Perfluoropropyl vinyl ether-acrylonitrile copolymer as well as preparation method and application thereof

Abstract

The invention discloses a perfluoropropyl vinyl ether-acrylonitrile copolymer as well as a preparation method and application thereof. The perfluoropropyl vinyl ether-acrylonitrile copolymer disclosed by the invention is formed by copolymerizing perfluoropropyl vinyl ether and acrylonitrile according to a molar ratio of 1: (2-4). The perfluoropropyl vinyl ether-acrylonitrile copolymer has the high pressure resistance of cyano groups, the flexibility of ether bonds and the hydrophobicity of fluorocarbon bonds, and has the advantages of being excellent in adhesive property, good in chemical stability, good in flexibility, balanced in hydrophilicity and hydrophobicity and the like; the lithium ion battery positive electrode material binder can improve the cycle life and the rate capability of the lithium ion battery under the conditions of high voltage and high load, and is suitable for large-scale industrial application.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a perfluoropropyl vinyl ether-acrylonitrile copolymer, its preparation method, and its application. Background Technology

[0002] In recent years, new energy vehicles, such as pure electric vehicles and plug-in hybrid electric vehicles, which use lithium-ion batteries as their core power source, have developed rapidly, and their market size and technological maturity have significantly improved. However, the actual driving range of pure electric vehicles is generally concentrated in the range of 400 to 600 kilometers, and the driving range will be further reduced under actual conditions such as low temperature and high speed, which makes it difficult to meet users' needs for long-distance travel and anxiety-free use.

[0003] Currently, to overcome the range bottleneck of electric vehicles, improving the energy density of lithium-ion batteries has become an industry consensus, and increasing the areal loading of active materials is one of the key technical paths. However, lithium-ion batteries with high-load, thick electrodes currently face prominent mechanical stress and transport dynamics problems, specifically: 1) The electrodes need to withstand large volume change stress during charging and discharging, which can easily lead to contact failure between electrode material particles and current collector peeling; 2) The solid-phase diffusion path of lithium ions in thick electrodes is significantly prolonged, which can lead to increased electrode polarization and limited capacity utilization.

[0004] Studies have shown that high-performance electrode material binders can improve the cycle stability of lithium-ion batteries under high voltage. However, existing electrode material binders still have many problems, such as low adhesion, easy hydrolysis of electrolyte, inability to effectively buffer mechanical stress under high load, poor wettability of electrode materials, poor interfacial bonding ability, and complex preparation process, making it difficult to fully meet the needs of practical applications.

[0005] Therefore, it is of great significance to develop an electrode material binder with excellent bonding performance, good chemical stability, good flexibility, balanced hydrophilicity and hydrophobicity, and simple preparation process. Summary of the Invention

[0006] The purpose of this invention is to provide a perfluoropropyl vinyl ether-acrylonitrile copolymer, its preparation method, and its application.

[0007] The technical solution adopted in this invention is:

[0008] A perfluoropropyl vinyl ether-acrylonitrile copolymer is copolymerized from perfluoropropyl vinyl ether (PPVE) and acrylonitrile (AN) in a molar ratio of 1:2 to 4.

[0009] Preferably, a perfluoropropyl vinyl ether-acrylonitrile copolymer is copolymerized from perfluoropropyl vinyl ether and acrylonitrile in a molar ratio of 1:3 to 4.

[0010] Preferably, the perfluoropropyl vinyl ether-acrylonitrile copolymer has a number-average molecular weight of 310kDa to 400kDa, a weight-average molecular weight of 450kDa to 530kDa, and a molecular weight distribution coefficient of 1.3 to 1.5.

[0011] A method for preparing the perfluoropropyl vinyl ether-acrylonitrile copolymer as described above includes the following steps: 1) Preparation of oil and aqueous phases: Preparation of the oil phase: Perfluoropropyl vinyl ether, acrylonitrile, initiator and stabilizer are mixed evenly to obtain the oil phase; Preparation of the aqueous phase: Dissolve the emulsifier in water to obtain the aqueous phase; 2) The oil phase is slowly added to the aqueous phase to form a crude emulsion, which is then subjected to ultrasonic treatment to form a fine emulsion, and then subjected to polymerization to obtain a polymer emulsion. 3) Add the demulsifier to the polymer emulsion to demulsify, then take the solid and wash, centrifuge and dry it to obtain perfluoropropyl vinyl ether-acrylonitrile copolymer.

[0012] Preferably, the initiator in step 1) is at least one of azobisisobutyronitrile and dimethyl azobisisobutyrate.

[0013] Preferably, the amount of initiator used in step 1) is 0.2% to 0.7% of the total mass of perfluoropropyl vinyl ether and acrylonitrile.

[0014] Preferably, the stabilizer in step 1) is at least one of n-hexadecane, n-octadecane, and stearic acid.

[0015] Preferably, the amount of stabilizer used in step 1) is 2% to 4% of the total mass of perfluoropropyl vinyl ether and acrylonitrile.

[0016] Preferably, the emulsifier in step 1) is at least one of sodium α-alkenyl sulfonate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate.

