Combination binder comprising a vinylidene fluoride copolymer and its use

Abstract

This invention discloses a composite binder comprising a vinylidene fluoride copolymer and its application. The composite binder includes a first copolymer and a second copolymer. The first copolymer is formed by copolymerizing a vinylidene fluoride monomer and a first monomer shown in Formula I, and the second copolymer is formed by copolymerizing a vinylidene fluoride monomer and a second monomer shown in Formula (II). Specific structures and limitations are detailed in the specification. The weight-average molecular weight of the first copolymer is 800,000 to 3,500,000, and the weight-average molecular weight of the second copolymer is ≤500,000. The mass ratio of the first copolymer to the second copolymer is 100:(0.2 to 10). When applied to the preparation of alkali metal battery electrode slurries, this composite binder not only exhibits excellent resistance to chemical gelation of the slurry and improves slurry stability, but also further enhances the adhesion of the electrode.

Description

Technical Field

[0001] This invention relates to the field of polymers, and more specifically to a composite binder comprising a vinylidene fluoride copolymer for use in alkali metal ion batteries, particularly sodium / lithium ion battery electrode slurries. Background Technology

[0002] Polyvinylidene fluoride (PVDF) is a common fluoropolymer widely used in petrochemicals, lithium batteries, coatings, water treatment, and other fields. In the lithium battery industry, the use of PVDF resin as a positive electrode binder and separator coating represents one of the fastest-growing markets for PVDF demand.

[0003] The positive electrode sheet is a crucial component of lithium-ion batteries. Its preparation typically involves placing the main positive electrode material, conductive agent, and binder together in a solvent (usually N-methylpyrrolidone) to form a homogeneous positive electrode slurry. This slurry is then coated onto a current collector (usually aluminum foil) and baked to obtain the positive electrode sheet. The positive electrode sheet must have a smooth, flat surface, free from bumps, depressions, uncovered areas, or cracks.

[0004] Polyvinylidene fluoride (PVDF) is currently the mainstream cathode binder, possessing excellent adhesion and electrochemical performance. However, PVDF itself has high molecular weight and chain regularity, making it prone to continuous dehydrofluorination reactions under alkaline conditions, forming double bonds. In lithium-ion battery layered oxide systems (such as lithium nickel cobalt manganese oxide (NCM)) or sodium-ion battery systems (such as sodium nickel iron manganese oxide (NFM) and sodium iron pyrophosphate (NFPP)), lithium or sodium hydroxides or carbonates are present, resulting in a high residual alkali content in the system. When PVDF is used as a binder in these systems with high residual alkali content, it easily undergoes a dehydrofluorination reaction under alkaline conditions to form double bonds. These double bonds further undergo chemical cross-linking, causing the slurry to lose its fluidity irreversibly; this phenomenon is called slurry chemical gelation.

[0005] Maurizio Biso et al. (Polymer Engineering and Science, 2016, 56(7):760-764) used NCM622 as the research object and compared the stability of three different slurries: suspension copolymer of vinylidene fluoride and acrylic acid, high molecular weight (Mw = 900,000) vinylidene fluoride emulsion homopolymer, and high molecular weight (Mw = 900,000) vinylidene fluoride and acrylic acid emulsion block copolymer. The results showed that the slurry prepared by suspension copolymer of vinylidene fluoride and acrylic acid had the best stability. After 30 days of storage, the slurry was still stable, and the stress-shear and viscosity-shear curves were basically the same as those on the day of preparation. However, the two PVDF samples prepared by emulsion polymerization both experienced irreversible slurry gelation. In addition, the lithium battery using suspension copolymer of vinylidene fluoride and acrylic acid as a binder had better internal resistance and battery capacity than the emulsion polymerized polyvinylidene fluoride sample. The main reason is that the carboxyl group in Solef5130 neutralizes some of the residual alkali in NCM622, reducing the amount of dehydrofluoric acid produced by the residual alkali in polyvinylidene fluoride. However, the suspension copolymer of vinylidene fluoride and acrylic acid exhibits poor stability in sodium-ion battery systems, with the slurry rapidly gelling within a short period. The main reason is likely that carboxyl groups can form coordination bonds with sodium ions, creating a network structure that causes the slurry to lose its fluidity. Lithium ions, with their small radius, cannot form coordination bonds with carboxyl groups. Therefore, the above approach is only effective in lithium-ion batteries and ineffective in sodium-ion batteries.

