Binder, and preparation method therefor and use thereof

Abstract

Disclosed in the present application is a binder, which comprises polyvinylidene fluoride having a weight-average molecular weight of 1,500,000-5,000,000, wherein optionally, the polyvinylidene fluoride has a weight-average molecular weight of 1,800,000-3,200,000.

Description

Binder and its preparation method and application Technical Field

[0001] The present application relates to the field of secondary batteries, and more specifically to a binder and a preparation method and application thereof. Background Art

[0002] The statements herein merely provide background information related to the present application and do not necessarily constitute prior art.

[0003] Secondary batteries, such as lithium-ion batteries, often require the addition of binders to the positive electrode active material layer to enhance the bonding strength between the active material layer and the current collector, as well as the bonding strength between the active material particles. Polyvinylidene fluoride is one of the most widely used binder types in lithium-ion batteries. It has the advantages of strong bonding, insolubility in electrolyte, no reaction with electrolyte, not prone to cracking after repeated charging, and no significant decrease in bonding strength. However, in traditional technologies, the amount of polyvinylidene fluoride used as a binder often accounts for more than 2.5% of the mass of the raw materials in the active material layer. If the amount of binder is too high, it will squeeze the proportion of active material in the active material layer, often resulting in brittle electrodes, poor toughness, and a decrease in the battery's cycle performance.

[0004] Summary of the Invention

[0005] According to various embodiments of the present application, an adhesive and a preparation method and application thereof are provided.

[0006] In a first aspect of the present application, a binder is provided, comprising polyvinylidene fluoride having a weight-average molecular weight of 1.5 to 5 million; optionally, the weight-average molecular weight of the polyvinylidene fluoride is 1.8 to 3.2 million.

[0007] By selecting polyvinylidene fluoride with a weight-average molecular weight of 1.5 to 5 million as a binder component, when the binder is used in the preparation of lithium-ion battery pole pieces, the binder content in the active material layer can be effectively reduced, thereby avoiding defects such as brittle pole pieces and impaired battery cycle performance caused by excessive use of binder in traditional technologies. At the same time, the bonding force between the components of the active material layer and between the active material layer and the current collector will not be reduced.

[0008] In some embodiments, the polydispersity index of the polyvinylidene fluoride is 1.5 to 2.5; alternatively, the polydispersity index of the polyvinylidene fluoride is 2 to 2.3. A suitable polydispersity index can improve the dispersion of polyvinylidene fluoride with a weight average molecular weight of 1.5 to 5 million in the solvent when preparing the positive electrode slurry, thereby effectively increasing the solid content of the slurry and reducing production costs.

[0009] In some embodiments, the number average molecular weight of the polyvinylidene fluoride is 600,000 to 3,000,000; alternatively, the number average molecular weight of the polyvinylidene fluoride is 1,000,000 to 2,000,000. By controlling the number average molecular weight of the polyvinylidene fluoride within a certain range, the polydispersity coefficient of the polyvinylidene fluoride can be within an appropriate range.

[0010] In some embodiments, the polyvinylidene fluoride has a Dv50 particle size of 50 μm to 150 μm; alternatively, the polyvinylidene fluoride has a Dv50 particle size of 60 μm to 100 μm. Controlling the Dv50 particle size of the polyvinylidene fluoride within an appropriate range allows polyvinylidene fluoride with a weight-average molecular weight of 1.5 to 5 million to have an appropriate solubility. When used to prepare positive electrode plates for lithium-ion batteries, this not only prevents the slow dissolution of high-molecular-weight polyvinylidene fluoride, thereby accelerating the preparation of positive electrode slurry, but most importantly, it also allows the amount of binder to be controlled at a low level without significantly negatively affecting the bonding performance, effectively improving the performance degradation of the plate and battery caused by high amounts of binder used in traditional technologies.

[0011] In some embodiments, the polyvinylidene fluoride has a crystallinity of 38% to 48%; alternatively, the polyvinylidene fluoride has a crystallinity of 40% to 45%. The crystallinity of polyvinylidene fluoride also affects its solubility. Therefore, the appropriate crystallinity range is also an important factor in maintaining the binder dosage at a low level.

[0012] In some embodiments, the viscosity of the binder solution prepared by dissolving the polyvinylidene fluoride in N-methylpyrrolidone is 3500mPa·s to 5000mPa·s, and the mass percentage of the binder in the binder solution is 3% to 5%. When preparing the positive electrode slurry, the binder solution needs to have a certain viscosity to prevent the sedimentation of the positive electrode active material and auxiliary agents such as the conductive agent, so that the slurry can be placed more stably. In traditional technology, to achieve a solution viscosity of 3500mPa·s to 5000mPa·s, at least 7% by mass of the binder is required to achieve it. However, by using the binder of the present application, the amount of the binder can be controlled within the range of 3% to 5%, which provides the possibility for the subsequent binder to have a lower content in the positive electrode active material layer.

[0013] In a second aspect of the present application, a method for preparing the binder according to one or more of the aforementioned embodiments is provided, comprising the following steps:

[0014] The vinylidene fluoride monomer is polymerized in a non-reactive gas atmosphere, a reaction pressure of 6 MPa to 8 MPa, and a reaction temperature of 45° C. to 60° C. for 6 h to 10 h;

[0015] A chain transfer agent is added, and when the pressure in the reaction system drops to 2MPa-2.5MPa, the reaction is stopped, the solid and liquid are separated, and the solid phase is retained.

