), lithium nickel oxide (e.g., LiNiO), lithium manganese oxide (e.g., LiMnO, LiMnO), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide (e.g., LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also simply called NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also simply called NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also simply called NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also simply called NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also simply called NCM811), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05 O2) and modified compounds thereof. Examples of the lithium-containing phosphate having an olivine structure include, but are not limited to, at least one of lithium iron phosphate (e.g., LiFePO4 (which can also be abbreviated as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (e.g., LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite material of lithium manganese iron phosphate and carbon.

[0108] In some embodiments, the positive electrode active material layer may optionally include a conventional adhesive other than the adhesive provided in the first aspect of the present application. For example, the conventional adhesive may include at least one of polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0109] In some embodiments, the positive electrode active material layer optionally further comprises a conductive agent, for example, at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0110] In some embodiments, the components for producing the above-described positive electrode sheet, such as the positive electrode active material, the conductive agent, the adhesive, and any other components, are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, and the positive electrode slurry is applied to a positive electrode current collector, followed by steps such as drying and cold pressing to obtain a positive electrode sheet, thereby producing a positive electrode sheet.

[0111] Negative electrode sheet

[0112] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer containing a negative electrode active material provided on at least one surface of the negative electrode current collector.

[0113] As an example, the negative electrode current collector has two surfaces that face each other in the thickness direction, and the negative electrode film layer is provided on one or both of the two facing surfaces of the negative electrode current collector.

[0114] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, copper foil can be used as the metal foil piece. The composite current collector can include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy) on a polymeric material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0115] In some embodiments, the negative electrode active material may be a battery negative electrode active material known in the art. For example, the negative electrode active material may include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be at least one selected from silicon elemental, silicon-oxygen compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be at least one selected from tin elemental, tin-oxygen compounds, and tin alloys. However, the present application is not limited to these materials, and other conventional materials usable as battery negative electrode active materials may also be used. These negative electrode active materials may be used alone or in combination.

[0116] In some embodiments, the negative electrode membrane layer may optionally include an adhesive, which may be at least one selected from styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0117] In some embodiments, the negative electrode film layer optionally further comprises a conductive agent, which may be at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0118] In some embodiments, the negative electrode membrane layer may further include other auxiliary agents, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)), if necessary.

[0119] In some embodiments, the components for producing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the adhesive, and any other components, are dispersed in a solvent (e.g., deionized water) to form a negative electrode slurry, and the negative electrode slurry is applied to a negative electrode current collector, followed by steps such as drying and cold pressing to obtain a negative electrode sheet, thereby producing a negative electrode sheet.

[0120] electrolyte

[0121] The electrolyte serves to conduct ions between the positive electrode sheet and the negative electrode sheet. The present application does not specifically limit the type of electrolyte, and it can be selected as needed. The electrolyte may be, for example, liquid, gel, or all-solid.

[0122] In some embodiments, the electrolyte is an electrolytic solution, which includes an electrolyte salt and a solvent.

[0123] In some embodiments, the electrolyte salt may be at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium bis(oxalato)borate, lithium difluorobis(oxalato)borate, and lithium tetrafluorooxalatoborate.

[0124] In some embodiments, the solvent may be at least one 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, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0125] In some embodiments, the electrolyte solution further contains additives as needed. For example, the additives may include a negative electrode film-forming additive, a positive electrode film-forming additive, or an additive that can improve certain battery performance, such as an additive that improves the overcharge performance of the battery or an additive that improves the high-temperature or low-temperature performance of the battery.

[0126] Separator

[0127] In some embodiments, the secondary battery further includes a separator. In the present application, the type of separator is not particularly limited, and any known porous structure separator having good chemical stability and mechanical stability can be selected.

[0128] In some embodiments, the separator may be made of at least one material selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multilayer composite film, and is not particularly limited. When the separator is a multilayer composite film, the materials of the layers may be the same or different, and are not particularly limited.

[0129] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be manufactured into an electrode assembly by a winding process or a lamination process.

