Secondary battery and preparation method thereof, energy storage system and electric equipment

Abstract

The invention provides a secondary battery and a preparation method thereof, an energy storage system and electric equipment, and relates to the technical field of energy storage. The method comprises the following steps: assembling a positive plate, a diaphragm and a negative plate, injecting an electrolyte to obtain a secondary battery, and sequentially mixing, coating and drying a first positive active material, nano SiO2, two-dimensional titanium carbide, a first conductive agent, a first binder and a first solvent to form a coating; mixing, coating and drying a second positive electrode active material, a carbon nanotube, a second conductive agent, a second binder and a second solvent in sequence to form a coating; a third positive electrode active material, an azobenzene group modified fluorinated acrylate copolymer, a third conductive agent, a third binder and a third solvent are sequentially mixed, coated, subjected to edge ultraviolet light curing treatment and dried, and the positive electrode plate is obtained. The preparation method is helpful for reducing the edge bulging phenomenon of the positive plate in the coating process, and is helpful for improving the capacity and the cycling stability of the secondary battery.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage, and in particular, to a secondary battery, a preparation method thereof, an energy storage system, and an electrical device. Background Art

[0002] With the rapid development of the energy storage market and lithium-ion batteries, the demand for high-energy and long-cycle batteries is increasing. The cathode material often determines the capacity of the battery. Therefore, an important means to develop high-capacity batteries is to increase the cathode capacity.

[0003] During the coating process of traditional cathodes, the "edge bulging" phenomenon is likely to occur due to the surface tension of the slurry. In order to eliminate the edge bulging phenomenon, low-viscosity slurries are often tried to reduce the surface tension and minimize the edge bulging. However, if the viscosity is too low, there are great risks for subsequent coating uniformity and surface density control. More importantly, when the viscosity is too low, the surface density of the electrode is small and the battery capacity is low, which is often highly antagonistic to the manufacture and production of high-capacity batteries. Summary of the Invention

[0004] The main object of the present invention is to provide a secondary battery, a preparation method thereof, an energy storage system, and an electrical device to solve the problems of edge bulging during the coating process of the cathode sheet and low capacity of the secondary battery in the prior art.

[0005] To achieve the above object, according to one aspect of the present invention, a preparation method of a secondary battery is provided. After a bare battery cell is made of a cathode sheet, a separator, and an anode sheet, it is assembled with a housing and a top cover, and then electrolyte is injected to obtain the secondary battery. The preparation steps of the cathode sheet include: S1, first mixing a first cathode active material, nano-SiO 2 , two-dimensional titanium carbide, a first conductive agent, a first binder, and a first solvent to obtain a first cathode slurry; S2, coating the first cathode slurry on the surface of a current collector and drying to form a first coating to obtain a first sheet; S3, second mixing a second cathode active material, carbon nanotubes, a second conductive agent, a second binder, and a second solvent to obtain a second cathode slurry; S4, coating the second cathode slurry on the surface of the first coating and drying to form a second coating to obtain a second sheet; S5, third mixing a third cathode active material, an azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent, a third binder, and a third solvent to obtain a third cathode slurry; S6, coating the third cathode slurry on the surface of the second coating, and successively performing edge ultraviolet curing treatment and drying to form a third coating to obtain the cathode sheet.

[0006] Furthermore, the mass of the above-mentioned first cathode active material, nano-SiO 2The mass ratio of the total mass of the two-dimensional titanium carbide, the first conductive agent, the first binder, and the first solvent is (96 - 97):(0.5 - 1):(0.5 - 1):(1 - 2):(60 - 65); and / or, the mass ratio of the second positive electrode active material, carbon nanotubes, the second conductive agent, the second binder, and the second solvent is (96 - 97):(0.5 - 1):(0.5 - 1):(1 - 2):(65 - 70); and / or, the mass ratio of the third positive electrode active material, the azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder, and the third solvent is (96 - 97):(0.5 - 1):(0.5 - 1):(1 - 2):(70 - 80).

[0007] Further, the mass ratio of the above-mentioned nano-SiO 2 and the two-dimensional titanium carbide is 1:(2 - 3).

[0008] Further, the D50 of the above-mentioned nano-SiO 2 is 40 - 100 nm; and / or, the average number of layers of the two-dimensional titanium carbide is 5 - 10 layers; and / or, the data molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35000 - 40000 Da.

[0009] Further, the above-mentioned azobenzene group-modified fluorinated acrylate copolymer is prepared by a polymerization reaction of azobenzene methacrylate and hexafluorobutyl methacrylate, and the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate is 1:(4 - 5).

[0010] Further, the viscosity of the above-mentioned first positive electrode paste is 8000 - 12000 mPa·s; and / or, the viscosity of the second positive electrode paste is 6000 - 8000 mPa·s; and / or, the viscosity of the third positive electrode paste is 4000 - 6000 mPa·s.

[0011] Further, the area of the ultraviolet light irradiation for the above-mentioned edge ultraviolet light curing treatment is within the range of extending 5 - 20 cm inward from the edge of the third coating; and / or, the wavelength of the ultraviolet light for the edge ultraviolet light curing treatment is 300 - 400 nm; and / or, the time for the edge ultraviolet light curing treatment is 0.05 - 0.1 min.

[0012] According to another aspect of the present invention, a secondary battery is provided, including a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet. The positive electrode sheet includes a current collector and a positive electrode active layer, and the positive electrode active layer includes a first coating, a second coating, and a third coating stacked in sequence; wherein, the material of the first coating includes the first positive electrode active material, nano-SiO 2, two-dimensional titanium carbide, a first conductive agent, and a first binder; the material of the second coating includes a second cathode active material, carbon nanotubes, a second conductive agent, and a second binder; the material of the third coating includes a third cathode active material, an azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent, and a third binder.

[0013] Further, the mass ratio of the first cathode active material, the total mass of nano-SiO 2 and two-dimensional titanium carbide, the mass of the first conductive agent, and the mass of the first binder is (96 - 97):(0.5 - 1):(0.5 - 1):(1 - 2); and / or, the mass ratio of the second cathode active material, carbon nanotubes, the second conductive agent, and the second binder is (96 - 97):(0.5 - 1):(0.5 - 1):(1 - 2); and / or, the mass ratio of the third cathode active material, the azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, and the third binder is (96 - 97):(0.5 - 1):(0.5 - 1):(1 - 2).

[0014] Further, the mass ratio of the above nano-SiO 2 and two-dimensional titanium carbide is 1:(2 - 3).

[0015] Further, the D50 of the above nano-SiO 2 is 40 - 100 nm; and / or, the average number of layers of two-dimensional titanium carbide is 5 - 10 layers; and / or, the number-average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35000 - 40000 Da.

[0016] Further, the azobenzene group-modified fluorinated acrylate copolymer is prepared by a polymerization reaction of azobenzene methacrylate and hexafluorobutyl methacrylate, and the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate is 1:(4 - 5).

[0017] Further, the total thickness of the first coating, the second coating, and the third coating is 160 - 200 μm; and / or, the thickness ratio of the first coating, the second coating, and the third coating is (80 - 100):(30 - 40):(50 - 60).

[0018] According to another aspect of the present invention, there is provided an energy storage system including a unit cell, and the unit cell is a secondary battery prepared by the preparation method of the secondary battery described above or the secondary battery described above.

[0019] According to another aspect of the present invention, there is provided an electrical device including the above energy storage system, and the energy storage system is used to provide power for the electrical device.

