Binder and preparation method thereof, positive plate, battery and electric equipment

Abstract

The invention provides a binder and a preparation method thereof, a positive plate, a battery and electric equipment, the binder comprises a polyvinylidene fluoride-g-polyvinylpyrrolidone graft copolymer, polyvinylidene fluoride is a main chain, and polyvinylpyrrolidone is a branched chain; the weight-average molecular weight of the polyvinylpyrrolidone is less than or equal to 60,000. According to the binder provided by the invention, the weight-average molecular weight of branched-chain polyvinylpyrrolidone in the polyvinylidene fluoride-g-polyvinylpyrrolidone grafted copolymer is limited to be less than or equal to 60,000, so that the physical network structure of an electrode active material and PVDF can be weakened, the viscosity of slurry is reduced, more hydrogen bonds can be provided, intramolecular hydrogen bonds formed by PVDF are reduced, and the service life of the electrode active material is prolonged. The hydrogen bond formed by PVDF and the current collector is improved, so that the peeling strength of the pole piece is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to a binder and a preparation method thereof, a positive electrode sheet, a battery and an electrical device. Background Art

[0002] Binders can improve peel strength by increasing branching or molecular weight, but this can lead to a sudden increase in slurry viscosity, severely impacting slurry processing performance. Adding dispersants can reduce slurry viscosity, but this also reduces the peel strength of the binder. Therefore, developing a binder that can effectively reduce slurry viscosity while simultaneously improving electrode peel strength is a pressing technical challenge. Summary of the Invention

[0003] The main purpose of the present invention is to provide an adhesive that can reduce the viscosity of the slurry and improve the peeling strength of the electrode.

[0004] The present invention also provides a method for preparing the adhesive, which can prepare the above-mentioned adhesive, has simple process and low cost.

[0005] The present invention also provides a positive electrode sheet comprising the above-mentioned binder, so that the positive electrode sheet has a greater peeling strength.

[0006] The present invention also provides a battery comprising the above-mentioned positive electrode sheet, so the positive electrode sheet of the battery has a greater peeling strength.

[0007] The present invention also provides an electrical device comprising the above-mentioned battery. Therefore, the performance of the battery associated with the electrical device is relatively excellent.

[0008] In a first aspect, the present invention provides a binder comprising a polyvinylidene fluoride-g-polyvinyl pyrrolidone graft copolymer, wherein the polyvinylidene fluoride is a main chain and the polyvinyl pyrrolidone is a side chain;

[0009] The weight average molecular weight of the polyvinyl pyrrolidone is less than or equal to 60,000.

[0010] In the binder as described above, the weight average molecular weight of the polyvinyl pyrrolidone is 15,000-40,000.

[0011] In the binder as described above, the weight average molecular weight of the polyvinylidene fluoride is 600,000-900,000.

[0012] In the binder as described above, the weight average molecular weight of the polyvinylidene fluoride is 600,000-700,000.

[0013] In the binder as described above, the mass percentage of the polyvinyl pyrrolidone in the binder is 7%-50%.

[0014] In the binder as described above, the mass percentage of the polyvinyl pyrrolidone in the binder is 15%-50%.

[0015] In a second aspect, the present invention provides a method for preparing the binder as described above, comprising the following steps:

[0016] The binder is obtained by grafting a raw material system including polyvinylidene fluoride, N-vinyl pyrrolidone and a polymerization inhibitor.

[0017] The preparation method of the binder as described above, wherein the mass ratio of the polyvinylidene fluoride to the N-vinyl pyrrolidone is 1:(0.1-2), preferably 1:(0.9-1.4);

[0018] and / or, the polymerization inhibitor comprises one or more of copper salts, ammonium salts, and iron salts, preferably ferrous ammonium sulfate;

[0019] And / or, the concentration of the polymerization inhibitor in the raw material system is 1 mmol / L-30 mmol / L, preferably 3 mmol / L-5 mmol / L.

[0020] The preparation method of the binder as described above, wherein the grafting treatment includes radiation grafting treatment;

[0021] The radiation dose of the radiation grafting treatment is 20kGy-45kGy, preferably 28kGy-40kGy;

[0022] And / or, the average dose rate of the radiation grafting treatment is 0.1 kGy / h-0.4 kGy / h, preferably 0.1 kGy / h-0.23 kGy / h.

[0023] In a third aspect, the present invention provides a positive electrode sheet comprising a positive electrode active material layer, wherein the positive electrode active material layer comprises the binder as described above or the binder prepared by the preparation method as described above.

[0024] In the positive electrode sheet as described above, the binder accounts for 2%-15% by mass of the positive electrode active material layer, preferably 2%-5% by mass.

[0025] The positive electrode sheet as described above, wherein the positive electrode active material layer further comprises a positive electrode active material and a conductive agent, and the positive electrode active material accounts for 65%-97% by mass of the positive electrode active material layer, preferably 90%-97% by mass;

[0026] And / or, the conductive agent accounts for 1%-20% by mass of the positive electrode active material layer, preferably 1%-5% by mass.

[0027] In a fourth aspect, the present invention provides a battery comprising the positive electrode sheet as described above.