[0006] Patent CN106299249A discloses a method for inhibiting gelation and improving the stability of high-nickel ternary material slurries using a modified polyvinylidene fluoride (PVDF) binder. The modified PVDF binder contains hydrophilic groups such as sulfonic acid groups, carboxyl groups, or amino groups, which inhibit the dehydrofluoric acid reaction of PVDF. However, this patent does not disclose the preparation method of the modified PVDF. In fact, introducing these groups into PVDF is extremely difficult. Industrially, PVDF is often synthesized using emulsion polymerization or suspension polymerization, with water being the main dispersed phase. Hydrophilic monomers are highly soluble in water and are difficult to dissolve in PVDF monomers and participate in the polymerization process of PVDF.

[0007] Patent CN106384816A discloses the application of a composite binder in the positive electrode slurry of a high-nickel ternary lithium-ion battery. The composite binder comprises a high-strength binder and an alkali-resistant binder, with a weight ratio of 1:1 to 4:1. The high-strength binder is a fluorinated or non-fluorinated polymer with a molecular weight of 1-1.5 million, requiring its side chains to contain substituents such as carboxyl, ester, or ketone groups, with a substitution rate of 20-150%, and can include polyvinylidene fluoride, polyacrylic acid resin, etc. The alkali-resistant binder is a fluorinated or non-fluorinated polymer with a molecular weight of 600,000-1,000,000, requiring its side chains to contain substituents such as sulfonic acid groups, sulfonate groups, or phosphate groups, with a substitution rate of 20-150%, and can include polyvinylidene fluoride, polyacrylic acid resin, etc. While this method can improve the alkali resistance and flowability of the positive electrode slurry, its adhesive strength still needs improvement.

[0008] Therefore, it is of great significance to develop polyvinylidene fluoride binders with good resistance to chemical gelation of slurries and high adhesion, which are also suitable for the preparation and performance improvement of positive electrode sheets for sodium-ion batteries or lithium-ion batteries. Summary of the Invention

[0009] To address the aforementioned technical problems, this invention proposes a simple preparation method for a combined binder containing vinylidene fluoride copolymer. When used in the preparation of alkali metal ion battery electrodes, it exhibits resistance to chemical gelation of slurry and can further improve the adhesion of the electrodes.

[0010] The objective of this invention is achieved through the following technical solution:

[0011] A composite adhesive comprising a vinylidene fluoride copolymer, comprising:

[0012] The first copolymer is formed by copolymerizing vinylidene fluoride monomer and a first monomer shown in Formula I below.

[0013]

[0014] In formula (I), R1 and R2 are independently selected from hydrogen, halogen, C1-C5 alkyl, C1-C5 haloalkyl or C1-C5 alkyl containing ester group, R3 is an electron-withdrawing group selected from halogen or C1-C5 haloalkyl, and R4 is selected from C1-C5 alkyl or C1-C5 haloalkyl.

[0015] The second copolymer is formed by copolymerizing vinylidene fluoride monomer and a second monomer of formula (II) below.

[0016]

[0017] In formula (II), R5, R6, and R7 are independently selected from hydrogen, halogen, C1-C5 alkyl, C1-C5 haloalkyl, ester-containing C1-C5 alkyl, or carboxyl-containing C1-C5 alkyl, and R8 is selected from hydrogen or a C1-C5 alkyl containing at least one carboxyl group.

[0018] The weight-average molecular weight of the first copolymer is 800,000 to 3,500,000, and the weight-average molecular weight of the second copolymer is ≤500,000; and the mass ratio of the first copolymer to the second copolymer is 100:(0.2 to 10).

[0019] Preferably, in formula (I), R1 and R2 are independently selected from hydrogen and C1-C3 alkyl, R3 is selected from halogen or C1-C3 haloalkyl, and R4 is selected from C1-C3 alkyl; in formula (II), R5, R6, and R7 are independently selected from hydrogen and C1-C3 alkyl, and R8 is selected from hydrogen or C1-C3 alkyl containing at least one carboxyl group.

[0020] More preferably, the first monomer is selected from methyl 2-trifluoromethacrylate or methyl 2-fluoroacrylate; the second monomer is selected from acrylic acid, monomethyl maleate or ethyl carboxyacrylate.

[0021] During the research process, this invention discovered that the first copolymer of this invention, by introducing electron-withdrawing groups into the polyvinylidene fluoride (PVDF) molecular chain, can reduce the electron cloud density of the double bonds formed by PVDF upon contact with alkaline substances, thereby inhibiting the further addition of double bonds to form a network crosslinked structure and achieving anti-gelling of the slurry. Electron-withdrawing groups can be introduced through a copolymerization reaction between the acrylate-based first monomer and PVDF. In the first copolymer, the ester itself possesses strong electron-withdrawing ability, and other electron-withdrawing substituents, such as halogen atoms or halogen-substituted alkyl groups, can be introduced into the double bond portion of the first monomer. It is important to emphasize that although carboxyl groups also have strong electron-withdrawing effects, in sodium-ion battery cathode materials, they easily form coordination bonds with sodium ions, promoting chemical gelation, especially in high molecular weight PVDF. The presence of carboxyl groups may lead to the formation of a large spatial crosslinking system, resulting in severe gelation of the slurry. Therefore, the first monomer of this invention does not contain carboxyl groups.