[0020] Applying the technical solution of the present application, the beneficial effects of the present application are as follows: First, the first positive electrode active material, nano-SiO 2 , two-dimensional titanium carbide, the first conductive agent, the first binder and the first solvent are mixed to obtain the first positive electrode slurry. The first positive electrode slurry is coated on the surface of the current collector and dried to form the first coating. There is a Si-O-Ti bond between nano-SiO 2 and two-dimensional titanium carbide, which helps to strengthen the bonding force between the two. The addition of nano-SiO 2 on the one hand helps to fill the microcracks, pores and other defects between the layers of two-dimensional titanium carbide and the contact surface between the first coating and the current collector, form a dense structure, reduce stress concentration, and help to improve the overall mechanical strength of the positive electrode sheet and enhance the adhesion between the first coating and the current collector; on the other hand, nano-SiO 2 can be used as a dispersant to reduce the agglomeration between two-dimensional titanium carbides, help to improve the distribution uniformity of two-dimensional titanium carbide, form a more efficient conductive network, and enhance the interface continuity at the same time. Then, the second positive electrode active material, carbon nanotubes, the second conductive agent, the second binder and the second solvent are mixed to obtain the second positive electrode slurry. The second positive electrode slurry is coated on the surface of the first coating and dried to form the second coating. The addition of carbon nanotubes helps to improve the conductivity of the positive electrode sheet. Finally, the third positive electrode active material, azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder and the third solvent are mixed to obtain the third positive electrode slurry. The third positive electrode slurry is coated on the surface of the second coating, and edge ultraviolet light curing treatment and drying are carried out in sequence to form the third coating. Under ultraviolet light irradiation, the azobenzene group in the azobenzene group-modified fluorinated acrylate copolymer changes from a trans structure to a cis structure, the molecular polarity increases, and the hydrogen bond and van der Waals force interactions between polymer segments weaken, resulting in a decrease in the slurry viscosity. During the coating process, by irradiating ultraviolet light in the edge area, the viscosity of this area is reduced, so that the slurry flows towards the central area under the drive of surface tension, reducing edge accumulation. After stopping the light irradiation, the viscosity of the edge area recovers and solidifies to form a stable coating. The fluorinated chain segments in the azobenzene group-modified fluorinated acrylate copolymer tend to enrich on the surface of the slurry, forming a low surface tension adsorption layer, further reducing the spreading rate of the slurry in the edge area and reducing the phenomenon of excessive thickness at the edge. Different from traditional positive electrode slurries and coating methods, the present application prepares a composite slurry and performs three-layer coating. On the one hand, it helps to eliminate the edge bulging phenomenon of the positive electrode sheet, prepare thick electrodes, and prepare large-capacity batteries under the same housing size; on the other hand, through the synergistic effect between the three-layer coating and the vertical carbon nanotube array, it helps to optimize the wettability, ion transport efficiency and surface interface stability of the positive electrode sheet, thereby helping to further improve the capacity and cycle stability of the secondary battery. Detailed implementation manners

[0021] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0022] As analyzed in the background art of the present application, in the prior art, there are problems such as edge bulging during the coating process of the positive electrode sheet and low capacity of the secondary battery. To solve this problem, the present application provides a secondary battery, its preparation method, an energy storage system, and an electrical device.

[0023] In a typical embodiment of the present application, a method for preparing a secondary battery is provided. After making a bare battery cell from a positive electrode sheet, a separator, and a negative electrode sheet, it is assembled with a casing and a top cover, and then electrolyte is injected to obtain the secondary battery. The preparation steps of the positive electrode sheet include: S1, mixing a first positive electrode active material, nano-SiO 2 , two-dimensional titanium carbide, a first conductive agent, a first binder, and a first solvent for a first mixing to obtain a first positive electrode slurry; S2, coating the first positive electrode slurry on the surface of a current collector and drying to form a first coating to obtain a first sheet; S3, mixing a second positive electrode active material, carbon nanotubes, a second conductive agent, a second binder, and a second solvent for a second mixing to obtain a second positive electrode slurry; S4, coating the second positive electrode slurry on the surface of the first coating and drying to form a second coating to obtain a second sheet; S5, mixing a third positive electrode active material, an azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent, a third binder, and a third solvent for a third mixing to obtain a third positive electrode slurry; S6, coating the third positive electrode slurry on the surface of the second coating, and successively performing edge ultraviolet curing treatment and drying to form a third coating to obtain the positive electrode sheet.

[0024] In the present application, first, a first positive electrode active material, nano-SiO 2 , two-dimensional titanium carbide, a first conductive agent, a first binder, and a first solvent are mixed to obtain a first positive electrode slurry. The first positive electrode slurry is coated on the surface of the current collector and dried to form a first coating. Between nano-SiO 2 and two-dimensional titanium carbide, Si-O-Ti bonding occurs, which helps to strengthen the bonding force between the two. The addition of nano-SiO 2 on the one hand helps to fill microcracks, pores and other defects between the layers of two-dimensional titanium carbide and at the contact surface between the first coating and the current collector, forming a dense structure, reducing stress concentration, and helping to improve the overall mechanical strength of the positive electrode sheet and enhance the adhesion between the first coating and the current collector; on the other hand, nano-SiO 2It can be used as a dispersant to reduce the agglomeration between two-dimensional titanium carbide, which helps to improve the distribution uniformity of two-dimensional titanium carbide, form a more efficient conductive network, and enhance the interface continuity. Then, the second positive electrode active material, carbon nanotubes, the second conductive agent, the second binder, and the second solvent are mixed to obtain the second positive electrode slurry. The second positive electrode slurry is coated on the surface of the first coating and dried to form the second coating. The addition of carbon nanotubes helps to improve the conductivity of the positive electrode sheet. Finally, the third positive electrode active material, azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder, and the third solvent are mixed to obtain the third positive electrode slurry. The third positive electrode slurry is coated on the surface of the second coating, and edge ultraviolet light curing treatment and drying are carried out in sequence to form the third coating. Under ultraviolet light irradiation, the azobenzene group in the azobenzene group-modified fluorinated acrylate copolymer changes from a trans structure to a cis structure, the molecular polarity increases, the hydrogen bond and van der Waals force interactions between polymer segments are weakened, and the viscosity of the slurry decreases. During the coating process, through ultraviolet light irradiation in the edge area, the viscosity of this area is reduced, so that the slurry flows towards the central area under the drive of surface tension, reducing edge accumulation. After stopping the light irradiation, the viscosity of the edge area recovers and solidifies to form a stable coating. The fluorinated segment in the azobenzene group-modified fluorinated acrylate copolymer tends to accumulate on the surface of the slurry, forming a low surface tension adsorption layer, further reducing the spreading rate of the slurry in the edge area and reducing the phenomenon of excessive thickness at the edge. Different from the traditional positive electrode slurry and coating method, in this application, a composite slurry is prepared and three-layer coating is carried out. On the one hand, it helps to eliminate the edge bulging phenomenon of the positive electrode sheet, prepare a thick electrode, and prepare a large-capacity battery under the same housing size; on the other hand, through the mutual synergistic effect between the three-layer coating and the vertical carbon nanotube array, it helps to optimize the wettability, ion transport efficiency, and surface interface stability of the positive electrode sheet, thereby helping to further improve the capacity and cycle stability of the secondary battery.

[0025] It should be noted that the positive electrode active material, separator, negative electrode sheet, and electrolyte of this application can all be obtained by purchase or through existing technologies.

[0026] Including but not limited to, the above first conductive agent, second conductive agent, and third conductive agent are each independently selected from any one or more of Super-P, graphene, and carbon fiber; the above first binder, second binder, and third binder are each independently selected from any one or more of polyvinylidene fluoride, polyvinylidene difluoride, styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid; the above first solvent, second solvent, and third solvent are each independently selected from any one or more of N-methylpyrrolidone, dimethyl sulfoxide, and dimethylacetamide.

[0027] In order to improve the uniformity and stability of the slurry, and thus improve the consistency of the positive electrode sheet, in one embodiment of the present application, the above-mentioned first mixing is carried out under a stirring state, the rotation speed of the first mixing is 20-50 rpm, and the time of the first mixing is 3-8 h; and / or, the second mixing is carried out under a stirring state, the rotation speed of the second mixing is 10-50 rpm, and the time of the second mixing is 3-8 h; and / or, the third mixing is carried out under a stirring state, the rotation speed of the third mixing is 10-50 rpm, and the time of the third mixing is 3-8 h.

[0028] In order to further optimize the wettability, ion transport efficiency and surface interface stability of the positive electrode sheet, and thus improve the capacity and cycle stability of the secondary battery, in one embodiment of the present application, the average tube length of the above-mentioned carbon nanotubes is 10-30 μm, and the average tube diameter is 15-25 nm.

[0029] In one embodiment of the present application, the mass ratio of the above-mentioned first positive electrode active material, nano-SiO 2 and the total mass of two-dimensional titanium carbide, the mass of the first conductive agent, the mass of the first binder and the mass of the first solvent is (96-97):(0.5-1):(0.5-1):(1-2):(60-65); and / or, the mass ratio of the second positive electrode active material, carbon nanotubes, the second conductive agent, the second binder and the second solvent is (96-97):(0.5-1):(0.5-1):(1-2):(65-70); and / or, the mass ratio of the third positive electrode active material, azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder and the third solvent is (96-97):(0.5-1):(0.5-1):(1-2):(70-80).

[0030] Controlling the mass ratio of the first positive electrode active material, nano-SiO 2 and the total mass of two-dimensional titanium carbide, the mass of the first conductive agent, the mass of the first binder and the mass of the first solvent within the above range helps, on the one hand, to control the viscosity of the first positive electrode slurry within a suitable range, thus helping to improve the adhesion between the first coating and the current collector; on the other hand, it helps to improve the interaction between the components, further enhance the bonding strength between the first coating and the current collector, form a dense bottom layer structure, helps to improve the mechanical stability of the positive electrode sheet, and reduce the loss of active substances during the cycling process. Controlling the mass ratio of the second positive electrode active material, carbon nanotubes, the second conductive agent, the second binder and the second solvent within the above range helps to improve the conductivity of the positive electrode sheet. Controlling the mass ratio of the third positive electrode active material, azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder and the third solvent within the above range helps, on the one hand, to control the viscosity of the third positive electrode slurry within a suitable range, thus helping to reduce the phenomenon of edge bulging.

