Positive electrode slurry, positive electrode sheet and lithium ion secondary battery

Abstract

The present invention discloses a positive electrode slurry, a positive electrode sheet, and a lithium-ion secondary battery, relating to the technical field of lithium-ion secondary batteries. The positive electrode slurry of the present invention contains a nickel-rich positive electrode active material and a 1,1-difluoroethylene copolymer binder modified with a sulfonic acid group. The sulfonic acid group reacts with the residual alkali on the surface of the nickel-rich positive electrode material to inhibit the HF elimination reaction between the residual alkali and the 1,1-difluoroethylene polymer, thereby preventing the formation of a conjugated polyene structure, avoiding gelation of the positive electrode slurry, and improving its viscosity and electrical performance. The positive electrode slurry of the present invention is suitable for preparing positive electrode sheets and lithium-ion secondary batteries, and the resulting battery has low internal resistance and a good constant current charge ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion secondary batteries, and more specifically, to a positive electrode slurry, a positive electrode sheet and a lithium-ion secondary battery. Background Art

[0002] With the rapid development of the new energy industry, lithium-ion secondary batteries (Li-ion batteries) have attracted significant attention as a key chemical energy source in this field. The positive electrode in a Li-ion secondary battery is one of its core components. The higher its energy density and capacity, the better the battery's kinetic performance. Nickel-rich cathode active materials, with their high energy density, are increasingly popular in long-range vehicles. However, increasing the nickel content in the cathode active material leads to high residual alkali on the surface of the cathode active material. This leads to a HF elimination reaction between the Ni-containing 1,1-difluoroethylene copolymer and the material, forming a conjugated polyene structure. This ultimately causes gelation during slurry preparation, preventing smooth electrode coating.

[0003] Prior art discloses a positive electrode mixture for a secondary battery, comprising a lithium-nickel composite oxide positive electrode active material, a vinylidene fluoride binder, and an organic acid. The mixture introduces carboxylic acid groups to inhibit gelation of the high-nickel positive electrode slurry. However, the binding energy between the carboxylic acid groups and lithium ions is greater, making lithium ions less likely to dissociate. This results in a high internal resistance, poor conductivity, and a low constant current charge ratio, hindering rapid charging and discharging. Battery kinetics still require improvement. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the existing lithium-ion secondary battery's positive electrode slurry, such as large internal resistance, low constant current charging ratio, and insufficient battery kinetic performance, and to provide a positive electrode slurry that prevents gelation of the positive electrode slurry by controlling the content of the positive electrode active material and the binder and the length of the sulfonic acid side chain in the binder, thereby reducing the internal resistance, increasing the constant current charging ratio, and improving its battery kinetic performance.

[0005] Another object of the present invention is to provide an application of a positive electrode slurry in a positive electrode sheet.

[0006] Another object of the present invention is to provide an application of a positive electrode slurry in a lithium-ion secondary battery.

[0007] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0008] A positive electrode slurry comprises a solid substance and a solvent, wherein the solid substance comprises a nickel-rich positive electrode active material, a conductive agent and a binder, wherein the mass content of the nickel-rich positive electrode active material in the solid substance is 90-98%, and the mass content of the binder is 0.5-5%;

[0009] The binder is a 1,1-difluoroethylene copolymer having a sulfonic acid group, and has the following sulfonic acid group-containing structural units:

[0010]

[0011] wherein R1, R2, and R3 are each independently a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 2 carbon atoms; and X is selected from one or more of ester, amide, and ether structures having a main chain of 2 to 20 carbon atoms;

[0012] The structural formula of the nickel-rich positive electrode active material is: Li x Ni y M 1-y O2, wherein 0.9≤X≤1.1, 0.6≤y≤1, and M is selected from one or more of Co, Mn, and Al.

[0013] It should be noted that:

[0014] Due to the high nickel content in the nickel-rich positive active material, the residual alkali on the surface of the active material increases, and after reacting with the 1,1-difluoroethylene copolymer, a conjugated polyene structure that is easily cross-linked is formed. The resulting positive electrode slurry will eventually undergo gelation, and the positive electrode sheet cannot be prepared. The nickel-rich positive active material in the positive electrode slurry of the present invention is rich in nickel and has a high energy density. Since the binder of the present invention is a 1,1-difluoroethylene copolymer having a sulfonic acid group, it can react with the residual alkali on the surface of the nickel-rich positive active material, inhibiting the HF elimination reaction between the residual alkali and the 1,1-difluoroethylene polymer, preventing the formation of a conjugated polyene structure, and avoiding the occurrence of chemical cross-linking. Therefore, the gelation phenomenon caused by the increase in nickel content in the positive electrode slurry is avoided.