[0028] In a fifth aspect, the present invention provides an electrical device comprising the battery as described above.

[0029] The binder provided by the present invention includes a polyvinylidene fluoride (PVDF)-g-polyvinyl pyrrolidone (PVP) graft copolymer, and the weight-average molecular weight of the branched polyvinyl pyrrolidone (PVP) is less than or equal to 60,000. It can weaken the physical network structure between the electrode active material and PVDF, reduce the viscosity of the slurry, and provide more hydrogen bonds, reduce the intramolecular hydrogen bonds formed by PVDF, and increase the hydrogen bonds formed between PVDF and the current collector, thereby improving the peel strength of the electrode. DETAILED DESCRIPTION

[0030] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0031] In the manufacturing process of lithium-ion batteries, the selection and optimization of cathode binders is a key factor affecting battery performance and production efficiency. Traditionally, polyvinylidene fluoride (PVDF) has been widely used as a cathode binder due to its excellent chemical stability and electrochemical properties. However, with the increasing requirements for battery energy density and production costs, reducing binder usage while maintaining or increasing battery power and capacity has become a major issue within the industry.

[0032] As a semi-crystalline polymer, the hydrogen bonds in PVDF's molecular structure are primarily used for intramolecular crystallization, which limits the formation of hydrogen bonds with active materials and current collectors, thereby affecting the bonding effect. To improve peel strength, common modification methods include increasing the branching degree or molecular weight of PVDF. However, while these methods can improve bonding strength, they often lead to a significant increase in slurry viscosity, which in turn affects the slurry's processing performance.

[0033] To address the problem of high slurry viscosity, dispersants such as polyvinylpyrrolidone (PVP) are often added to reduce the viscosity. However, the incompatibility of PVDF and PVP in N-methylpyrrolidone (NMP) solvent leads to macroscopic phase separation, weakening the respective functions of the binder and dispersant, further limiting the performance optimization of the electrode.

[0034] The inventors of the present application have found that increasing the number of adsorption sites of the binder, reducing the formation of hydrogen bonds within the binder's molecules, and inhibiting its crystallization behavior are beneficial for reducing the amount of binder used while maintaining or improving the peel strength of the electrode and reducing the slurry viscosity.

[0035] Based on this, in the first aspect, the present invention provides a binder comprising a polyvinylidene fluoride (PVDF)-g-polyvinyl pyrrolidone (PVP) graft copolymer, wherein polyvinylidene fluoride (PVDF) is the main chain and polyvinyl pyrrolidone (PVP) is the side chain; the weight-average molecular weight of polyvinyl pyrrolidone (PVP) is less than or equal to 60,000, for example, it can be 10,000, 20,000, 30,000, 40,000, 50,000, 60,000 or a range consisting of any two of them.

[0036] The binder provided by the present invention includes a PVDF-g-PVP graft copolymer, the weight-average molecular weight of its branched PVP is less than or equal to 60,000, which can reduce the viscosity of the slurry and improve the peeling strength of the electrode. This is because the use of PVP with a weight-average molecular weight less than or equal to 60,000 as a branch suppresses the gel risk brought by PVDF. The PVP branch can weaken the physical network structure of the electrode active material and PVDF through competitive adsorption, thereby reducing the viscosity of the slurry. At the same time, the carbonyl groups contained in the PVP branch can form more hydrogen bonds with the active material and the current collector surface. The introduction of the PVP branch reduces the regularity of the PVDF molecular chain structure, making it difficult for PVDF to form intramolecular hydrogen bonds, which in turn makes it difficult for PVDF to crystallize. More fluorine atoms in the molecular chain are able to form hydrogen bonds with the current collector, ultimately increasing the number of sites for hydrogen bond adsorption on the entire polymer chain, thereby improving the peeling strength of the electrode.

[0037] Therefore, the binder provided by the present invention includes a polyvinylidene fluoride (PVDF)-g-polyvinyl pyrrolidone (PVP) graft copolymer, and the weight-average molecular weight of the branched polyvinyl pyrrolidone (PVP) is less than or equal to 60,000, which can weaken the physical network structure between the electrode active material and PVDF, reduce the viscosity of the slurry, and provide more hydrogen bonds, reduce the intramolecular hydrogen bonds formed by PVDF, and increase the hydrogen bonds formed between PVDF and the current collector, thereby improving the peel strength of the electrode.

[0038] In the present invention, the weight average molecular weight of PVP can be tested by two methods:

[0039] A. Nuclear Magnetic Resonance (NMR)

[0040] Comparing the grafted copolymer and the unmodified PVDF sample, the ratio of the hydrogen spectrum signal integrated area of ​​the -CH2CF2- unit in PVDF to the pyrrolidone ring in PVP was analyzed, and the weight-average molecular weight of PVP was calculated based on the grafting rate. PVP weight-average molecular weight = (grafting rate × molecular weight of PVDF repeating unit) / (PVP hydrogen spectrum signal peak integrated area / PVDF hydrogen spectrum signal peak integrated area)

[0041] X-ray photoelectron spectroscopy (XPS)

[0042] The branch density and branch length structural characteristics of the grafted copolymer were obtained by X-ray photoelectron spectroscopy (XPS) and grafting rate determination.