[0031] In one embodiment of the present application, the mass ratio of the above-mentioned nano-SiO 2 to two-dimensional titanium carbide is 1:(2 - 3).

[0032] Controlling the mass ratio of nano-SiO 2 to two-dimensional titanium carbide within the above range helps to improve the interaction between the two. On the one hand, it helps to further enhance the overall mechanical strength of the positive electrode sheet and strengthen the adhesion between the first coating and the current collector; on the other hand, it helps to further improve the distribution uniformity of two-dimensional titanium carbide, form a more efficient conductive network, and enhance the interface continuity at the same time.

[0033] In one embodiment of the present application, the D50 of the above-mentioned nano-SiO 2 is 40 - 100 nm; and / or, the average number of layers of two-dimensional titanium carbide is 5 - 10 layers; and / or, the number-average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35000 - 40000 Da.

[0034] Controlling the D50 of nano-SiO 2 within the above range helps to increase the contact area between nano-SiO 2 and two-dimensional titanium carbide, thereby helping to further improve the dispersibility of two-dimensional titanium carbide and fill pores and microcracks, forming a denser structure, which helps to improve the mechanical strength and toughness of the positive electrode sheet, reduce structural damage during the cycling process, and extend the life of the secondary battery. Controlling the average number of layers of two-dimensional titanium carbide within the above range, on the one hand, helps to reduce the influence of interlayer static charges and improve the interlayer stability of the material, thereby helping to improve the stable structure of the positive electrode sheet; on the other hand, it helps to provide more open channels, which is beneficial to the rapid diffusion of lithium ions between the layers, shortens the transmission path of lithium ions, and improves the charge and discharge rate of the battery. Controlling the number-average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer within the above range helps to rapidly change its viscosity under UV light irradiation, facilitate the precise control of the behavior of the positive electrode slurry during the coating process, and at the same time helps to form a denser network structure, which helps to increase the durability and stability of the coating.

[0035] In one embodiment of the present application, the above-mentioned azobenzene group-modified fluorinated acrylate copolymer is prepared by a polymerization reaction of azobenzene methacrylate and hexafluorobutyl methacrylate, and the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate is 1:(4 - 5).

[0036] Controlling the molar ratio of azobenzene methacrylate and hexafluorobutyl methacrylate within the above range helps to control the content of azobenzene groups and the fluorinated chain segments within an appropriate range, thereby further helping to reduce the viscosity of the third positive electrode slurry and form an adsorption layer with a low surface tension on the slurry surface, and further helping to reduce the edge swelling phenomenon of the positive electrode sheet.

[0037] In an embodiment of the present application, the viscosity of the above-mentioned first positive electrode slurry is 8000~12000 mPa·s; and / or, the viscosity of the second positive electrode slurry is 6000~8000 mPa·s; and / or, the viscosity of the third positive electrode slurry is 4000~6000 mPa·s.

[0038] Controlling the viscosity of the first positive electrode slurry within the above range helps to enhance the adhesion between the slurry and the current collector and form a firm first layer of substrate. Controlling the viscosity of the second positive electrode slurry within the above range helps to improve the bonding force between the second coating and the first coating and reduce edge swelling. Controlling the viscosity of the third positive electrode slurry within the above range helps to reduce the surface tension and reduce the migration of the slurry to the edge, thereby helping to reduce edge swelling.

[0039] In order to further improve the stability of the positive electrode sheet and thus improve the capacity and cycle stability of the secondary battery, in an embodiment of the present application, the viscosity of the above-mentioned first positive electrode slurry is 2000~6000 mPa·s higher than the viscosity of the second positive electrode slurry; and / or, the viscosity of the second positive electrode slurry is 2000~4000 mPa·s higher than the viscosity of the third positive electrode slurry.

[0040] In order to further reduce the edge swelling phenomenon and improve the capacity and cycle stability of the secondary battery, in an embodiment of the present application, it is preferably that the ultraviolet light irradiation area of the above-mentioned edge ultraviolet light curing treatment is within the range of extending 5~20 cm inward from the edge of the third coating; and / or, the ultraviolet light wavelength of the edge ultraviolet light curing treatment is 300~400 nm; and / or, the time of the edge ultraviolet light curing treatment is 0.05~0.1 min.

[0041] In an embodiment of the present application, the above-mentioned nano-SiO 2The preparation method includes adjusting the pH of the mixed solution of ethanol and water to 9 - 11, dropping tetraethyl orthosilicate into the above - mentioned mixed solution at a dropping rate of 0.5 - 2 mL / min, then adding 3 - aminopropyltriethoxysilane to reduce the agglomeration phenomenon after calcination, continuously stirring for 1 - 2 h to form a transparent sol, continuing slow stirring for 12 - 48 h, and the sol gradually transforms into a wet gel. Immerse the wet gel in ethanol for 24 - 72 hours to enhance the network structure strength and reduce drying cracks. Place the wet gel in an oven and dry it at 60 - 100 °C for 16 - 48 h to obtain a dry gel. Put the dry gel into a muffle furnace, heat it at a rate of 2 - 5 °C / min to 500 - 800 °C, keep it warm for 2 - 4 h, and then naturally cool it to room temperature to obtain white nano - SiO 2 powder.

[0042] In an embodiment of the present application, the preparation method of the above - mentioned two - dimensional titanium carbide includes: dissolving LiF in an HCl solution, adding Ti 3 AlC 2 powder (purity ≥ 98%, particle size ≤ 38 μm) into the above - mentioned solution, stirring and reacting at 30 - 50 °C with a rotation speed of 500 rpm for 24 hours, then centrifuging (8000 rpm, 10 minutes, five times) with deionized water until the pH ≥ 6, collecting and dispersing the centrifuged precipitate in dimethyl sulfoxide, performing ultrasonic treatment, centrifuging again, and collecting the upper - layer dispersion liquid to obtain a Ti 3 C 2 T x dispersion liquid. Freeze - dry the dispersion liquid (-60 - -30 °C, 36 - 48 h) to obtain a fluffy Ti 3 C 2 T x powder.

[0043] In an embodiment of the present application, the preparation method of the above azobenzene group-modified fluorinated acrylate copolymer includes: dissolving azophenol and methacryloyl chloride in tetrahydrofuran, adding triethylamine as an acid-binding agent, and reacting for 4 - 6 h under an ice bath. The reaction solution is filtered to remove the generated TEA·HCl salt, the collected filtrate is rotary evaporated to remove THF to obtain a yellow oil. The product is precipitated by slowly dropping it into cold methanol (-20 °C) with a small amount of dichloromethane (10 mL), filtered and then vacuum dried at 40 - 80 °C for 10 - 18 h to obtain azobenzene methacrylate. Hexafluorobutyl methacrylate, azobenzene methacrylate, and azodiisobutyronitrile are dissolved in N-methylpyrrolidone, deoxygenated by passing nitrogen for 30 minutes, then heated to 70 °C, stirred at a rate of 500 rpm, and polymerized for 12 - 14 hours. The reaction solution is cooled to room temperature, slowly dropped into cold methanol (-20 °C) for precipitation, the white flocculent product is collected by filtration, washed 3 times with methanol, and then vacuum dried at 60 - 80 °C for 12 - 24 h to obtain a white azobenzene group-modified fluorinated acrylate copolymer.

[0044] In another typical embodiment of the present application, a secondary battery is provided, including a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet. The positive electrode sheet includes a current collector and a positive electrode active layer, and the positive electrode active layer includes a first coating, a second coating, and a third coating stacked in sequence; wherein, the material of the first coating includes a first positive electrode active material, nano-SiO 2 , two-dimensional titanium carbide, a first conductive agent, and a first binder; the material of the second coating includes a second positive electrode active material, carbon nanotubes, a second conductive agent, and a second binder; the material of the third coating includes a third positive electrode active material, an azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent, and a third binder.