[0015] In the binder, the length of X affects the degree of swing of the sulfonic acid group. When the main chain of X is multiple atoms, the sulfonic acid group is easy to swing and easily migrate to the aluminum foil interface, thereby enhancing the bonding strength of the binder. However, when the main chain of X has too many atoms and the main chain is too long, the content of sulfonic acid groups in the binder of the same mass is too low, which is not conducive to preventing the gelation of the positive electrode slurry. Moreover, when the content of sulfonic acid groups is too low, there are also fewer lithium sulfonate groups that react with the electrolyte to have high lithium ion conductivity, which is not conducive to reducing the internal resistance of the battery and improving the constant current charging ratio.

[0016] In addition, when the mass content of the binder is too low or the content of the positive electrode active material is too high, the sulfonic acid group cannot completely react the residual alkali on the surface of the nickel-rich positive electrode active material, and the bonding force between the active material and the conductive agent is poor. The active material is easy to fall off from the current collector during the battery cycle, resulting in a battery cycle drop; when the mass content of the binder is too high or the content of the nickel-rich positive electrode active material is too low, the effective contact between the nickel-rich positive electrode active material and the conductive agent is reduced, and the obtained positive electrode slurry is used in the battery. The capacity of the battery is reduced, the internal resistance is increased, and the battery kinetic performance is also reduced. When the mass content of the binder is too high, the molecular chains are entangled with each other, and gelation will also occur.

[0017] The solvent of the positive electrode slurry of the present invention is preferably N-methylpyrrolidone; the conductive agent of the positive electrode slurry is preferably one or more of conductive graphite, conductive carbon black, acetylene black, carbon nanotubes, and graphene, wherein the mass content of the conductive agent in the solid material is preferably 0.1 to 5%.

[0018] The binder can be obtained by copolymerizing 1,1-difluoroethylene with one or more of 2-acrylamido-2-methylpropanesulfonic acid, 2-methyl-2-acrylate-2-sulfoethyl ester, 2-trifluoromethyl-2-acrylate-2-sulfoethyl ester, 3-allyl-2-hydroxy-1-propanesulfonic acid and perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonic acid.

[0019] Preferably, the mass content of the nickel-rich positive electrode active material in the solid material is 95-98%, and the mass content of the binder is 1-2%.

[0020] Preferably, the molar content of the structural units containing sulfonic acid groups in the binder is 0.1 to 5%.

[0021] If the content of sulfonic acid groups in the binder is too low, it cannot fully react with the residual alkali, thereby inhibiting the gelation of the positive electrode slurry; while if the content of sulfonic acid groups is too high, the crystallinity of 1,1-difluoroethylene copolymer will be reduced, and the binder's resistance to electrolyte corrosion in the battery will be reduced, which is not conducive to long-term cycle performance.

[0022] More preferably, the molar content of the structural units containing sulfonic acid groups in the binder is 0.8 to 1.5%, and even more preferably 1 to 1.2%.

[0023] Specifically, the weight average molecular weight of the binder is 6.5 to 3 million.

[0024] More preferably, the weight average molecular weight of the binder is 1 to 1.5 million, and even more preferably 1.22 million.

[0025] The binder of the present invention is a 1,1-difluoroethylene copolymer having a sulfonic acid group. When the molecular weight is too low, the molecular chain length is short, which is not conducive to improving the bonding strength; when the molecular weight is too large, the copolymer dissolves slowly, affecting production efficiency.

[0026] Preferably, the viscosity of the positive electrode slurry is 5000 to 15000 mPa·s, more preferably 6000 to 8000 mPa·s.

[0027] Low slurry viscosity not only causes the slurry to settle easily, but also generally results in a high solvent content, requiring high energy consumption to evaporate the solvent, impacting production efficiency. Excessive slurry viscosity results in poor fluidity in pipelines, hindering production efficiency. Furthermore, controlling the viscosity of the positive electrode slurry can control the performance of the resulting battery. Low positive electrode slurry viscosity generally results in low peel strength for the resulting positive electrode sheet, impacting battery life and stability.