[0017] Preferably, the amount of emulsifier used in step 1) is 10% to 15% of the total mass of perfluoropropyl vinyl ether and acrylonitrile.

[0018] Preferably, the solid content of the fine emulsion in step 2) is 20% to 25%.

[0019] Preferably, the polymerization reaction in step 2) is carried out at a temperature of 70℃ to 90℃ for a reaction time of 10h to 15h.

[0020] Preferably, the polymerization reaction in step 2) is carried out in a protective atmosphere.

[0021] Preferably, the protective atmosphere is a nitrogen atmosphere.

[0022] Preferably, the polymerization reaction in step 2) is carried out at a stirring rate of 150 rpm to 200 rpm.

[0023] Preferably, the demulsifier in step 3) is at least one of calcium chloride, sodium chloride, and aluminum sulfate.

[0024] Preferably, the amount of demulsifier used in step 3) is 1% to 2.5% of the mass of the polymer in the polymer emulsion.

[0025] Preferably, the centrifugation in step 3) is carried out at a centrifugation rate of 3000 rpm to 4000 rpm, and the time for a single centrifugation is 3 min to 5 min.

[0026] Preferably, the drying method in step 3) is freeze drying.

[0027] Preferably, the freeze-drying is carried out at a temperature of -60℃ to -40℃ for a drying time of 20h to 30h.

[0028] An adhesive comprising a perfluoropropyl vinyl ether-acrylonitrile copolymer and a solvent.

[0029] Preferably, the solvent is at least one of N-methylpyrrolidone and dimethyl sulfoxide.

[0030] Preferably, the perfluoropropyl vinyl ether-acrylonitrile copolymer in the adhesive contains 6% to 10% by mass.

[0031] A positive electrode sheet comprising the above-mentioned perfluoropropyl vinyl ether-acrylonitrile copolymer.

[0032] Preferably, the positive electrode sheet comprises a current collector and a positive electrode coating.

[0033] Preferably, the current collector is one of aluminum foil, single-sided carbon-coated aluminum foil, or double-sided carbon-coated aluminum foil.

[0034] Preferably, the positive electrode coating comprises the following raw materials in weight percentage: Positive electrode active material: 80%–94%; Conductive agent: 3%–10%; Adhesive: 1%–10%.

[0035] Preferably, the positive electrode active material is at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary material, and lithium nickel-manganese oxide.

[0036] Preferably, the conductive agent is at least one of carbon black, graphite, carbon nanotubes, graphene, and carbon fiber.

[0037] Preferably, the positive electrode active material loading of the positive electrode sheet is 3 mg / cm³. 2 ~30mg / cm 2 .

[0038] A method for preparing a positive electrode sheet as described above includes the following steps: a) The positive electrode active material, conductive agent and binder are mixed and ball-milled to obtain a slurry; b) The slurry is coated on the surface of the current collector, then dried and cut to obtain the positive electrode sheet.

[0039] Preferably, the drying in step b) is carried out at a temperature of 60°C to 90°C for a drying time of 6 hours to 48 hours.

[0040] A lithium-ion battery comprising the aforementioned positive electrode.

[0041] The beneficial effects of the present invention are: the perfluoropropyl vinyl ether-acrylonitrile copolymer of the present invention combines the high-pressure resistance of cyano groups, the flexibility of ether bonds, and the hydrophobicity of carbon-fluorine bonds. It has the advantages of excellent bonding performance, good chemical stability, good flexibility, and a balance of hydrophilicity and hydrophobicity. When used as a binder for positive electrode materials, it can improve the cycle life and rate performance of lithium-ion batteries under high voltage and high load conditions, and is suitable for large-scale industrial applications.

[0042] Specifically: 1) This invention achieves a synergistic effect of multiple functions through molecular structure design: the cyano group in the acrylonitrile unit has strong polarity and high electrochemical stability, which can endow the perfluoropropyl vinyl ether-acrylonitrile copolymer with excellent high voltage resistance (>4.5V); the carbon-fluorine bond in the perfluoropropyl vinyl ether has excellent hydrophobicity and chemical inertness, which can inhibit electrolyte decomposition and transition metal dissolution; the ether bond in the perfluoropropyl vinyl ether increases the rotational freedom of the perfluoropropyl vinyl ether-acrylonitrile copolymer backbone, so that the perfluoropropyl vinyl ether-acrylonitrile copolymer has excellent flexibility while maintaining high strength, and thus can buffer the volume change stress of the high-load electrode during charge and discharge cycles; 2) This invention ingeniously achieves a balance between hydrophilicity and hydrophobicity, as well as a balance between soft and hard segments: the moderate hydrophilicity of the cyano group and the strong hydrophobicity of the carbon-fluorine bond complement each other, ensuring good processability of the electrode material in polar solvents while reducing the electrode's sensitivity to moisture; the synergistic effect of the rigid cyano group and the flexible ether bond gives the perfluoropropyl vinyl ether-acrylonitrile copolymer both high strength and high toughness, providing sufficient adhesion to maintain the integrity of the electrode structure while also possessing good strain dissipation capability; 3) This invention innovatively employs fine emulsion polymerization technology to successfully prepare perfluoropropyl vinyl ether-acrylonitrile copolymers with a well-defined soft and hard segment microphase separation structure. This polymerization method forms stable micron-sized droplets through ultrasonic emulsification, which not only solves the problem of oil-soluble monomers being insoluble in aqueous solutions (easily leading to stratification), but also provides an ideal nanoreactor for the monomers, ensuring that the composition of the perfluoropropyl vinyl ether-acrylonitrile copolymer is uniform and the molecular weight is controllable. This significantly improves the stability and repeatability of the polymerization reaction, laying a solid foundation for industrial production. 4) The preparation process of the perfluoropropyl vinyl ether-acrylonitrile copolymer of the present invention is simple, the reaction conditions are mild, the reaction process is stable and controllable, the raw materials are readily available, the production cost is reasonable, the process is environmentally friendly and efficient, and large-scale production can be achieved without special equipment. It has a very broad application prospect in the field of high voltage and high energy density lithium-ion batteries. Attached Figure Description