[0045] In the first coating, nano-SiO 2 and two-dimensional titanium carbide are bonded through Si-O-Ti bonds, which helps to strengthen the binding force between the two. The addition of nano-SiO 2 on the one hand helps to fill the microcracks, pores and other defects between the two-dimensional titanium carbide layers and the contact surface between the first coating and the current collector, forming a dense structure, reducing stress concentration, and helping to improve the overall mechanical strength of the positive electrode sheet and enhance the adhesion between the first coating and the current collector; on the other hand, nano-SiO 2It can be used as a dispersant to reduce the agglomeration between two-dimensional titanium carbide, which helps to improve the distribution uniformity of two-dimensional titanium carbide, form a more efficient conductive network, and enhance the interface continuity at the same time. The addition of carbon nanotubes in the second coating helps to improve the conductivity of the positive electrode sheet. The presence of the azobenzene group-modified fluorinated acrylate copolymer in the third coating helps to reduce the phenomenon of edge swelling of the positive electrode sheet. Under ultraviolet light irradiation, the azobenzene group in the azobenzene group-modified fluorinated acrylate copolymer changes from a trans structure to a cis structure, the molecular polarity increases, the hydrogen bond and van der Waals force interactions between polymer segments weaken, and the slurry viscosity decreases. During the coating process, by irradiating ultraviolet light on the edge area, the viscosity of this area is reduced, so that the slurry flows towards the central area under the drive of surface tension, reducing edge accumulation. After stopping the light irradiation, the viscosity of the edge area recovers and solidifies to form a stable coating. The fluorinated segment in the azobenzene group-modified fluorinated acrylate copolymer tends to accumulate on the surface of the slurry, forming a low surface tension adsorption layer, further reducing the spreading rate of the slurry in the edge area and reducing the phenomenon of excessive thickness at the edge. Different from the traditional single-layer coating, the three-layer coatings in this application play a synergistic role with each other. On the one hand, it helps to eliminate the edge swelling phenomenon of the positive electrode sheet, prepare a thick electrode, and prepare a large-capacity battery under the same housing size; on the other hand, the synergistic effect between the three-layer coatings and the vertical carbon nanotube array helps to optimize the wettability, ion transport efficiency and surface interface stability of the positive electrode sheet, thereby helping to improve the capacity and cycle stability of the secondary battery.

[0046] In an embodiment of the present application, the mass ratio of the above-mentioned first positive electrode active material, nano-SiO 2 and the total mass of two-dimensional titanium carbide, the mass of the first conductive agent and the mass of the first binder is (96-97):(0.5-1):(0.5-1):(1-2); and / or, the mass ratio of the second positive electrode active material, carbon nanotubes, the second conductive agent and the second binder is (96-97):(0.5-1):(0.5-1):(1-2); and / or, the mass ratio of the third positive electrode active material, the azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent and the third binder is (96-97):(0.5-1):(0.5-1):(1-2).

[0047] Control the mass of the first positive electrode active material, nano-SiO 2When the ratio of the total mass of nano - SiO₂ and two - dimensional titanium carbide, the mass of the first conductive agent, the mass of the first binder, and the mass of the first solvent is within the above - mentioned range, it helps to improve the adhesion between the first coating and the current collector; on the other hand, it helps to enhance the interaction between the components, further strengthen the bonding strength between the first coating and the current collector, form a dense bottom - layer structure, contribute to improving the mechanical stability of the positive electrode sheet, and reduce the loss of active materials during the cycling process. Controlling the mass ratio of the second positive - electrode active material, carbon nanotubes, the second conductive agent, the second binder, and the second solvent within the above - mentioned range helps to improve the conductivity of the positive electrode sheet. Controlling the mass ratio of the third positive - electrode active material, azobenzene - group - modified fluorinated acrylate copolymer, the third conductive agent, the third binder, and the third solvent within the above - mentioned range helps to reduce the phenomenon of edge swelling.

[0048] In an embodiment of the present application, the above - mentioned nano - SiO₂ 2 and the mass ratio of two - dimensional titanium carbide is 1:(2 - 3).

[0049] Controlling the mass ratio of nano - SiO₂ 2 and two - dimensional titanium carbide within the above - mentioned range helps to improve the interaction between the two. On the one hand, it helps to further enhance the overall mechanical strength of the positive electrode sheet and strengthen the adhesion between the first coating and the current collector; on the other hand, it helps to further improve the distribution uniformity of two - dimensional titanium carbide, form a more efficient conductive network, and enhance the interface continuity at the same time.

[0050] In an embodiment of the present application, the D50 of the above - mentioned nano - SiO₂ 2 is 40 - 100 nm; and / or, the average number of layers of two - dimensional titanium carbide is 5 - 10 layers; and / or, the number - average molecular weight of the azobenzene - group - modified fluorinated acrylate copolymer is 35000 - 40000 Da.

[0051] Controlling the D50 of nano - SiO₂ 2 within the above - mentioned range helps to improve nano - SiO₂ 2The contact area with two-dimensional titanium carbide, which helps to further improve the dispersion of two-dimensional titanium carbide and fill pores and microcracks, forming a denser structure, thus contributing to improving the mechanical strength and toughness of the positive electrode sheet, reducing structural damage during cycling, and prolonging the life of the secondary battery. Controlling the average number of layers of two-dimensional titanium carbide within the above range helps to reduce the influence of interlayer static charges on the one hand, enhance the interlayer stability of the material, and thus contribute to improving the stable structure of the positive electrode sheet; on the other hand, it helps to provide more open channels, facilitating the rapid diffusion of lithium ions between layers, shortening the lithium ion transport path, and enhancing the charge and discharge rate of the battery. Controlling the number average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer within the above range helps to rapidly change its viscosity under UV light irradiation, facilitating the precise control of the behavior of the positive electrode slurry during the coating process, and at the same time helps to form a denser network structure, contributing to increasing the durability and stability of the coating.

[0052] In an embodiment of the present application, the above-mentioned azobenzene group-modified fluorinated acrylate copolymer is prepared by a polymerization reaction of azobenzene methacrylate and hexafluorobutyl methacrylate, and the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate is 1:(4 - 5).

[0053] Controlling the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate within the above range helps to control the content of azobenzene groups and fluorinated segments within a suitable range, thereby contributing to further reducing the viscosity of the third positive electrode slurry and forming an adsorption layer with a low surface tension on the surface of the slurry, and further contributing to reducing the edge swelling phenomenon of the positive electrode sheet.

[0054] In an embodiment of the present application, the total thickness of the above-mentioned first coating, second coating, and third coating is 160 - 200 μm; and / or, the thickness ratio of the first coating, second coating, and third coating is (80 - 100):(30 - 40):(50 - 60).

[0055] Controlling the total thickness of the first coating, second coating, and third coating within the above range helps to prepare a thick electrode and manufacture a large-capacity battery under the same housing size. Controlling the thickness ratio of the first coating, second coating, and third coating within the above range helps to improve the interaction between the coatings, thus contributing to forming a positive electrode sheet with a stable structure, and contributing to improving the capacity and cycle stability of the secondary battery.

[0056] In order to reduce the phenomenon of edge bulging and cracking of the positive electrode sheet during winding, in an embodiment of the present application, the edge thickness of the above-mentioned positive electrode active layer is 10 - 7 μm lower than the center thickness ratio.

[0057] In still another exemplary embodiment of the present application, an energy storage system is provided, including a unit cell, which is a secondary battery prepared by the aforementioned method for preparing a secondary battery or the aforementioned secondary battery.

[0058] Since the energy storage system described above contains the secondary battery of the present application, this energy storage system has a relatively high energy density and a long service life.

[0059] In still another exemplary embodiment of the present application, an electrical device is provided, including the aforementioned energy storage system, and the energy storage system is used to provide power for the electrical device.

[0060] Since the energy storage system in the electrical device described above contains the secondary battery of the present application, this electrical device has a relatively high power and a long service life.

[0061] The beneficial effects of the present application will be further described below in conjunction with embodiments.

[0062] Example 1

[0063] Preparation of nano-SiO 2 : Mix 40 mL of ethanol and 40 mL of water to obtain a mixed solution. Adjust the pH of the mixed solution to 10 using ammonia water. Drop 10 mL of tetraethyl orthosilicate into the above mixed solution at a dropping rate of 1 mL / min. Subsequently, add 2 mL of 3-aminopropyltriethoxysilane and continuously stir for 1 h to form a transparent sol. Continue slow stirring for 48 h, and the sol gradually turns into a wet gel. Immerse the wet gel in ethanol for 48 hours, then place it in an oven and dry it at 60 °C for 16 h to obtain a dry gel. Put the dry gel into a muffle furnace and calcine it. Heat it to 600 °C at a rate of 3 °C / min, hold for 3 h, and then naturally cool to room temperature to obtain white nano-SiO 2 powder, and the D50 of nano-SiO 2 is 50 nm.

[0064] Ti 3 C 2 T x Preparation of : Dissolve LiF (1.98 g) in 20 mL of HCl solution (concentration: 2 mol / L). Add 1 g of Ti 3 AlC 2Powder (with a purity of 99% and an average particle size of 20 μm) was added to the above solution, and stirred and reacted at 40 °C with a rotation speed of 500 rpm for 24 hours. Subsequently, it was centrifuged (8000 rpm, 10 minutes, five times) with deionized water until the pH reached 6. The centrifuged precipitate was collected and dispersed in dimethyl sulfoxide, and ultrasonic treated (with a power of 500 W and a time of 1 hour). Then it was centrifuged again (with a rotation speed of 3000 rpm, a time of 10 minutes, and twice), and the upper layer dispersion was collected to obtain a Ti 3 C 2 T x dispersion. The dispersion was freeze-dried (-60 °C, 48 h) to obtain a fluffy Ti 3 C 2 T x powder. The average number of layers of Ti 3 C 2 T x was 8 layers.

