Fluoride-free binder material for positive electrode, and preparation method therefor and use thereof

Abstract

The present disclosure relates to a fluoride-free binder material for a positive electrode, and a preparation method therefor and the use thereof. Provided are a fluoride-free binder material for a positive electrode and a preparation method therefor. A copolymer of acrylonitrile and an acrylate is used as a main body structure, an ionic monomer is introduced, and a polyfunctional olefin monomer is introduced during polymerization to increase the branching degree of a polymer, thereby preparing a highly-branched acrylonitrile polymer modified via copolymerization, which polymer is used as a fluorine-free binder material for a positive electrode. The modified binder can provide more sites for contact between active materials and between a current collector and the active materials so as to further enhance the bonding ability, thereby increasing the overall cohesive force and improving the adhesion to the current collector; and the binder can effectively inhibit the fracture and delamination of an electrode sheet during processing, ensures the compactness of a positive electrode material even in the case of a low use amount, and retains structural integrity and cycling stability during cycling.

Description

A positive electrode fluorine-free binder material and its preparation method and application

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure claims priority to application number 2024102142848 filed with the Patent Office of China on February 27, 2024, entitled “A positive electrode fluorine-free binder material, its preparation method and application”, the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present disclosure relates to the technical field of positive electrode binder materials for new energy lithium-ion batteries, and in particular to a positive electrode fluorine-free binder material and a preparation method and application thereof. Background Art

[0004] With the development of battery technology, the requirements for battery energy density are becoming increasingly higher. In addition to developing high-energy-density electrode material systems, reducing the proportion of inactive materials by coating ultra-thick electrodes or reducing the amount of binder used is also a major solution. Therefore, more stringent requirements are put forward for the polyvinylidene fluoride (PVDF) system, the main product type of electrode materials, to further improve its bonding strength and increase the softness of the electrodes.

[0005] In order to achieve this requirement, PVDF needs to be further copolymerized and modified. On the one hand, the orderly arrangement of the polymer chains is prevented, thereby reducing the crystallinity of the PVDF system and improving the softness of the electrode, so that the processing and use requirements can still be met under ultra-thick electrode coating. On the other hand, the grafted polar groups can increase the adhesion with the foil to ensure that the electrode will not be demolded due to the reduction of the adhesive dosage. However, the copolymerization modification of vinylidene fluoride (VDF) is more difficult. Foreign companies have more mature technology in selecting suitable comonomers and dosages, and there are certain technical barriers. At the same time, due to the large difference in the competitive polymerization rate between VDF and olefin monomers, only a small proportion of olefin monomers can be copolymerized with VDF (the proportion of olefin copolymerization generally does not exceed 5%). If you want to achieve a significant improvement in performance, the modification is difficult. The currently used PVDF does not have functional groups and cannot play the role of conducting lithium ions and electrons. It only has weak van der Waals forces with the electrode materials, which makes it unable to achieve strong adhesion properties. It has limitations in its use in high energy / high power density and long cycle life batteries. If it is modified, it is difficult to modify due to the reactivity problem of the VDF material itself, and functionalization cannot be achieved. In addition, the VDF raw material for synthesizing PVDF is a high-risk environmental pollutant. The supply of raw materials is limited and cannot meet the growing demand for lithium-ion batteries. In actual application, there are also problems such as gelation. At the same time, in the ternary positive electrode, due to the inherent properties of the PVDF system, the main chain of the polymer is easily de-HF to form double bonds in a strong alkaline environment. The water in the slurry or the amines in the solvent attack the double bonds, which eventually causes the slurry to easily gel, affecting the normal ingredients, coating and subsequent processes, and seriously reducing production capacity.

[0006] PVDF is a fluorine-containing binder, and its polymer monomer VDF is prepared by dehydrochlorination of 1,1-difluoro-1-chloroethane (R142B). However, R142B is a highly dangerous environmental pollutant and does not meet the requirements of green and sustainable development of the new energy industry. Developed countries such as the European Union are gradually restricting its use in lithium batteries.

[0007] In response to the above problems, the market now hopes to develop a new technology that can solve the above technical problems, and polyacrylonitrile adhesives are the most promising direction. However, the existing polyacrylonitrile adhesives have poor solubility during direct use and need to be stirred and dissolved in N-methylpyrrolidone (NMP) for a long time or pre-dispersed with the addition of organic solvents. The processing process is too complicated. At the same time, when preparing the glue and slurry, the solid content cannot be too high, otherwise it will lead to high viscosity, which is not conducive to industrial production. In addition, the toughness of acrylonitrile polymers is poor, and they are prone to cracking during rolling, winding, and assembly processes.