[0043] Figure 1 The infrared spectrum of the perfluoropropyl vinyl ether-acrylonitrile copolymer in Example 2 is shown. Detailed Implementation

[0044] The present invention will be further explained and described below with reference to specific embodiments.

[0045] Example 1: A perfluoropropyl vinyl ether-acrylonitrile copolymer is prepared by the following method: 1) Preparation of oil and aqueous phases: Preparation of the oil phase: 8.35 g of perfluoropropyl vinyl ether, 6.65 g of acrylonitrile (the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:4), 0.06 g of azobisisobutyronitrile and 0.4 g of n-hexadecane were mixed and stirred at 25 °C for 20 min at a stirring rate of 150 rpm to obtain the oil phase. Preparation of the aqueous phase: 2g of sodium α-alkenylsulfonate was added to 80g of ultrapure water, and then stirred at 25℃ for 10min at a stirring rate of 180rpm to obtain the aqueous phase. 2) The oil phase was added dropwise to the aqueous phase under stirring over 30 minutes to form a crude emulsion. The stirring speed was 180 rpm. The emulsion was then sonicated for 10 minutes to form a fine emulsion (which showed a blue opalescent color). The emulsion was then placed in a nitrogen atmosphere and stirred at 80°C for 12 hours at a stirring speed of 150 rpm to obtain a polymer emulsion (which was milky white). 3) Add 0.5g of calcium chloride to the polymer emulsion to break the emulsion, then take the solid and wash it three times each with methanol and ultrapure water. After each washing, centrifuge at a speed of 3500rpm for 4min. Then freeze-dry it at -50℃ for 24h to obtain perfluoropropyl vinyl ether-acrylonitrile copolymer (white powder).

[0046] An adhesive, the preparation method of which is as follows: 0.6 g of the perfluoropropyl vinyl ether-acrylonitrile copolymer of this example was dissolved in 5.4 g of N-methylpyrrolidone to obtain the adhesive.

[0047] A positive electrode sheet is prepared by the following method: a) Add the nickel-cobalt-manganese ternary material, carbon black Super P, and the binder in this embodiment to a planetary ball mill at a mass ratio of 85:10:5 and ball mill until uniform to obtain a slurry; b) Coat the slurry onto the surface of a single-sided carbon-coated aluminum foil (the side containing the carbon layer), then vacuum dry it at 85°C for 24 hours, allow it to cool naturally to room temperature, and then cut it into circular pieces with a diameter of 12 mm to obtain the positive electrode sheet (the loading of the nickel-cobalt-manganese ternary material is 3 mg / cm³). 2 ).

[0048] Example 2: A perfluoropropyl vinyl ether-acrylonitrile copolymer is identical to Example 1 except that the amount of perfluoropropyl vinyl ether in step 1) is adjusted from "8.35g" to "9.39g", the amount of acrylonitrile in step 1) is adjusted from "6.65g" to "5.61g" (the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:3), and the amount of azobisisobutyronitrile in step 1) is adjusted from "0.06g" to "0.1g".

[0049] The infrared spectrum of the perfluoropropyl vinyl ether-acrylonitrile copolymer in this embodiment is as follows: Figure 1 As shown.

[0050] Depend on Figure 1 We know that: 2243cm -1 The peak at 993 cm⁻¹ represents the characteristic stretching vibration absorption peak of the cyano group (-CN) in the acrylonitrile unit. -1 1150cm -1 ~1250cm -1 The range represents the characteristic vibrational absorption peaks of the perfluoroether bond (-O-CF3) in the perfluoropropyl vinyl ether (PPVE) unit and the multiple absorption peaks of the CF bond, respectively. The coexistence of these characteristic peaks confirms the successful copolymerization reaction between acrylonitrile and PPVE. Furthermore, at 1620 cm⁻¹... -1~1680cm -1 No obvious olefinic C=C double bond characteristic absorption was observed within the range, indicating that both perfluoropropyl vinyl ether and acrylonitrile had reacted sufficiently.

[0051] An adhesive is identical to that of Example 1, except that it is prepared using the perfluoropropyl vinyl ether-acrylonitrile copolymer of this example.

[0052] A positive electrode (nickel-cobalt-manganese ternary material with a loading of 3 mg / cm³) 2 Except for the adhesive used in this embodiment, it is completely the same as in Example 1.