[0065] Preparation of azobenzene group-modified fluorinated acrylate copolymer: 10 mmol of azophenol and 12 mmol of methacryloyl chloride were dissolved in 50 mL of tetrahydrofuran, and 15 mmol of triethylamine (TEA) was added. The reaction was carried out at 0 °C in an ice bath for 5 h. The reaction solution was filtered to remove the generated TEA·HCl salt. The collected filtrate was rotary evaporated to remove tetrahydrofuran to obtain a yellow oil. The yellow oil was slowly dropped into cold methanol (-20 °C) with 10 mL of dichloromethane for precipitation, and after filtration, it was vacuum dried at 60 °C for 15 h to obtain azobenzene methacrylate. 16 mmol of hexafluorobutyl methacrylate, 4 mmol of azobenzene methacrylate, and 0.052 g of azobisisobutyronitrile were dissolved in 50 mL of N-methylpyrrolidone, and nitrogen was passed for 30 minutes to remove oxygen. Then the temperature was raised to 70 °C, and the stirring rate was 500 rpm for a polymerization reaction of 12 hours. The reaction solution was cooled to room temperature and slowly dropped into cold methanol (-20 °C) for precipitation. The white flocculent product was collected by filtration, washed 3 times with methanol, and then vacuum dried at 60 °C for 12 h to obtain a white azobenzene group-modified fluorinated acrylate copolymer. The number-average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer was 35000 Da.

[0066] Preparation of the positive electrode sheet: The first lithium iron phosphate material, the above-mentioned nano-SiO 2 , the above-mentioned Ti 3 C 2 T x , the first Super-P, the first polyvinylidene fluoride, and the first N-methylpyrrolidone were stirred and mixed for the first time at a rotation speed of 20 rpm for 8 h. The mass of the first lithium iron phosphate material, the nano-SiO 2 and Ti 3 C 2 Tx The mass ratio of the total mass, the mass of the first Super - P, the mass of the first polyvinylidene fluoride, and the mass of the first N - methylpyrrolidone is 96:1:0.5:2:60, and the mass ratio of nano - SiO 2 and Ti 3 C 2 T x is 1:2, obtaining a first positive electrode slurry with a viscosity of 12000 mPa·s; coating the first positive electrode slurry on the surface of a copper foil, drying at 60 °C for 2 h to form a first coating, obtaining a first sheet; mixing a second lithium iron phosphate material, carbon nanotubes (average tube length is 30 μm, average tube diameter is 25 nm), a second Super - P, a second polyvinylidene fluoride, and a second N - methylpyrrolidone at a rotation speed of 10 rpm for 8 h for a second mixing, and the mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super - P, the second polyvinylidene fluoride, and the second N - methylpyrrolidone is 96:1:0.5:2:65, obtaining a second positive electrode slurry with a viscosity of 8000 mPa·s; coating the second positive electrode slurry on the surface of the first coating, drying at 60 °C for 2 h to form a second coating, obtaining a second sheet; mixing a third lithium iron phosphate material, the above - mentioned azobenzene - group - modified fluorinated acrylate copolymer, a third Super - P, a third polyvinylidene fluoride, and a third N - methylpyrrolidone at a rotation speed of 10 rpm for 8 h for a third mixing, and the mass ratio of the third lithium iron phosphate material, the azobenzene - group - modified fluorinated acrylate copolymer, the third Super - P, the third polyvinylidene fluoride, and the third N - methylpyrrolidone is 96:1:0.5:2:70, obtaining a third positive electrode slurry with a viscosity of 6000 mPa·s; coating the third positive electrode slurry on the surface of the second coating, successively performing edge ultraviolet - light curing treatment and drying to form a third coating, obtaining a positive electrode sheet, wherein the ultraviolet - light irradiation area of the edge ultraviolet - light curing treatment is within the range from the edge of the third coating to 20 cm inward, the ultraviolet - light wavelength is 365 nm, the time of the edge ultraviolet - light curing treatment is 0.1 min, the total thickness of the first coating, the second coating, and the third coating is 180 μm, and the thickness ratio of the first coating, the second coating, and the third coating is 80:40:50.

[0067] Preparation of the secondary battery: After making the above - mentioned positive electrode sheet, polyethylene separator, and graphite negative electrode sheet into a bare battery core, assembling it with a shell and a top cover, and then injecting a lithium hexafluorophosphate electrolyte to obtain a secondary battery.

[0068] Example 2

[0069] The difference from Example 1 is that the mass of the first lithium iron phosphate material, nano - SiO 2 and Ti 3 C 2 T xThe mass ratio of the total mass, the mass of the first Super-P, the mass of the first polyvinylidene fluoride, and the mass of the first N-methylpyrrolidone is 96:1:0.5:2:65, and the viscosity of the first positive electrode slurry is 8000 mPa·s; the mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super-P, the second polyvinylidene fluoride, and the second N-methylpyrrolidone is 96:1:0.5:2:70, and the viscosity of the second positive electrode slurry is 6000 mPa·s; the mass ratio of the third lithium iron phosphate material, the azobenzene group-modified fluorinated acrylate copolymer, the third Super-P, the third polyvinylidene fluoride, and the third N-methylpyrrolidone is 96:1:0.5:2:80, and the viscosity of the third positive electrode slurry is 4000 mPa·s, and a secondary battery is finally obtained.

[0070] Example 3

[0071] The difference from Example 1 is that the mass of the first lithium iron phosphate material, nano-SiO 2 and Ti 3 C 2 T x The mass ratio of the total mass, the mass of the first Super-P, the mass of the first polyvinylidene fluoride, and the mass of the first N-methylpyrrolidone is 96:1:0.5:2:65, and the viscosity of the first positive electrode slurry is 8000 mPa·s; the mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super-P, the second polyvinylidene fluoride, and the second N-methylpyrrolidone is 96:1:0.5:2:65, and the viscosity of the second positive electrode slurry is 8000 mPa·s; the mass ratio of the third lithium iron phosphate material, the azobenzene group-modified fluorinated acrylate copolymer, the third Super-P, the third polyvinylidene fluoride, and the third N-methylpyrrolidone is 96:1:0.5:2:80, and the viscosity of the third positive electrode slurry is 4000 mPa·s, and a secondary battery is finally obtained.

[0072] Example 4

[0073] The difference from Example 1 is that the mass of the first lithium iron phosphate material, nano-SiO 2 and Ti 3 C 2 T xThe mass ratio of the total mass, the mass of the first Super-P, the mass of the first polyvinylidene fluoride, and the mass of the first N-methylpyrrolidone is 96:1:0.5:2:65, and the viscosity of the first positive electrode slurry is 8000 mPa·s; the mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super-P, the second polyvinylidene fluoride, and the second N-methylpyrrolidone is 96:1:0.5:2:65, and the viscosity of the second positive electrode slurry is 8000 mPa·s; the mass ratio of the third lithium iron phosphate material, azobenzene group-modified fluorinated acrylate copolymer, the third Super-P, the third polyvinylidene fluoride, and the third N-methylpyrrolidone is 96:1:0.5:2:65, and the viscosity of the third positive electrode slurry is 8000 mPa·s, and finally a secondary battery is obtained.

[0074] Example 5

[0075] The difference from Example 1 is that the mass ratio of the mass of the first lithium iron phosphate material, nano-SiO 2 and Ti 3 C 2 T x The total mass, the mass of the first Super-P, the mass of the first polyvinylidene fluoride, and the mass of the first N-methylpyrrolidone is 96:0.5:0.5:2:65; the mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super-P, the second polyvinylidene fluoride, and the second N-methylpyrrolidone is 96:0.5:0.5:2:65; the mass ratio of the third lithium iron phosphate material, azobenzene group-modified fluorinated acrylate copolymer, the third Super-P, the third polyvinylidene fluoride, and the third N-methylpyrrolidone is 96:0.5:0.5:2:65, and finally a secondary battery is obtained.

[0076] Example 6

[0077] The difference from Example 1 is that the mass ratio of the mass of the first lithium iron phosphate material, nano-SiO 2 and Ti 3 C 2 T x The total mass, the mass of the first Super-P, the mass of the first polyvinylidene fluoride, and the mass of the first N-methylpyrrolidone is 96:0.2:0.5:2:65; the mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super-P, the second polyvinylidene fluoride, and the second N-methylpyrrolidone is 96:0.2:0.5:2:65; the mass ratio of the third lithium iron phosphate material, azobenzene group-modified fluorinated acrylate copolymer, the third Super-P, the third polyvinylidene fluoride, and the third N-methylpyrrolidone is 96:0.2:0.5:2:65, and finally a secondary battery is obtained.