[0008] Therefore, developing a new modified acrylonitrile-based green and environmentally friendly fluorine-free positive electrode binder to replace the use of PVDF in the positive electrode, solving its problems in the existing processing and application process, and providing better processing performance, higher energy density and more stable cycle performance, has great social and economic significance and market prospects. Summary of the Invention

[0009] The technical problem to be solved by the present disclosure is to overcome the deficiencies and defects mentioned in the above background technology, provide a polymer with a copolymer of acrylonitrile and acrylate as the main structure, introduce an ionic monomer, and introduce a multifunctional olefin monomer during the polymerization process to increase the branching degree of the polymer, thereby preparing a highly branched copolymer-modified acrylonitrile polymer as a fluorine-free binder material for the positive electrode and its application.

[0010] In order to solve the above technical problems, the technical solutions proposed in this disclosure are:

[0011] A positive electrode fluorine-free binder material comprises the following raw materials in parts by weight:

[0012] 40-60 parts of a first monomer, wherein the first monomer is acrylonitrile and / or methacrylonitrile;

[0013] 30-60 parts of the second monomer, which is an acrylate and / or methacrylate monomer containing a long-chain alkane;

[0014] 5-15 parts of the third monomer, wherein the third monomer is a combination of one or more of an olefinic acidic monomer, a sodium salt corresponding to an olefinic acidic monomer, or a lithium salt corresponding to an olefinic acidic monomer.

[0015] Preferably, the second monomer is a combination of one or more of ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl methacrylate, butyl acrylate, butyl methacrylate, isobutyl methacrylate, isodecyl methacrylate, amyl acrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, isooctyl acrylate, isooctyl methacrylate, lauryl acrylate, lauryl methacrylate, octadecyl acrylate, octadecyl methacrylate, methoxypolyethylene glycol methacrylate and ethoxyethyl acrylate. The cyano group provided by the first monomer can give the binder stronger cohesion, and the second monomer provides a long-chain alkane structure for the binder molecule. The long-chain alkane can give the binder better flexibility and electrolyte resistance.

[0016] Preferably, the third monomer is a combination of one or more of acrylic acid, sodium acrylate, lithium acrylate, methacrylic acid, sodium methacrylate, lithium methacrylate, β-acryloyloxypropionic acid, 2-acrylamide-2-methylpropanesulfonic acid, sodium allyl sulfonate, sodium vinyl sulfonate, sodium allyl sulfonate and sodium styrene sulfonate. The third monomer is an ionic monomer whose purpose is to ionize ions and to complex with Li+ during battery cycling to increase ionic conductivity. It can be an propylene carboxylic acid monomer, an propylene sulfonic acid monomer, and their corresponding lithium salts, sodium salts and the like.

[0017] Preferably, the raw material further comprises 2.5-5 parts by mass of a fourth functional monomer, and the fourth functional monomer is a multiolefin monomer.

[0018] Preferably, the fourth functional monomer is a combination of one or more of 1,6-hexanediol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol polymethyl ethylene oxide diacrylate, ethoxylated bisphenol A diacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate and propoxylated glycerol triacrylate.

[0019] Based on the general inventive concept, the present disclosure also provides a method for preparing a positive electrode fluorine-free binder material, comprising the following steps:

[0020] (1) Pre-emulsification: adding an emulsifier, an initiator, and deionized water to a portion of the first monomer, the second monomer, and the third monomer, and pre-emulsifying to obtain a mixed emulsion;

[0021] (2) heating the mixed emulsion pre-emulsified in step (1) to a reaction temperature and stirring the mixture to react; and when the emulsion begins to change color to blue, adding dropwise a fourth functional monomer and a mixture of the first monomer, the second monomer, and the third monomer to obtain a mixed emulsion;

[0022] (3) adding an initiator dropwise to the mixed emulsion obtained in step (2), and reacting at the same temperature to obtain a preliminary emulsion;

[0023] (4) filtering, neutralizing, and cooling the preliminary emulsion obtained in step (3) to obtain a polymer emulsion;

[0024] (5) drying the polymer emulsion obtained in step (4) to obtain a solid powder;

[0025] (6) The solid powder obtained in step (5) is washed, purified, dried, and sieved to obtain a positive electrode fluorine-free binder material.