[0053] Example 3: A perfluoropropyl vinyl ether-acrylonitrile copolymer is identical to Example 1 except that the amount of perfluoropropyl vinyl ether in step 1) is adjusted from "8.35g" to "10.72g", the amount of acrylonitrile in step 1) is adjusted from "6.65g" to "4.28g" (the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:2), and the amount of azobisisobutyronitrile in step 1) is adjusted from "0.06g" to "0.08g".

[0054] An adhesive is identical to that of Example 1, except that it is prepared using the perfluoropropyl vinyl ether-acrylonitrile copolymer of this example.

[0055] A positive electrode (nickel-cobalt-manganese ternary material with a loading of 3 mg / cm³) 2 Except for the adhesive used in this embodiment, it is completely the same as in Example 1.

[0056] Comparative Example 1: A perfluoropropyl vinyl ether-acrylonitrile copolymer is prepared by the following method: 1) Preparation of pre-emulsion and emulsifier solution: Preparation of preemulsion: 8.35 g of perfluoropropyl vinyl ether, 6.65 g of acrylonitrile (the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:4), 1 g of sodium dodecyl sulfate and 5 g of ultrapure water were mixed and stirred at 25 °C for 20 min at a stirring rate of 150 rpm to obtain the preemulsion. Preparation of emulsifier solution: Add 2g of sodium α-alkenylsulfonate to 75g of ultrapure water, and stir at 25℃ for 10min at a stirring rate of 180rpm to obtain emulsifier solution; 2) The pre-emulsion was added dropwise to the emulsifier solution under stirring over 45 minutes to form a crude emulsion at a stirring rate of 180 rpm. The mixture was then mechanically stirred for 1 hour at 80°C and 200 rpm to form an emulsion (milky white). 0.06 g of potassium persulfate was then added, and the mixture was placed under a nitrogen atmosphere and stirred at 80°C for 12 hours at a stirring rate of 150 rpm to obtain a polymer emulsion (milky white). 3) Add 0.5g of calcium chloride to the polymer emulsion to break the emulsion, then take the solid and wash it three times each with methanol and ultrapure water. After each washing, centrifuge at a speed of 3500rpm for 4min. Then freeze-dry it at -50℃ for 24h to obtain perfluoropropyl vinyl ether-acrylonitrile copolymer (white powder).

[0057] An adhesive is identical to that of Example 1, except that it is prepared using the perfluoropropyl vinyl ether-acrylonitrile copolymer of this comparative example.

[0058] A positive electrode (nickel-cobalt-manganese ternary material with a loading of 3 mg / cm³) 2 Except for the binder used in this comparative example, it is completely the same as Example 1.

[0059] Comparative Example 2: A perfluoropropyl vinyl ether-acrylonitrile copolymer is identical to Comparative Example 1 except that the amount of perfluoropropyl vinyl ether in step 1) is adjusted from "8.35g" to "9.39g", the amount of acrylonitrile in step 1) is adjusted from "6.65g" to "5.61g" (the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:3) and the amount of potassium persulfate in step 2) is adjusted from "0.06g" to "0.08g".

[0060] An adhesive is identical to that of Example 1, except that it is prepared using the perfluoropropyl vinyl ether-acrylonitrile copolymer of this comparative example.

[0061] A positive electrode (nickel-cobalt-manganese ternary material with a loading of 3 mg / cm³) 2 Except for the binder used in this comparative example, it is completely the same as Example 1.

[0062] Comparative Example 3: A perfluoropropyl vinyl ether-acrylonitrile copolymer is identical to Comparative Example 1 except that the amount of perfluoropropyl vinyl ether in step 1) is adjusted from "8.35g" to "10.24g", the amount of acrylonitrile in step 1) is adjusted from "6.65g" to "4.76g" (the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:2), and the amount of potassium persulfate in step 2) is adjusted from "0.06g" to "0.1g".

[0063] An adhesive is identical to that of Example 1, except that it is prepared using the perfluoropropyl vinyl ether-acrylonitrile copolymer of this comparative example.

[0064] A positive electrode (nickel-cobalt-manganese ternary material with a loading of 3 mg / cm³) 2 Except for the binder used in this comparative example, it is completely the same as Example 1.

[0065] Comparative Example 4: A positive electrode sheet is prepared by the following method: a) Add nickel-cobalt-manganese ternary material, carbon black Super P and polyvinylidene fluoride (PVDF) solution (mass fraction of 10%) to a planetary ball mill at a mass ratio of 85:10:5 and ball mill until uniform to obtain slurry; b) Coat the slurry onto the surface of a single-sided carbon-coated aluminum foil (the side containing the carbon layer), then vacuum dry it at 80℃ for 24 hours, allow it to cool naturally to room temperature, and then cut it into circular pieces with a diameter of 12mm to obtain the positive electrode sheet (the loading of the nickel-cobalt-manganese ternary material is 3mg / cm³). 2 ).