[0078] Example 7

[0079] The difference from Example 1 is that the mass ratio of nano-SiO 2 and Ti 3 C 2 T x is 1:3, and a secondary battery is finally obtained.

[0080] Example 8

[0081] The difference from Example 1 is that the mass ratio of nano-SiO 2 and Ti 3 C 2 T x is 1:4, and a secondary battery is finally obtained.

[0082] Example 9

[0083] The difference from Example 1 is that the pH of the mixed solution is adjusted to 9 with ammonia water to obtain nano-SiO 2 with a D50 of 40 nm; the ultrasonic time is 2 h to obtain Ti 3 C 2 T x with an average number of layers of 5, and a secondary battery is finally obtained.

[0084] Example 10

[0085] The difference from Example 1 is that the pH of the mixed solution is adjusted to 11 with ammonia water to obtain nano-SiO 2 with a D50 of 100 nm; the ultrasonic time is 0.5 h to obtain Ti 3 C 2 T x with an average number of layers of 10, and a secondary battery is finally obtained.

[0086] Example 11

[0087] The difference from Example 1 is that the pH of the mixed solution is adjusted to 5 with hydrochloric acid to obtain nano-SiO 2 with a D50 of 20 nm; the ultrasonic time is 3 h to obtain single-layer Ti 3 C 2 T x and a secondary battery is finally obtained.

[0088] Example 12

[0089] The difference from Example 1 is that the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate is 1:5, and the polymerization reaction time is 14 h to obtain an azobenzene group-modified fluorinated acrylate copolymer with a number-average molecular weight of 40000 Da, and a secondary battery is finally obtained.

[0090] Example 13

[0091] The difference from Example 1 is that the molar ratio of azobenzene methacrylate to hexafluorobutyl methacrylate is 1:6, the polymerization reaction time is 16 h, an azobenzene group-modified fluorinated acrylate copolymer with a number average molecular weight of 50,000 Da is obtained, and finally a secondary battery is obtained.

[0092] Example 14

[0093] The difference from Example 1 is that the thickness ratio of the first coating, the second coating and the third coating in the positive electrode sheet is 100:30:60, and finally a secondary battery is obtained.

[0094] Example 15

[0095] The difference from Example 1 is that the thickness ratio of the first coating, the second coating and the third coating in the positive electrode sheet is 70:50:40, and finally a secondary battery is obtained.

[0096] Example 16

[0097] The difference from Example 1 is that for the preparation of nano-SiO 2 : 40 mL of ethanol and 40 mL of water are mixed to obtain a mixed solution. The pH of the mixed solution is adjusted to 10 with ammonia water. 10 mL of tetraethyl orthosilicate is added dropwise to the above mixed solution at a dropping rate of 1 mL / min, and then 2 mL of 3-aminopropyltriethoxysilane is added. Stir continuously for 1 h to form a transparent sol, and continue to stir slowly for 48 h. The sol gradually turns into a wet gel. The wet gel is immersed in ethanol for 48 hours, and then placed in an oven and dried at 60 °C for 16 h to obtain a dry gel. The dry gel is placed in a muffle furnace and calcined. It is heated to 600 °C at a rate of 3 °C / min and held for 3 h, and then naturally cooled to room temperature to obtain white nano-SiO 2 powder, and the D50 of nano-SiO 2 is 50 nm.

[0098] Ti 3 C 2 T x For the preparation of : LiF (1.98 g) is dissolved in 20 mL of HCl solution (concentration 2 mol / L), and 1 g of Ti 3 AlC 2Powder (with a purity of 99% and an average particle size of 20 μm) was added to the above solution, and the mixture was stirred and reacted at 40 °C with a rotation speed of 500 rpm for 24 hours. Subsequently, it was centrifuged (8000 rpm, 10 minutes, five times) with deionized water and washed until the pH reached 6. The centrifuged precipitate was collected and dispersed in dimethyl sulfoxide, and ultrasonic treatment was performed (power: 500 W, time: 1 hour). Then, it was centrifuged again (rotation speed: 3000 rpm, time: 10 minutes, twice), and the upper-layer dispersion was collected to obtain a Ti 3 C 2 T x dispersion. The dispersion was freeze-dried (-60 °C, 48 h) to obtain a fluffy Ti 3 C 2 T x powder. The average number of layers of Ti 3 C 2 T x was 8 layers.

[0099] Preparation of azobenzene-group modified fluorinated acrylate copolymer: 10 mmol of azophenol and 12 mmol of methacryloyl chloride were dissolved in 50 mL of tetrahydrofuran, and 15 mmol of triethylamine (TEA) was added. The reaction was carried out at 0 °C in an ice bath for 5 h. The reaction solution was filtered to remove the generated TEA·HCl salt. The collected filtrate was rotary-evaporated to remove tetrahydrofuran, obtaining a yellow oil. The yellow oil was slowly dropped into cold methanol (-20 °C) with 10 mL of dichloromethane for precipitation, and after filtration, it was dried in vacuo at 60 °C for 15 h to obtain azobenzene methacrylate. 16 mmol of hexafluorobutyl methacrylate, 4 mmol of azobenzene methacrylate, and 0.052 g of azobisisobutyronitrile were dissolved in 50 mL of N-methylpyrrolidone, and nitrogen was passed for 30 minutes to remove oxygen. Then, the temperature was raised to 70 °C, and the stirring rate was 500 rpm for a polymerization reaction for 12 hours. The reaction solution was cooled to room temperature and slowly dropped into cold methanol (-20 °C) for precipitation. The white flocculent product was collected by filtration, washed 3 times with methanol, and then dried in vacuo at 60 °C for 12 h to obtain a white azobenzene-group modified fluorinated acrylate copolymer. The number-average molecular weight of the azobenzene-group modified fluorinated acrylate copolymer was 35000 Da.

[0100] Preparation of the positive electrode sheet: The first lithium iron phosphate material, the above-mentioned nano-SiO 2 , the above-mentioned Ti 3 C 2 T x , the first Super-P, the first polyvinylidene fluoride, and the first N-methylpyrrolidone were stirred at a rotation speed of 50 rpm for 3 h for the first mixing. The mass of the first lithium iron phosphate material, nano-SiO 2 and Ti 3 C 2 Tx The mass ratio of the total mass, the mass of the first Super - P, the mass of the first polyvinylidene fluoride, and the mass of the first N - methylpyrrolidone is 97:1:1:1:60, and the mass ratio of nano - SiO 2 and Ti 3 C 2 T x is 1:2, obtaining a first positive electrode slurry with a viscosity of 12000 mPa·s; the first positive electrode slurry is coated on the surface of a copper foil and dried at 60 °C for 2 h to form a first coating, obtaining a first sheet; the second lithium iron phosphate material, carbon nanotubes (average tube length is 10 μm, average tube diameter is 15 nm), the second Super - P, the second polyvinylidene fluoride, and the second N - methylpyrrolidone are stirred at a rotation speed of 50 rpm for 3 h for a second mixing. The mass ratio of the second lithium iron phosphate material, carbon nanotubes, the second Super - P, the second polyvinylidene fluoride, and the second N - methylpyrrolidone is 97:1:1:1:65, obtaining a second positive electrode slurry with a viscosity of 8000 mPa·s; the second positive electrode slurry is coated on the surface of the first coating and dried at 60 °C for 2 h to form a second coating, obtaining a second sheet; the third lithium iron phosphate material, the above - mentioned azobenzene - group - modified fluorinated acrylate copolymer, the third Super - P, the third polyvinylidene fluoride, and the third N - methylpyrrolidone are stirred at a rotation speed of 50 rpm for 3 h for a third mixing. The mass ratio of the third lithium iron phosphate material, the azobenzene - group - modified fluorinated acrylate copolymer, the third Super - P, the third polyvinylidene fluoride, and the third N - methylpyrrolidone is 97:1:1:1:70, obtaining a third positive electrode slurry with a viscosity of 6000 mPa·s; the third positive electrode slurry is coated on the surface of the second coating, and edge ultraviolet curing treatment and drying are carried out in sequence to form a third coating, obtaining a positive electrode sheet. Among them, the ultraviolet light irradiation area of the edge ultraviolet curing treatment is within the range from the edge of the third coating to 5 cm inward extension, the ultraviolet light wavelength is 365 nm, the time of the edge ultraviolet curing treatment is 0.05 min, the total thickness of the first coating, the second coating, and the third coating is 200 μm, and the thickness ratio of the first coating, the second coating, and the third coating is 80:40:50.