[0016] By controlling the temperature, pressure and time of the polymerization reaction within an appropriate range, vinylidene fluoride can react in the form of homopolymerization, thereby obtaining polyvinylidene fluoride with higher regularity, and the weight-average molecular weight of polyvinylidene fluoride can be controlled within a preset range to achieve a low amount of binder in the positive electrode slurry.

[0017] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0018] In some embodiments, the amount of the chain transfer agent is 1.5% to 3% of the mass of the vinylidene fluoride monomer. Controlling the amount of the chain transfer agent within a suitable range can control the polymer chain length, thereby obtaining a polyvinylidene fluoride product with a suitable molecular weight range.

[0019] In some embodiments, the polymerization reaction comprises the following steps:

[0020] Adding a solvent and a dispersant into a container, evacuating the container and then filling it with a non-reactive gas;

[0021] Adding an initiator and a pH adjuster to the container, adjusting the pH value to 6.5-7, and then adding vinylidene fluoride monomer to make the pressure in the container reach 6MPa-8MPa;

[0022] After stirring for 30 to 60 minutes, the temperature is raised to 45 to 60°C to carry out the polymerization reaction.

[0023] Before heating for polymerization reaction, the materials are mixed evenly, which can make the reaction proceed more thoroughly and the polydispersity coefficient, crystallinity and particle size of the obtained polyvinylidene fluoride more appropriate.

[0024] In some embodiments, the amount of the solvent used is 2 to 8 times the mass of the vinylidene fluoride monomer.

[0025] In some embodiments, the dispersant includes one or more of cellulose ether and polyvinyl alcohol; alternatively, the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

[0026] In some embodiments, the amount of the dispersant is 0.1% to 0.3% by mass of the vinylidene fluoride monomer.

[0027] In some embodiments, the initiator is an organic peroxide; optionally, the organic peroxide includes one or more of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate and tert-butyl peroxypivalate.

[0028] In some embodiments, the amount of the initiator is 0.15% to 1% by weight of the vinylidene fluoride monomer.

[0029] In some embodiments, the pH adjuster includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and ammonia solution.

[0030] In some embodiments, the amount of the pH adjuster is 0.05% to 0.2% by mass of the vinylidene fluoride monomer.

[0031] The third aspect of the present application provides the use of polyvinylidene fluoride defined in one or more of the aforementioned embodiments in the preparation of a binder.

[0032] In a fourth aspect of the present application, a secondary battery is provided, comprising a positive electrode sheet, a separator, and a negative electrode sheet, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet;

[0033] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes the binder described in one or more of the aforementioned embodiments.

[0034] In some embodiments, the binder content in the positive electrode active material layer is 0.6% to 1.2% by mass; alternatively, the binder content is 0.6% to 0.8% by mass. In conventional technology, the amount of binder used in the positive electrode active material layer is often as high as 2.5% or more. When the amount drops to 2% or less, it will significantly affect the bonding between the particles in the positive electrode active material layer, or the bonding between the active material layer and the current collector. However, the use of the binder in this application can greatly reduce the amount of binder used to 1.2% or less compared to conventional technology without significantly affecting the bonding performance. The low binder content effectively improves the brittleness and cycle performance of the electrode.

[0035] In a fifth aspect of the present application, a battery module is provided, comprising the secondary battery described in one or more of the aforementioned embodiments.

[0036] In a sixth aspect of the present application, a battery pack is provided, comprising the battery module as claimed in the preceding claims.

[0037] In a seventh aspect of the present application, an electrical device is provided, comprising one or more of the secondary battery described in one or more of the aforementioned embodiments, the aforementioned battery module, and the aforementioned battery pack.

[0038] The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to better describe and illustrate the embodiments or examples of the applications disclosed herein, reference may be made to one or more of the accompanying drawings. The additional details or examples used to describe the accompanying drawings should not be considered to limit the scope of the disclosed applications, the embodiments or examples currently described, and any of the best modes currently understood for these applications.

[0040] FIG1 is a scanning electron microscope image of polyvinylidene fluoride particles according to one embodiment of the present application.

[0041] [Corrected 21.08.2023 according to Rule 91] Figure 2 is a scanning electron microscope image of polyvinylidene fluoride particles according to one embodiment of the present application.

[0042] [Corrected 21.08.2023 according to Rule 91] Figure 3 is a scanning electron microscope image of polyvinylidene fluoride particles according to one embodiment of the present application.

[0043] [Corrected 21.08.2023 according to Rule 91] Figure 4 is a scanning electron microscope image of polyvinylidene fluoride particles according to one embodiment of the present application.

[0044] FIG5 is a scanning electron microscope image of Comparative Example 1 of the present application.

[0045] FIG6 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0046] FIG. 7 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 6 .

[0047] FIG8 is a schematic diagram of a battery module according to an embodiment of the present application.

[0048] FIG9 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0049] FIG10 is an exploded view of the battery pack shown in FIG9 according to an embodiment of the present application.

[0050] FIG. 11 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0051] Description of reference numerals:

[0052] 1: Battery pack; 2: Upper case; 3: Lower case; 4: Battery module; 5: Secondary battery; 51: Housing; 52: Electrode assembly; 53: Cover. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solutions and advantages of this application more clearly understood, this application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0054] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0055] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of 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 all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0056] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0057] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0058] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, 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.