[0022] However, the adhesive strength of ester groups is insufficient. Therefore, this invention introduces a second copolymer to form a combined binder. By introducing carboxyl groups through copolymerization of a second monomer containing carboxyl groups and vinylidene fluoride (PVDF), the adhesion of the electrode can be significantly improved. However, the presence of carboxyl groups can form coordination bonds with sodium ions, causing PVDF to form a network structure in the sodium-ion battery slurry, leading to gelation. Further research revealed that when the weight-average molecular weight of the second copolymer is ≤500,000, the carboxyl content in the second monomer is low, effectively preventing the formation of this network structure. Although the molecular weight of the carboxyl-containing second copolymer is low, in the electrode, this portion of PVDF can bond with the PVDF in the first copolymer through eutectic action, thereby significantly improving the electrode adhesion of the combined binder.

[0023] In the first copolymer, the suitable content of the first monomer repeating unit is 0.1–10 mol%. When the proportion of the first monomer repeating unit is less than 0.1 mol%, the number of electron-withdrawing groups is insufficient, failing to adequately reduce the electron cloud density of the double bonds; when the proportion of the first monomer repeating unit is greater than 10 mol%, the crystallinity of polyvinylidene fluoride decreases, leading to increased swelling of the combined binder in the electrolyte and deterioration of battery cycle life. Preferably, the molar content of the first monomer repeating unit in the first copolymer is 0.5–5 mol%.

[0024] In the second copolymer, the appropriate content of the second monomer repeating unit is 0.05–10 mol%. When the proportion of the second monomer repeating unit is less than 0.05 mol%, the number of carboxyl groups is insufficient, and the improvement in adhesion is not significant. When the proportion of the second monomer repeating unit is greater than 10 mol%, a large number of carboxyl groups will form coordination bonds with sodium ions, causing polyvinylidene fluoride to form a network structure in the sodium-ion battery slurry, resulting in gelation. Simultaneously, the mass content of the second copolymer in the combined binder should not exceed 10%, otherwise it will lead to a decrease in electrode adhesion. Preferably, the molar content of the second monomer repeating unit in the second copolymer is 1–5 mol%.

[0025] The appropriate molecular weight range is one of the important factors affecting the high adhesive strength of vinylidene fluoride copolymers. When the molecular weight is low, it is insufficient to provide strong interaction forces between the cathode material particles and the electrode sheet; when the molecular weight is high, the solubility of vinylidene fluoride copolymers in N-methylpyrrolidone decreases, which also leads to a significant deterioration in parameters such as the solid content of the slurry.

[0026] Therefore, the weight-average molecular weight of the first copolymer of the present invention is preferably 1.2 million to 2 million, and the weight-average molecular weight of the second copolymer is preferably 100,000 to 300,000; and the mass ratio of the first copolymer to the second copolymer is 100:(1 to 6).

[0027] The present invention provides a simple method for preparing a composite adhesive containing a vinylidene fluoride copolymer. First, a first copolymer and a second copolymer are prepared, and then the first copolymer and the second copolymer are blended in a specific mass ratio to obtain the composite adhesive.

[0028] The first copolymer and the second copolymer of the present invention can be obtained by existing processes. From an industrialization perspective, emulsion polymerization and suspension polymerization are two preferred preparation methods.

[0029] In one embodiment, the present invention uses emulsion polymerization to prepare vinylidene fluoride copolymer, comprising the following steps:

[0030] (1) Add pure water and emulsifier to the reactor, and after purging with nitrogen, raise the temperature to 30-100℃;

[0031] (2) Add vinylidene fluoride, comonomer, chain transfer agent and initiator to start the reaction;

[0032] (3) Add the remaining vinylidene fluoride, comonomer, chain transfer agent and initiator in a segmented or continuous manner, and maintain the polymerization reaction pressure at 3.0 to 10.0 MPa;

[0033] (4) When the pressure inside the reactor drops to 2.0 to 9.0 MPa, and the total mass of vinylidene fluoride and comonomers reaches the preset value, the polymerization reaction ends.

[0034] The emulsifiers used in this invention are commonly used emulsifiers in the art, such as perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, perfluoropolyethers, polyoxyethylene, polyacrylates, etc.