[0043] 1) In the XPS energy spectrum of polyvinylidene fluoride-g-polyvinyl pyrrolidone graft copolymer, the peaks at 688.5eV and 290.5eV correspond to the fluorine element and carbon element (-CF2) on the PVDF main chain, respectively, and the corresponding element percentages [F] and [C] can be obtained. The peaks at 399eV and 531eV correspond to the nitrogen element (pyrrolidone ring) and oxygen element (C=O) on the PVP side chain, respectively, and the element percentages [N] and [O] can also be obtained. The branch density of the substance to be characterized can be obtained as follows: Γ = ([N] / [F]) × ((ρ PVDF ×N A ) / (M PVP单元 ×d XPS ));

[0044] Among them, ρ PVDF is the density of PVDF; N A is Avogadro's constant; M PVP单元 is the molecular weight of the PVP repeating unit (calculated based on the molecular structure of the repeating unit and the relative molecular mass of the corresponding atoms); d XPS XPS detection depth is 5-10nm.

[0045] 2) Divide the grafting rate by the branch density to obtain the branch length of the substance to be characterized. The formula is as follows: branch length = GR / Γ, PVP weight average molecular weight = M PVP单元 × branch length. The grafting rate GR is determined by referring to the mass percentage of PVP in the binder (grafting rate).

[0046] In some embodiments of the present invention, the weight average molecular weight of polyvinylpyrrolidone (PVP) is 15,000-40,000, for example, 15,000, 20,000, 25,000, 30,000, 35,000, 40,000, or any two thereof.

[0047] As a preferred solution, when the weight-average molecular weight of PVP is 15,000-40,000, the slurry viscosity can be further reduced and the electrode peeling strength can be improved.

[0048] In some embodiments of the present invention, the weight average molecular weight of polyvinylidene fluoride (PVDF) is 600,000-900,000, for example, 600,000, 650,000, 700,000, 750,000, 800,000, 850,000, 900,000, or any two thereof.

[0049] The PVDF in this invention has a weight-average molecular weight of 600,000-900,000, providing stronger adhesion, ensuring a secure bond between the active material and the current collector, improving the electrode peel strength, and enhancing the battery's cycle life and reliability. It also improves the material's mechanical strength and toughness, and offers high chemical stability.

[0050] In the present invention, the weight-average molecular weight of polyvinylidene fluoride (PVDF) can be determined using gel permeation chromatography (GPC) coupled with thermogravimetric analysis (TGA). GPC measures the molecular weight W of the graft copolymer, which is then measured using TGA in a nitrogen atmosphere by heating the temperature from room temperature to the decomposition temperature of PVP (~300°C), recording the mass loss as W3. The weight loss ratio, A, is calculated as W3 / W, and the weight-average molecular weight of PVDF is calculated as (1-A) × W.

[0051] In some embodiments of the present invention, the weight average molecular weight of polyvinylidene fluoride (PVDF) is 600,000-700,000, for example, 600,000, 610,000, 620,000, 630,000, 640,000, 650,000, 660,000, 670,000, 680,000, 690,000, 700,000, or any two thereof.

[0052] As a preferred solution, it can provide sufficient bonding force to ensure a strong bond between the active material and the current collector, improve the peel strength of the electrode sheet, and enhance the cycle life and reliability of the battery. It can also improve the mechanical strength and toughness of the material and increase its chemical stability.

[0053] In some embodiments of the present invention, the mass percentage of polyvinylpyrrolidone (PVP) in the binder, that is, the grafting rate, is 7%-50%. For example, it can be 7%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or any two thereof.

[0054] In the present invention, the mass percentage of PVP in the binder within the above range can effectively weaken the physical network structure between the electrode active material and PVDF, reducing the slurry viscosity. It can also provide more hydrogen bonds, reduce the intramolecular hydrogen bonds formed by PVDF, and enhance the hydrogen bonds formed between PVDF and the current collector, thereby improving the electrode peel strength.

[0055] In the present invention, the mass percentage of polyvinylpyrrolidone (PVP) in the binder can be determined using thermogravimetric analysis (TGA). In a nitrogen atmosphere, the temperature is raised from room temperature to the PVP decomposition temperature (~300°C), and the mass loss W1 is recorded. The temperature is then raised to the PVDF decomposition temperature (~450°C), and the mass loss W2 is recorded. The mass percentage of PVP in the binder is calculated based on the weight loss ratios of the two stages, i.e., the grafting ratio GR = W1 / (W1 + W2).

[0056] In some embodiments of the present invention, the mass percentage of polyvinylpyrrolidone (PVP) in the binder is 15%-50%, for example, it can be 15%, 17%, 20%, 22%, 25%, 27%, 30%, 35%, 40%, 45%, 50% or any two thereof.

[0057] As a preferred solution, the slurry viscosity can be further reduced and the electrode peeling strength can be improved.

[0058] The preparation method of the binder of the present invention can be radiation grafting treatment and / or ozone activated grafting treatment, preferably radiation grafting treatment.