[0059] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0060] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0061] Generally speaking, a binder is an indispensable raw material for preparing the positive electrode active material layer. It provides adhesion between the raw material particles of the active material layer, and between the active material layer and the current collector layer, ensuring that the secondary battery can operate normally during multiple charge and discharge processes. In order to ensure sufficient adhesion, traditional technologies generally require the use of a binder with a mass percentage of more than 2.5% of the positive electrode active material layer. However, excessive amounts of binder will cause the finished electrode to be brittle and prone to cracking during use and processing, affecting the production yield and safety of the battery. Moreover, excessive binder content is not conducive to the cycle performance of the battery. Therefore, how to reduce the amount of binder in the positive electrode active material layer without negatively affecting its bonding performance is an urgent problem to be solved in the field of secondary batteries.

[0062] Based on this, the inventors of the present application have provided a binder through extensive research, which includes polyvinylidene fluoride with a weight average molecular weight of 1.5 to 5 million; optionally, the weight average molecular weight of the polyvinylidene fluoride is 1.8 to 3.2 million. The weight average molecular weight of polyvinylidene fluoride can also be, for example, 1.6 million, 1.7 million, 1.8 million, 1.9 million, 2 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 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 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 it can be within the range of any two of the above points.

[0063] By selecting polyvinylidene fluoride with a weight-average molecular weight of 1.5 to 5 million as a binder component, when the binder is used in the preparation of lithium-ion battery pole pieces, the binder content in the active material layer can be effectively reduced, thereby avoiding defects such as brittle pole pieces and impaired battery cycle performance caused by excessive use of binder in traditional technologies. At the same time, the bonding force between the components of the active material layer and between the active material layer and the current collector will not be reduced.

[0064] In some embodiments, the polydispersity index of polyvinylidene fluoride is 1.5 to 2.5; alternatively, the polydispersity index of polyvinylidene fluoride is 2 to 2.3. The polydispersity index of polyvinylidene fluoride can also be, for example, 1.6, 1.7, 1.8, 1.9, 2, 2.1 or 2.2, or a range consisting of any two of the above values. A suitable polydispersity index can make the polyvinylidene fluoride with a weight average molecular weight of 1.5 to 5 million more dispersible in the solvent when preparing the positive electrode slurry, thereby effectively increasing the solid content of the slurry and reducing production costs.

[0065] In this application, the polydispersity coefficient can be measured using methods known in the art, for example, a Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141). A 3.0% polystyrene solution sample is used as a reference, and a matching chromatographic column is selected (oil-based: Styragel HT5 DMF 7.8*300mm + Styragel HT4). A 3.0% adhesive solution is prepared using purified N-methylpyrrolidone (NMP) solvent. The prepared solution is allowed to stand for one day before use. During testing, tetrahydrofuran is first drawn into a syringe and rinsed several times. Then, 5 ml of the experimental solution is drawn, air is expelled from the syringe, and the needle tip is wiped dry. Finally, the sample solution is slowly injected into the injection port. After the reading stabilizes, the data is acquired. The weight-average molecular weight a and number-average molecular weight b are read separately. Polydispersity coefficient = a / b.

[0066] In some embodiments, the number average molecular weight of the polyvinylidene fluoride is 600,000 to 3,000,000; alternatively, the number average molecular weight of the polyvinylidene fluoride is 1,000,000 to 2,000,000. The number average molecular weight of the polyvinylidene fluoride can also 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 a range between any two of the above values. By controlling the number average molecular weight of the polyvinylidene fluoride within a certain range, the polydispersity coefficient of the polyvinylidene fluoride can be within a suitable range.

[0067] In some embodiments, the Dv50 particle size of the polyvinylidene fluoride is 50 μm to 150 μm. Alternatively, the Dv50 particle size of the polyvinylidene fluoride is 60 μm to 100 μm. The Dv50 particle size of the polyvinylidene fluoride can also be, for example, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, or 140 μm, or any two of the above values. Controlling the Dv50 particle size of polyvinylidene fluoride within an appropriate range can ensure that polyvinylidene fluoride with a weight-average molecular weight of 1.5 to 5 million has an appropriate solubility. When used to prepare positive electrode plates for lithium-ion batteries, it can not only prevent the slow dissolution of large-molecular-weight polyvinylidene fluoride, thereby speeding up the preparation of positive electrode slurry, but most importantly, it can also control the amount of binder used at a lower level without causing excessive negative impact on the bonding performance, thereby effectively improving the situation in which high amounts of binder used in traditional technologies cause damage to the plate and battery performance.

[0068] In this application, Dv50 refers to the particle size at which the cumulative particle size distribution reaches 50% in a particle size distribution curve. Its physical meaning is that 50% of the particles have a particle size smaller (or larger) than this value. For example, Dv50 can be conveniently measured using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd. in the United Kingdom, in accordance with GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0069] In some embodiments, the crystallinity of the polyvinylidene fluoride is 38% to 48%; alternatively, the crystallinity of the polyvinylidene fluoride is 40% to 45%. The crystallinity of the polyvinylidene fluoride can also be, for example, 39%, 40%, 41%, 42%, 43%, or 44%, or a range between any two of the aforementioned values. The crystallinity of the polyvinylidene fluoride also affects its solubility. Therefore, an appropriate crystallinity range is also an important factor in maintaining a low binder dosage.

[0070] In the present application, the crystallinity can be tested by methods known in the art. For example, the crystallinity can be measured by differential scanning calorimetry (DSC). DSC can measure the heat released when a crystalline polymer melts. This heat is the melting heat ΔHf of the crystalline portion of the polymer. The melting heat of the polymer is proportional to its crystallinity. Therefore, the crystallinity θ = partial crystalline melting heat of the polymer / 100% crystalline melting heat of the polymer × 100%, where the 100% crystalline melting heat of the polymer is known.