[0035] In another embodiment, the preparation of vinylidene fluoride copolymers is carried out by suspension polymerization, including the following steps:

[0036] (1) Add pure water and dispersant to the reactor, and after purging with nitrogen, raise the temperature to 20-80℃;

[0037] (2) Add vinylidene fluoride, comonomer, chain transfer agent and initiator to start the reaction;

[0038] (3) Add the remaining vinylidene fluoride, comonomer, chain transfer agent and initiator in a segmented or continuous manner, and maintain the polymerization reaction pressure at 3.0 to 15.0 MPa;

[0039] (4) When the pressure inside the reactor drops to 2.0 to 14.0 MPa, and the total mass of vinylidene fluoride and comonomers reaches the preset value, the polymerization reaction ends.

[0040] The dispersant used in this invention is a commonly used dispersant in the art, such as methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, magnesium hydroxide, etc.

[0041] The chain transfer agent and initiator involved in the emulsion polymerization and suspension polymerization processes of this invention can be commonly used reagents in the field.

[0042] The initiator is generally an organic or inorganic peroxide, such as diisopropyl peroxide, diisobutyl peroxide, diethyl peroxide, potassium persulfate, ammonium persulfate, etc. The amount of the initiator is 0.01–0.5 wt% of the total vinylidene fluoride monomer, preferably 0.05–0.2 wt%. A small amount of reducing agent (such as sodium sulfite, oxalic acid, etc.) may also be added during polymerization to accelerate the reaction rate or lower the polymerization temperature.

[0043] For chain transfer agents, commonly used reagents in this field can be selected, such as methanol, ethanol, isopropanol, ethyl acetate, diethyl malonate, etc.

[0044] The weight-average molecular weight of the copolymer can be adjusted by controlling the amount of initiator and chain transfer agent added.

[0045] The present invention also provides an electrode composition comprising a binder, a conductive agent, and an electrode material, wherein the binder is selected from any of the aforementioned composite binders containing vinylidene fluoride copolymers.

[0046] Specifically, the electrode composition includes:

[0047] (1) A combined binder, the amount of which accounts for 0.2 to 10.0 wt% of the electrode composition;

[0048] (2) The conductive agent is selected from carbon black, carbon nanotubes or mixtures thereof, and the amount used accounts for 0.5 to 10.0 wt% of the electrode composition;

[0049] (3) Powdered electrode material, accounting for 80-98.5 wt% of the electrode composition, selected from general formula AMY2 or general formula AB(XO4). f E 1-f The compound is represented by A, where A is Li or Na, M is selected from at least one of Co, Ni, Fe, Mn, Cr, and V, Y is O or S, B is selected from at least one of Fe, Mn, and Ni, X is selected from at least one of P, S, V, Si, Nb, and Mo, E is selected from F, OH, or Cl, and 0.75 ≤ f ≤ 1.0.

[0050] In lithium-ion battery systems, the powdered electrode materials include lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide, and lithium nickel cobalt manganese oxide.

[0051] In sodium-ion battery systems, the powdered electrode materials include sodium iron phosphate pyrophosphate, sodium iron sulfate, sodium nickel iron manganate, etc.

[0052] The present invention also provides an alkali metal ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the above-described electrode composition.

[0053] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0054] 1. The vinylidene fluoride copolymer in the composite adhesive of the present invention can be prepared by conventional industrial processes using vinylidene fluoride, a first monomer and / or a second monomer as raw materials. Then, the composite adhesive can be obtained by blending the vinylidene fluoride copolymers of the corresponding weight-average molecular weights obtained in the preparation process at a specific mass. The preparation method is simple and suitable for industrial production.

[0055] 2. The composite binder containing vinylidene fluoride copolymer described in this invention has excellent alkali resistance. When applied to the preparation of lithium / sodium-ion battery electrodes with strong alkalinity, especially in the preparation of positive electrode slurries for lithium-ion battery layered oxide main materials and sodium-ion battery main materials, it not only has excellent anti-slurry chemical gelation properties, but also further improves the adhesion of the electrode. Detailed Implementation

[0056] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternatives, improvements, and equivalents that may be included within the scope of the claims.

[0057] The test methods involved in the embodiments and comparative examples of this invention are described below:

[0058] Method for testing weight-average molecular weight: Dissolve the sample to be tested in N,N-dimethylformamide, use polystyrene as a standard, and use gel permeation chromatography to test the weight-average molecular weight.

[0059] Test method for comonomer content: Characterization is performed by determining the DMF solution state of the polymer using nuclear magnetic resonance.

[0060] The method for testing carboxyl groups: The content of carboxyl groups in the polymer was tested by nuclear magnetic resonance. The main method is as follows: The resin sample to be tested was fully dissolved in deuterated DMSO and its 1H NMR was measured. Based on the integrated intensity of the signal peaks observed at 12-14 ppm, which mainly comes from carboxyl groups, and the signals observed at 2.25 ppm and 2.89 ppm, which mainly come from 1,1-difluoroethylene, the amount of carboxyl-related copolymer structural units in the polymer was calculated.