[0026] The preparation method disclosed herein is applicable to various polymerization methods, such as emulsion polymerization, microemulsion polymerization, suspension polymerization, microsuspension polymerization, solution polymerization, bulk polymerization and the like.

[0027] Preferably, the portion of the first monomer, the second monomer, and the third monomer in step (1) accounts for 85-90% of the mass of the total first monomer, the second monomer, and the third monomer.

[0028] Preferably, in step (1), the amount of the emulsifier added is 0.5-5 parts by mass, and the amount of the initiator added is 0.05-3.75 parts by mass; the emulsifier is a combination of one or more of calcium dodecylbenzoate, sodium dodecylsulfonate, sodium dodecyl sulfate, SR-10, SE-10, SN-10 and OP-10, and the initiator is a combination of one or more of sodium persulfate, potassium persulfate, ammonium persulfate and azobisisobutylamidine hydrochloride.

[0029] Preferably, the amount of the initiator added in step (3) is 0.05 to 1.25 parts by mass.

[0030] Preferably, the emulsification time of the pre-emulsification in step (1) is 0.5 h to 2 h; the temperature-raising reaction in step (2) is to raise the temperature to 50 to 80° C.; the heat-keeping reaction in step (3) is 2 to 3 h; the pH adjuster added for neutralization in step (4) is any one of ammonia water, lithium salt, and sodium salt, and the pH value adjusted by neutralization is 6.8 to 7.2; the drying in step (5) is any one or more of demulsification drying, vacuum drying, spray drying, and forced air drying. The particle size is controlled by sieving to be less than 1 mm to prevent the particles from agglomerating during the drying process, resulting in a reduction in specific surface area, thereby affecting the dissolution rate.

[0031] Based on the overall inventive concept, the present disclosure also provides an application of a positive electrode fluorine-free binder material, wherein the positive electrode fluorine-free binder material is used to prepare a lithium ion battery positive electrode, wherein the lithium ion battery positive electrode includes a current collector and a positive electrode active coating coated on the surface of the current collector.

[0032] Preferably, a lithium ion soft pack battery is prepared, wherein the lithium ion soft pack battery includes the lithium ion battery positive electrode.

[0033] The positive electrode current collector includes but is not limited to one or more combinations of lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide and lithium iron manganese phosphate.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The main structure of the positive electrode binder provided by this disclosure consists of an alkane polymer backbone and three functional groups: cyano groups, ionic groups, and long-chain alkanes. The cyano groups impart strong cohesion to the binder; the long-chain alkanes provide flexibility and electrolyte resistance; and the ionic groups improve the binder's adhesion and ionic conductivity. The addition of a fourth functional monomer introduces a branched structure to further optimize performance. After the branching modification, the viscosity of the glue is greatly reduced. The low viscosity of the glue improves the solubility of the binder in NMP, and the positive electrode slurry configured therewith has excellent fluidity and stability. The viscosity of the glue is greatly reduced after the branching modification because, on the one hand, the molecular chain segments are not completely stretched out after the branching modification, and the long-chain branching will increase the viscous flow activation energy of the polymer, thereby further reducing the viscosity of the polymer; on the other hand, when the molecular weight is equivalent, the relaxation time of the material with a branched structure is relatively short compared to the material with an ordinary long-chain structure, which is also beneficial to reducing the viscosity of the polymer; furthermore, the molecular chain structure of the branched structure is more compact, and the probability of entanglement between molecular chains is greatly reduced compared to the segment type. All of these contribute to reducing the viscosity of the polymer; at the same time, the branched modified binder can provide more contact sites between the active substances and the active substances, and between the current collector and the active substances. After the branched modification, the molecular chain segments are not completely stretched out. Per unit area, the -CN content is higher and the polarity is stronger. The number of contact sites forming a bonding structure between the positive electrode active material and the conductive agent is increased, the bonding efficiency is higher, and the bonding ability is further enhanced, which increases the overall cohesion and the bonding to the current collector. It can effectively inhibit the breakage and peeling of the pole pieces during the processing process, and can still make the positive electrode materials in close contact with each other at a lower dosage, maintaining the structural integrity and cycle stability during the cycle. DETAILED DESCRIPTION

[0036] To facilitate understanding of the present disclosure, the present disclosure will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments, but the scope of protection of the present disclosure is not limited to the following specific embodiments.