[0071] In some embodiments, the viscosity of the binder solution prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone is 3500 mPa·s to 5000 mPa·s, and the binder content in the binder solution is 3% to 5% by weight. The viscosity of the binder solution can also be, for example, 3750 mPa·s, 4000 mPa·s, 4250 mPa·s, 4500 mPa·s, or 4750 mPa·s, or a range between any two of the above values. When preparing the positive electrode slurry, the binder solution needs to have a certain viscosity to prevent sedimentation of the positive electrode active material and additives such as the conductive agent, so that the slurry can be placed more stably. In traditional technology, to achieve a solution viscosity of 3500mPa·s to 5000mPa·s, at least 7% by mass of the binder is required. However, by using the binder of the present application, the amount of the binder can be controlled within the range of 3% to 5%, which provides the possibility for the subsequent binder to have a lower content in the positive electrode active material layer.

[0072] In this application, the viscosity of the binder solution can be tested using methods known in the art. As an example, the viscosity of the binder solution can be tested using an Anton Paar rotational rheometer to measure the dynamic viscosity of the material at a shear rate of 0.1s. -1 ~100s -1 , 31 points are measured and the consistency coefficient is obtained after fitting, which can characterize the viscosity of the binder solution.

[0073] In a second aspect of the present application, a method for preparing the binder of one or more of the aforementioned embodiments is provided, comprising the following steps:

[0074] The vinylidene fluoride monomer is polymerized in a non-reactive gas atmosphere, a reaction pressure of 6 MPa to 8 MPa, and a reaction temperature of 45° C. to 60° C. for 6 h to 10 h;

[0075] A chain transfer agent is added, and when the pressure in the reaction system drops to 2MPa-2.5MPa, the reaction is stopped, the solid and liquid are separated, and the solid phase is retained.

[0076] It is understood that the non-reactive gas refers to a gas that does not react with the reactants in the reaction system. Common non-reactive gases may include, for example, inert gases such as argon and nitrogen.

[0077] In some embodiments, the reaction pressure may also be, for example, 6.5 MPa, 7 MPa, or 7.5 MPa.

[0078] In some embodiments, the reaction temperature may be, for example, 50°C or 55°C.

[0079] In some embodiments, the polymerization reaction time may be, for example, 7 h, 8 h, or 9 h.

[0080] By controlling the temperature, pressure and time of the polymerization reaction within an appropriate range, vinylidene fluoride can react in the form of homopolymerization, thereby obtaining polyvinylidene fluoride with higher regularity, and the weight-average molecular weight of polyvinylidene fluoride can be controlled within a preset range to achieve a low amount of binder in the positive electrode slurry.

[0081] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.

[0082] In some embodiments, the amount of chain transfer agent used is 1.5% to 3% of the weight of the vinylidene fluoride monomer, and the amount of chain transfer agent used can also be, for example, 2% or 2.5%. Controlling the amount of chain transfer agent within a suitable range can control the polymer chain length, thereby obtaining a polyvinylidene fluoride product with a suitable molecular weight range.

[0083] In some embodiments, the polymerization reaction comprises the following steps:

[0084] Adding a solvent and a dispersant into a container, evacuating the container and then filling it with a non-reactive gas;

[0085] Add an initiator and a pH adjuster to the container, adjust the pH value to 6.5-7, and then add vinylidene fluoride monomer to make the pressure in the container reach 6MPa-8MPa;

[0086] After stirring for 30 to 60 minutes, the temperature is raised to 45 to 60°C to carry out the polymerization reaction.

[0087] Before heating for polymerization reaction, the materials are mixed evenly, which can make the reaction proceed more thoroughly and the polydispersity coefficient, crystallinity and particle size of the obtained polyvinylidene fluoride more appropriate.

[0088] In some embodiments, the amount of solvent used is 2 to 8 times the mass of the vinylidene fluoride monomer. The amount of solvent used can also be, for example, 3, 4, 5, 6, or 7 times the mass of the vinylidene fluoride monomer. Suitable solvents can include, for example, water, preferably deionized water.

[0089] In some embodiments, the dispersant includes one or more of cellulose ether and polyvinyl alcohol; alternatively, the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

[0090] In some embodiments, the amount of the dispersant used is 0.1% to 0.3% of the mass of the vinylidene fluoride monomer, and can also be 0.2%.

[0091] In some embodiments, the initiator is an organic peroxide; optionally, the organic peroxide includes one or more of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethylperoxydicarbonate, diisopropyl peroxydicarbonate, and tert-butyl peroxypivalate.

[0092] In some embodiments, the amount of the initiator is 0.15% to 1% by weight of the vinylidene fluoride monomer, and can also be 0.2%, 0.4%, 0.6%, or 0.8%.

[0093] In some embodiments, the pH adjuster includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and aqueous ammonia.

[0094] In some embodiments, the amount of the pH adjuster is 0.05% to 0.2% of the mass of the vinylidene fluoride monomer, and can also be 0.1% or 0.15%.

[0095] The third aspect of the present application provides the use of polyvinylidene fluoride defined in one or more of the aforementioned embodiments in the preparation of a binder.

[0096] In a fourth aspect of the present application, a secondary battery is provided, comprising a positive electrode sheet, a separator, and a negative electrode sheet, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet;

[0097] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes the binder of one or more of the aforementioned embodiments.