[0130] In some embodiments, the secondary battery may have an exterior packaging, which is used to seal the electrode assembly and electrolyte.

[0131] In some embodiments, the exterior of the secondary battery may be a hard case, such as a hard plastic case, an aluminum case, or a steel case. The exterior of the secondary battery may be a soft pack, such as a bag-type soft pack. The soft bag may be made of plastic, such as polypropylene, polybutylene terephthalate, or polybutylene succinate.

[0132] In the present application, the shape of the secondary battery is not particularly limited, and may be cylindrical, rectangular, or any other shape. For example, Fig. 6 shows a secondary battery 5 having a rectangular structure as an example.

[0133] In some embodiments, referring to FIG. 7 , the exterior may include a case 51 and a cover plate 53. The case 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and side plate together form a housing cavity. The case 51 has an opening communicating with the housing cavity, and the cover plate 53 can cover the opening and seal the housing cavity. The positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is packaged in the housing cavity. The electrode assembly 52 is impregnated with an electrolyte. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, and those skilled in the art can select the number according to specific actual needs.

[0134] In some embodiments, the secondary batteries may be assembled into a battery module, and the number of secondary batteries included in the battery module may be one or more, and the specific number can be selected by those skilled in the art depending on the application and capacity of the battery module.

[0135] Fig. 8 shows an example of a battery module 4. Referring to Fig. 8, in the battery module 4, the plurality of secondary batteries 5 may be arranged in order along the longitudinal direction of the battery module 4. Of course, they may be arranged in any other manner. Furthermore, the plurality of secondary batteries 5 may be fixed by fasteners.

[0136] Preferably, the battery module 4 may further include a case having a housing space for housing the plurality of secondary batteries 5.

[0137] In some embodiments, the battery modules may be assembled into a battery pack, and the number of battery modules included in the battery pack may be one or more, and the specific number can be selected by those skilled in the art depending on the application and capacity of the battery pack.

[0138] 9 and 10 show an example of a battery pack 1. Referring to FIGS. 9 and 10, the battery pack 1 may include a battery box and a plurality of battery modules 4 provided in the battery box. The battery box includes an upper housing 2 and a lower housing 3, and the upper housing 2 is covered by the lower housing 3 to form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0139] The present application also provides a power consuming device including at least one of the secondary battery, battery module, or battery pack according to the present application. The secondary battery, battery module, or battery pack may be used as a power source for the power consuming device or as an energy storage unit for the power consuming device. The power consuming device may include, but is not limited to, a mobile device, an electric vehicle, an electric train, a ship, a satellite, an energy storage system, etc. Here, the mobile device may be, for example, a mobile phone, a laptop, etc., and the electric vehicle may be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but is not limited to these.

[0140] The power consumption device can be selected as a secondary battery, a battery module, or a battery pack depending on the usage needs.

[0141] 11 shows an example of a power consuming device 6. The power consuming device may be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. To meet the power consuming device's need for high power output and high energy density of secondary batteries, a battery pack or battery module may be employed.

[0142] Other example devices may be mobile phones, tablet computers, laptop computers, etc. These devices are generally required to be thin and can use secondary batteries as their power source.

[0143] The details of one or more embodiments of the present disclosure are set forth in the drawings and description below. Other features, objects, and advantages of the present disclosure will become apparent from the description, drawings, and claims.

[0144] The present application will be described in more detail below with reference to specific examples and comparative examples. Experimental parameters not described in the following specific examples should be primarily referenced to the guides provided in the present application documents, and may refer to experimental manuals or other experimental methods known in the art, or to the experimental conditions recommended by manufacturers. The equipment and raw materials used in the following examples are relatively specific, and other specific examples are not limited thereto. The weights of relevant components described in the examples of the present application not only indicate the specific content of each component, but also the proportional weight relationship between each component. Therefore, if scaled proportionally according to the content of the relevant components in the examples, all will fall within the ranges disclosed in the examples of the present application. Specifically, the weights described in the examples of the present application may be in mass units known in the chemical industry, such as μg, mg, g, or kg. The reagents and equipment used are not specified by manufacturer, and are all commercially available, conventional products.