[0037] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present disclosure.

[0038] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present disclosure can be purchased from the market or prepared by existing methods.

[0039] Example 1:

[0040] A method for preparing a positive electrode fluorine-free binder material:

[0041] In a four-necked reaction flask equipped with a stirrer, a thermometer, a reflux condenser, and a constant pressure dropping funnel, an emulsifier (SR-10, 0.5 g), an initiator (sodium persulfate, 0.4 g), and a solvent (H2O, 553.3 g) were added. After stirring to completely dissolve, nitrogen was introduced. Then, the first monomer (acrylonitrile, 32 g), the second monomer (lauryl acrylate, 42.5 g), and the third monomer (lithium methacrylate, 5 g) were added and pre-emulsified for 0.5 h. Then, under the protection of inert gas N2, the temperature was raised to the reaction temperature of 60°C, and the reaction was stirred. When the emulsion turned blue, the reaction was continued. The reaction was continued for about 10 minutes, and then a mixture of the monomer (acrylonitrile, 8 g) and the fourth functionalized monomer (1,6-hexanediol diacrylate, 5 g) was slowly added dropwise under stirring for 1 hour. After the addition was completed, an initiator (potassium persulfate, 0.1 g) was added, and the reaction was continued for 4 hours. After the reaction was completed, an appropriate amount of ammonia water was added to neutralize the mixture to a pH of 7. The product was filtered to obtain a polymer emulsion. The obtained polymer emulsion was spray-dried to obtain a solid powder. The solid powder was washed with deionized water, dried, crushed, and sieved to prepare the positive electrode fluorine-free binder material.

[0042] Example 2:

[0043] A method for preparing a positive electrode fluorine-free binder material:

[0044] In a four-necked reaction flask equipped with a stirrer, a thermometer, a reflux condenser, and a constant pressure dropping funnel, an emulsifier (sodium lauryl sulfate, 0.5 g), an initiator (ammonium persulfate, 0.4 g), and a solvent (H2O, 553.3 g) were added. After stirring to completely dissolve, nitrogen was introduced, and then the first monomer (acrylonitrile, 32.5 g), the second monomer (lauryl acrylate, 40 g), and the third monomer (lithium methacrylate, 5 g) were added and pre-emulsified for 0.5 h. Then, under the protection of inert gas N2, the temperature was raised to the reaction temperature of 60°C, and the reaction was stirred. When the emulsion turned blue, the reaction was continued for about 10 min. Then, a mixture of the first monomer (acrylonitrile, 7.5g), the second monomer (lauryl acrylate, 2.5g) and the fourth functionalized monomer (1,6-hexanediol diacrylate, 2.5g) was slowly added dropwise under stirring, and the addition time was 1h. After the addition was complete, an initiator (potassium persulfate, 0.1g) was added, and the reaction was continued for 4h. After the reaction was completed, an appropriate amount of ammonia was added and neutralized to a pH of 7. The product was filtered, and the resulting polymer emulsion was spray-dried to obtain a solid powder. The solid powder was washed with deionized water, dried, crushed, and sieved to prepare the positive electrode fluorine-free binder material. Comparative Example 1, this embodiment changes the feeding method of the subsequent branched modification, and can also optimize the unbranched modified material.

[0045] Example 3:

[0046] A method for preparing a positive electrode fluorine-free binder material:

[0047] The difference between this embodiment and embodiment 1 is that the first monomer is replaced by methacrylonitrile, and the amount of the second monomer, lauryl acrylate, used in the pre-emulsification process is increased by 10 g. The rest is the same as in embodiment 1. Because methacrylonitrile has one more methyl group than acrylonitrile monomer, the presence of the methyl group significantly restricts the rotational movement of the CC main chain after polymerization. In order to maintain good flexibility of the film, the amount of the soft monomer needs to be increased.

[0048] Example 4:

[0049] A method for preparing a positive electrode fluorine-free binder material:

[0050] The difference between this embodiment and Example 1 is that the second monomer is replaced with octadecyl acrylate, the amount used during pre-emulsification is reduced to 30 g, and the amount added dropwise remains unchanged. The rest is the same as Example 1. Because octadecyl acrylate has a longer carbon chain, it can provide better toughness at a smaller ratio.