[0098] In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 0.6% to 1.2%; optionally, the mass percentage of the binder is 0.6% to 0.8%. The amount of the binder can also be, for example, 0.7%, 0.8%, 0.9%, 1% or 1.1%. In traditional technology, the amount of binder in the positive electrode active material layer is often as high as 2.5% or more. When the amount drops to 2% or less, it will significantly affect the bonding between the particles of the positive electrode active material layer, or the bonding between the active material layer and the current collector. The use of the binder in this application can greatly reduce the amount of binder to 1.2% or less compared to traditional technology, without significantly affecting the bonding performance. The low amount of binder effectively improves the brittleness and cycle performance of the electrode.

[0099] In a fifth aspect of the present application, a battery module is provided, comprising a secondary battery according to one or more of the aforementioned embodiments.

[0100] In a sixth aspect of the present application, a battery pack is provided, comprising the battery module as claimed in the preceding claims.

[0101] In a seventh aspect of the present application, an electrical device is provided, comprising one or more of the secondary batteries of one or more of the aforementioned embodiments, the aforementioned battery modules, and the aforementioned battery packs.

[0102] In addition, the secondary battery, battery module, battery pack, and electric device of the present application will be described below with reference to the drawings as appropriate.

[0103] In one embodiment of the present application, a secondary battery is provided.

[0104] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0105] Positive electrode

[0106] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, and the positive electrode active material layer contains the binder provided in the first aspect of the present application.

[0107] As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is provided on either or both of the two facing surfaces of the positive electrode current collector.

[0108] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. 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 may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0109] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used 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 may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), 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 O2 (also referred to as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0110] In some embodiments, the positive electrode active material layer may further optionally include other conventional binders in addition to the binder provided in the first aspect of the present application. As an example, the conventional binder may include at least one of polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0111] In some embodiments, the positive electrode active material layer may further include a conductive agent. For example, 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.

[0112] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed 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 sheet can be obtained.

[0113] Negative electrode

[0114] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0115] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0116] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0117] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode 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, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0118] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of 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).

[0119] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0120] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0121] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0122] electrolytes

[0123] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0124] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0125] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0126] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl 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, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0127] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0128] Isolation film

[0129] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0130] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0131] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0132] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0133] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0134] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG6 shows a secondary battery 5 with a square structure as an example.

[0135] In some embodiments, referring to Figure 7, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0136] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0137] Figure 8 shows an example battery module 4. Referring to Figure 8 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0138] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0139] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0140] Figures 9 and 10 illustrate an example battery pack 1. Referring to Figures 9 and 10 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0141] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto. Among them, mobile devices may be, for example, mobile phones, laptops, etc.; electric vehicles may be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited thereto.

[0142] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0143] Figure 11 shows an example of an electric device 6. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the electric device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module can be used.

[0144] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0145] The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims.

[0146] The present application is further described in detail below with reference to specific examples and comparative examples. For experimental parameters not specified in the following specific examples, reference is made to the guidance given in the application documents, and reference may also be made to the experimental manual of this field or other experimental methods known in the art, or to the experimental conditions recommended by the manufacturer. It is understandable that the instruments and raw materials used in the following examples are more specific, and in other specific embodiments, they are not limited thereto; the weights of the relevant components mentioned in the examples of this application specification can not only refer to the specific content of each component, but can also represent the proportional relationship of the weights between the components. Therefore, as long as the content of the relevant components of the description of the examples of this application is proportionally amplified or reduced, it is within the scope disclosed in the examples of this application specification. Specifically, the weights described in the description of the examples of this application can be mass units known in the chemical and chemical fields such as μg, mg, g, and kg. The reagents or instruments used that do not indicate the manufacturer are conventional products that can be obtained commercially.

[0147] Example 1

[0148] (1) Preparation of polyvinylidene fluoride binder:

[0149] 4 kg of deionized water and 2 g of methyl cellulose ether were added to a 10 L autoclave, and the autoclave was evacuated and O2 was replaced with N2 three times. 5 g of tert-butyl peroxypivalate and 2 g of sodium bicarbonate were added again, and 1 kg of vinylidene fluoride was added to increase the pressure to 7 MPa. The mixture was mixed and stirred for 30 minutes, and the temperature was raised to 45° C. to carry out a polymerization reaction. After 6 hours of polymerization, 30 g of cyclohexane was added to continue the reaction. The reaction was stopped when the pressure in the autoclave dropped to 2 MPa. The reaction system was centrifuged, and the solid phase was collected, washed, and dried to obtain a polyvinylidene fluoride binder.

[0150] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight average molecular weight 1.8 million, polydispersity index 2.1, crystallinity 42%, Dv50 particle size 60 μm. The polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 3600 mPa·s.

[0151] (2) Preparation of positive electrode slurry:

[0152] 3.99 kg of lithium iron phosphate, 32.8 g of the polyvinylidene fluoride binder prepared in step (1), 57.4 g of acetylene black, 12.3 g of a 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 binder in the solute of the positive electrode slurry was 0.8%;

[0153] (3) The positive electrode slurry prepared in step (2) is scraped onto the carbon-coated aluminum foil, baked at 110°C for 15 minutes, cold pressed and cut into discs with a diameter of 15 mm, and then made into a buckle battery with a metal lithium sheet, an isolation membrane, and an electrolyte.

[0154] Examples 2 to 5

[0155] The method is basically the same as Example 1, except that the mass percentages of the polyvinylidene fluoride binder in step (2) are 0.9%, 1%, 1.1% and 1.2% respectively.