[0061] Solution viscosity test method: The solution viscosity was tested according to GB / T 2794, specifically as follows: 14.0g of polymer resin powder was added to 186g of N-methylpyrrolidone (NMP), and stirred at 1500rpm for 2-4 hours using a high-speed disperser to obtain a homogeneous solution. After defoaming the prepared solution, it was kept at 30℃ in a water bath for 0.5 hours. The solution viscosity was tested using a Bollerfeld DV2TRVTJ0 rotational viscometer with the RV2# rotor selected. Tests were performed at 2rpm and 5rpm, maintaining a constant temperature of 30℃ throughout the test. Data was recorded after the readings stabilized. If the instrument data was unstable, the value recorded 5 minutes after the start of the test was taken as the solution viscosity.

[0062] Test method for slurry stability: Under low humidity and room temperature conditions, 1.5g of polymer resin powder, 1g of conductive carbon black, and 97.5g of sodium nickel cobalt manganese oxide (NCM, nickel content 83wt%) or sodium nickel iron manganese oxide (NFM, nickel content 33%) were uniformly dispersed in a mixing tank for 0.5h under mechanical stirring. Then, 42.8g of NMP was added and stirred thoroughly to dissolve, forming a homogeneous slurry. NMP was then added to adjust the viscosity to 3000–7000 cP. The slurry was then filtered through a 200-mesh metal filter to obtain the desired slurry. The flow state of the slurry was observed at 5, 24, and 48 hours.

[0063] Test method for electrode adhesion: The slurry prepared in the slurry stability test is degassed under vacuum and then uniformly coated onto a 14-micron-thick carbon-coated aluminum foil using a coating machine at a coating speed of 0.8–1.2 m / min. The aluminum foil coated with the electrode material mixture is dried in a three-stage vacuum oven (temperature 90℃→110℃→90℃) to obtain the electrode sheet, which can be further rolled using a roller press. Using a tensile testing machine, the adhesion strength of the polymer after being prepared into an electrode is determined according to ISO 4624 standard (adhesion pull-off test) at 25℃ and 50% relative humidity. Each value is the average of at least 5 current collector measurements.

[0064] Example 1

[0065] (1) Preparation of the first copolymer: 3100g of deionized water and 1g of methylcellulose were added to a 5L vertical polymerization reactor. The reactors were combined, evacuated, and purged with nitrogen until the oxygen content inside the reactor was less than 10ppm. The temperature inside the reactor was raised to 40℃, and 1500g of vinylidene fluoride monomer was added. The pressure inside the reactor was raised to 8.0MPa. 4g of diisopropyl peroxide dicarbonate was added. Then, stirring was started at a rate of 450rpm to begin the polymerization reaction. The pressure inside the reactor was maintained at 7.5-8.0MPa by adding methyl 2-fluoroacrylate aqueous dispersion (prepared by 32g of methyl 2-fluoroacrylate and 968g of deionized water). The reaction ended when 1000g of methyl 2-fluoroacrylate aqueous dispersion was added. The resin was discharged, filtered, collected, washed, and dried to obtain vinylidene fluoride copolymer resin powder, namely copolymer A-1, with a weight-average molecular weight of 1.751 million and a molar content of methyl 2-fluoroacrylate repeating units of 1.48 mol.

[0066] (2) Preparation of the second copolymer: 2800g of deionized water and 1g of methylcellulose were added to a 5L vertical polymerization reactor. The reactors were combined, evacuated, and purged with nitrogen until the oxygen content inside the reactor was less than 10ppm. The temperature inside the reactor was raised to 50℃, and 1000g of vinylidene fluoride monomer was added. The pressure inside the reactor was raised to 6.0MPa. 9g of diisopropyl peroxide and 25g of ethyl acetate were added. Then, stirring was started at 450rpm to begin the polymerization reaction. The pressure inside the reactor was maintained at 5.5-6.0MPa by adding an acrylic acid aqueous dispersion (prepared from 32g of acrylic acid and 1568g of deionized water). The reaction ended when 1600g of acrylic acid aqueous dispersion was added. The product was discharged, filtered, the resin was collected, washed, and dried to obtain vinylidene fluoride copolymer resin powder, namely copolymer B-1, with a weight-average molecular weight of 113,000 and a molar content of 3.5mol% of repeating acrylic acid units.

[0067] (3) Preparation of composite adhesive: Copolymer A-1 and copolymer B-1 are blended at a mass ratio of 97.5:2.5 to obtain composite adhesive.

[0068] Example 2

[0069] The operation of this embodiment is the same as that of embodiment 1, except that in step (3), copolymer A-1 and copolymer B-1 are blended at a mass ratio of 95:5 to obtain a combined adhesive.