[0145] Example 1

[0146] (1) Manufacture of polyvinylidene fluoride adhesives

[0147] Add 4kg of deionized water and 2g of methyl cellulose ether to a 10L autoclave, evacuate, and replace O2 with N2 three times, then add 5g of t-butyl peroxypivalate and 2g of sodium bicarbonate, add 1kg of vinylidene fluoride monomer until the pressure reaches 7MPa, mix and stir for 30 minutes, heat to 45℃, carry out polymerization reaction, after 6 hours of polymerization reaction, add 30g of cyclohexane to continue the reaction, and stop the reaction when the pressure in the reactor drops to 2MPa, centrifuge the reaction system, collect the solid phase, wash and dry to obtain polyvinylidene fluoride adhesive.

[0148] The relevant parameters of the polyvinylidene fluoride adhesive produced in this example were a weight-average molecular weight of 1.8 million, a polydispersity coefficient of 2.1, a crystallinity of 42%, and a Dv50 particle size of 60 μm. The polyvinylidene fluoride adhesive produced in this example was dissolved in N-methylpyrrolidone to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 3600 mPa·s.

[0149] (2) Preparation of cathode slurry

[0150] 3.99 kg of lithium iron phosphate, 32.8 g of the polyvinylidene fluoride adhesive prepared in step (1), 57.4 g of acetylene black, 12.3 g of dispersant, and 2.4 kg of N-methylpyrrolidone (NMP) were mixed uniformly to obtain a positive electrode slurry. The mass percentage of the polyvinylidene fluoride adhesive in the solute of the positive electrode slurry was 0.8%.

[0151] (3) The positive electrode slurry prepared in step (2) was blade-coated onto a carbon-aluminum foil, baked at 110°C for 15 minutes, cold-pressed, and then cut into a disk with a diameter of 15 mm. This was then assembled into a button battery together with a metallic lithium sheet, a separator, and an electrolyte.

[0152] Examples 2-5

[0153] The mass percentages of the polyvinylidene fluoride adhesive in step (2) were basically the same as in Example 1, except that they were 0.9%, 1%, 1.1%, and 1.2%, respectively.

[0154] Example 6

[0155] In step (1), the polymerization reaction was carried out for 8 hours, and then 25 g of cyclohexane was added to continue the reaction.

[0156] The relevant parameters of the polyvinylidene fluoride adhesive prepared in this example were a weight average molecular weight of 2.5 million, a polydispersity coefficient of 2.1, a crystallinity of 44%, and a particle size of 80 μm with a Dv50. The polyvinylidene fluoride adhesive prepared in this example was dissolved in N-methylpyrrolidone to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 4300 mPa·s.

[0157] Examples 7-10

[0158] The procedures are basically the same as in Example 6, except that the mass percentages of the polyvinylidene fluoride adhesive in step (2) are 0.9%, 1%, 1.1%, and 1.2%, respectively.

[0159] Example 11

[0160] In step (1), the polymerization reaction was carried out for 10 hours, and then 20 g of cyclohexane was added to continue the reaction.

[0161] The relevant parameters of the polyvinylidene fluoride adhesive prepared in this example were a weight average molecular weight of 3.2 million, a polydispersity coefficient of 2.2, a crystallinity of 45%, and a Dv50 particle size of 100 μm. The polyvinylidene fluoride adhesive prepared in this example was dissolved in N-methylpyrrolidone to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 4900 mPa·s.

[0162] Examples 12-15

[0163] The examples are basically the same as in Example 11, except that the mass percentages of the polyvinylidene fluoride adhesive in step (2) are 0.9%, 1%, 1.1%, and 1.2%, respectively.

[0164] Example 16

[0165] In step (1), the polymerization reaction was carried out for 12 hours, and then 15 g of cyclohexane was added to continue the reaction, which was basically the same as in Example 1.