[0028] The positive electrode slurry of the present invention can be prepared by the following preparation method:

[0029] The nickel-rich positive electrode active material, the conductive agent and the sulfonic acid monomer copolymerized 1,1,-difluoroethylene are evenly mixed, and a solvent is added and stirred to synthesize a slurry.

[0030] The present invention also protects the use of the positive electrode slurry in preparing positive electrode sheets.

[0031] The present invention specifically protects a positive electrode sheet, which is prepared from the positive electrode slurry of the present invention.

[0032] The binder in the positive electrode slurry described in this invention is a vinylidene fluoride copolymer with sulfonic acid groups. The sulfonic acid groups react with residual alkali to inhibit gelation of the positive electrode slurry, allowing the positive electrode sizing material to be smoothly applied to the electrode sheet. Furthermore, the present invention controls the binder content in the positive electrode slurry and the length of X in the binder, resulting in a strong adhesive bond and excellent peelability of the resulting positive electrode sheet.

[0033] The present invention particularly protects a lithium-ion secondary battery, wherein the positive electrode sheet of the lithium-ion secondary battery is prepared with the positive electrode slurry of the present invention.

[0034] The present invention introduces sulfonic acid groups as binders into 1,1-difluoroethylene copolymers, which can react with residual alkali on the surface of nickel-rich positive electrode active materials to prevent the formation of conjugated polyene structures, avoid chemical cross-linking, and prevent the gelation of the positive electrode slurry. In addition, since the sulfonic acid groups can react with the electrolyte to be converted into lithium sulfonate groups with high lithium ion conductivity, the prepared battery has lower internal resistance and higher constant current charging ratio, which can give the battery better dynamics.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The binder of the positive electrode slurry of the present invention has a sulfonic acid group side chain of a certain length, which can avoid the gel phenomenon. The positive electrode slurry does not gel after being left to stand for 12 hours, and the slurry can flow. The positive electrode sheet prepared from the positive electrode slurry has a peel strength of 68 to 168 N / m. The prepared lithium-ion secondary battery has a capacity greater than 2.28 Ah, an internal resistance less than 4.96 mΩ, and a constant current charging ratio greater than 80.4%. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a picture of the positive electrode slurry of Example 1 after standing for 12 hours;

[0038] Figure 2 This is a picture of the positive electrode slurry of Comparative Example 1 after it was left to stand for 12 hours. DETAILED DESCRIPTION

[0039] The present invention will be further described below in conjunction with specific embodiments, but the examples do not limit the present invention in any form. Unless otherwise specified, the raw materials and reagents used in the examples of the present invention are conventionally purchased raw materials and reagents.

[0040] Example 1

[0041] A positive electrode slurry, comprising a solid substance and a solvent, wherein the solid substance is a positive electrode active material, a conductive agent and a binder, and the positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 O2, the binder is 2-acrylamide-2-methylpropanesulfonic acid modified 1,1-difluoroethylene copolymer, the conductive agent is high-purity conductive carbon black (SUPER P Li), the solvent is N-methylpyrrolidone, and the solid material contains the nickel-rich positive electrode active material and the binder, and the balance is the conductive agent;

[0042] The parameters of the positive electrode slurry are detailed in Table 1.

[0043] Table 1. Parameters of positive electrode slurry in Example 1

[0044]

[0045]

[0046] The positive electrode slurry can be prepared by the following method, which includes the following steps:

[0047] Adding LiNi in a double planetary mixer 0.8 Co 0.1 Mn 0.1O2 active material, conductive agent and binder, gradually add some N-methylpyrrolidone (NMP), and stir at 1500rpm to synthesize slurry. Finally, by adding NMP solvent and testing with Brookfield rotational viscometer, the initial viscosity of the positive electrode slurry is adjusted to 7230mPa·s.

[0048] Examples 2 to 9

[0049] A positive electrode slurry, the difference from Example 1 is shown in Table 2.

[0050] Table 2. Parameters of positive electrode slurry for Examples 1 to 9

[0051]

[0052] The preparation method of the positive electrode slurry is the same as that of Example 1.

[0053] Comparative Example 1

[0054] A positive electrode slurry is prepared by the same method as in Example 1, except that no 2-acrylamide-2-methylpropanesulfonic acid monomer is added in the preparation of the 1,1-difluoroethylene copolymer, and the molecular weight Mw of the binder is 1.18 million.