[0156] Example 6

[0157] The process is basically the same as Example 1, except that, in step (1), after the polymerization reaction is carried out for 8 h, 25 g of cyclohexane is added to continue the reaction;

[0158] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 2.5 million, polydispersity coefficient of 2.1, crystallinity of 44%, and Dv50 particle size of 80 μm. The polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 4300 mPa·s.

[0159] Examples 7 to 10

[0160] The method is basically the same as Example 6, except that the mass percentages of the polyvinylidene fluoride binder in step (2) are 0.9%, 1%, 1.1% and 1.2% respectively.

[0161] Example 11

[0162] The process is basically the same as Example 1, except that, in step (1), after the polymerization reaction is carried out for 10 h, 20 g of cyclohexane is added to continue the reaction;

[0163] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 3.2 million, polydispersity coefficient of 2.2, crystallinity of 45%, and Dv50 particle size of 100 μm. The polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 4900 mPa·s.

[0164] Examples 12 to 15

[0165] Basically the same as Example 11, except that the mass percentages of the polyvinylidene fluoride binder in step (2) are 0.9%, 1%, 1.1% and 1.2% respectively.

[0166] Example 16

[0167] The method is basically the same as Example 1, except that, in step (1), after the polymerization reaction is carried out for 12 h, 15 g of cyclohexane is added to continue the reaction;

[0168] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 5 million, polydispersity coefficient of 2.2, crystallinity of 46%, and Dv50 particle size of 150 μm. The polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 6500 mPa·s.

[0169] Examples 17-18

[0170] [Corrected 21.08.2024 according to Rule 91] Basically the same as Example 16, except that the mass percentages of the polyvinylidene fluoride binder in step (2) are 0.6% and 0.7% respectively.

[0171] Example 19

[0172] The method is basically the same as Example 1, except that, in step (1), after the polymerization reaction is carried out for 5 h, 33 g of cyclohexane is added to continue the reaction;

[0173] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 1.5 million, polydispersity coefficient of 2.18, crystallinity of 41.5%, and Dv50 particle size of 45 μm. The polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 3000 mPa·s.

[0174] Example 20

[0175] Basically the same as Example 19, except that the mass percentage of the polyvinylidene fluoride binder in step (2) is 1.8%.

[0176] Example 21

[0177] The process is substantially the same as in Example 1, except that the polymerization temperature in step (1) is 65°C;

[0178] [Corrected 21.08.2023 according to Rule 91] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 2 million, polydispersity coefficient of 2.4, crystallinity of 52%, Dv50 particle size of 50 μm; the polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 4000 mPa·s.

[0179] Example 22

[0180] The process is basically the same as Example 1, except that the polymerization reaction time in step (1) is 12 h;

[0181] [Corrected 21.08.2023 according to Rule 91] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 1.83 million, polydispersity coefficient of 2.1, crystallinity of 42%, Dv50 particle size of 60 μm; the polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 3700 mPa·s.

[0182] Example 23

[0183] The process is substantially the same as in Example 1, except that the polymerization pressure in step (1) is 9 MPa;

[0184] [Corrected 21.08.2023 according to Rule 91] The relevant parameters of the polyvinylidene fluoride binder prepared in this example are as follows: weight-average molecular weight of 1.6 million, polydispersity coefficient of 2.15, crystallinity of 42%, Dv50 particle size of 50 μm; the polyvinylidene fluoride binder prepared in this example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 2500 mPa·s.

[0185] Comparative Example 1

[0186] The method is basically the same as Example 1, except that in step (2), the polyvinylidene fluoride binder prepared in step (1) is replaced by commercially available polyvinylidene fluoride with a weight-average molecular weight of 800,000 (701A brand produced by East Sunshine Company), so that the mass percentage of polyvinylidene fluoride in the slurry solute is 2.5%.

[0187] Comparative Example 2

[0188] The process is basically the same as Example 1, except that the polymerization temperature in step (1) is 35°C;

[0189] [Corrected on 21.08.2023 according to Rule 91] The relevant parameters of the polyvinylidene fluoride binder prepared in this comparative example are as follows: weight-average molecular weight of 1.2 million, polydispersity coefficient of 1.8, crystallinity of 40%, Dv50 particle size of 30 μm; the polyvinylidene fluoride binder prepared in this comparative example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 450 mPa·s.

[0190] Comparative Example 3

[0191] The process is basically the same as Example 1, except that the polymerization reaction time in step (1) is 4 h;

[0192] [Corrected on 21.08.2023 according to Rule 91] The relevant parameters of the polyvinylidene fluoride binder prepared in this comparative example are as follows: weight-average molecular weight of 1.1 million, polydispersity coefficient of 1.9, crystallinity of 38%, Dv50 particle size of 25 μm; the polyvinylidene fluoride binder prepared in this comparative example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 232 mPa·s.

[0193] Comparative Example 4

[0194] The method is basically the same as Example 1, except that the polymerization pressure in step (1) is 5 MPa;

[0195] [Corrected on 21.08.2023 according to Rule 91] The relevant parameters of the polyvinylidene fluoride binder prepared in this comparative example are as follows: weight-average molecular weight of 1.4 million, polydispersity coefficient of 2.2, crystallinity of 41%, Dv50 particle size of 40 μm; the polyvinylidene fluoride binder prepared in this comparative example was dissolved in N-methylpyrrolidone solution to prepare a solution with a mass percentage concentration of 4 wt%, and the solution viscosity was measured to be 1800 mPa·s.