[0051] Example 5:

[0052] A method for preparing a positive electrode fluorine-free binder material:

[0053] The difference between this embodiment and Example 1 is that the second monomer is replaced by butyl acrylate in an amount of 60 g, and the rest is the same as Example 1. Because the carbon chain of butyl acrylate is shorter, the toughness of the film is insufficient when the amount is small, and its amount needs to be increased to provide better toughness for the electrode.

[0054] Example 6:

[0055] A method for preparing a positive electrode fluorine-free binder material:

[0056] The difference between this embodiment and embodiment 1 is that the third monomer is replaced by sodium methacrylate, and the rest is the same as embodiment 1.

[0057] Example 7:

[0058] A method for preparing a positive electrode fluorine-free binder material:

[0059] The difference between this embodiment and embodiment 1 is that the fourth functional monomer is replaced by trimethylolpropane triacrylate and the amount used is reduced to 3 g. The rest is the same as embodiment 1.

[0060] Comparative Example 1:

[0061] A positive electrode binder material: selected from commercial product Solvay PVDF5130 on the market.

[0062] Comparative Example 2:

[0063] A positive electrode binder material: In a four-necked reaction flask equipped with a stirrer, a thermometer, a reflux condenser, and a constant pressure dropping funnel, an emulsifier (sodium lauryl sulfate, 0.5g), an initiator (potassium persulfate, 0.5g), and a solvent (H2O, 533.3g) are added. After stirring and dissolving completely, nitrogen is introduced, and then a first monomer (acrylonitrile, 40g), a second monomer (lauryl acrylate, 42.5g), and a third monomer (lithium methacrylate, 5g) are added and pre-emulsified for 0.5h. Then, under the protection of inert gas N2, the temperature is raised to a reaction temperature of 60°C, stirred for reaction, and kept warm for 5h. After the reaction is completed, an appropriate amount of ammonia water is added to neutralize to a pH of 7, and the product is filtered to obtain a polymer emulsion. The obtained polymer emulsion is spray-dried to obtain a solid powder. The solid powder is washed with deionized water, dried, crushed, and sieved to prepare the lithium ion battery positive electrode binder. The difference from Example 1 is that no fourth functional monomer is added for branching modification.

[0064] Comparative Example 3:

[0065] A positive electrode binder material: In a four-necked reaction flask equipped with a stirrer, a thermometer, a reflux condenser, and a constant pressure dropping funnel, an emulsifier (sodium lauryl sulfate, 0.5g), an initiator (ammonium persulfate, 0.5g), and a solvent (H2O, 533.3g) are added. After stirring and dissolving completely, nitrogen is introduced, and then a first monomer (acrylonitrile, 40g), a second monomer (octadecyl acrylate, 42.5g), and a third monomer (lithium methacrylate, 5g) are added and pre-emulsified for 0.5h. Then, under the protection of inert gas N2, the temperature is raised to a reaction temperature of 60°C, stirred for reaction, and kept warm for 5h. After the reaction is completed, an appropriate amount of ammonia water is added to neutralize to a pH of 7, and the product is filtered to obtain a polymer emulsion. The obtained polymer emulsion is spray-dried to obtain a solid powder. The solid powder is washed with deionized water, dried, crushed, and sieved to prepare the lithium ion battery positive electrode binder. The difference from Example 4 is that no fourth functional monomer is added for branching modification.

[0066] Comparative Example 4:

[0067] A method for preparing a positive electrode fluorine-free binder material:

[0068] The difference between this comparative example and Example 1 is that the fourth functional monomer is increased by 5 g, and the rest are the same as Example 1. Increasing the amount of the fourth functional monomer causes the binder to be excessively cross-linked during the polymerization process, resulting in poor toughness of the electrode.