[0059] In a second aspect, the present invention provides a method for preparing the binder as described above, comprising the following steps:

[0060] A raw material system including polyvinylidene fluoride, N-vinyl pyrrolidone (NVP) and a polymerization inhibitor is grafted to obtain a binder.

[0061] The present invention adopts a grafting treatment method to modify PVDF powder, uses NVP monomer as a grafting monomer, and the presence of a polymerization inhibitor can control the grafting rate and the weight average molecular weight of PVP in the final binder.

[0062] The preparation method of the adhesive of the present invention can prepare the above-mentioned adhesive, and the adhesive can reduce the viscosity of the slurry and improve the peeling strength of the electrode piece.

[0063] In some embodiments of the present invention, the mass ratio of polyvinylidene fluoride to N-vinyl pyrrolidone is 1:(0.1-2), for example, it can be 1:0.1, 1:0.3, 1:0.9, 1:1.2, 1:1.4, 1:1.7, 1:2 or a range consisting of any two thereof, preferably 1:(0.9-1.4).

[0064] The mass ratio of polyvinylidene fluoride to N-vinyl pyrrolidone (NVP) is within the above range, which can ensure that NVP forms sufficient branches on the PVDF main chain to improve the performance of the binder, while avoiding phase separation or unnecessary viscosity increase caused by excessive NVP.

[0065] In some embodiments, the polymerization inhibitor includes one or more of copper salts, ammonium salts, and iron salts, preferably ferrous ammonium sulfate; the iron salt includes ferrous ammonium sulfate. Ferric ammonium sulfate is a polymerization inhibitor with a wide source and mild reaction, which is conducive to the smooth progress of the reaction.

[0066] In some embodiments, the concentration of the polymerization inhibitor in the raw material system is 1 mmol / L-30 mmol / L, for example, it can be 1 mmol / L, 3 mmol / L, 5 mmol / L, 8 mmol / L, 10 mmol / L, 15 mmol / L, 20 mmol / L, 25 mmol / L, 30 mmol / L or any two thereof, preferably 3 mmol / L-5 mmol / L.

[0067] The concentration of the polymerization inhibitor in the raw material system is within the above range, which can control the reaction rate of the grafting treatment and prevent excessive homopolymerization of NVP or uneven grafting. An appropriate amount of polymerization inhibitor can ensure the controllability of the reaction and the uniformity of the product.

[0068] In some embodiments of the present invention, the grafting process comprises an irradiation grafting process.

[0069] The present invention employs an irradiation grafting process, using gamma rays generated by cobalt as an initiator. After irradiation, the PVDF molecular chains generate free radicals, which initiate polymerization of NVP monomers to produce a PVDF-g-PVP graft copolymer. Specifically, purified and dried PVDF powder is added to an irradiation tube, followed by distilled water, a polymerization inhibitor, and NVP monomers. The resulting aqueous solution is then allowed to stand in a nitrogen atmosphere. The tube is then sealed and irradiated at room temperature. After irradiation, the sample is purified to produce the PVDF-g-PVP graft copolymer.

[0070] The radiation dose for the irradiation grafting treatment is 20 kGy-45 kGy, for example, 20 kGy, 22 kGy, 24 kGy, 28 kGy, 30 kGy, 34 kGy, 38 kGy, 40 kGy, 45 kGy or any two thereof, preferably 28 kGy-40 kGy.

[0071] In some embodiments, the average dose rate of the irradiation grafting treatment is 0.1 kGy / h-0.4 kGy / h, for example, it can be 0.1 kGy / h, 0.14 kGy / h, 0.18 kGy / h, 0.23 kGy / h, 0.26 kGy / h, 0.32 kGy / h, 0.4 kGy / h or a range consisting of any two thereof, preferably 0.1 kGy / h-0.23 kGy / h.

[0072] The irradiation dose and average dose rate of the irradiation grafting treatment are within the above ranges, which can ensure sufficient energy input to initiate the polymerization reaction while avoiding excessively high dose rates that may cause material degradation or unnecessary side reactions.

[0073] In a third aspect, the present invention provides a positive electrode sheet comprising a positive electrode active material layer, wherein the positive electrode active material layer comprises the binder as described above or the binder prepared by the preparation method as described above.

[0074] The positive electrode sheet of the present invention includes a positive electrode active material layer, and the positive electrode active material layer includes the binder as described above. Therefore, the positive electrode sheet has a relatively high peeling strength.

[0075] In some embodiments of the present invention, the binder accounts for 2%-15% by mass of the positive electrode active material layer, for example, it can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any two thereof, preferably 2%-5%.

[0076] In the present invention, the mass ratio of the binder in the positive electrode active material layer is within the above range, which can further improve the peel strength of the electrode sheet.

[0077] In some embodiments, the positive electrode active material layer further includes a positive electrode active material and a conductive agent, and the mass percentage of the positive electrode active material in the positive electrode active material layer is 65%-97%, for example, it can be 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97% or any two thereof, preferably 90%-97%.