[0066] Comparative Example 5: A positive electrode sheet is prepared by the following method: a) Add nickel-cobalt-manganese ternary material, carbon black Super P and polyacrylonitrile (PAN) solution (mass fraction of 10%) to a planetary ball mill at a mass ratio of 85:10:5 and ball mill until uniform to obtain slurry; b) Coat the slurry onto the surface of a single-sided carbon-coated aluminum foil (the side containing the carbon layer), then vacuum dry it at 80℃ for 24 hours, allow it to cool naturally to room temperature, and then cut it into circular pieces with a diameter of 12mm to obtain the positive electrode sheet (the loading of the nickel-cobalt-manganese ternary material is 3mg / cm³). 2 ).

[0067] Performance testing: 1) The gelation rate and swelling rate test results of the perfluoropropyl vinyl ether-acrylonitrile copolymers in Examples 1-3 and Comparative Examples 1-3 are shown in the table below: Table 1. Test results of gel rate and swelling rate of perfluoropropyl vinyl ether-acrylonitrile copolymer Test Project gelation rate ( % ） Swelling rate ( % ） Example 1 2.12 15.73 Example 2 1.54 8.12 Example 3 2.63 22.63 Comparative Example 1 15.37 18.19 Comparative Example 2 14.28 12.54 Comparative Example 3 16.79 28.06 Note: Gel ratio (Soxhlet extraction): 1g of perfluoropropyl vinyl ether-acrylonitrile copolymer was placed in a 400-mesh stainless steel filter and extracted continuously by reflux for 24h under nitrogen protection with N-methylpyrrolidone (NMP) as solvent, with the reflux frequency controlled at 6 to 8 times / h. After extraction, the insoluble matter was vacuum dried at 85℃ to constant weight, and the gel ratio was calculated according to the following formula: Gel ratio (%) = mass of insoluble matter after extraction / initial mass of sample × 100%.

[0068] Swelling rate (tested under simulated battery environment, with strict temperature control at 25℃±0.5℃; detection range 1%~500%): A circular membrane with a diameter of 15mm and a thickness of 0.5mm was made from perfluoropropyl vinyl ether-acrylonitrile copolymer (the perfluoropropyl vinyl ether-acrylonitrile copolymer was first dissolved with N-methylpyrrolidone, then coated onto the substrate surface and dried to form a film). The circular membrane was then immersed in a 1mol / L LiPF6 / EC-DEC-EMC electrolyte and placed in an argon-filled glove box at 25℃ for 72h. The circular membrane was then removed and the surface liquid was wiped off and weighed within 30s. The swelling rate was then calculated according to the following formula: Swelling rate (%) = (mass after immersion - mass before immersion) / mass before immersion × 100%.

[0069] As shown in Table 1: a) The gel rate of the perfluoropropyl vinyl ether-acrylonitrile copolymer (prepared by fine emulsion polymerization) in Examples 1-3 was 1.54%-2.63% (for binder applications, a low gel rate means better solubility and processing performance, the slurry is less likely to clump and clog the coating die, and at the same time ensures the uniform distribution of the binder on the surface of the active material, which is the basis for building a stable electrode structure), which is much lower than the 14.28%-16.79% of the perfluoropropyl vinyl ether-acrylonitrile copolymer (prepared by conventional emulsion polymerization) in Comparative Examples 1-3. The reason is that the ultrasonic emulsification process in fine emulsion polymerization can form uniform micron-sized droplets, which, as an independent nanoreactor, can effectively avoid cross-linking side reactions caused by excessive local concentration of monomers, ensuring the controllability and uniformity of the polymerization reaction; b) The perfluoropropyl vinyl ether-acrylonitrile copolymer in Example 2 achieved a minimum swelling rate of 8.12% because: under the condition that the molar ratio of perfluoropropyl vinyl ether to acrylonitrile is 1:3, an appropriate amount of cyano groups provides the necessary polarity, while sufficient perfluoropropyl vinyl ether monomers form a continuous and dense hydrophobic barrier. The two work together to construct an effective electrolyte barrier network at the molecular level. c) Although the perfluoropropyl vinyl ether-acrylonitrile copolymer in Example 3 was also prepared by a fine emulsion polymerization process, the excessively high content of perfluoropropyl vinyl ether led to an overly dense concentration of hydrophobic groups in the copolymer molecular chain, resulting in increased rigidity and loose stacking of the copolymer molecular chain, which in turn formed microscopic defect channels, making it easier for the electrolyte to penetrate. At the same time, the excessively high fluorine content weakened the proper interaction between the cyano group and the electrode components, resulting in decreased interfacial stability, and ultimately leading to a high swelling rate (22.63%) of the copolymer.