[0166] The relevant parameters of the polyvinylidene fluoride adhesive prepared in this example were a weight average molecular weight of 5 million, a polydispersity coefficient of 2.2, a crystallinity of 46%, and a particle size of 150 μm at Dv50. The polyvinylidene fluoride adhesive prepared in this example was dissolved in N-methylpyrrolidone to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 6500 mPa·s.

[0167] Examples 17-18

[0168] Polyvinylidene fluoride adhesive in step (2) of The results are basically the same as in Example 16, except that the mass percentages are 0.6% and 0.7%, respectively.

[0169] Example 19

[0170] In step (1), the polymerization reaction was carried out for 5 hours, and then 33 g of cyclohexane was added to continue the reaction, which was basically the same as in Example 1.

[0171] The relevant parameters of the polyvinylidene fluoride adhesive prepared in this example were a weight average molecular weight of 1.5 million, a polydispersity coefficient of 2.18, a crystallinity of 41.5%, and a particle size of 45 μm with a Dv50. The polyvinylidene fluoride adhesive prepared in this example was dissolved in N-methylpyrrolidone to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 3000 mPa·s.

[0172] Example 20

[0173] This is almost the same as Example 19, except that the mass percentage of the polyvinylidene fluoride adhesive in step (2) is 1.8%.

[0174] Example 21

[0175] The procedure was basically the same as in Example 1, except that the polymerization reaction temperature in step (1) was 65°C.

[0176] Book implementation The relevant parameters of the polyvinylidene fluoride adhesive produced in this example were a weight average molecular weight of 2 million, a polydispersity coefficient of 2.4, a crystallinity of 52%, and a Dv50 particle size of 50 μm. The polyvinylidene fluoride adhesive produced in this example was dissolved in an N-methylpyrrolidone solution to produce a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 4000 mPa·s.

[0177] Example 22

[0178] The procedure was basically the same as in Example 1, except that the polymerization reaction time in step (1) was 12 hours.

[0179] Book implementation The relevant parameters of the polyvinylidene fluoride adhesive produced in this example were a weight average molecular weight of 1.83 million, a polydispersity coefficient of 2.1, a crystallinity of 42%, and a Dv50 particle size of 60 μm. The polyvinylidene fluoride adhesive produced in this example was dissolved in N-methylpyrrolidone solution to produce a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 3700 mPa·s.

[0180] Example 23

[0181] The procedure was basically the same as in Example 1, except that the pressure of the polymerization reaction in step (1) was 9 MPa.

[0182] Book implementation The relevant parameters of the polyvinylidene fluoride adhesive produced in this example were a weight average molecular weight of 1.6 million, a polydispersity coefficient of 2.15, a crystallinity of 42%, and a Dv50 particle size of 50 μm. The polyvinylidene fluoride adhesive produced in this example was dissolved in an N-methylpyrrolidone solution to produce a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 2500 mPa·s.

[0183] Comparative Example 1

[0184] In step (2), the polyvinylidene fluoride adhesive produced in step (1) was replaced with commercially available polyvinylidene fluoride with a weight average molecular weight of 800,000 (product number 701A manufactured by Tosei Corporation), and the mass percentage of polyvinylidene fluoride in the slurry solute was set to 2.5%, but this was essentially the same as Example 1.

[0185] Comparative Example 2

[0186] The procedure was basically the same as in Example 1, except that the temperature of the polymerization reaction in step (1) was 35°C.

[0187] The relevant parameters of the polyvinylidene fluoride adhesive produced in this comparative example are a weight average molecular weight of 1.2 million, a polydispersity coefficient of 1.8, a crystallinity of 40%, and a particle size of Dv50 of 30 μm. comparison The polyvinylidene fluoride adhesive produced in the example was dissolved in an N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 450 mPa·s.

[0188] Comparative Example 3

[0189] The procedure was basically the same as in Example 1, except that the polymerization reaction time in step (1) was 4 hours.

