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

Abstract

The present invention provides a secondary battery and its preparation method, energy storage system and electrical equipment, which relate to the field of energy storage technology. A positive electrode sheet, a separator and a negative electrode sheet are assembled, and then an electrolyte is injected to obtain a secondary battery, wherein a first positive electrode active material, nano-SiO2, two-dimensional titanium carbide, a first conductive agent, a first binder and a first solvent are sequentially mixed, coated and dried to form a coating; a second positive electrode active material, carbon nanotubes, a second conductive agent, a second binder and a second solvent are sequentially mixed, coated and dried 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, edge-ultraviolet cured and dried to obtain a positive electrode sheet. The above preparation method helps to reduce the bulging phenomenon of the positive electrode sheet during the coating process and helps to improve the capacity and cycle stability of the secondary battery.

Description

Technical Field

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

[0002] With the rapid development of the energy storage market and lithium-ion batteries, the demand for high-energy, long-cycle batteries is increasing. Since cathode materials often determine battery capacity, increasing cathode capacity is a key approach to developing high-capacity batteries.

[0003] Traditional cathode coating processes are prone to edge bulging due to the surface tension of the slurry. To eliminate this, low-viscosity slurries are often used to reduce surface tension and minimize edge bulging. However, excessively low viscosity poses significant risks to subsequent coating consistency and areal density control. More importantly, low viscosity results in low electrode areal density and, consequently, low battery capacity, which can be challenging to manufacture and produce high-capacity batteries. Summary of the Invention

[0004] The main purpose of the present invention is to provide a secondary battery and its preparation method, energy storage system and electrical equipment to solve the problems of bulging edges of positive electrode sheets during coating and low capacity of secondary batteries in the prior art.

[0005] In order to achieve the above-mentioned object, according to one aspect of the present invention, a method for preparing a secondary battery is provided, wherein a positive electrode sheet, a separator and a negative electrode sheet are made into a bare battery cell, which is then assembled with a shell and a top cover, and then an electrolyte is injected to obtain a secondary battery. The steps for preparing the positive electrode sheet include: S1, first 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 a current collector, drying it to form a first coating to obtain a first sheet; S3, coating a second positive electrode The active material, carbon nanotubes, a second conductive agent, a second binder and a second solvent are mixed for a second time to obtain a second positive electrode slurry; S4, the second positive electrode slurry is coated on the surface of the first coating layer, and dried to form a second coating layer to obtain a second sheet; S5, the third positive electrode active material, an azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent, a third binder and a third solvent are mixed for a third time to obtain a third positive electrode slurry; S6, the third positive electrode slurry is coated on the surface of the second coating layer, and edge UV curing treatment and drying are carried out in sequence to form a third coating layer to obtain a positive electrode sheet.

[0006] Furthermore, the mass ratio of the first positive electrode active material, 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 (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] Furthermore, the mass ratio of the above-mentioned nano-SiO2 and two-dimensional titanium carbide is 1:(2~3).

[0008] Furthermore, 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 35,000-40,000 Da.

[0009] Furthermore, the azobenzene group-modified fluorinated acrylate copolymer is prepared by polymerization of azophenyl methacrylate and hexafluorobutyl methacrylate, and the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate is 1:(4~5).

[0010] Furthermore, 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.

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

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

[0013] Furthermore, the mass ratio of the first positive electrode active material, the total mass of nano-SiO2 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 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).

[0014] Furthermore, the mass ratio of the above-mentioned nano-SiO2 and two-dimensional titanium carbide is 1:(2~3).

[0015] Furthermore, 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 35,000-40,000 Da.

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

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

[0018] According to another aspect of the present invention, there is provided an energy storage system, comprising a unit cell, wherein the unit cell is a secondary battery prepared by the aforementioned method for preparing a secondary battery or the aforementioned secondary battery.

[0019] According to another aspect of the present invention, there is provided an electrical device comprising the aforementioned energy storage system, wherein the energy storage system is used to provide power to the electrical device.