[0070] Example 3

[0071] The operation of this embodiment is the same as that of embodiment 1, except that in step (3), copolymer A-1 and copolymer B-1 are blended at a mass ratio of 90:10 to obtain a combined adhesive.

[0072] Example 4

[0073] The operation of this embodiment is the same as that of embodiment 1, except that in step (1), the 1000g of methyl 2-fluoroacrylate aqueous dispersion is prepared by 160g of methyl 2-fluoroacrylate and 840g of deionized water. Other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer A-2, is obtained. The weight average molecular weight is measured to be 1.655 million, and the molar content of methyl 2-fluoroacrylate repeating units is 7.01 mol%.

[0074] Copolymer A-2 and copolymer B-1 were blended at a mass ratio of 97.5:2.5 to obtain a composite adhesive.

[0075] Example 5

[0076] The operation of this embodiment is the same as that of embodiment 1, except that in step (1), the first monomer added is replaced with methyl 2-trifluoromethyl acrylate aqueous dispersion (prepared from 43g of methyl 2-trifluoromethyl acrylate and 957g of deionized water), the total amount added remains unchanged, and all other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer A-3, is obtained, the weight average molecular weight is measured to be 1.565 million, and the molar content of methyl 2-trifluoromethyl acrylate repeating units is 1.32 mol%.

[0077] Copolymer A-3 and copolymer B-1 were blended at a mass ratio of 97.5:2.5 to obtain a composite adhesive.

[0078] Example 6

[0079] The operation of this embodiment is the same as that of embodiment 1, except that in step (1), the first monomer added is replaced with methyl 2-bromoacrylate aqueous dispersion (prepared by 51g methyl 2-bromoacrylate and 949g deionized water), the total amount added remains unchanged, and all other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer A-4, is obtained, the weight average molecular weight is measured to be 1.852 million, and the molar content of methyl 2-bromoacrylate repeating units is 1.70 mol%.

[0080] Copolymer A-4 and copolymer B-1 were blended at a mass ratio of 97.5:2.5 to obtain a composite adhesive.

[0081] Example 7

[0082] The operation of this embodiment is the same as that of embodiment 1, except that in step (2), the second monomer added is replaced with an aqueous dispersion of 2-carboxyethyl acrylate (prepared from 65g of 2-carboxyethyl acrylate and 936g of deionized water), the total amount added remains unchanged, and all other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer B-2, is obtained, with a weight-average molecular weight of 139,000 and a molar content of 3.2 mol% of repeating units of 2-carboxyethyl acrylate.

[0083] Copolymer A-1 and copolymer B-2 were blended at a ratio of 97.5:2.5 to obtain a composite adhesive.

[0084] Example 8

[0085] The operation of this embodiment is the same as that of embodiment 1, except that in step (2), the second monomer added is replaced with a maleic acid monomethyl ester dispersion (prepared from 60g of maleic acid monomethyl ester and 936g of deionized water), the total amount added remains unchanged, and all other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer B-3, is obtained, the weight average molecular weight is measured to be 156,000, and the molar content of repeating units of maleic acid monomethyl ester is 2.8mol%.

[0086] Copolymer A-1 and copolymer B-3 were blended at a ratio of 97.5:2.5 to obtain a composite adhesive.

[0087] Example 9

[0088] Preparation of the second copolymer: 3200g of deionized water and 0.5g of polyethylene glycol-propylene glycol copolymer were added to a 5L vertical polymerization reactor. The reactor was evacuated and replaced with nitrogen three times until the oxygen content in the reactor was less than 10ppm.

[0089] Add 330g of vinylidene fluoride monomer to the reactor, raise the reactor temperature to 80℃, add 12g of isopropanol and 5g of ammonium persulfate, maintain the temperature for 10 minutes, then start stirring at 350rpm. Add acrylic acid solution (6g acrylic acid to make 360g acrylic acid solution) to the reactor at a constant rate of approximately 3g / min, and replenish vinylidene fluoride monomer to maintain stable pressure until the cumulative amount of vinylidene fluoride added reaches 800g, at which point the reaction is stopped. After the reaction is complete, collect the emulsion, filter, demulsify, wash, and dry to obtain polyvinylidene fluoride resin powder, i.e., copolymer B-4, with a weight-average molecular weight of 210,000 and a molar content of 0.07mol% of repeating acrylic acid units.

[0090] The rest is the same as in Example 1, except that copolymer A-1 and copolymer B-4 are blended at a ratio of 97.5:2.5 to obtain a combined adhesive.