[0078] In some embodiments, the conductive agent accounts for 1%-20% by mass of the positive electrode active material layer. For example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or a range consisting of any two of them, preferably 1%-5%.

[0079] The present invention does not particularly limit the specific type of positive electrode active material, and conventional materials in the field can be selected. For example, it can be selected from at least one of lithium cobalt oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich manganese-based, ternary active materials, sulfur, sulfur complexes, lithium iron sulfate, lithium ion fluorophosphate, lithium vanadium fluorophosphate, lithium iron fluorophosphate, and lithium manganese oxide; preferably, lithium iron phosphate.

[0080] The present invention does not particularly limit the specific type of the conductive agent, and conventional materials in the art can be selected, for example, at least one selected from carbon black, graphite, carbon nanotubes, Super P, and acetylene black, preferably graphite and / or carbon nanotubes.

[0081] The method for preparing the positive electrode sheet of the present invention may include the following steps:

[0082] First, mix the binder and organic solvent at a stirring speed of 600-1000 rpm for 0.5-1.5 hours. Then, add the conductive agent, stirring at a speed of 800-1000 rpm, and mix for 15-40 minutes. Then, add a portion of the positive electrode active material, for example, 30%-50% by weight, to the solution and continue mixing for 0.3-0.6 hours, maintaining a constant stirring speed. Finally, add the remaining positive electrode active material and continue mixing for 0.4-1 hour to form a positive electrode slurry. The organic solvent includes at least one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide, preferably N-methylpyrrolidone. The organic solvent may comprise 10%-80% of the total mass of the positive electrode slurry, for example, 10%, 20%, 30%, 35%, 40%, 50%, 60%, 70%, 80%, or any combination thereof, preferably 35%-50%.

[0083] The positive electrode slurry is applied to at least one surface of the positive electrode current collector at a coating rate of 10 mm / min to 80 mm / min, for example, 10 mm / min, 20 mm / min, 25 mm / min, 30 mm / min, 40 mm / min, 50 mm / min, 60 mm / min, 70 mm / min, 80 mm / min, or a range consisting of any two thereof, preferably 25 mm / min to 40 mm / min. Drying and roll pressing are performed to form a positive electrode sheet. The drying temperature is 40°C to 90°C, for example, 40°C, 50°C, 60°C, 65°C, 70°C, 80°C, 85°C, 90°C, or a range consisting of any two thereof, preferably 65°C to 85°C; the drying time is 10 min to 60 min, for example, 10 min, 15 min, 20 min, 25 min, 30 min, 40 min, 50 min, 60 min, or a range consisting of any two thereof, preferably 15 min to 25 min. The positive electrode current collector includes aluminum foil and / or carbon-coated aluminum foil, preferably carbon-coated aluminum foil.

[0084] In a fourth aspect, the present invention provides a battery comprising the positive electrode sheet as described above. The battery has advantages corresponding to the positive electrode sheet described above, which will not be described in detail.

[0085] The battery of the present invention includes, in addition to the positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. The composition of the negative electrode sheet can refer to conventional negative electrode sheets in the art, and the separator can also be a separator commonly used in the art, such as PP film, PE film, etc.

[0086] The battery of the present invention can be prepared by conventional methods in the field. Specifically, the positive electrode sheet, the separator and the negative electrode sheet can be stacked in sequence, and then the battery core can be obtained through a lamination or winding process, and then through baking, liquid injection, formation, packaging and other processes to obtain the above-mentioned battery.

[0087] The batteries of the present application may include battery cells, battery modules, and battery packs. In some embodiments, battery cells may be assembled into a battery module, which may contain one or more battery cells, with the specific number selected by those skilled in the art based on the application and capacity of the battery module. In some embodiments, battery modules may also be assembled into a battery pack, which may contain one or more battery modules, with the specific number selected by those skilled in the art based on the application and capacity of the battery pack.

[0088] There is no particular restriction on the specific type of battery in this application. For example, from the perspective of shape, the battery includes but is not limited to square shell batteries, soft pack batteries and cylindrical batteries, etc., and this application does not impose any particular restrictions. From the perspective of the core structure, the core of the battery can be a wound core (i.e., the positive electrode sheet, the negative electrode sheet and the separator are stacked and then wound to form the core), or it can be a laminated core (i.e., multiple positive electrode sheets, negative electrode sheets and separators are stacked to form the core). The outer shell can be a hard shell (such as a steel shell, a hard plastic shell, etc.), or a soft shell (such as an aluminum plastic film, a bag-type soft shell, etc.). This application does not impose any particular restrictions.

[0089] In a fifth aspect, the present invention provides an electrical device comprising the battery as described above. The electrical device has advantages corresponding to those of the positive electrode sheet described above, which will not be described in detail.

[0090] The electrical equipment of the present invention can be conventional electrical equipment in the field, such as power equipment (such as electric vehicles), electronic equipment (such as computers, mobile phones, digital cameras, printers, fax machines, etc.), wearable devices (such as watches, bracelets, VR glasses, etc.), household appliances (such as air conditioners, refrigerators, washing machines, microwave ovens, etc.), etc., without special restrictions.