[0020] Applying the technical solution of the present application, the beneficial effects of the present application are as follows: the present application first mixes the first positive electrode active material, nano-SiO2, two-dimensional titanium carbide, the first conductive agent, the first binder and the first solvent to obtain a first positive electrode slurry, and the first positive electrode slurry is coated on the surface of the current collector and dried to form a first coating. Nano-SiO2 and two-dimensional titanium carbide are bonded through Si-O-Ti, which helps to strengthen the bonding force between the two. On the one hand, the addition of nano-SiO2 helps to fill the microcracks, holes 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 of the first coating to the current collector; on the other hand, nano-SiO2 can act 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 interface continuity. Then, a second positive electrode active material, carbon nanotubes, a second conductive agent, a second binder, and a 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 layer and dried to form a second coating layer. The addition of carbon nanotubes helps to improve the conductivity of the positive electrode sheet. Finally, a third positive electrode active material, an azobenzene-modified fluorinated acrylate copolymer, a third conductive agent, a third binder, and a 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 layer. The edge UV curing treatment and drying are sequentially performed to form a third coating layer. Under UV irradiation, the azobenzene groups in the azobenzene-modified fluorinated acrylate copolymer transform from a trans structure to a cis structure, increasing the molecular polarity, weakening the hydrogen bonds and van der Waals interactions between the polymer segments, and decreasing the viscosity of the slurry. During the coating process, the viscosity of the edge area is reduced by UV irradiation, so that the slurry flows toward the center area under the drive of 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-modified fluorinated acrylate copolymer tend to accumulate on the slurry surface, forming 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 traditional positive electrode slurries and coating methods, this application prepares a composite slurry and performs a 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 with the same shell size; on the other hand, the 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. DETAILED DESCRIPTION

[0021] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application 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 technology of this application, in the prior art, there are problems of bulging edges of positive electrode sheets during the coating process and low capacity of secondary batteries. In order to solve this problem, this application provides a secondary battery and its preparation method, energy storage system and electrical equipment.

[0023] In a typical embodiment of the present application, a method for preparing a secondary battery is provided, wherein a positive electrode sheet, a separator and a negative electrode sheet are made into a bare cell, which is then assembled with a shell and a top cover, and then an electrolyte is injected to obtain a secondary battery. The steps for preparing the positive electrode sheet include: S1, first 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 a current collector, drying it to form a first coating to obtain a first sheet; S3, coating a second positive electrode active material, a first conductive agent, a first binder and a first solvent to obtain a first positive electrode slurry; The material, carbon nanotubes, a second conductive agent, a second binder and a second solvent are mixed for a second time to obtain a second positive electrode slurry; S4, the second positive electrode slurry is coated on the surface of the first coating layer, and dried to form a second coating layer to obtain a second sheet; S5, the third positive electrode active material, azobenzene group-modified fluorinated acrylate copolymer, a third conductive agent, a third binder and a third solvent are mixed for a third time to obtain a third positive electrode slurry; S6, the third positive electrode slurry is coated on the surface of the second coating layer, and edge UV curing treatment and drying are carried out in sequence to form a third coating layer to obtain a positive electrode sheet.

[0024] In this application, a first positive electrode active material, nano-SiO2, two-dimensional titanium carbide, a first conductive agent, a first binder and a first solvent are first 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. Nano-SiO2 and two-dimensional titanium carbide are bonded through Si-O-Ti, which helps to strengthen the bonding between the two. On the one hand, the addition of nano-SiO2 helps to fill the microcracks, holes 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 of the first coating to the current collector; on the other hand, nano-SiO2 can act 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 interface continuity. Then, a second positive electrode active material, carbon nanotubes, a second conductive agent, a second binder, and a 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 layer and dried to form a second coating layer. The addition of carbon nanotubes helps to improve the conductivity of the positive electrode sheet. Finally, a third positive electrode active material, an azobenzene-modified fluorinated acrylate copolymer, a third conductive agent, a third binder, and a 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 layer. The edge UV curing treatment and drying are sequentially performed to form a third coating layer. Under UV irradiation, the azobenzene groups in the azobenzene-modified fluorinated acrylate copolymer transform from a trans structure to a cis structure, increasing the molecular polarity, weakening the hydrogen bonds and van der Waals interactions between the polymer segments, and decreasing the viscosity of the slurry. During the coating process, the viscosity of the edge area is reduced by UV irradiation, so that the slurry flows toward the center area under the drive of 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-modified fluorinated acrylate copolymer tend to accumulate on the slurry surface, forming 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 traditional positive electrode slurries and coating methods, this application prepares a composite slurry and performs a 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 with the same shell size; on the other hand, the 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.

[0025] It should be noted that the positive electrode active material, separator, negative electrode sheet and electrolyte of the present application can all be purchased or prepared using existing technologies.

[0026] Including but not limited to, the above-mentioned 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-mentioned first binder, second binder and third binder are each independently selected from any one or more of polyvinylidene fluoride, polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose and polyacrylic acid; the above-mentioned 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, thereby improving the consistency of the positive electrode sheet, in one embodiment of the present application, the above-mentioned first mixing is carried out in a stirring state, the rotation speed of the first mixing is 20~50rpm, and the time of the first mixing is 3~8h; and / or, the second mixing is carried out in a stirring state, the rotation speed of the second mixing is 10~50rpm, and the time of the second mixing is 3~8h; and / or, the third mixing is carried out in a stirring state, the rotation speed of the third mixing is 10~50rpm, and the time of the third mixing is 3~8h.