[0196] Characterization Tests:

[0197] The polyvinylidene fluoride binders, pole pieces or button cells prepared in the examples and comparative examples were subjected to the following characterization tests:

[0198] (1) Scanning the polyvinylidene fluoride binder with a scanning electron microscope to determine the morphology and particle size;

[0199] (2) Conduct a brittleness test on the electrode. The test steps are as follows:

[0200] The cold pressed electrode is sampled in the horizontal direction, which can be (4cm×25cm vertically) 100cm 2 The number of samples is 3, and the sample area is 100cm 2 (Fold in half lengthwise at 25cm) Pre-fold, place the pre-folded test film on the test bench, roll it once with a 2kg cylindrical roller, and check under the light to see if there are any light leaks. If there are any light leaks, the sample does not meet the requirements. If there are no light leaks, record the number of rolls. Fold the test sample back along the longitudinal crease, observe the crease against the light until a light-transmitting point appears and record the number of rolls n1. Repeat the above operation to obtain the number of light-transmitting rolls n2 and n3 for the second and third strips, and calculate the average number of rolls = (n1 + n2 + n3) / 3;

[0201] (3) Adhesion test, the test steps are as follows:

[0202] Refer to the national standard "Adhesive 180° Peel Strength Test Method" and use a blade to cut a sample with a width of 30mm and a length of 100-160mm. Apply special double-sided tape with a width of 20mm and a length of 90-150mm to the steel plate. Attach the electrode sample cut earlier to the double-sided tape with the test surface facing down, and then use a pressure roller to roll it three times in the same direction.

[0203] Insert a paper tape with the same width as the electrode and 80-200mm longer than the sample length under the electrode and fix it with wrinkle glue;

[0204] Turn on the Sansi tensile testing machine (sensitivity is 1N) and the indicator light will light up. Adjust the limit block to the appropriate position and secure the end of the steel plate without the electrode with the lower clamp. Fold the paper tape upward and secure it with the upper clamp. Use the "up" and "down" buttons on the manual controller included with the tensile testing machine to adjust the position of the upper clamp, then perform the test and read the value.

[0205] (4) Battery capacity retention test, the test steps are as follows:

[0206] The battery is charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C, left for 5 minutes, and then discharged at 1 / 3C to 2.5V. 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 after each cycle is Pn = Cn / C0*100%. With the 100 point values ​​P1, P2...P100 as the vertical coordinates and the corresponding number of cycles as the horizontal coordinates, a curve graph of battery capacity retention rate and cycle number is obtained;

[0207] During the test, the first cycle corresponds to n=1, the second cycle corresponds to n=2, ... and the 100th cycle corresponds to n=100; the battery capacity retention rate data is the data measured after 500 cycles under the above test conditions, that is, the P500 value.

[0208] The above test data are listed in Table 1:

[0209] Table 1

[0210]

[0211] [Corrected 21.08.2023 in accordance with Article 91]

[0212] Analyzing the data in Table 1 and comparing Example 1, Example 6, Example 11, Example 16, and Example 19, it can be seen that as the weight-average molecular weight increases, the Dv50 particle size, bonding force, and cycle performance of the binder generally show an upward trend, while the average number of rolling times of the pole piece will gradually decrease. As can be seen from Example 17, when the weight-average molecular weight of polyvinylidene fluoride reaches 5 million, only 0.6% of the dosage can provide a bonding force of 30.7N / m, and the battery cycle performance is good, but the pole piece brittleness is slightly higher, but it is still better than the commercially available binder (Comparative Example 1). It can be seen that the binder provided in this application can effectively reduce the amount of binder in the positive electrode active material and provide good pole piece toughness, bonding force, and battery cycle performance.

[0213] Comparing Example 1 and Example 21, although the molecular weights of the two are close, the reaction temperature of Example 21 is relatively high, and the crystallinity and polydispersity coefficient of polyvinylidene fluoride prepared by the polymerization reaction are also relatively high, which affects the brittleness of the electrode and significantly reduces the average number of rolling times. Therefore, the polymerization reaction temperature is preferably controlled at 45°C to 60°C, so that the crystallinity can be controlled within the appropriate range of 38% to 48%, and the polydispersity coefficient can be controlled within the more preferred range of 2 to 2.3.

[0214] Comparing Examples 1 to 5, 6 to 10, 11 to 15, 16 to 18, and 19 to 20, it can be seen that as the amount of polyvinylidene fluoride increases, the adhesion and cycle performance increase, but the electrode will gradually become brittle, and the average number of rolling times decreases. In particular, in Example 20, after the amount of binder is greatly increased to 1.8%, the improvement in cycle performance and adhesion becomes limited, but the number of rolling times will be significantly reduced.

[0215] Compared with Example 1, in Example 22, the polymerization reaction time is longer. With the continuous consumption of the monomer and the decrease in pressure, the polymerization conditions are no longer met. Prolonging the reaction time cannot continue the polymerization reaction. Therefore, for energy consumption and efficiency considerations, it is more appropriate to control the reaction time within 10 hours.

[0216] Compared with Example 1, in Example 23, the reaction pressure is higher, the pressure of the monomer entering the reaction liquid is higher, and more monomers enter the reaction liquid, which can lead to the occurrence of a large-scale polymerization reaction and an increase in the amount of polyvinylidene fluoride produced. As the monomer decreases, the polyvinylidene fluoride lacks the supply of monomers and the molecular weight generated is relatively small, which has a certain impact on battery performance.

[0217] Compared with Example 1, in Comparative Example 2, the reaction temperature is relatively low, the driving force of copolymerization is relatively small, the polymerization reaction is insufficient, and the prepared molecular weight is relatively small, resulting in a significant decrease in adhesion at the same addition amount and a significant decrease in cycle performance.