[0190] The relevant parameters of the polyvinylidene fluoride adhesive produced in this comparative example were a weight average molecular weight of 1.1 million, a polydispersity coefficient of 1.9, a crystallinity of 38%, and a Dv50 particle size of 25 μm. comparison The polyvinylidene fluoride adhesive produced in the example was dissolved in an N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 232 mPa·s.

[0191] Comparative Example 4

[0192] The procedure was basically the same as in Example 1, except that the pressure of the polymerization reaction in step (1) was 5 MPa.

[0193] The relevant parameters of the polyvinylidene fluoride adhesive produced in this comparative example are a weight average molecular weight of 1.4 million, a polydispersity coefficient of 2.2, a crystallinity of 41%, and a particle size of Dv50 of 40 μm. comparison The polyvinylidene fluoride adhesive prepared in the examples was dissolved in an N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the measured solution viscosity was 1800 mPa·s.

[0194] Evaluation and Measurement

[0195] The polyvinylidene fluoride adhesive, electrode sheet, and button battery produced in each of the examples and comparative examples were subjected to the following evaluation measurements.

[0196] (1) Polyvinylidene fluoride adhesive is scanned with a scanning electron microscope to determine the morphology and particle size.

[0197] (2) The electrode sheet is subjected to a brittleness measurement, and the measurement steps are as follows:

[0198] The electrode sheet after cold pressing was sampled along the horizontal direction and divided into 100cm sections (4cm x 25cm lengthwise). 2The number of samples of the area can be three, and the sample is 100 cm 2 The test film (25cm long, folded in half) is pre-folded, placed on a flat surface on the test bench, and pressed once with a 2kg cylindrical roller. After checking for light leakage spots under lamp light, samples with light leakage spots do not meet the requirements, and the number of roller presses is recorded as no light leakage spots. The test sample is then folded back along the vertical fold, and the fold is observed under light. Record the number of roller presses (n1) until a light transmission spot appears on the fold. Repeat the above process to obtain the number of light-transmitting roller presses (n2 and n3) for the second and third rolls, and calculate the average number of roller presses = (n1 + n2 + n3) / 3.

[0199] (3) The measurement steps for adhesive strength measurement are as follows:

[0200] Refer to the Chinese national standard "Method for Measuring 180° Peel Strength of Adhesives," cut a sample 30mm wide x 100-160mm long with a blade, attach it to a steel plate with special double-sided tape, and attach the tape 20mm wide x 90-150mm long. Attach the electrode sheet sample cut out above to the double-sided tape, place the measurement surface down, and then roll it three times in the same direction with a press roll.

[0201] A paper tape with the same width as the electrode sheet and a length greater than the sample length (80-200 mm) is inserted under the electrode sheet and fixed with adhesive.

[0202] Turn on the power (sensitivity 1N) of the Sansi tensioning machine, turn on the indicator light, adjust the position limiting block to the appropriate position, and fix the end of the steel plate that does not have the electrode sheet attached with the lower jig. Fold the paper tape up and fix it with the upper jig, and adjust the position of the upper jig using the "up" and "down" buttons on the manual controller attached to the tensioning machine, then test and read the numerical value.

[0203] (4) The method for measuring the battery capacity retention rate is as follows.

[0204] The battery was charged at a constant current of 1 / C to 3.65V, then charged at a constant voltage of 0.05C at 3.65V, left for 5 minutes, and then discharged at 1 / C to 2.5V, with the resulting capacity designated as the initial capacity C0. The above steps were repeated for the same battery, recording the discharge capacity Cn of the battery at the nth cycle. The battery capacity retention rate after each cycle was calculated as Pn = Cn / C0 * 100%, and the 100 point values ​​P1, P2...P100 were used as the ordinate and the corresponding cycle number as the abscissa to create a graph of battery capacity retention rate versus cycle number.

[0205] In the measurement process, the first cycle corresponds to n=1, the second cycle corresponds to n=2, ... the 100th cycle corresponds to n=100, and the battery capacity retention rate data is the data measured after 500 cycles under the above measurement conditions, i.e., the P500 value.