[0028] In order to further optimize the wettability, ion transfer efficiency and surface interface stability of the positive electrode sheet, thereby improving 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 10~30μm, and the average tube diameter is 15~25nm.

[0029] In one embodiment of the present application, the mass ratio of the first positive electrode active material, 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 (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).

[0030] Controlling the mass ratio of the first positive electrode active material, 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 ratio of the first solvent within the above ranges helps, on the one hand, to control the viscosity of the first positive electrode slurry within an appropriate range, thereby helping to improve the adhesion between the first coating layer 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 layer and the current collector, form a dense underlying structure, help improve the mechanical stability of the positive electrode sheet, and reduce the loss of active materials during cycling. 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 ranges helps to improve the conductivity of the positive electrode sheet. Controlling the mass ratio of the third positive electrode active material, azobenzene-modified fluorinated acrylate copolymer, the third conductive agent, the third binder, and the third solvent within the above ranges helps, on the one hand, to control the viscosity of the third positive electrode slurry within an appropriate range, thereby helping to reduce the phenomenon of bulging.

[0031] In one embodiment of the present application, the mass ratio of the above-mentioned nano-SiO2 and two-dimensional titanium carbide is 1:(2~3).

[0032] Controlling the mass ratio of nano-SiO2 and 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 and strengthen the adhesion of the first coating layer to the current collector; on the other hand, it helps to further improve the distribution uniformity of the two-dimensional titanium carbide, forming a more efficient conductive network, while enhancing interface continuity.

[0033] In one embodiment of the present application, the D50 of the above-mentioned nano-SiO2 is 40~100nm; 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 35000~40000Da.

[0034] Controlling the D50 of nano-SiO2 within the above range helps to increase the contact area between nano-SiO2 and two-dimensional titanium carbide, thereby helping to further improve the dispersion of two-dimensional titanium carbide and fill pores and microcracks, forming a denser structure, thereby helping to improve the mechanical strength and toughness of the positive electrode sheet, reduce structural damage during the cycle process, and extend the life of the secondary battery. Controlling the average number of two-dimensional titanium carbide layers within the above range, on the one hand, helps to reduce the impact of interlayer static charge 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 conducive to the rapid diffusion of lithium ions between layers, shortening the lithium ion transmission path, and improving 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 quickly change its viscosity under UV light irradiation, facilitates 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 azobenzene group-modified fluorinated acrylate copolymer is prepared by polymerization of azophenyl methacrylate and hexafluorobutyl methacrylate, and the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate is 1:(4~5).

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

[0037] In one embodiment of the present application, 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.

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

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

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

[0041] In one embodiment of the present application, the preparation method of the above-mentioned nano-SiO2 includes adjusting the pH of a mixture of ethanol and water to 9-11, adding ethyl orthosilicate dropwise to the above-mentioned mixture 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 hours to form a transparent sol, continuing to slowly stir for 12-48 hours, the sol gradually transforms into a wet gel, immersing the wet gel in ethanol for 24-72 hours to enhance the network structure strength and reduce drying cracking, placing the wet gel in an oven and drying it at 60-100°C for 16-48 hours to obtain a dry gel, placing the dry gel in a muffle furnace for calcination, heating it to 500-800°C at 2-5°C / min, keeping it warm for 2-4 hours, and then naturally cooling it to room temperature to obtain a white nano-SiO2 powder.

[0042] In one embodiment of the present application, the preparation method of the above-mentioned two-dimensional titanium carbide includes: dissolving LiF in HCl solution, adding Ti3AlC2 powder (purity ≥98%, particle size ≤38μm) to the above-mentioned solution, stirring at 30~50°C and a speed of 500rpm for 24 hours, then washing with deionized water by centrifugation (8000rpm, 10 minutes, five times) to pH ≥6, collecting the precipitate after centrifugation and dispersing it in dimethyl sulfoxide, ultrasonically treating it, centrifuging it again, collecting the upper dispersion, and obtaining Ti3C2T x The dispersion was freeze-dried (-60~-30℃, 36~48h) to obtain fluffy Ti3C2T x powder.