[0218] Compared with Example 1, in Comparative Example 3, the polymerization reaction time is shorter, the polymerization reaction cannot be continued, and the prepared molecular weight is relatively small, which also causes a decrease in adhesion and cycle performance.

[0219] Compared with Example 1, in Comparative Example 4, the reaction pressure is lower, the pressure of the monomer entering the reaction liquid is lower, the reaction monomer cannot be continuously replenished, which is not conducive to the continuous polymerization. The molecular weight of the obtained product is too low to meet the requirements for adhesion, and the battery cycle performance is also reduced.

[0220] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0221] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and such modifications and improvements are all within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be determined by the appended claims.

Claims

1. A binder comprising polyvinylidene fluoride having a weight average molecular weight of 1.5 to 5 million; optionally, the weight average molecular weight of the polyvinylidene fluoride is 1.8 to 3.2 million.

2. The adhesive according to claim 1, characterized in that The polydispersity coefficient of the polyvinylidene fluoride is 1.5 to 2.5; optionally, the polydispersity coefficient of the polyvinylidene fluoride is 2 to 2.

3.

3. The adhesive according to any one of claims 1 to 2, characterized in that The number average molecular weight of the polyvinylidene fluoride is 600,000 to 3,000,000; optionally, the number average molecular weight of the polyvinylidene fluoride is 1,000,000 to 2,000,000.

4. The adhesive according to any one of claims 1 to 3, characterized in that The Dv50 particle size of the polyvinylidene fluoride is 50 μm to 150 μm; optionally, the Dv50 particle size of the polyvinylidene fluoride is 60 μm to 100 μm.

5. The adhesive according to any one of claims 1 to 4, characterized in that: The crystallinity of the polyvinylidene fluoride is 38% to 48%; optionally, the crystallinity of the polyvinylidene fluoride is 40% to 45%.

6. The adhesive according to any one of claims 1 to 5, characterized in that: The viscosity of the binder solution prepared by dissolving the polyvinylidene fluoride in N-methylpyrrolidone is 3500mPa·s to 5000mPa·s, and the mass percentage of the binder in the binder solution is 3% to 5%.

7. The method for preparing the binder according to any one of claims 1 to 6, comprising the following steps: The vinylidene fluoride monomer is polymerized in a non-reactive gas atmosphere, a reaction pressure of 6MPa to 8MPa, and a reaction temperature of 45°C to 60°C for 6h to 10h; A chain transfer agent is added, and when the pressure in the reaction system drops to 2MPa-2.5MPa, the reaction is stopped, the solid and liquid are separated, and the solid phase is retained.

8. The preparation method according to claim 7, characterized in that: The chain transfer agent includes one or more of cyclohexane, isopropanol, methanol and acetone.

9. The preparation method according to any one of claims 7 to 8, characterized in that: The amount of the chain transfer agent used is 1.5% to 3% of the mass of the vinylidene fluoride monomer.

10. The preparation method according to any one of claims 7 to 9, characterized in that: The polymerization reaction comprises the following steps: Adding a solvent and a dispersant into a container, evacuating the container and then filling it with a non-reactive gas; Adding an initiator and a pH adjuster into the container, adjusting the pH value to 6.5-7, and then adding vinylidene fluoride monomer to make the pressure in the container reach 6MPa-8MPa; After stirring for 30 to 60 minutes, the temperature is raised to 45 to 60°C to carry out the polymerization reaction.

11. The preparation method according to claim 10, characterized in that: The amount of the solvent used is 2 to 8 times the mass of the vinylidene fluoride monomer.

12. The preparation method according to any one of claims 10 to 11, characterized in that: The dispersant includes one or more of cellulose ether and polyvinyl alcohol; optionally, the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.

13. The preparation method according to any one of claims 10 to 12, characterized in that: The amount of the dispersant used is 0.1% to 0.3% of the mass of the vinylidene fluoride monomer.

14. The preparation method according to any one of claims 10 to 13, characterized in that: The initiator is an organic peroxide; optionally, the organic peroxide includes one or more of tert-amyl peroxypivalate, tert-amyl peroxypivalate, 2-ethyl peroxydicarbonate, diisopropyl peroxydicarbonate and tert-butyl peroxypivalate.

15. The preparation method according to any one of claims 10 to 14, characterized in that: The amount of the initiator used is 0.15% to 1% of the mass of the vinylidene fluoride monomer.

16. The preparation method according to any one of claims 10 to 15, characterized in that: The pH adjuster includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate and ammonia water.

17. The preparation method according to any one of claims 10 to 16, characterized in that: The amount of the pH regulator is 0.05% to 0.2% of the mass of the vinylidene fluoride monomer.

18. Use of polyvinylidene fluoride as defined in any one of claims 1 to 6 in the preparation of an adhesive.

19. A secondary battery, comprising a positive electrode sheet, a separator and a negative electrode sheet, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet; The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the binder according to any one of claims 1 to 6.

20. The secondary battery according to claim 19, characterized in that: In the positive electrode active material layer, the mass percentage of the binder is 0.6% to 1.2%; optionally, the mass percentage of the binder is 0.6% to 0.8%.

21. A battery module comprising the secondary battery according to any one of claims 19 to 20.

22. A battery pack comprising the battery module according to claim 21.

23. An electrical device comprising one or more of the secondary battery according to any one of claims 19 to 20, the battery module according to claim 21, and the battery pack according to claim 22.

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