[0070] 2) The molecular weight and molecular weight distribution index of the perfluoropropyl vinyl ether-acrylonitrile copolymers in Examples 1-3 and Comparative Examples 1-3 are shown in the table below: Table 2. Molecular weight and molecular weight distribution index of perfluoropropyl vinyl ether-acrylonitrile copolymers Test Project Mn （ kDa ） Mw （ kDa ） PDI Example 1 352.4 496.8 1.41 Example 2 398.7 528.4 1.33 Example 3 315.6 457.9 1.45 Comparative Example 1 253.6 612.3 2.41 Comparative Example 2 267.1 702.6 2.63 Comparative Example 3 235.8 713.4 3.03 Note: Number-average molecular weight (Mn) and weight-average molecular weight (Mw) were determined using gel permeation chromatography (GPC) on a GPC system equipped with a differential refractive index detector. The system used three polystyrene gel columns in series (to ensure coverage of a wide molecular weight separation range). The mobile phase was chromatographically pure tetrahydrofuran, and the flow rate was strictly controlled at 1.00 mL / min ± 0.01 mL / min. The column oven and detector temperatures were maintained at 35.0℃ ± At 0.1℃, a molecular weight-elution time standard calibration curve was established using a series of narrow-distribution polystyrene standards (molecular weight range from 1.0 kDa to 2000 kDa, a total of 12 points) before testing. During sample preparation, 5.0 mg ± 0.1 mg of purified perfluoropropyl vinyl ether-acrylonitrile copolymer was completely dissolved in 2.5 mL of tetrahydrofuran (THF) to obtain a test solution with a concentration of 2.0 mg / mL. After filtration through a 0.22 μm polytetrafluoroethylene syringe filter, 100 μL was injected into the solution using an autosampler for analysis. The obtained chromatogram was analyzed using data processing software to directly calculate the number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution index (PDI, Mw / Mn).

[0071] As shown in Table 2: a) The perfluoropropyl vinyl ether-acrylonitrile copolymers (prepared by fine emulsion polymerization) in Examples 1-3 (prepared by conventional emulsion polymerization) have an average Mn that is about 40% higher and a significantly lower PDI (the PDI of the perfluoropropyl vinyl ether-acrylonitrile copolymers in Examples 1-3 is all below 1.5, while the PDI of the perfluoropropyl vinyl ether-acrylonitrile copolymers in Comparative Examples 1-3 is all above 2.4; high molecular weight and narrow molecular weight distribution polymers are the fundamental way to obtain high cohesive strength and high processing stability). The reason is that the uniform nano-sized droplets in fine emulsion polymerization act as independent reactors, eliminating the monomer diffusion step and allowing the chain growth reaction to proceed at a constant concentration, which greatly suppresses the chain transfer side reactions that lead to a decrease in molecular weight and a widening of molecular weight distribution. (b) In the fine emulsion polymerization system, the monomer molar ratio is the core lever for regulating performance. When the molar ratio of AN to PPVE is 3:1 (Example 2), the optimal kinetic balance is achieved: AN provides the necessary polar adhesion, while PPVE contributes hydrophobicity and chain rigidity. At this ratio, the copolymerization reaction of the two monomers is most synergistic, effectively avoiding the steric hindrance and chain transfer effects that may be caused by an excessively high PPVE ratio (e.g., a molar ratio of AN to PPVE of 2:1), or the increased brittleness caused by an excessively high AN ratio (e.g., a molar ratio of AN to PPVE of 4:1). Therefore, Example 2 achieved the highest molecular weight (Mn=398.7kDa) and the narrowest molecular weight distribution (PDI=1.33) among all samples, which directly corresponds to its optimal comprehensive adhesion performance, proving that a molar ratio of AN to PPVE of 3:1 is the golden ratio in this system.

[0072] 3) Cut the positive electrode sheets from Examples 1-3 and Comparative Examples 1-5 into strips with a size of 20mm × 200mm. Then, firmly adhere the uncoated side of the positive electrode sheet to the surface of a glass slide, and then firmly adhere 3M tape to the surface of the positive electrode coating. Finally, perform a peel strength test using a tensile testing machine. The test results are shown in the table below: Table 3. Peel strength test results of positive electrode sheet Test Project Average peel force ( N ） Maximum peeling force ( N ） Example 1 8.85 10.05 Example 2 8.51 9.64 Example 3 6.57 7.21 Comparative Example 1 7.12 7.73 Comparative Example 2 6.89 7.26 Comparative Example 3 5.51 5.92 Comparative Example 4 1.24 1.56 Comparative Example 5 8.92 10.18 As shown in Table 3: a) The average peel force of the positive electrode in Example 1 reached 8.85N, which is higher than 7.12N of the positive electrode in Comparative Example 1, representing an improvement of 24.3%. This is because the fine emulsion polymerization formed a more regular molecular structure and a more uniform distribution of functional groups, which reduced the formation of gel and thus improved the bonding efficiency. (b) Under the same active material (nickel-cobalt-manganese ternary material) and the same ratio, the average peel force of the positive electrode sheet in Example 1 reached 8.85 N, which is much higher than the 1.24 N of the positive electrode sheet in Comparative Example 4. This indicates that the perfluoropropyl vinyl ether-acrylonitrile copolymer of the present invention has a significant bonding advantage compared with traditional PVDF. This performance improvement is mainly due to the strong polarity of the cyano group in the copolymer molecule, which can form a stronger dipole-dipole interaction with the electrode components, while PVDF only relies on weaker van der Waals forces. The difference between the maximum peel force and the average peel force of the positive electrode sheet in Example 1 was 1.20 N (relative difference of 13.6%), while the difference of the positive electrode sheet in Comparative Example 4 was 0.32 N (relative difference of 25.8%). This indicates that the electrode components of the positive electrode sheet in Example 1 are more uniformly distributed and the slurry dispersion is better. This characteristic is of great significance for ensuring the structural stability of the electrode in long-term cycling. c) The positive electrode sheet (pure polyacrylonitrile) in Comparative Example 5 achieved the best adhesion (average peel force of 8.92 N) due to the highest cyano density. The positive electrode sheet in Example 1 (molar ratio of AN to PPVE of 4:1, average peel force of 8.85 N) and the positive electrode sheet in Example 2 (molar ratio of AN to PPVE of 3:1, average peel force of 8.51 N) followed closely by a small difference. This shows that when the molar ratio of AN to PPVE is 4:1 and 3:1, the overall performance can be improved by introducing an appropriate amount of fluorinated groups while almost completely retaining the adhesion advantage of cyano groups. d) The average peeling force (6.57 N) of the positive electrode in Example 3 is lower than that of the positive electrode in Example 1 and Example 2. The reason is that the excessive PPVE content leads to increased rigidity of the molecular chain and decreased flexibility. At the same time, the spatial shielding effect of a large number of fluorinated groups on the cyano group weakens the polarity.