[0043] In one embodiment of the present application, the preparation method of the above-mentioned azobenzene group-modified fluorinated acrylate copolymer includes: dissolving azophenol and methacryloyl chloride in tetrahydrofuran, adding triethylamine as an acid binding agent, reacting in an ice bath for 4 to 6 hours, filtering the reaction solution to remove the generated TEA·HCl salt, collecting the obtained filtrate, and rotary evaporating to remove THF to obtain a yellow oil. The product is slowly dripped into cold methanol (-20°C) with a small amount of dichloromethane (10 mL) to precipitate, and after filtering, vacuum drying at 40 to 80°C for 10 to 18 hours to obtain azophenyl methacrylate. Hexafluorobutyl methacrylate, azophenyl methacrylate, and azobisisobutyronitrile were dissolved in N-methylpyrrolidone, and nitrogen was passed through for 30 minutes to deoxygenate. The temperature was then raised to 70°C, the stirring rate was 500 rpm, and the polymerization reaction was carried out for 12 to 14 hours. The reaction solution was cooled to room temperature and slowly added dropwise to cold methanol (-20°C) for precipitation. The white flocculent product was collected by filtration, washed with methanol three times, and then vacuum dried at 60 to 80°C for 12 to 24 hours to obtain a white azobenzene group-modified fluorinated acrylate copolymer.

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

[0045] The Si-O-Ti bond between the nano-SiO2 and the two-dimensional titanium carbide in the first coating helps strengthen the bonding between them. The addition of nano-SiO2 helps fill microcracks and holes between the two-dimensional titanium carbide layers and at the interface between the first coating and the current collector, forming a dense structure and reducing stress concentration. This helps improve the overall mechanical strength of the positive electrode and enhances the adhesion of the first coating to the current collector. Furthermore, nano-SiO2 acts as a dispersant to reduce agglomeration between the two-dimensional titanium carbides, helping to improve their uniformity, form a more efficient conductive network, and enhance interfacial continuity. The addition of carbon nanotubes in the second coating helps improve the conductivity of the positive electrode. The presence of an azobenzene-modified fluorinated acrylate copolymer in the third coating helps reduce bulging of the positive electrode edge. Under ultraviolet light, the azobenzene groups in the azobenzene-modified fluorinated acrylate copolymer transform from a trans structure to a cis structure, increasing the molecular polarity, weakening hydrogen bonding and van der Waals interactions between the polymer segments, and reducing the slurry viscosity. 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 under the driving force of 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 surface of the slurry to form a low surface tension adsorption layer, which further reduces the slurry spreading rate in the edge area and reduces the phenomenon of excessive thickness at the edge. Different from the traditional one-layer coating, the three-layer coating of the present application plays a synergistic role. On the one hand, it helps to eliminate the bulging edge phenomenon of the positive electrode, 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, thereby helping to improve the capacity and cycle stability of the secondary battery.

[0046] In one embodiment of the present application, the mass ratio of the first positive electrode active material, the total mass of nano-SiO2 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 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] Controlling the mass ratio of the first positive electrode active material, 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 ratio of the first solvent within the above ranges helps 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 underlying structure, help improve the mechanical stability of the positive electrode sheet, and reduce the loss of active materials during the cycle. 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 ranges 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 ranges helps to reduce the phenomenon of bulging edges.

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

[0049] Controlling the mass ratio of nano-SiO2 and 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 and strengthen the adhesion of the first coating layer to the current collector; on the other hand, it helps to further improve the distribution uniformity of the two-dimensional titanium carbide, forming a more efficient conductive network, while enhancing interface continuity.

[0050] In one embodiment of the present application, the D50 of the above-mentioned nano-SiO2 is 40~100nm; 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 35000~40000Da.

[0051] Controlling the D50 of nano-SiO2 within the above range helps to increase the contact area between nano-SiO2 and two-dimensional titanium carbide, thereby helping to further improve the dispersion of two-dimensional titanium carbide and fill pores and microcracks, forming a denser structure, thereby helping to improve the mechanical strength and toughness of the positive electrode sheet, reduce structural damage during the cycle process, and extend the life of the secondary battery. Controlling the average number of two-dimensional titanium carbide layers within the above range, on the one hand, helps to reduce the impact of interlayer static charge 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 conducive to the rapid diffusion of lithium ions between layers, shortening the lithium ion transmission path, and improving 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 quickly change its viscosity under UV light irradiation, facilitates 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.

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

[0053] Controlling the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate within the above range helps to control the content of azophenyl groups and fluorinated segments within an appropriate range, thereby helping to further reduce the viscosity of the third positive electrode slurry, forming an adsorption layer with low surface tension on the surface of the slurry, and further helping to further reduce the bulging phenomenon of the positive electrode sheet.

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

[0055] Controlling the total thickness of the first, second, and third coatings within the above range facilitates the preparation of thick electrodes and, with the same housing size, high-capacity batteries. Controlling the thickness ratio of the first, second, and third coatings within the above range helps enhance the interaction between the coatings, thereby contributing to the formation of a structurally stable positive electrode sheet, thereby improving the capacity and cycle stability of the secondary battery.