[0091] The technical solution of the present invention is further described below with reference to specific embodiments.

[0092] Example 1

[0093] The preparation method of the binder of this embodiment comprises the following steps:

[0094] PVDF with a weight-average molecular weight of 600,000 was selected. Purified and dried PVDF powder was added to an irradiation tube. Distilled water, the polymerization inhibitor ammonium ferrous sulfate, and NVP monomer were added to form an aqueous solution (i.e., the raw material system) and allowed to stand in a nitrogen atmosphere. The irradiation tube was then sealed and irradiated at room temperature. After completion of the irradiation, the sample was purified to obtain a PVDF-g-PVP graft copolymer. Irradiation was initiated using cobalt-generated gamma rays. The mass ratio of PVDF to NVP was 1:0.9, the irradiation dose for the grafting treatment was 36 kGy, and the average dose rate was 0.23 kGy / h. The concentration of the polymerization inhibitor ammonium ferrous sulfate in the raw material system was 3 mmol / L. In the PVDF-g-PVP graft copolymer, PVDF served as the backbone, and PVP served as the side chains. The weight-average molecular weight of the PVP side chains was 40,000, and the PVP side chains accounted for 15% of the total weight of the PVDF-g-PVP graft copolymer.

[0095] Preparation of the positive electrode slurry: Mix the binder PVDF-g-PVP graft copolymer with N-methylpyrrolidone at a stirring speed of 600 rpm for 1.5 hours. Then, add the conductive carbon black and carbon nanotube mixture at a stirring speed of 800 rpm and mix for 30 minutes. Then, add 50% by weight of the positive electrode active material lithium iron phosphate to the solution and continue mixing for 0.5 hours, maintaining a constant speed. Finally, add the remaining 50% of the positive electrode active material lithium iron phosphate and continue mixing until a positive electrode slurry is formed. The mass ratio of the positive electrode active material, binder, conductive agent, and solvent is 96.17:2.68:1.15:35.

[0096] Preparation of the positive electrode sheet: The positive electrode slurry was applied to both surfaces of the positive electrode current collector carbon-coated aluminum foil, dried at 85°C for 25 minutes, and roll-pressed to obtain the positive electrode sheet. The positive electrode sheet includes a positive electrode active material layer, which includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material accounts for 96.17% by mass of the positive electrode active material layer, the binder accounts for 2.68% by mass of the positive electrode active material layer, and the conductive agent accounts for 1.15% by mass of the positive electrode active material layer.

[0097] Example 2

[0098] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 2 are basically the same as those of Example 1, except that PVDF with a weight-average molecular weight of 650,000 is selected as the raw material.

[0099] Example 3

[0100] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 3 are basically the same as those of Example 1, except that PVDF with a weight-average molecular weight of 700,000 is selected as the raw material.

[0101] Example 4

[0102] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 4 are basically the same as those of Example 1, except that PVDF with a weight-average molecular weight of 900,000 is selected as the raw material.

[0103] Example 5

[0104] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 5 are basically the same as those of Example 1, except that the mass ratio of PVDF to NVP is 1:1.4, the irradiation dose of the irradiation grafting treatment is 38 kGy, the average dose rate of the irradiation grafting treatment is 0.23 kGy / h, the concentration of the inhibitor ammonium ferrous sulfate in the raw material system is 5 mmol / L, and the weight average molecular weight of PVP in the obtained binder is 50,000.

[0105] Example 6

[0106] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 6 are basically the same as those of Example 1, except that the mass ratio of PVDF to NVP is 1:1.4, the irradiation dose of the irradiation grafting treatment is 40 kGy, the average dose rate of the irradiation grafting treatment is 0.18 kGy / h, the concentration of the inhibitor ammonium ferrous sulfate in the raw material system is 5 mmol / L, and the weight average molecular weight of PVP in the obtained binder is 60,000.

[0107] Example 7

[0108] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 7 are basically the same as those of Example 1, except that the irradiation dose of the irradiation grafting treatment is 24 kGy, the concentration of the inhibitor ammonium ferrous sulfate in the raw material system is 2 mmol / L, and the mass percentage of the PVP side chains in the obtained binder to the mass percentage of the PVDF-g-PVP graft copolymer is 7%.

[0109] Example 8

[0110] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 8 are basically the same as those of Example 1, except that the mass ratio of PVDF to NVP is 1:1.2, the irradiation dose of the irradiation grafting treatment is 28 kGy, the average dose rate of the irradiation grafting treatment is 0.18 kGy / h, and the mass percentage of the PVP side chains in the obtained binder to the mass percentage of the PVDF-g-PVP graft copolymer is 30%.

[0111] Example 9

[0112] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 9 are basically the same as those of Example 1, except that the mass ratio of PVDF to NVP is 1:1.2, the concentration of the inhibitor ammonium ferrous sulfate in the raw material system is 4.5 mmol / L, and the mass percentage of the PVP side chain to the mass percentage of the PVDF-g-PVP graft copolymer is 50%.