[0055] Comparative Example 2

[0056] A positive electrode slurry is different from the embodiment in that the binder used is Arkema Kynar HSV900.

[0057] Comparative Example 3

[0058] A positive electrode slurry is different from Example 1 in that the binder is acrylic acid copolymer PVDF, wherein the molecular weight of the binder is Mw 1.16 million.

[0059] Comparative Example 4

[0060] A positive electrode slurry is different from Example 1 in that the mass content of the binder in the positive electrode slurry is 0.1%.

[0061] Comparative Example 5

[0062] A positive electrode slurry is different from Example 1 in that the mass content of the binder in the positive electrode slurry is 7%.

[0063] Result detection

[0064] Detection of sulfonic acid group content in 1,1-difluoroethylene copolymer:

[0065] Acid-base titration: Dissolve 0.2 g of vinylidene fluoride copolymer in 19.8 g of acetone at approximately 80°C, then add 1 g of pure water to prepare the vinylidene fluoride copolymer solution to be titrated. Neutralization titration is performed using 0.01 mol / L sodium hydroxide solution at room temperature, using phenolphthalein as the indicator.

[0066] Binder weight average molecular weight test:

[0067] The weight average molecular weight of the copolymer was determined by gel permeation chromatography (GPC) using polymethyl methacrylate as the standard and N,N-dimethylacetamide (DMAc) as the mobile phase.

[0068] Cathode slurry viscosity test:

[0069] 120 g of the slurry was placed in a 150 ml beaker, sealed with parafilm, and placed in a 2520.2°C water bath for 1 hour. Viscosity was measured using a Brookfield rotational viscometer, model DV2TLVTJ0, rotor 63, at 12 rpm.

[0070] Positive electrode peeling force test:

[0071] Use a scraper to evenly coat the slurry on both sides of a 12um thick aluminum foil, bake it in a blast oven at 120°C for 40 minutes, and the single-sided coating density is 160g / m2, and the double-sided density is 320g / m2. Then, it is rolled by a roller press, and the compaction density is controlled to 3.5g / cm3 to obtain the positive electrode sheet. Then, a 3M HVB double-sided tape (19mm*60mm) is attached to one end of the steel plate. After that, the negative electrode sheet is cut into 20mm*200mm strips, and its positive active layer is attached to the double-sided tape. In an atmosphere of 25°C and 50% relative humidity, the stress of peeling the aluminum foil in the 180° direction at a speed of 100mm / min is measured as the bonding strength.

[0072] Lithium-ion secondary battery performance test:

[0073] (1) Preparation of lithium-ion secondary batteries:

[0074] The negative electrode material, artificial graphite, the conductive agent, acetylene black, the adhesive, butadiene-styrene emulsion, and the dispersant, sodium carboxymethyl cellulose, were thoroughly stirred and mixed in a deionized water solvent system. The mixture was then coated onto a copper foil negative electrode current collector to form a negative electrode sheet. A porous PE membrane was used as the separator. The electrolyte was a 1 mol / L LiPF6 solution, and the solvents were ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate. Finally, the positive electrode sheet containing the positive electrode slurry of the present invention, the negative electrode sheet, and the separator were assembled into a soft-pack battery with a capacity of approximately 3Ah (1C constant volume).

[0075] (2) Internal resistance test:

[0076] When the battery cell is charged to 50% capacity, use a 1kHz voltage internal resistance tester to detect the internal resistance of the battery.

[0077] (3) Constant current charging ratio:

[0078] At 25 2 5 ℃, discharge the battery cell at a constant current of 0.33C to 3.0V, let it stand for 10 minutes, and then charge it at a constant current of 2C to 4.35V; then change to constant voltage charging until the charging current is ≤ 0.02C; among them, the capacity of the constant current charging stage is C1, and the capacity of the constant voltage charging stage is C2, then the constant current charging ratio = C1 / (C1+C2).

[0079] (4) Battery capacity:

[0080] At 25°C, the cell was discharged at a constant current of 0.33C to 3.0V and allowed to rest for 10 minutes. It was then charged at a constant current of 1.0C to 4.35V. The cell was then switched to constant voltage charging until the charging current was ≤0.02C. The capacity during the constant current charging phase is Q1, and the capacity during the constant voltage charging phase is Q2. Therefore, the battery capacity is Q1 + Q2. The test results are detailed in Tables 3 and 4.