[0206] The above measurement data is entered in Table 1.

[0207] [Table 1]

[0208] Analyzing the data in Table 1 and comparing Examples 1, 6, 11, 16, and 19, it can be seen that as the weight-average molecular weight increases, the adhesive's Dv50 particle size, adhesive strength, and cycle performance generally tend to increase, while the average number of roll presses for the electrode sheet gradually decreases. As can be seen from Example 17, when the weight-average molecular weight of polyvinylidene fluoride reaches 5 million, a usage amount of only 0.6% can provide an adhesive strength of 30.7 N / m, and good battery cycle performance. However, although the electrode sheet is somewhat brittle, this is still superior to the commercially available adhesive (Comparative Example 1). As can be seen, the adhesive provided herein can reliably reduce the amount of adhesive used in the positive electrode active material and provide good electrode sheet toughness, adhesive strength, and battery cycle performance.

[0209] Comparing Example 1 and Example 21, the molecular weights of the two are similar. However, the reaction temperature in Example 21 is high, which increases the crystallinity and polydispersity coefficient of the polyvinylidene fluoride produced by the polymerization reaction, affecting the brittleness of the electrode sheet and significantly reducing the average number of roll presses. Therefore, it is preferable to control the polymerization reaction temperature to 45°C-60°C, which allows the crystallinity to be controlled within an appropriate range of 38%-48% and the polydispersity coefficient to be controlled within a more preferable range of 2-2.3.

[0210] As can be seen from the comparison of Examples 1-5, 6-10, 11-15, 16-18, and 19-20, as the amount of polyvinylidene fluoride used increases, the adhesive strength and cycle performance increase, but the electrode sheet gradually becomes more brittle and the average number of roll presses decreases. In particular, in Example 20, after the amount of adhesive used is significantly increased to 1.8%, there is a limit to the improvement in cycle performance and adhesive strength, but the number of roll presses decreases significantly.

[0211] Compared with Example 1, in Example 22, the polymerization reaction time is long, and due to continuous consumption of monomer and decrease in pressure, the polymerization conditions are not reached, and the polymerization reaction cannot be continued by extending the reaction time. Therefore, from the viewpoint of energy consumption and efficiency, it is more preferable to control the reaction time within 10 hours.

[0212] Compared with Example 1, in Example 23, the reaction pressure was high, the pressure at which the monomer entered the reaction solution was high, and the monomer entered the reaction solution more frequently, which led to the occurrence of a wide range of polymerization reactions, and the amount of polyvinylidene fluoride produced was large. With the decrease in monomer, the molecular weight of polyvinylidene fluoride produced was relatively small due to the lack of monomer supply, which had a certain impact on battery performance.

[0213] Compared with Example 1, Comparative Example 2 had a lower reaction temperature, a weaker copolymerization promotion force, an insufficient polymerization reaction, and a smaller prepared molecular weight. As a result, the adhesive strength was significantly reduced at the same addition amount, and the cycle performance was obviously reduced.

[0214] Compared with Example 1, Comparative Example 3 has a shorter polymerization reaction time, the polymerization reaction is not continuous, and the prepared molecular weight is small, which also leads to a decrease in adhesive strength and cycle performance.

[0215] Compared with Example 1, in Comparative Example 4, the reaction pressure was low, and the pressure at which the monomer entered the reaction solution was low, making it impossible to continuously replenish the reaction monomer, which was unfavorable for the continuous progress of polymerization. The molecular weight of the prepared product was too low, which could not meet the requirements for adhesive strength, and the cycle performance of the battery was also reduced.

[0216] The technical features of the above-described embodiments can be combined in any manner, and for the sake of brevity, not all possible combinations of the technical features in the above-described embodiments are described, but as long as there is no contradiction in the combination of these technical features, they should be considered within the scope described in this specification.