[0101] Preparation of the secondary battery: After making the above - mentioned positive electrode sheet, polyethylene separator, and graphite negative electrode sheet into a bare battery core, it is assembled with a shell and a top cover, and then a lithium hexafluorophosphate electrolyte is injected to obtain a secondary battery.

[0102] Comparative Example 1

[0103] The difference from Example 1 is that the addition of the azobenzene - group - modified fluorinated acrylate copolymer is cancelled, and finally a secondary battery is obtained.

[0104] Comparative Example 2

[0105] The difference from Example 1 is that the addition of nano-SiO 2 and Ti 3 C 2 T x is cancelled, and a secondary battery is finally obtained.

[0106] Comparative Example 3

[0107] The difference from Example 1 is that the addition of carbon nanotubes is cancelled, and a secondary battery is finally obtained.

[0108] Comparative Example 4

[0109] The difference from Example 1 is that the segmented coating is cancelled. After mixing the first positive electrode paste, the second positive electrode paste and the third positive electrode paste, they are coated on the copper foil and then subjected to edge ultraviolet light curing treatment and drying in sequence to obtain a positive electrode sheet, and finally a secondary battery is obtained.

[0110] Comparative Example 5

[0111] The difference from Example 1 is that the addition of nano-SiO 2 is cancelled, and a secondary battery is finally obtained.

[0112] Comparative Example 6

[0113] The difference from Example 1 is that the addition of Ti 3 C 2 T x is cancelled, and a secondary battery is finally obtained.

[0114] Comparative Example 7

[0115] The difference from Example 1 is that the coating of the first positive electrode paste on the surface of the copper foil is cancelled. After mixing the first positive electrode paste and the second positive electrode paste, they are coated on the surface of the copper foil, and finally a secondary battery is obtained.

[0116] Comparative Example 8

[0117] The difference from Example 1 is that the coating of the second positive electrode paste on the surface of the first coating is cancelled. After mixing the second positive electrode paste and the third positive electrode paste, they are coated on the surface of the first coating, and finally a secondary battery is obtained.

[0118] Comparative Example 9

[0119] The difference from Example 1 is that the edge ultraviolet light curing treatment is cancelled, and a secondary battery is finally obtained.

[0120] Performance Test

[0121] Observe whether edge bulging occurs during the coating process of the observation examples and comparative examples, measure the thickness of the central region and the edge region of the positive electrode sheets prepared in the observation examples and comparative examples, calculate the thickness difference, where thickness difference = thickness of the edge region - thickness of the middle region; test the initial discharge specific capacity at 0.1C and the capacity retention rate after 300 cycles at 0.3C of the secondary batteries prepared in the observation examples and comparative examples. The test results of whether edge bulging occurs, thickness difference, initial discharge specific capacity, and capacity retention rate are shown in Table 1.

[0122] Table 1

[0123]

[0124] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0125] From the data in Table 1, it can be known that in Examples 1 to 4, the positive electrode sheets are prepared by regulating the viscosities of the first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry. In Examples 1 and 2, the viscosities of the first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry decrease in sequence. In Example 3, the viscosities of the first positive electrode slurry and the second positive electrode slurry are the same and higher than that of the third positive electrode slurry. In Example 4, the viscosities of the first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry are all the same. From the results, it can be seen that the viscosities of the first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry have a certain influence on the edge thickness of the positive electrode sheet. By setting the viscosities of the first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry to decrease in sequence, it is helpful to reduce the edge thickness of the positive electrode sheet, thereby helping to improve the initial discharge specific capacity and cycling stability of the battery; in Examples 1, 5, and 6, by adjusting the proportions of nano-SiO 2 、Ti 3 C 2 T x 、carbon nanotubes, and azobenzene group-modified fluorinated acrylate copolymer in the slurry respectively to prepare the positive electrode sheets. From the data, it can be seen that the proportions of nano-SiO 2 、Ti 3 C 2 T x 、carbon nanotubes, and azobenzene group-modified fluorinated acrylate copolymer mainly affect the initial discharge specific capacity and capacity retention rate of the battery. Appropriate proportions of nano-SiO 2 、Ti 3 C 2 T x 、carbon nanotubes, and azobenzene group-modified fluorinated acrylate copolymer are helpful to improve the mutual synergy between the coatings, thereby helping to improve the initial discharge specific capacity and capacity retention rate of the battery; the differences between Examples 1, 7, and 8 lie in nano-SiO 2 and Ti 3 C2 T x The mass ratio of 2 and Ti 3 C 2 T x does not have an obvious impact on the edge thickness of the positive electrode sheet. However, an appropriate mass ratio of 2 and Ti 3 C 2 T x helps to improve the first discharge specific capacity and capacity retention rate of the battery; the differences between Examples 1, 9 to 11 lie in the particle size of 2 and the number of layers of Ti 3 C 2 T x . It can be seen from the data that the particle size of 2 and the number of layers of Ti 3 C 2 T x do not affect the edge thickness of the positive electrode sheet. Appropriate particle size of 2 and the number of layers of Ti 3 C 2 T x help to improve the first discharge specific capacity and capacity retention rate of the battery; the differences between Examples 1, 12 and 13 lie in the molar ratio of azobenzene methacrylate and hexafluorobutyl methacrylate and the number average molecular weight of the azobenzene group modified fluorinated acrylate copolymer. It can be seen from the data that appropriate molar ratio of azobenzene methacrylate and hexafluorobutyl methacrylate and the number average molecular weight of the azobenzene group modified fluorinated acrylate copolymer help to generate a better ultraviolet light response, further reduce the viscosity of the slurry, and thus help to reduce the thickness of the edge of the positive electrode sheet; the differences between Examples 1, 14 and 15 lie in the thickness ratio of the first coating, the second coating and the third coating. It can be known from the data results that appropriate thickness ratio of the first coating, the second coating and the third coating helps to improve the mutual synergy among the three, and thus helps to improve the first discharge specific capacity and cycle stability of the battery.

[0126] It can be known from the data results of Example 1 and Comparative Example 1 that without adding the azobenzene group modified fluorinated acrylate copolymer, the viscosity of the third slurry cannot be reduced by ultraviolet light irradiation, which is not conducive to reducing the thickness of the edge of the positive electrode sheet and leads to serious edge swelling; in Comparative Example 2, without 2 and Ti 3 C 2 T xThe addition of 2 will not significantly affect the thickness of the edge of the positive electrode sheet, but it is not conducive to improving the conductivity and ion transport efficiency of the positive electrode sheet, thereby being not conducive to improving the first discharge specific capacity and cycle stability of the battery; in Comparative Example 3, the addition of carbon nanotubes is cancelled, which will not significantly affect the thickness of the edge of the positive electrode sheet, but it is not conducive to optimizing the wettability, ion transport efficiency and surface interface stability of the positive electrode sheet, resulting in a decrease in the first discharge specific capacity and cycle stability of the battery; in Comparative Example 4, the segmented coating is cancelled, and the overall viscosity of the positive electrode slurry is relatively large, and it is difficult to significantly reduce the viscosity of the positive electrode slurry through ultraviolet light irradiation, resulting in obvious bulging of the edge of the positive electrode sheet; in Comparative Example 5, the addition of nano-SiO 2 is cancelled, and it is difficult to fill defects such as microcracks and holes between the Ti 3 C 2 T x layers and on the contact surface between the first coating and the current collector. Ti 3 C 2 T x is prone to agglomeration, resulting in a decrease in the first discharge specific capacity and cycle stability of the battery; in Comparative Example 6, the addition of Ti 3 C 2 T x is cancelled, the conductivity and lithium ion transport efficiency of the positive electrode sheet decrease. Although it does not affect the thickness of the edge of the positive electrode sheet, it is not conducive to improving the first discharge specific capacity and cycle stability of the battery; in Comparative Example 7, after mixing the first positive electrode slurry and the second positive electrode slurry and then coating it on the surface of the copper foil, the viscosity of the slurry is too low, which is not conducive to the combination between the coating and the current collector, thereby being not conducive to improving the cycle stability of the battery; in Comparative Example 8, the coating of the second positive electrode slurry on the surface of the first coating is cancelled. After mixing the second positive electrode slurry and the third positive electrode slurry and then coating it on the surface of the first coating, the viscosity of the slurry is too high, and the concentration of the azobenzene group-modified fluorinated acrylate copolymer is diluted, which is not conducive to the progress of ultraviolet light irradiation, thereby being not conducive to reducing the thickness of the positive electrode edge, resulting in bulging; in Comparative Example 9, the edge ultraviolet light curing treatment is cancelled, and the azobenzene group-modified fluorinated acrylate copolymer in the third positive electrode slurry does not undergo an ultraviolet light response, making it difficult to reduce the viscosity of the third positive electrode slurry and having a high surface tension, resulting in serious bulging of the edge of the positive electrode sheet, thereby causing a significant decrease in the first discharge specific capacity and capacity retention rate of the battery.