[0113] Example 10

[0114] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 10 are basically the same as those of Example 1, except that the mass percentage of the positive electrode active material in the positive electrode active material layer is 93.85%, the mass percentage of the binder in the positive electrode active material layer is 5%, and the mass percentage of the conductive agent in the positive electrode active material layer is 1.15%.

[0115] Example 11

[0116] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Example 11 are basically the same as those of Example 1, except that the mass percentage of the positive electrode active material in the positive electrode active material layer is 83.85%, the mass percentage of the binder in the positive electrode active material layer is 15%, and the mass percentage of the conductive agent in the positive electrode active material layer is 1.15%.

[0117] Comparative Example 1

[0118] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Comparative Example 1 are basically the same as those of Example 1, except that the binder is a PVDF homopolymer (selected from H60-R) with a weight-average molecular weight of 600,000.

[0119] Comparative Example 2

[0120] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Comparative Example 2 are basically the same as those of Example 1, except that the binder is a PVDF-PVP block copolymer.

[0121] Comparative Example 3

[0122] The preparation method of the binder, positive electrode slurry, and positive electrode sheet of Comparative Example 3 is basically the same as that of Example 1, except that the mass ratio of PVDF to NVP is 1:1.4, the irradiation dose of the irradiation grafting treatment is 45 kGy, the average dose rate of the irradiation grafting treatment is 0.13 kGy / h, the concentration of the inhibitor ammonium ferrous sulfate in the raw material system is 5 mmol / L, and the weight average molecular weight of the branched PVP in the obtained binder is 70,000.

[0123] Comparative Example 4

[0124] The preparation methods of the binder, positive electrode slurry, and positive electrode sheet of Comparative Example 4 are basically the same as those of Example 1, except that the binder is a PVDF homopolymer with a weight-average molecular weight of 900,000.

[0125] Test example:

[0126] 1. PVP mass percentage in binder: Using thermogravimetric analysis (TGA), in a nitrogen atmosphere, heat the sample from room temperature to the PVP decomposition temperature (~300°C), recording the mass loss (W1). Continue heating the sample to the PVDF decomposition temperature (~450°C), recording the mass loss (W2). Calculate the PVP mass percentage in the binder based on the weight loss ratio of the two stages, i.e., the grafting ratio (GR) = W1 / (W1+W2).

[0127] 2. PVP weight average molecular weight (two methods):

[0128] A. Nuclear Magnetic Resonance (NMR)

[0129] (1) Compare the NMR spectra of the graft copolymer and homopolymer PVDF samples, analyze the ratio of the integrated area of ​​the hydrogen spectrum signal of the -CH2CF2- unit in PVDF and the pyrrolidone ring in PVP, and calculate the side chain molecular weight based on the grafting rate.

[0130] (2) Side chain molecular weight = (grafting rate × molecular weight of PVDF repeating unit) / (PVP hydrogen spectrum signal peak integral area / PVDF hydrogen spectrum signal peak integral area)

[0131] X-ray photoelectron spectroscopy (XPS)

[0132] The graft copolymer was subjected to X-ray photoelectron spectroscopy (XPS) and grafting rate determination, and its branch density and branch length structural characteristics were obtained through analysis and calculation.

[0133] (1) In the XPS energy spectrum of polyvinylidene fluoride-g-polyvinyl pyrrolidone graft copolymer, the peaks at 688.5 eV and 290.5 eV correspond to the fluorine element and carbon element (-CF2) on the PVDF main chain, respectively, and the corresponding element percentages [F] and [C] can be obtained. The peaks at 399 eV and 531 eV correspond to the nitrogen element (pyrrolidone ring) and oxygen element (C=O) on the PVP side chain, respectively, and the element percentages [N] and [O] can also be obtained. The branch density of the substance to be characterized can be obtained as follows:

[0134] Γ = ([N] / [F]) × ((ρ PVDF ×N A ) / (M PVP单元 ×d XPS )

[0135] Among them, ρ PVDFis the density of PVDF; N A is Avogadro's constant; M PVP单元 is the molecular weight of the PVP repeating unit (calculated based on the molecular structure of the repeating unit and the relative molecular mass of the corresponding atoms); d XPS XPS detection depth is 5-10nm.

[0136] (2) The branch length of the substance to be characterized can be obtained by dividing the grafting rate by the branch density. The formula is as follows: branch length = GR / Γ, branch molecular weight = M PVP单元 × branch length.

[0137] 3. PVDF weight-average molecular weight: Gel permeation chromatography (GPC) coupled with thermogravimetric analysis (TGA) was used to determine the molecular weight (W) of the graft copolymer. TGA was then used to measure the mass loss (W3) by heating the polymer from room temperature to the decomposition temperature of PVP (~300°C) in a nitrogen atmosphere. The weight loss ratio (A) was calculated as W3 / W. PVDF weight-average molecular weight = (1-A) × W.

[0138] 4. Viscosity test: The viscosity of the positive electrode slurry was tested using a rotational rheometer. The test conditions were: room temperature (25°C), shear rate range (0.01s -1 -800s -1 ), rotor diameter (20 mm), and parallel plate spacing (1000 μm).