[0081] Table 3. Molecular weight of 1,1-difluoroethylene copolymers and molar content of sulfonic acid group-containing structural units

[0082]

[0083]

[0084] Table 4. Positive electrode slurry, positive electrode sheet and its lithium ion secondary battery performance

[0085]

[0086]

[0087] It can be seen from Table 3 and Table 4 that in the positive electrode slurry prepared by the present invention, the content of the nickel-rich positive electrode active material and the binder, the source of the sulfonic acid group in the binder, the molar content of the structural unit containing the sulfonic acid group, and the molecular weight of the binder will affect the performance of the obtained positive electrode slurry, the positive electrode sheet, and the lithium ion secondary battery. And it can be seen from the data in Table 4 that the greater the viscosity of the positive electrode slurry, the greater the peel strength of the positive electrode sheet (such as Example 1 and Example 3), indicating that the effect of the viscosity of the positive electrode slurry on the peeling force of the positive electrode sheet is not a simple linear trend. The binder in Comparative Examples 1 and 2 does not contain a sulfonic acid group, and the positive electrode slurry gels. Comparative Example 3 is modified with a carboxylic acid group, and the internal resistance of the obtained lithium ion secondary battery is higher and the constant current charge ratio is worse. The lower the internal resistance of the battery, the higher the constant current charge ratio, indicating that the battery kinetics performance is better and more conducive to rapid charge and discharge, that is, modified with a carboxylic acid group, and its battery kinetics performance is not as good as that of the sulfonic acid group modification. The slurry in Comparative Example 5 gelled after standing for 12 hours, making it difficult to coat the positive electrode sheet in actual production.

[0088] Figure 1 and Figure 2 The pictures shown are of the positive electrode slurries of Example 1 and Comparative Example 1 after standing for 12 hours. It can be seen that the positive electrode slurry of Example 1 has not gelled and is in a flowable state, and can be used for positive electrode sheet coating, while the positive electrode slurry of Comparative Example 1 has gelled and cannot flow, making it difficult to coat the positive electrode sheet in actual production.

[0089] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the claims of the present invention.

Claims

1. A positive electrode slurry comprising a solid substance and a solvent, wherein the solid substance is a nickel-rich positive electrode active material, a conductive agent and a binder, characterized in that: The mass content of the nickel-rich positive electrode active material in the solid material is 90-98%, and the mass content of the binder is 0.5-5%; The binder is obtained by copolymerizing 1,1-difluoroethylene with one or more of 2-acrylamido-2-methylpropanesulfonic acid, 2-methyl-2-acrylic acid-2-sulfoethyl ester, 2-trifluoromethyl-2-acrylic acid-2-sulfoethyl ester, 3-allyl-2-hydroxy-1-propanesulfonic acid and perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonic acid; The structural formula of the nickel-rich positive electrode active material is: Li x Ni y M 1-y O2, wherein 0.9≤X≤1.1, 0.6≤y≤1, and M is selected from one or more of Co, Mn, and Al; The molar content of the structural unit containing sulfonic acid groups in the binder is 0.1-4.5%.

2. The positive electrode slurry according to claim 1, characterized in that The mass content of the nickel-rich positive electrode active material in the solid material is 95-98%, and the mass content of the binder is 1-2%.

3. The positive electrode slurry according to claim 1, characterized in that The molar content of the structural unit containing sulfonic acid groups in the binder is 0.8-1.5%.

4. The positive electrode slurry according to claim 1, characterized in that The weight average molecular weight of the binder is 6.5 to 3 million.

5. The positive electrode slurry according to claim 4, characterized in that The weight average molecular weight of the binder is 1 to 1.5 million.

6. The positive electrode slurry according to claim 5, characterized in that The viscosity of the positive electrode slurry is 5000-15000 mPa·s.

7. Use of the positive electrode slurry according to any one of claims 1 to 6 in preparing a positive electrode sheet.

8. A positive electrode sheet, characterized in that: The positive electrode sheet is prepared from the positive electrode slurry according to any one of claims 1 to 6.

9. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises a positive electrode sheet prepared with the positive electrode slurry according to any one of claims 1 to 6.

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