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

[0057] In another typical embodiment of the present application, an energy storage system is provided, including a unit cell, wherein the unit cell 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 contains the secondary battery of the present application, the energy storage system has a higher energy density and a longer service life.

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

[0060] Since the energy storage system in the above-mentioned electrical equipment contains the secondary battery of the present application, the electrical equipment has greater power and longer service life.

[0061] The beneficial effects of the present application will be further illustrated below with reference to examples.

[0062] Example 1

[0063] Preparation of nano-SiO2: Mix 40 mL of ethanol and 40 mL of water to obtain a mixed solution, adjust the pH of the mixed solution to 10 with ammonia water, add 10 mL of ethyl orthosilicate dropwise to the mixed solution at a dropping rate of 1 mL / min, then add 2 mL of 3-aminopropyltriethoxysilane, continue stirring for 1 hour to form a transparent sol, continue slow stirring for 48 hours, 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 hours to obtain a dry gel, place the dry gel in a muffle furnace and calcine it, raise the temperature to 600°C at 3°C / min, keep it warm for 3 hours, and then naturally cool to room temperature to obtain white nano-SiO2 powder, the D50 of nano-SiO2 is 50 nm.

[0064] Ti3C2T x Preparation: LiF (1.98 g) was dissolved in 20 mL of HCl solution (concentration of 2 mol / L), 1 g of Ti3AlC2 powder (purity of 99%, average particle size of 20 μm) was added to the above solution, and the mixture was stirred at 500 rpm at 40 ° C for 24 hours. It was then washed with deionized water by centrifugation (8000 rpm, 10 minutes, five times) until the pH was 6. The precipitate after centrifugation was collected and dispersed in dimethyl sulfoxide, ultrasonically treated (power of 500 W, time for 1 hour), and centrifuged again (speed of 3000 rpm, time for 10 minutes, times twice), and the upper dispersion was collected to obtain Ti3C2T x The dispersion was freeze-dried (-60 ° C, 48 h) to obtain fluffy Ti3C2T x Powder, Ti3C2T x The average number of floors is 8.

[0065] Preparation of azophenyl group-modified fluorinated acrylate copolymer: Azophenol (10 mmol) and methacryloyl chloride (12 mmol) were dissolved in 50 mL of tetrahydrofuran. Triethylamine (TEA, 15 mmol) was added and the mixture was allowed to react in an ice bath (0°C) for 5 h. The resulting TEA·HCl salt was removed by filtration. The filtrate was collected and rotary evaporated to remove the tetrahydrofuran, yielding a yellow oil. This oil was precipitated by slowly adding 10 mL of dichloromethane to cold methanol (-20°C). After filtration, the mixture was dried under vacuum at 60°C for 15 h to yield azophenyl methacrylate. 16 mmol of hexafluorobutyl methacrylate, 4 mmol of azophenyl methacrylate, and 0.052 g of azobisisobutyronitrile were dissolved in 50 mL of N-methylpyrrolidone, and nitrogen was passed through for 30 minutes to deoxygenate. The temperature was then raised to 70°C, the stirring rate was 500 rpm, and the polymerization reaction was carried out for 12 hours. The reaction solution was cooled to room temperature and slowly added dropwise to cold methanol (-20°C) for precipitation. The white flocculent product was collected by filtration, washed with methanol three times, and then dried in vacuo at 60°C for 12 hours to obtain a white azobenzene group-modified fluorinated acrylate copolymer. The number average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35,000 Da.