[0139] 5. Peel Strength Test: Peel strength testing was performed using a tensile testing machine in accordance with the national standard GB / T2792-2014. The positive electrode sheets were cut into 100 mm long and 40 mm wide specimens to prepare test pieces. Adhesive tape was applied to the electrode-coated surface of the test pieces, using aluminum foil as the "flexible adhesive material." A 180-degree peel test was performed at a tensile speed of 100 mm / min to measure the peel strength.

[0140] Table 1

[0141]

[0142] As can be seen from Table 1, compared with the comparative example, the binder provided by the present invention includes a polyvinylidene fluoride (PVDF)-g-polyvinyl pyrrolidone (PVP) graft copolymer, and the weight average molecular weight of the branched polyvinyl pyrrolidone (PVP) is less than or equal to 60,000, which can weaken the physical network structure of the electrode active material and PVDF, reduce the slurry viscosity, and provide more hydrogen bonds, reduce the intramolecular hydrogen bonds formed by PVDF, and increase the hydrogen bonds formed between PVDF and the current collector, thereby improving the electrode peeling strength.

[0143] Compared with Comparative Example 1, the PVDF-g-PVP graft copolymer of Example 1 can reduce the viscosity of the slurry and improve the peel strength of the electrode.

[0144] It can be seen from Examples 1 to 4 that as the weight-average molecular weight of PVDF increases, the electrode peeling strength gradually increases.

[0145] It can be seen from Example 1, Example 7 and Example 8 that as the mass percentage of the PVP side chains in the PVDF-g-PVP graft copolymer increases, the slurry viscosity gradually decreases.

[0146] Finally, it should be noted that those skilled in the art will readily identify other embodiments of the present invention after considering the specification and practicing the invention disclosed herein. The present invention is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary techniques in the art not disclosed herein. The present invention is not limited to the precise structure described above and may be modified and altered in various ways without departing from the scope thereof. The scope of the present invention is limited solely by the appended claims.

Claims

1. A binder, characterized in that: It includes polyvinylidene fluoride-g-polyvinyl pyrrolidone graft copolymer, wherein polyvinylidene fluoride is the main chain and polyvinyl pyrrolidone is the side chain; The weight average molecular weight of the polyvinyl pyrrolidone is less than or equal to 60,000.

2. The adhesive according to claim 1, characterized in that The weight average molecular weight of the polyvinyl pyrrolidone is 15,000-40,000.

3. The adhesive according to claim 1 or 2, characterized in that The weight average molecular weight of the polyvinylidene fluoride is 600,000-900,000.

4. The adhesive according to any one of claims 1 to 3, characterized in that The weight average molecular weight of the polyvinylidene fluoride is 600,000-700,000.

5. The adhesive according to any one of claims 1 to 4, characterized in that The mass percentage of the polyvinyl pyrrolidone in the binder is 7%-50%.

6. The adhesive according to any one of claims 1 to 5, characterized in that The mass percentage of the polyvinyl pyrrolidone in the binder is 15%-50%.

7. A method for preparing the adhesive according to any one of claims 1 to 6, characterized in that: The following steps are involved: The binder is obtained by grafting a raw material system including polyvinylidene fluoride, N-vinyl pyrrolidone and a polymerization inhibitor.

8. The method for preparing the adhesive according to claim 7, wherein: The mass ratio of the polyvinylidene fluoride to the N-vinyl pyrrolidone is 1:(0.1-2), preferably 1:(0.9-1.4); and / or, the polymerization inhibitor comprises one or more of copper salts, ammonium salts, and iron salts, preferably ferrous ammonium sulfate; And / or, the concentration of the polymerization inhibitor in the raw material system is 1 mmol / L-30 mmol / L, preferably 3 mmol / L-5 mmol / L.

9. The method for preparing the adhesive according to claim 7 or 8, characterized in that: The grafting treatment includes irradiation grafting treatment; The radiation dose of the radiation grafting treatment is 20kGy-45kGy, preferably 28kGy-40kGy; And / or, the average dose rate of the radiation grafting treatment is 0.1 kGy / h-0.4 kGy / h, preferably 0.1 kGy / h-0.23 kGy / h.

10. A positive electrode sheet, characterized in that: The positive electrode active material layer comprises a positive electrode active material layer, wherein the positive electrode active material layer comprises the binder according to any one of claims 1 to 6 or the binder prepared by the preparation method according to any one of claims 7 to 9.

11. The positive electrode sheet according to claim 10, characterized in that: The binder accounts for 2%-15% by mass of the positive electrode active material layer, preferably 2%-5% by mass.

12. The positive electrode sheet according to claim 10 or 11, characterized in that: The positive electrode active material layer further comprises a positive electrode active material and a conductive agent, wherein the positive electrode active material accounts for 65% to 97% by mass of the positive electrode active material layer, preferably 90% to 97% by mass; And / or, the conductive agent accounts for 1%-20% by mass of the positive electrode active material layer, preferably 1%-5% by mass.

13. A battery, characterized in that: The positive electrode sheet comprises the positive electrode sheet according to any one of claims 10 to 12.

14. An electrical device, characterized in that: Including the battery according to claim 13.

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