[0127] In this application, first, the first positive electrode active material, nano-SiO 2 , two-dimensional titanium carbide, the first conductive agent, the first binder and the first solvent are mixed to obtain the first positive electrode slurry. The first positive electrode slurry is coated on the surface of the current collector and dried to form the first coating. The Si-O-Ti bonding between nano-SiO 2 and two-dimensional titanium carbide helps to strengthen the bonding force between the two. Nano-SiO 2On the one hand, the addition of nano-SiO helps to fill the defects such as microcracks and holes between the two-dimensional titanium carbide layers and the contact surface between the first coating and the current collector, forming a dense structure, reducing stress concentration, and helping to improve the overall mechanical strength of the positive electrode and enhance the adhesion between the first coating and the current collector; on the other hand, nano-SiO 2 It can be used as a dispersant to reduce the agglomeration between two-dimensional titanium carbide, which helps to improve the distribution uniformity of two-dimensional titanium carbide, form a more efficient conductive network, and enhance the continuity of the interface. Then the second positive electrode active material, carbon nanotubes, second conductive agent, second binder and second solvent are mixed to obtain a second positive electrode slurry, the second positive electrode slurry is coated on the surface of the first coating, and dried to form a second coating. The addition of carbon nanotubes helps to improve the conductivity of the positive electrode sheet. Finally, the third positive electrode active material, azobenzene group-modified fluorinated acrylate copolymer, third conductive agent, third binder and third solvent are mixed to obtain a third positive electrode slurry, the third positive electrode slurry is coated on the surface of the second coating, and edge ultraviolet light curing and drying are performed in sequence to form a third coating. The azobenzene group in the azobenzene group-modified fluorinated acrylate copolymer is transformed from a trans structure to a cis structure under ultraviolet light irradiation, the molecular polarity increases, the hydrogen bonds and van der Waals interactions between the polymer segments are weakened, and the slurry viscosity decreases. During the coating process, the edge area is irradiated with ultraviolet light to reduce the viscosity of the area, so that the slurry flows to the central area driven by surface tension, reducing edge accumulation. After the light is stopped, the viscosity of the edge area is restored and solidified to form a stable coating. The fluorinated segments in the azobenzene group-modified fluorinated acrylate copolymer tend to be enriched on the slurry surface to form a low surface tension adsorption layer, further reducing the slurry spreading rate in the edge area and reducing the phenomenon of excessive thickness at the edge. Different from the traditional positive electrode slurry and coating method, the present application prepares a composite slurry and performs three-layer coating. On the one hand, it helps to eliminate the bulging edge phenomenon of the positive electrode sheet, prepare thick electrodes, and prepare large-capacity batteries under the same shell size; on the other hand, the mutual synergistic effect between the three-layer coating and the vertical carbon nanotube array helps to optimize the wettability, ion transfer efficiency and surface interface stability of the positive electrode sheet, thereby helping to further improve the capacity and cycle stability of the secondary battery.

[0128] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a secondary battery, comprising: preparing a positive electrode sheet, a separator and a negative electrode sheet into a bare cell, assembling the cells with a housing and a top cover, and injecting an electrolyte to obtain the secondary battery, wherein: The steps of preparing the positive electrode sheet include: S1, mixing a first positive electrode active material, nano-SiO2, two-dimensional titanium carbide, a first conductive agent, a first binder and a first solvent to obtain a first positive electrode slurry; S2, coating the first positive electrode slurry on the surface of the current collector, and drying to form a first coating layer to obtain a first sheet; S3, performing a second mixing of the second positive electrode active material, the carbon nanotubes, the second conductive agent, the second binder and the second solvent to obtain a second positive electrode slurry; S4, coating the second positive electrode slurry on the surface of the first coating layer, and drying to form a second coating layer to obtain a second sheet; S5, performing a third mixing of the third positive electrode active material, the azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder and the third solvent to obtain a third positive electrode slurry; S6, coating the third positive electrode slurry on the surface of the second coating layer, and sequentially performing edge UV curing treatment and drying to form a third coating layer to obtain the positive electrode sheet.

2. The method for preparing a secondary battery according to claim 1, characterized in that: The ratio of the mass of the first positive electrode active material, the total mass of the nano-SiO2 and the two-dimensional titanium carbide, the mass of the first conductive agent, the mass of the first binder and the mass of the first solvent is (96-97): (0.5-1): (0.5-1): (1-2): (60-65); And / or, the mass ratio of the second positive electrode active material, the carbon nanotubes, the second conductive agent, the second binder and the second solvent is (96-97): (0.5-1): (0.5-1): (1-2): (65-70); And / or, the mass ratio of the third positive electrode active material, the azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent, the third binder and the third solvent is (96-97):(0.5-1):(0.5-1):(1-2):(70-80).

3. The method for preparing a secondary battery according to claim 2, characterized in that: The mass ratio of the nano-SiO2 to the two-dimensional titanium carbide is 1:(2-3).

4. The method for preparing a secondary battery according to claim 1, characterized in that: The D50 of the nano-SiO2 is 40-100 nm; and / or the average number of layers of the two-dimensional titanium carbide is 5-10 layers; and / or the molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35000-40000 Da.

5. The method for preparing a secondary battery according to claim 1, characterized in that: The azobenzene group-modified fluorinated acrylate copolymer is prepared by polymerization of azophenyl methacrylate and hexafluorobutyl methacrylate, wherein the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate is 1:(4-5).

6. The method for preparing a secondary battery according to any one of claims 1 to 5, characterized in that: The viscosity of the first positive electrode slurry is 8000-12000 mPa·s; and / or the viscosity of the second positive electrode slurry is 6000-8000 mPa·s; and / or the viscosity of the third positive electrode slurry is 4000-6000 mPa·s.

7. The method for preparing a secondary battery according to any one of claims 1 to 5, characterized in that: The ultraviolet light irradiation of the edge ultraviolet light curing treatment is within the range of 5 to 20 cm extending inward from the edge of the third coating; and / or, the ultraviolet light wavelength of the edge ultraviolet light curing treatment is 300 to 400 nm; and / or, the time of the edge ultraviolet light curing treatment is 0.05 to 0.1 min.

8. A secondary battery comprising a positive electrode sheet, a separator, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet comprises a current collector and a positive electrode active layer, characterized in that: The positive electrode active layer includes a first coating layer, a second coating layer and a third coating layer stacked in sequence; wherein, The material of the first coating layer includes a first positive electrode active material, nano-SiO2, two-dimensional titanium carbide, a first conductive agent and a first binder; The material of the second coating layer includes a second positive electrode active material, carbon nanotubes, a second conductive agent and a second binder; The materials of the third coating layer include a third positive electrode active material, an azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent and a third binder.

9. The secondary battery according to claim 8, characterized in that: The ratio of the mass of the first positive electrode active material, the total mass of the nano-SiO2 and the two-dimensional titanium carbide, the mass of the first conductive agent and the mass of the first binder is (96-97): (0.5-1): (0.5-1): (1-2); And / or, the mass ratio of the second positive electrode active material, the carbon nanotubes, the second conductive agent and the second binder is (96-97): (0.5-1): (0.5-1): (1-2); And / or, the mass ratio of the third positive electrode active material, the azobenzene group-modified fluorinated acrylate copolymer, the third conductive agent and the third binder is (96-97):(0.5-1):(0.5-1):(1-2).

10. The secondary battery according to claim 9, characterized in that: The mass ratio of the nano-SiO2 to the two-dimensional titanium carbide is 1:(2-3).

11. The secondary battery according to claim 8, characterized in that: The D50 of the nano-SiO2 is 40-100 nm; and / or the average number of layers of the two-dimensional titanium carbide is 5-10 layers; and / or the number average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35000-40000 Da.

12. The secondary battery according to claim 8, characterized in that: The azobenzene group-modified fluorinated acrylate copolymer is prepared by polymerization of azophenyl methacrylate and hexafluorobutyl methacrylate, wherein the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate is 1:(4-5).

13. The secondary battery according to any one of claims 8 to 12, characterized in that: The total thickness of the first coating, the second coating and the third coating is 160-200 μm; and / or the thickness ratio of the first coating, the second coating and the third coating is (80-100):(30-40):(50-60).

14. An energy storage system, comprising a unit battery, characterized in that: The unit cell is a secondary battery prepared by the method for preparing a secondary battery according to any one of claims 1 to 7 or a secondary battery according to any one of claims 8 to 13.

15. An electrical equipment, characterized in that: Including the energy storage system as described in claim 14, the energy storage system is used to provide power for the electrical equipment.

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