[0073] 4) Using the positive electrode sheets from Examples 1-3 and Comparative Examples 1-5 as the positive electrode, lithium metal sheets as the negative electrode, polypropylene film as the battery separator, and a 1 mol / L lithium hexafluorophosphate solution as the electrolyte (the solvent is composed of ethylene carbonate, propylene carbonate, and diethyl carbonate in a volume ratio of 1:1:1), lithium-ion coin cells were assembled in a glove box. Electrochemical impedance spectroscopy was then performed on an electrochemical workstation. The test results are shown in the table below: Table 4 Electrochemical impedance test results of lithium-ion coin cells Test Project Charge transfer impedance ( Ω ） Example 1 52.7 Example 2 45.2 Example 3 85.3 Comparative Example 1 68.9 Comparative Example 2 57.4 Comparative Example 3 92.6 Comparative Example 4 153.6 Comparative Example 5 138.2 As shown in Table 4: a) The charge transfer impedances of the lithium-ion coin cells assembled with the positive electrode sheets (the binder being a perfluoropropyl vinyl ether-acrylonitrile copolymer prepared by fine emulsion polymerization) in Examples 1-3 were 52.7Ω, 45.2Ω, and 85.3Ω, respectively, which were much smaller than the 153.6Ω of the lithium-ion coin cell assembled with the positive electrode sheet (the binder being PVDF) in Comparative Example 4 and the 138.2Ω of the lithium-ion coin cell assembled with the positive electrode sheet (the binder being pure polyacrylonitrile) in Comparative Example 5. The reason is that the cyano groups in the perfluoropropyl vinyl ether-acrylonitrile copolymer significantly improve the charge transport efficiency of the electrode interface through their strong electron-withdrawing effect, while the fluorinated groups suppress the occurrence of side reactions by constructing a stable electrode / electrolyte interface. b) The charge transfer impedance of the lithium-ion coin cell assembled with the positive electrode in Example 3 is 85.3Ω, which is higher than that of the lithium-ion coin cells assembled with the positive electrode in Comparative Examples 1 and 2 (68.9Ω and 57.4Ω). The reason is that the content of PPVE was too high when preparing the perfluoropropyl vinyl ether-acrylonitrile copolymer, which disrupted the hydrophilic-hydrophobic balance of the copolymer molecular structure. c) The charge transfer impedances of the lithium-ion coin cells assembled from the positive electrode sheets (the binder being a perfluoropropyl vinyl ether-acrylonitrile copolymer prepared by conventional emulsion polymerization) in Examples 1-3 were 68.9Ω, 57.4Ω, and 92.6Ω, respectively, which were much lower than those of the lithium-ion coin cells assembled from the positive electrode sheets in Comparative Examples 4-5. The reason is that the high impedance of the lithium-ion coin cells assembled from the positive electrode sheets in Comparative Example 4 is attributed to the weak van der Waals forces of PVDF and the tendency for electrochemical decomposition under high voltage, while the high impedance of the lithium-ion coin cells assembled from the positive electrode sheets in Comparative Example 5 is due to the excessive rigidity of the pure polyacrylonitrile molecular chain and the interfacial instability caused by its strong hydrophilicity.