[0066] Preparation of positive electrode: the first lithium iron phosphate material, the above nano-SiO2, the above Ti3C2T x , the first Super-P, the first polyvinylidene fluoride and the first N-methylpyrrolidone are stirred at a speed of 20 rpm for 8 hours for the first mixing, the mass of the first lithium iron phosphate material, the nano-SiO2 and Ti3C2T x The ratio of the total 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. The nano-SiO2 and Ti3C2T xThe mass ratio of the second lithium iron phosphate material, the carbon nanotubes (average tube length of 30 μm, average tube diameter of 25 nm), the second Super-P, the second polyvinylidene fluoride and the second N-methylpyrrolidone was 1:2 to obtain a first positive electrode slurry with a viscosity of 12000 mPa·s; the first positive electrode slurry was coated on the surface of the copper foil and dried at 60°C for 2 hours to form a first coating to obtain a first sheet; the second lithium iron phosphate material, the carbon nanotubes (average tube length of 30 μm, average tube diameter of 25 nm), the second Super-P, the second polyvinylidene fluoride and the second N-methylpyrrolidone were stirred at a speed of 10 rpm for 8 hours for a second mixing, the mass ratio of the second lithium iron phosphate material, the carbon nanotubes, the second Super-P, the second polyvinylidene fluoride and the second N-methylpyrrolidone was 96:1:0.5:2:65 to obtain a second positive electrode slurry with a viscosity of 8000 mPa·s; the second positive electrode slurry was coated on the surface of the first coating and dried at 60°C for 2 hours to form a second coating to obtain a second sheet; the third lithium iron phosphate material, the above-mentioned azobenzene group-modified fluorinated acrylate copolymer, the third S Super-P, a third polyvinylidene fluoride and a third N-methyl pyrrolidone are stirred at a speed of 10 rpm for 8 hours 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-methyl pyrrolidone is 96:1:0.5:2:70 to obtain 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 the edge UV curing treatment and drying are carried out in sequence to form a third coating to obtain a positive electrode sheet, wherein the UV light irradiation area of ​​the edge UV curing treatment is from the edge of the third coating to the range extending inward by 20 cm, the UV wavelength is 365 nm, the edge UV curing treatment time 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 secondary battery: After the above-mentioned positive electrode sheet, polyethylene separator and graphite negative electrode sheet are made into a bare battery cell, they are assembled with the shell and top cover, and then lithium hexafluorophosphate electrolyte is injected to obtain a secondary battery.

[0068] Example 2

[0069] The difference from Example 1 is that the quality of the first lithium iron phosphate material, nano-SiO2 and Ti3C2T xThe total mass ratio 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 8000mPa·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 6000mPa·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 4000mPa·s, and finally a secondary battery is obtained.

[0070] Example 3

[0071] The difference from Example 1 is that the quality of the first lithium iron phosphate material, nano-SiO2 and Ti3C2T x The total mass ratio 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 8000mPa·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 8000mPa·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 4000mPa·s, and finally a secondary battery is obtained.

[0072] Example 4

[0073] The difference from Example 1 is that the quality of the first lithium iron phosphate material, nano-SiO2 and Ti3C2T xThe total mass ratio 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 8000mPa·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 8000mPa·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 96:1:0.5:2:65, and the viscosity of the third positive electrode slurry is 8000mPa·s, and finally a secondary battery is obtained.

[0074] Example 5

[0075] The difference from Example 1 is that the quality of the first lithium iron phosphate material, nano-SiO2 and Ti3C2T x The mass ratio of the total 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, the 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, the 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 quality of the first lithium iron phosphate material, nano-SiO2 and Ti3C2T x The mass ratio of the total 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, the 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, the 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 nano-SiO2 and Ti3C2T xThe mass ratio of is 1:3, and a secondary battery is finally obtained.

[0080] Example 8

[0081] The difference from Example 1 is that nano-SiO2 and Ti3C2T x The mass ratio of 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 by using ammonia water to obtain nano-SiO2 with a D50 of 40 nm; the ultrasonic time is 2 h, and the average number of layers of Ti3C2T is 5. x , and finally a secondary battery is obtained.

[0084] Example 10

[0085] The difference from Example 1 is that the pH of the mixed solution is adjusted to 11 by using ammonia water to obtain nano-SiO2 with a D50 of 100 nm; the ultrasonic time is 0.5 h, and the average number of layers of Ti3C2T is 10. x , and finally a secondary battery is 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-SiO2 with a D50 of 20 nm; the ultrasonic time is 3 h to obtain a single layer of Ti3C2T x , and finally a secondary battery is obtained.

[0088] Example 12

[0089] The difference from Example 1 is that the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate is 1:5, the polymerization reaction time is 14 h, and an azobenzene group-modified fluorinated acrylate copolymer with a number average molecular weight of 40,000 Da is obtained, and finally a secondary battery is obtained.

[0090] Example 13

[0091] The difference from Example 1 is that the molar ratio of azophenyl methacrylate to hexafluorobutyl methacrylate is 1:6, the polymerization reaction time is 16 h, and 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 layer, the second coating layer and the third coating layer in the positive electrode sheet is 100:30:60, and a secondary battery is finally obtained.

[0094] Example 15

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

[0096] Example 16

[0097] The difference from Example 1 is that, nano-SiO2 is prepared by mixing 40 mL of ethanol and 40 mL of water to obtain a mixed solution, adjusting the pH of the mixed solution to 10 using ammonia water, adding 10 mL of tetraethyl orthosilicate dropwise to the mixed solution at a rate of 1 mL / min, subsequently adding 2 mL of 3-aminopropyltriethoxysilane, stirring continuously for 1 h to form a transparent sol, continuing to slowly stir for 48 h, and the sol gradually transforms into a wet gel, which is immersed in ethanol for 48 hours, then placed in an oven, and dried at 60 ° C for 16 h to obtain a dry gel, which is placed in a muffle furnace and calcined, heated to 600 ° C at 3 ° C / min, kept warm for 3 h, and then naturally cooled to room temperature to obtain white nano-SiO2 powder, with a D50 of nano-SiO2 of 50 nm.