[0091] Comparative Example 1

[0092] This comparative example provides the preparation of vinylidene fluoride homopolymer A'-1: 3100g of deionized water and 1g of methylcellulose were added to a 5L vertical polymerization reactor. The reactors were combined, evacuated, and purged with nitrogen until the oxygen content inside the reactor was less than 10ppm. The temperature inside the reactor was raised to 40℃, and 1500g of vinylidene fluoride monomer was added. The pressure inside the reactor was raised to 8.0MPa. 3.5g of diisopropyl peroxide and 1g of ethyl acetate were added. Then, stirring was started at a rate of 450rpm to begin the polymerization reaction. By adding 1000g of deionized water, the pressure inside the reactor was maintained at 7.5-8.0MPa. The reaction ended after the total amount of deionized water (1000g) was added. The product was discharged, filtered, the resin was collected, washed, and dried to obtain vinylidene fluoride homopolymer resin powder, with a weight-average molecular weight of 1.805 million.

[0093] Comparative Example 2

[0094] Copolymer A-1 is used only as a binder.

[0095] Comparative Example 3

[0096] This comparative example provides the preparation of vinylidene fluoride-acrylic acid copolymer A'-2: 3100g of deionized water and 1g of methylcellulose were added to a 5L vertical polymerization reactor. The reactors were combined, evacuated, and purged with nitrogen until the oxygen content inside the reactor was less than 10ppm. The temperature inside the reactor was raised to 40℃, and 1500g of vinylidene fluoride monomer was added. The pressure inside the reactor was raised to 8.0MPa. 7.5g of diisopropyl peroxide dicarbonate was added. Then, stirring was started at a rate of 450rpm to begin the polymerization reaction. By adding 22g of acrylic acid / 1000g of deionized water dispersion, the pressure inside the reactor was maintained at 7.5-8.0MPa. When the total amount of 1000g of acrylic acid aqueous dispersion was added, the reaction was stopped. The product was discharged, filtered, the resin was collected, washed, and dried to obtain vinylidene fluoride-acrylic acid copolymer resin powder. The weight-average molecular weight was measured to be 1.689 million, and the molar content of repeating acrylic acid units was 1.56mol%.

[0097] Comparative Example 4

[0098] The operation of this comparative example is the same as that of Example 1, except that in step (2), the 1600g acrylic acid solution added is prepared by 3g acrylic acid and 1597g deionized water. The total amount added remains unchanged, and all other operations remain unchanged. Polyvinylidene fluoride copolymer resin powder, namely copolymer B'-1, is obtained. The weight average molecular weight is 155,000 and the molar content of repeating acrylic acid units is 0.03mol.

[0099] Copolymer A-1 and copolymer B'-1 were blended at a ratio of 97.5:2.5 to obtain a composite adhesive.

[0100] Comparative Example 5

[0101] The operation of this comparative example is the same as that of Example 1, except that in step (2), diisopropyl peroxide dicarbonate is changed to 6g and ethyl acetate is changed to 15g, while other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer B'-2, is obtained. The weight average molecular weight is measured to be 711,000 and the molar content of repeating acrylic acid units is 3.5mol%.

[0102] Copolymer A-1 and copolymer B'-2 were blended at a ratio of 97.5:2.5 to obtain a composite adhesive.

[0103] Comparative Example 6

[0104] The operation of this comparative example is the same as that of Example 1, except that in step (3), copolymer A-1 and copolymer B-1 are blended at a mass ratio of 85:15 to obtain a combined adhesive.

[0105] Comparative Example 7

[0106] The operation of this comparative example is the same as that of Example 1, except that in step (1), the first comonomer is changed to methyl acrylate (R3 does not contain electron-withdrawing groups), and the diisopropyl peroxide is changed to 7g. All other operations remain unchanged, and vinylidene fluoride copolymer resin powder, namely copolymer A'-3, is obtained. The weight average molecular weight is measured to be 1.551 million, and the molar content of methyl acrylate repeating units is 1.5 mol%.

[0107] Copolymer A'-3 and copolymer B-1 were blended at a mass ratio of 97.5:2.5 to obtain a composite adhesive. The adhesives from Examples 1-9 and Comparative Examples 1-7 were subjected to slurry stability and adhesion tests in NCM and NFM active material systems. The test results are shown in Table 1.

[0108] Table 1. Results of Adhesive Application Performance Tests

[0109]

[0110]

[0111] Examples 1-9 demonstrate that the combined binder provided by this invention exhibits significant advantages in slurry stability and electrode adhesion in both lithium-ion NCM and sodium-ion NFM systems. Comparison of Example 1 and Comparative Examples 4-6 shows that excellent slurry stability and electrode adhesion can only be achieved simultaneously in both lithium-ion NCM and sodium-ion NFM systems under specific conditions of the second copolymer (specific second monomer content, specific weight-average molecular weight, specific second copolymer content in the combined binder, etc.). If the molar content of the second monomer in the second copolymer is insufficient, or the weight-average molecular weight of the second copolymer is too high, or the mass ratio of the second copolymer to the first copolymer is different, the aforementioned properties cannot be simultaneously satisfied.