[0069] Example 8:

[0070] Application of a fluorine-free positive electrode binder material:

[0071] The positive and negative electrode slurries and pole pieces and their soft-pack batteries of Examples 1 to 8 and Comparative Examples 1 to 3 were prepared according to the following method. The positive pole piece of Comparative Example 4 was not prepared into a soft-pack battery because it was prone to cracking during the winding process:

[0072] (1) Preparation of positive electrode slurry and electrode: including but not limited to the following preparation methods

[0073] The positive electrode fluorine-free binder material prepared by the present invention is prepared into a binder glue solution with a solid content of 5wt% and NMP (N-methylpyrrolidone) solution, and then the positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 After O2, conductive agent Super-Li, conductive agent CNT and configured binder glue are fully dispersed and mixed according to the solid content of 97:1.5:0.5:1, the solvent NMP is added to adjust the solid content to 65%. After high-speed dispersion, the mixed slurry is filtered through a 200-mesh screen and evenly coated on both sides of aluminum foil. After drying, rolling and cutting, the required positive electrode sheet is obtained. The surface density of the positive electrode active layer is 35.5 mg / cm 2 , compacted density is about 3.3g / cm 3 .

[0074] (2) Negative electrode slurry and electrode preparation: including but not limited to the following preparation methods

[0075] The negative electrode active material graphite, conductive agent Super-Li, binder SBR and dispersant CMC were added to deionized water solvent in a ratio of 97.8:0.4:1.2:0.6 to adjust the solid content to 50%, and stirred thoroughly to obtain a mixed slurry. The mixed slurry was then filtered through a 200-mesh screen and evenly coated on a copper foil. After drying, rolling and cutting, the desired negative electrode sheet was obtained. The surface density of the negative electrode sheet was about 16.0 mg / cm 2 , compacted density is about 1.5g / cm 3 .

[0076] The diaphragm uses a PE porous polymer film with a thickness of 20 μm as the isolation membrane.

[0077] Ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) were mixed evenly in a mass ratio of 3:5:2, and then the electrolyte lithium salt LiPF6 was dissolved in the above-configured solvent at 1 mol / L to obtain an electrolyte solution.

[0078] (3) Preparation of lithium-ion batteries: The positive electrode sheet, negative electrode sheet and separator are wound, and then welded, baked, injected, packaged, formed, and divided into different volumes to obtain an experimental battery cell.

[0079] (IV) Test method:

[0080] 1. Viscosity test:

[0081] To measure the viscosity of rubber and slurry materials using a kinematic viscometer, place a certain amount of sample in a sample cup and slowly immerse the rotor in the sample. Connect the viscometer to the water bath system. Wait until the sample temperature stabilizes at 25°C before starting the measurement. The test lasts 120 seconds, with a data acquisition interval of 20 seconds. Six data points are collected, and the average value is the viscosity of the test sample.

[0082] 2. Pole flexibility:

[0083] The test is conducted using a cylindrical mandrel bending tester. The electrode is cut into 2cm*15cm rectangular strips, then wound around a 2mm diameter metal cylinder. The strip is then slowly pulled 180° to observe whether the electrode has cracks or powder loss. The electrode's flexibility is determined by observing the presence of cracks and powder loss on the surface. No cracks or powder loss indicates good performance, while the presence of cracks or powder loss indicates poor performance.

[0084] 3. Peel force:

[0085] Use a tensile tester to test the bonding strength between the diaphragm and the current collector. First, adhere the 3M special peeling tape to the stainless steel plate, then adhere the electrode to the tape. The tensile tester clamps the electrode and peels it 180°. The stroke speed is 100mm / min. The test data is judged based on the average value of the stable period.

[0086] The prepared positive electrode sheet and soft pack battery were tested, and the obtained data are shown in Table 1:

[0087] Table 1: Examples and Comparative Examples Performance Test

[0088] Solubility is the dissolution of a solute per unit time. During the dissolution of a polymer, the lower the solvent viscosity, the freer the solvent molecules will move in the solvent, which is conducive to better penetration of the solvent molecules into the polymer molecules. The solvent molecules are more likely to infiltrate the polymer, accelerating the dissolution process of the polymer. The faster the dissolution rate per unit time, and the higher the solubility in the same amount of solvent, the lower the 5wt% glue viscosity, and the better the solubility. It can be seen from the above table that the binder solubility performance provided by Examples 1 to 7 is better than that of PVDF and Comparative Examples 2 and 3, and the slurry viscosity is greatly reduced at the same solid content. By using the branched modified binder, the toughness of the electrode is significantly optimized compared to the unmodified one, which can provide excellent processing performance for the processing of the electrode, and the peeling performance of the positive electrode prepared based on the modified binder is not less than 30N / m, which is significantly higher than PVDF513. 0 Compared with Comparative Examples 2 and 3, Comparative Example 4 may be due to the addition of more fourth functional monomers, which leads to cross-linking and entanglement between the binder molecular chains, thereby making the pole piece insufficiently flexible and prone to cracking during the winding process. The branched structure introduced by the fourth monomer in the present disclosure can improve the solubility of the binder in NMP, which is manifested in a significant reduction in the viscosity of the glue. Compared with Comparative Examples 2 and 3, Examples 1 and 2 have a viscosity reduction of about 65% at the same solid content, and a viscosity reduction of 50% compared with the PVDF-5130 in Comparative Example 1, which brings certain conveniences to the configuration of the glue in the actual production process. In Comparative Examples 2 and 3, it was found that the unbranched modified binder had a higher viscosity in the positive electrode slurry and needed to be further diluted during actual use, which brought certain complexity to the processing. The modified binder had a significantly reduced viscosity in the positive electrode slurry at the same solid content and could be used directly, thereby improving work efficiency.

[0089] 4. Electrochemical cycle stability:

[0090] The prepared soft-pack test battery was subjected to electrochemical cycle performance testing of constant current and constant voltage charge and discharge at 25°C, with test rates of 0.5C and 1C.

[0091] Capacity decay rate after 500 cycles = (1st cycle capacity - 500th cycle capacity) / 1st cycle capacity × 100%

[0092] Table 2: Electrochemical cycle performance test results of the disclosed embodiments and comparative examples

[0093] As can be seen from Table 2 above, the modified binders provided in Examples 1 to 7 have a capacity attenuation rate of less than 9.1% after 500 cycles at a current density of 0.5C, and a capacity attenuation rate of less than 14.6% after 500 cycles at a current density of 1C. Compared with PVDF and the comparative example, it is found that the modified binders effectively improve the cycle life of the battery. Industrial Applicability

[0094] The positive electrode binder provided by the present invention is composed of three functional side groups, namely cyano, ionic groups and long-chain alkanes, and a hyperbranched polymer main chain, wherein the cyano group can give the binder stronger cohesion; the long-chain alkanes can give the binder better flexibility and electrolyte resistance; the ionic groups can improve the adhesion and ionic conductivity of the binder; the branched structure has a higher functional group content per unit area, which can provide more contact sites between active substances and active substances, and between current collectors and active substances, and has stronger adhesion. It can effectively inhibit the breakage and peeling of the pole pieces during the processing process, and can still make the positive electrode materials in close contact with each other at a lower dosage, maintain the structural integrity and cycle stability during the cycle, and has very good practicality as a positive electrode binder.

Claims

1. A positive electrode fluorine-free binder material, characterized in that: Made from the following raw materials in parts by weight: 40-60 parts of a first monomer, wherein the first monomer is acrylonitrile and / or methacrylonitrile; 30-60 parts of a second monomer, wherein the second monomer is an acrylate and / or methacrylate monomer; 5-15 parts of a third monomer, wherein the third monomer is a combination of one or more of an olefinic acidic monomer, a sodium salt corresponding to an olefinic acidic monomer, or a lithium salt corresponding to an olefinic acidic monomer; The raw materials also include 2.5-5 parts by mass of the fourth functional monomer; The second monomer is a combination of one or more of ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl methacrylate, butyl acrylate, butyl methacrylate, isobutyl methacrylate, isodecyl methacrylate, amyl acrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, isooctyl acrylate, isooctyl methacrylate, lauryl acrylate, lauryl methacrylate, octadecyl acrylate, octadecyl methacrylate, methoxypolyethylene glycol methacrylate and ethoxyethyl acrylate; The third monomer is a combination of one or more of acrylic acid, sodium acrylate, lithium acrylate, methacrylic acid, sodium methacrylate, lithium methacrylate, β-acryloxypropionic acid, 2-acrylamide-2-methylpropanesulfonic acid, sodium allyl sulfonate, sodium vinyl sulfonate, sodium allyl sulfonate and sodium styrene sulfonate; The fourth functional monomer is a combination of one or more of 1,6-hexanediol diacrylate, neopentyl glycol polymethyl ethylene oxide diacrylate, ethoxylated bisphenol A diacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate and propoxylated glycerol triacrylate; The preparation method of the positive electrode fluorine-free binder material comprises the following steps: (1) Pre-emulsification: adding part of the first monomer, the second monomer, and the third monomer to an emulsifier, an initiator, and deionized water to pre-emulsify the mixture to obtain a mixed emulsion; (2) heating the mixed emulsion pre-emulsified in step (1) to a reaction temperature and stirring the mixture to react; and when the emulsion begins to change color to blue, adding dropwise the fourth functional monomer and the remaining mixture of the first monomer, the second monomer, and the third monomer to obtain a mixed emulsion; (3) adding an initiator dropwise to the mixed emulsion obtained in step (2), and reacting at the same temperature to obtain a preliminary emulsion; (4) filtering, neutralizing, and cooling the preliminary emulsion obtained in step (3) to obtain a polymer emulsion; (5) drying the polymer emulsion obtained in step (4) to obtain a solid powder; (6) The solid powder obtained in step (5) is washed, purified, dried, and sieved to obtain a positive electrode fluorine-free binder material.