[0217] The above examples only represent some embodiments of the present application, and although the descriptions are more specific and detailed, they should not be understood as limiting the scope of the patent. Those skilled in the art may make minor modifications and improvements without departing from the spirit of the present application, and all of these fall within the scope of protection of the present invention. Therefore, the scope of protection of the present application is defined by the appended claims.

Claims

1. A secondary battery, a positive electrode sheet, a separator, and a negative electrode sheet, the separator being provided between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, the positive electrode active material layer including an adhesive; The adhesive comprises polyvinylidene fluoride having a weight average molecular weight of 1.5 million to 5 million, and the polyvinylidene fluoride has a polydispersity coefficient of 1.5 to 2.5, where the polydispersity coefficient is the weight average molecular weight / number average molecular weight; In the positive electrode active material layer, the mass percentage of the adhesive is 0.6% to 1.2%.

2. 2. The secondary battery according to claim 1, wherein the polyvinylidene fluoride has a polydispersity coefficient of 2 to 2.

3.

3. 3. The secondary battery according to claim 1, wherein the polyvinylidene fluoride has a number average molecular weight of 600,000 to 3,000,000.

4. 3. The secondary battery according to claim 1, wherein the particle size of the polyvinylidene fluoride is 50 μm to 150 μm in terms of Dv50.

5. 3. The secondary battery according to claim 1, wherein the polyvinylidene fluoride has a crystallinity of 38% to 48%.

6. 3. The secondary battery according to claim 1, wherein the viscosity of the adhesive solution prepared by dissolving the polyvinylidene fluoride in N-methylpyrrolidone is 3500 mPa·s to 5000 mPa·s, and the mass percentage of the adhesive in the adhesive solution is 3% to 5%.

7. A method for manufacturing a secondary battery according to claim 1, Polymerizing vinylidene fluoride monomer in a non-reactive gas atmosphere at a reaction pressure of 6 MPa-8 MPa and a reaction temperature of 45°C-60°C for 6 hours-10 hours; Adding a chain transfer agent, reducing the pressure in the reaction system to 2 MPa-2.5 MPa, stopping the reaction, separating the solid phase, and retaining the solid phase; A manufacturing method for producing an adhesive by a method comprising the steps of:

8. 8. The process of claim 7, wherein the chain transfer agent comprises one or more of cyclohexane, isopropanol, methanol, and acetone.

9. 9. The method according to claim 7, wherein the amount of the chain transfer agent used is 1.5% to 3% of the mass of the vinylidene fluoride monomer.

10. The polymerization reaction is adding a solvent and a dispersant to a vessel, evacuating the vessel, and then filling the vessel with a non-reactive gas; Adding an initiator and a pH adjuster to the vessel to adjust the pH to 6.5-7, and then adding vinylidene fluoride monomer to adjust the pressure in the vessel to 6 MPa-8 MPa; After stirring for 30-60 minutes, the temperature is raised to 45-60°C to carry out a polymerization reaction; The method according to claim 7 or 8, comprising:

11. 11. The method according to claim 10, wherein the amount of the solvent used is 2 to 8 times the mass of the vinylidene fluoride monomer.

12. The method of claim 10, wherein the dispersant comprises one or more of a cellulose ether and a polyvinyl alcohol.

13. 11. The method according to claim 10, wherein the amount of the dispersant used is 0.1% to 0.3% of the mass of the vinylidene fluoride monomer.

14. 11. The method according to claim 10, wherein the initiator is an organic peroxide.

15. 11. The method of claim 10, wherein the amount of the initiator used is 0.15%-1% of the mass of the vinylidene fluoride monomer.

16. The method according to claim 10, wherein the pH adjuster comprises one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and aqueous ammonia.

17. 11. The method according to claim 10, wherein the amount of the pH adjuster used is 0.05% to 0.2% of the mass of the vinylidene fluoride monomer.

18. A battery module comprising the secondary battery according to claim 1 .

19. A battery pack comprising the battery module according to claim 18.

20. A power consuming device comprising one or more of the secondary battery according to claim 1, the battery module according to claim 18, and the battery pack according to claim 19.

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