[0098] Ti3C2T x Preparation: LiF (1.98 g) was dissolved in 20 mL of HCl solution (concentration of 2 mol / L), 1 g of Ti3AlC2 powder (purity of 99%, average particle size of 20 μm) was added to the above solution, and the mixture was stirred at 500 rpm at 40 ° C for 24 hours. It was then washed with deionized water by centrifugation (8000 rpm, 10 minutes, five times) until the pH was 6. The precipitate after centrifugation was collected and dispersed in dimethyl sulfoxide, ultrasonically treated (power of 500 W, time for 1 hour), and centrifuged again (speed of 3000 rpm, time for 10 minutes, times twice), and the upper dispersion was collected to obtain Ti3C2T x The dispersion was freeze-dried (-60 ° C, 48 h) to obtain fluffy Ti3C2T x Powder, Ti3C2T x The average number of floors is 8.

[0099] Preparation of azophenyl group-modified fluorinated acrylate copolymer: Azophenol (10 mmol) and methacryloyl chloride (12 mmol) were dissolved in 50 mL of tetrahydrofuran. Triethylamine (TEA, 15 mmol) was added and the mixture was allowed to react in an ice bath (0°C) for 5 h. The resulting TEA·HCl salt was removed by filtration. The filtrate was collected and rotary evaporated to remove the tetrahydrofuran, yielding a yellow oil. This oil was precipitated by slowly adding 10 mL of dichloromethane to cold methanol (-20°C). After filtration, the mixture was dried under vacuum at 60°C for 15 h to yield azophenyl methacrylate. 16 mmol of hexafluorobutyl methacrylate, 4 mmol of azophenyl methacrylate, and 0.052 g of azobisisobutyronitrile were dissolved in 50 mL of N-methylpyrrolidone, and nitrogen was passed through for 30 minutes to deoxygenate. The temperature was then raised to 70°C, the stirring rate was 500 rpm, and the polymerization reaction was carried out for 12 hours. The reaction solution was cooled to room temperature and slowly added dropwise to cold methanol (-20°C) for precipitation. The white flocculent product was collected by filtration, washed with methanol three times, and then dried in vacuo at 60°C for 12 hours to obtain a white azobenzene group-modified fluorinated acrylate copolymer. The number average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer is 35,000 Da.

[0100] Preparation of positive electrode: the first lithium iron phosphate material, the above nano-SiO2, the above Ti3C2T x , the first Super-P, the first polyvinylidene fluoride and the first N-methylpyrrolidone are stirred at a speed of 50 rpm for 3 hours for the first mixing, the mass of the first lithium iron phosphate material, the nano-SiO2 and Ti3C2T x The ratio of the total 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. The mass of nano-SiO2 and Ti3C2T xThe mass ratio of the second lithium iron phosphate material, the carbon nanotubes, the second Super-P, the second polyvinylidene fluoride and the second N-methylpyrrolidone was 97:1:1:1:65 to obtain a second positive electrode slurry with a viscosity of 8000 mPa·s; the second positive electrode slurry was coated on the surface of the copper foil and dried at 60°C for 2h to form a first coating to obtain a first sheet; the second lithium iron phosphate material, the carbon nanotubes (average tube length of 10 μm, average tube diameter of 15 nm), the second Super-P, the second polyvinylidene fluoride and the second N-methylpyrrolidone were stirred at a speed of 50 rpm for 3h for a second mixing, and the mass ratio of the second lithium iron phosphate material, the carbon nanotubes, the second Super-P, the second polyvinylidene fluoride and the second N-methylpyrrolidone was 97:1:1:1:65 to obtain a second positive electrode slurry with a viscosity of 8000 mPa·s; the second positive electrode slurry was coated on the surface of the first coating and dried at 60°C for 2h to form a second coating to obtain a second sheet; the third lithium iron phosphate material, the above-mentioned azobenzene group-modified fluorinated acrylate copolymer, the third S Super-P, a third polyvinylidene fluoride and a third N-methylpyrrolidone are stirred at a speed of 50 rpm for 3 hours 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 97:1:1:1:70 to obtain 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 the edge UV curing treatment and drying are carried out in sequence to form a third coating to obtain a positive electrode sheet, wherein the UV light irradiation area of ​​the edge UV curing treatment is from the edge of the third coating to the range extending inward by 5 cm, the UV wavelength is 365 nm, the edge UV curing treatment time 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 secondary battery: After the above-mentioned positive electrode sheet, polyethylene separator and graphite negative electrode sheet are made into a bare battery cell, they are assembled with the shell and top cover, and then 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 eliminated, and a secondary battery is finally obtained.