2. A method for preparing a positive electrode fluorine-free binder material according to claim 1, characterized in that: The following steps are involved: (1) Pre-emulsification: adding part of the first monomer, the second monomer, and the third monomer to an emulsifier, an initiator, and deionized water to pre-emulsify the mixture to obtain a mixed emulsion; (2) heating the mixed emulsion pre-emulsified in step (1) to a reaction temperature and stirring the mixture to react; and when the emulsion begins to change color to blue, adding dropwise the fourth functional monomer and the remaining mixture of the first monomer, the second monomer, and the third monomer to obtain a mixed emulsion; (3) adding an initiator dropwise to the mixed emulsion obtained in step (2), and reacting at the same temperature to obtain a preliminary emulsion; (4) filtering, neutralizing, and cooling the preliminary emulsion obtained in step (3) to obtain a polymer emulsion; (5) drying the polymer emulsion obtained in step (4) to obtain a solid powder; (6) The solid powder obtained in step (5) is washed, purified, dried, and sieved to obtain a positive electrode fluorine-free binder material.

3. The preparation method according to claim 2, characterized in that The portion of the first monomer, the second monomer, and the third monomer in step (1) accounts for 85 to 90% of the mass of the total first monomer, the second monomer, and the third monomer.

4. The preparation method according to claim 2 or 3, characterized in that In step (1), the emulsifier is added in an amount of 0.5-5 parts by mass, and the initiator is added in an amount of 0.05-3.75 parts by mass; the emulsifier is a combination of one or more of calcium dodecylbenzoate, sodium dodecylsulfonate, sodium dodecyl sulfate, SR-10, SE-10, SN-10 and OP-10, and the initiator is a combination of one or more of sodium persulfate, potassium persulfate, ammonium persulfate and azobisisobutylamidine hydrochloride.

5. The preparation method according to any one of claims 2 to 4, characterized in that The amount of the initiator added in step (3) is 0.05 to 1.25 parts by mass.

6. The preparation method according to any one of claims 2 to 5, characterized in that The emulsification time of the pre-emulsification in step (1) is 0.5h-2h.

7. The preparation method according to any one of claims 2 to 6, characterized in that The reaction temperature in step (2) is 50-80° C., and the dropwise addition time is 0.5-1.5 h.

8. The preparation method according to any one of claims 2 to 7, characterized in that The heat preservation reaction in step (3) is 2 to 3 hours.

9. The preparation method according to any one of claims 2 to 8, characterized in that The pH regulator added for neutralization in step (4) is any one of ammonia water, lithium salt, and sodium salt, and the pH value adjusted by neutralization is 6.8 to 7.

2.

10. The preparation method according to any one of claims 2 to 9, characterized in that: The drying in step (5) is any one or more combinations of demulsification drying, vacuum drying, spray drying and blast drying.

11. A use of the positive electrode fluorine-free binder material according to claim 1 or the positive electrode fluorine-free binder material prepared by the preparation method according to any one of claims 2 to 10, characterized in that: The positive electrode fluorine-free binder material is used to prepare a positive electrode of a lithium ion battery. The positive electrode of the lithium ion battery comprises a current collector and a positive electrode active coating coated on the surface of the current collector.

12. The use of the positive electrode fluorine-free binder material according to claim 11, characterized in that: A lithium ion soft pack battery is prepared, wherein the lithium ion soft pack battery includes the lithium ion battery positive electrode.

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