[0074] 5) The loading of the nickel-cobalt-manganese ternary material prepared according to the methods of Examples 1-3 and Comparative Examples 1-5 was 15 mg / cm³. 2 The positive electrode sheet (high-load thick electrode) is then assembled into a lithium-ion coin cell (specifically as above), and a charge-discharge cycle test is performed at a current of 0.5C and a voltage of 2.7V to 4.6V, with 200 cycles. The test results are shown in the table below: Table 5. Charge-discharge cycle test results of lithium-ion button batteries Test Project Specific capacity / First lap ( mAh / g ） Specific capacity / cycle 200 lock up( mAh / g ） Capacity retention rate ( % ） Example 1 198.56 150.23 75.65 Example 2 199.81 175.34 87.75 Example 3 195.63 122.56 62.64 Comparative Example 1 196.53 140.96 71.72 Comparative Example 2 197.42 148.39 75.16 Comparative Example 3 192.51 110.63 57.45 Comparative Example 4 190.56 95.62 50.17 Comparative Example 5 191.26 84.23 44.03 As shown in Table 5: a) The lithium-ion coin cell assembled with the positive electrode sheet in Example 2 achieved a capacity retention rate of 87.75%, demonstrating excellent cycle performance and reflecting the optimal molecular structure design: the cyano group provides high voltage resistance, the fluorinated group constructs the electrolyte barrier layer, and the ether bond ensures moderate flexibility. The three work together to achieve electrode structure stability under high load. b) The capacity retention rates of the lithium-ion coin cells assembled with positive electrode sheets in Examples 1 to 3 are higher than those of the lithium-ion coin cells assembled with positive electrode sheets in Comparative Examples 1 to 3 (e.g., comparing the lithium-ion coin cells assembled with positive electrode sheets in Example 1 with those assembled with positive electrode sheets in Comparative Example 1). The reason is that the fine emulsion polymerization method can form a more regular molecular structure compared with the traditional emulsion polymerization method. c) The capacity retention rate of the lithium-ion coin cell assembled with the positive electrode in Example 3 is lower than that of the lithium-ion coin cells assembled with the positive electrode in Comparative Example 1 and Comparative Example 2. The reason is that the content of PPVE is too high, which leads to strong molecular chain rigidity, disrupts the hydrophilic-hydrophobic balance, and weakens the ability to buffer volume changes. d) The capacity retention rate of the lithium-ion coin cell assembled with the positive electrode in Comparative Example 4 is only 50.17%. The reason is that PVDF has only weak van der Waals properties and is highly unstable under high voltage conditions. e) The capacity retention rate of the lithium-ion coin cell assembled with the positive electrode in Comparative Example 5 is only 44.03%. The reason is that the molecular chain of PAN is too rigid and has high hydrophilicity, which cannot meet the high load requirements. In summary, this invention achieves optimal synergy between functional groups through the combination of optimal formulation and fine emulsion polymerization, providing an ideal binder solution for high-voltage, high-load batteries.

[0075] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A perfluoropropyl vinyl ether-acrylonitrile copolymer, characterized in that, It is copolymerized from perfluoropropyl vinyl ether and acrylonitrile in a molar ratio of 1:2 to 4.

2. The perfluoropropyl vinyl ether-acrylonitrile copolymer according to claim 1, characterized in that: The perfluoropropyl vinyl ether-acrylonitrile copolymer has a number-average molecular weight of 310 kDa to 400 kDa, a weight-average molecular weight of 450 kDa to 530 kDa, and a molecular weight distribution coefficient of 1.3 to 1.

5.

3. A method for preparing the perfluoropropyl vinyl ether-acrylonitrile copolymer as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Preparation of oil and aqueous phases: Preparation of the oil phase: Perfluoropropyl vinyl ether, acrylonitrile, initiator and stabilizer are mixed evenly to obtain the oil phase; Preparation of the aqueous phase: Dissolve the emulsifier in water to obtain the aqueous phase; 2) The oil phase is slowly added to the aqueous phase to form a crude emulsion, which is then subjected to ultrasonic treatment to form a fine emulsion, and then subjected to polymerization to obtain a polymer emulsion. 3) Add the demulsifier to the polymer emulsion to demulsify, then take the solid and wash, centrifuge and dry it to obtain perfluoropropyl vinyl ether-acrylonitrile copolymer.

4. The preparation method according to claim 3, characterized in that: Step 1) The initiator is at least one of azobisisobutyronitrile (AIB) and dimethyl azobisisobutyrate (DIBO). The amount of the initiator in Step 1) is 0.2% to 0.7% of the total mass of perfluoropropyl vinyl ether and acrylonitrile. Step 1) The stabilizer is at least one of n-hexadecane, n-octadecane, and stearic acid. The amount of the stabilizer in Step 1) is 2% to 4% of the total mass of perfluoropropyl vinyl ether and acrylonitrile. Step 1) The emulsifier is at least one of sodium α-alkenyl sulfonate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate. The amount of the emulsifier in Step 1) is 10% to 15% of the total mass of perfluoropropyl vinyl ether and acrylonitrile.

5. The preparation method according to claim 3, characterized in that: Step 2) The solid content of the fine emulsion is 20% to 25%.

6. The preparation method according to claim 3 or 5, characterized in that: Step 2) The polymerization reaction is carried out at a temperature of 70℃~90℃ for a reaction time of 10h~15h.

7. The preparation method according to claim 3, characterized in that: Step 3) The demulsifier is at least one of calcium chloride, sodium chloride, and aluminum sulfate; the amount of the demulsifier used in step 3) is 1% to 2.5% of the mass of the polymer in the polymer emulsion.

8. An adhesive, characterized in that, The composition includes the perfluoropropyl vinyl ether-acrylonitrile copolymer as described in claim 1 or 2 and a solvent.

9. A positive electrode sheet, characterized in that, It comprises the perfluoropropyl vinyl ether-acrylonitrile copolymer as described in claim 1 or 2.

10. A lithium-ion battery, characterized in that, It includes the positive electrode sheet as described in claim 9.

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