[0112] Comparative Example 1, using only PVDF homopolymer as a binder, exhibited poor slurry stability and weak adhesion. Comparative Example 2, using only the first copolymer as a binder, showed good slurry stability, but weak adhesion in both lithium-ion NCM and sodium-ion NFM systems. Comparative Example 3, using a traditional vinylidene fluoride-acrylic acid copolymer as a binder, while possessing excellent electrode adhesive properties, showed poor slurry stability in the sodium-ion NFM system. Comparative Example 7, synthesizing the first copolymer using a first monomer without electron-withdrawing groups, even when combined with a specific second copolymer, showed slurry stability inferior to the combined binder of this invention.

[0113] Therefore, only by simultaneously using a specific first monomer to form a first copolymer and a specific second monomer to form a second copolymer, and ensuring that the ratio of the two comonomers and their proportions in the combined binder are within a suitable range, can good slurry stability be maintained and electrode adhesion improved in both lithium-ion NCM systems and sodium-ion NFM systems.

Claims

1. A composite adhesive comprising a vinylidene fluoride copolymer, comprising: The first copolymer is formed by copolymerizing vinylidene fluoride monomer and a first monomer shown in Formula I below. In formula (I), R1 and R2 are independently selected from hydrogen, halogen, C1-C5 alkyl, C1-C5 haloalkyl or C1-C5 alkyl containing ester group, R3 is an electron-withdrawing group selected from halogen or C1-C5 haloalkyl, and R4 is selected from C1-C5 alkyl or C1-C5 haloalkyl. The second copolymer is formed by copolymerizing vinylidene fluoride monomer and a second monomer of formula (II) below. In formula (II), R5, R6, and R7 are independently selected from hydrogen, halogen, C1-C5 alkyl, C1-C5 haloalkyl, ester-containing C1-C5 alkyl, or carboxyl-containing C1-C5 alkyl, and R8 is selected from hydrogen or a C1-C5 alkyl containing at least one carboxyl group. The weight-average molecular weight of the first copolymer is 800,000 to 3,500,000, and the weight-average molecular weight of the second copolymer is ≤500,000; and the mass ratio of the first copolymer to the second copolymer is 100:(0.2 to 10).

2. The composite adhesive comprising vinylidene fluoride copolymer according to claim 1, characterized in that: In formula (I), R1 and R2 are independently selected from hydrogen and C1-C3 alkyl, R3 is selected from halogen or C1-C3 haloalkyl, and R4 is selected from C1-C3 alkyl; in formula (II), R5, R6, and R7 are independently selected from hydrogen and C1-C3 alkyl, and R8 is selected from hydrogen or C1-C3 alkyl containing at least one carboxyl group.

3. The composite adhesive comprising vinylidene fluoride copolymer according to claim 2, characterized in that: The first monomer is selected from methyl 2-trifluoromethacrylate or methyl 2-fluoroacrylate; the second monomer is selected from acrylic acid, monomethyl maleate or ethyl carboxyacrylate.

4. The composite adhesive comprising vinylidene fluoride copolymer according to claim 1, characterized in that: In the first copolymer, the molar content of the first monomer repeating unit is 0.1 to 10%; in the second copolymer, the molar content of the second monomer repeating unit is 0.05 to 10%.

5. The composite adhesive comprising vinylidene fluoride copolymer according to claim 4, characterized in that: In the composite adhesive, the mass ratio of the first copolymer and the second copolymer is 100:(1-6).

6. An electrode composition comprising a binder, a conductive agent, and an electrode material, characterized in that: The adhesive is selected from the combined adhesives containing vinylidene fluoride copolymers according to any one of claims 1-5.

7. The electrode composition according to claim 6, characterized in that: The electrode composition comprises: (1) A combined binder, the amount of which accounts for 0.2 to 10.0 wt% of the electrode composition; (2) The conductive agent is selected from carbon black, carbon nanotubes or mixtures thereof, and the amount used accounts for 0.5 to 10.0 wt% of the electrode composition; (3) a powder electrode material, in an amount of 80 to 98.5 wt% of the mass of the electrode composition, selected from the group consisting of a compound of the general formula AMY2 or a compound of the general formula AB(XO4) f E 1-f denotes a compound, wherein A is Li or Na, M is selected from at least one of Co, Ni, Fe, Mn, Cr, V, Y is O or S; B is selected from at least one of Fe, Mn, Ni, X is selected from at least one of P, S, V, Si, Nb, Mo, E is selected from F, OH or Cl, 0.75 < f < 1.

0.

8. An alkali metal ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that: The positive electrode comprises the electrode composition of claim 6 or 7.

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