[0104] Comparative Example 2

[0105] The difference from Example 1 is that nano-SiO2 and Ti3C2T are eliminated. x By adding, a secondary battery is finally obtained.

[0106] Comparative Example 3

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

[0108] Comparative Example 4

[0109] The difference from Example 1 is that the segmented coating is eliminated, and the first positive electrode slurry, the second positive electrode slurry and the third positive electrode slurry are mixed and coated on the copper foil, and then edge UV curing and drying are carried out in sequence to obtain a positive electrode sheet and finally a secondary battery.

[0110] Comparative Example 5

[0111] The difference from Example 1 is that the addition of nano-SiO2 is eliminated, and a secondary battery is finally obtained.

[0112] Comparative Example 6

[0113] The difference from Example 1 is that Ti3C2T x By adding, 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 slurry on the surface of the copper foil is omitted, and the first positive electrode slurry and the second positive electrode slurry are mixed and then coated on the surface of the copper foil to finally obtain a secondary battery.

[0116] Comparative Example 8

[0117] The difference from Example 1 is that the second positive electrode slurry is not coated on the surface of the first coating layer, and the second positive electrode slurry and the third positive electrode slurry are mixed and then coated on the surface of the first coating layer to finally obtain a secondary battery.

[0118] Comparative Example 9

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

[0120] Performance Testing

[0121] The coating process of the examples and comparative examples was observed to see whether bulging occurred. The thickness of the center area and the edge area of ​​the positive electrode sheets prepared in the examples and comparative examples were measured, and the thickness difference was calculated (thickness difference = thickness of the edge area - thickness of the middle area). The secondary batteries prepared in the examples and comparative examples were tested for initial discharge capacity in grams at 0.1C and capacity retention after 300 cycles at 0.3C. The test results for bulging, thickness difference, initial discharge capacity in grams, and capacity retention 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 seen that in Examples 1 to 4, the positive electrode sheet is prepared by regulating the viscosity 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 are reduced in sequence. In Example 3, the viscosity of the first positive electrode slurry and the second positive electrode slurry is the same, and higher than that of the third positive electrode slurry. In Example 4, the viscosity of the first positive electrode slurry, the second positive electrode slurry and the third positive electrode slurry is the same. From the results, it can be seen that the viscosity of the first positive electrode slurry, the second positive electrode slurry and the third positive electrode slurry has a certain influence on the edge thickness of the positive electrode sheet. By setting the viscosity 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 first discharge gram capacity and cycle stability of the battery; in Examples 1, 5 and 6, by adjusting the viscosity of nano-SiO2, Ti3C2T x , carbon nanotubes and azobenzene group modified fluorinated acrylate copolymer in the slurry respectively to prepare the positive electrode sheet. From the data, it can be seen that nano-SiO2, Ti3C2T x The proportion of carbon nanotubes and azobenzene group modified fluorinated acrylate copolymers mainly affects the first discharge capacity and capacity retention rate of the battery. Suitable nano-SiO2, Ti3C2T x The proportion of carbon nanotubes and azobenzene group modified fluorinated acrylate copolymer helps to improve the mutual synergy between the coatings, thereby helping to improve the first discharge capacity and capacity retention rate of the battery; the difference between Examples 1, 7 and 8 is that nano-SiO2 and Ti3C2T x The mass ratio of nano-SiO2 and Ti3C2T is different. From the data results, we can see that nano-SiO2 and Ti3C2T x The mass ratio of nano-SiO2 and Ti3C2T does not have a significant effect on the edge thickness of the positive electrode, but the appropriate nano-SiO2 and Ti3C2T x The mass ratio of SiO2 and Ti3C2T is helpful to improve the initial discharge capacity and capacity retention rate of the battery; the difference between Examples 1 and 9 to 11 is that the particle size of SiO2 and Ti3C2T x The number of layers is different. From the data, it can be seen that the particle size of SiO2 and Ti3C2T x The number of layers will not affect the edge thickness of the positive electrode sheet, the appropriate particle size of SiO2 and Ti3C2T xThe number of layers helps to improve the first discharge gram capacity and capacity retention rate of the battery; the difference between Examples 1, 12 and 13 is that the molar ratio of azophenyl methacrylate and hexafluorobutyl methacrylate and the number average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer are different. It can be seen from the data that the appropriate molar ratio of azophenyl methacrylate and hexafluorobutyl methacrylate and the number average molecular weight of the azobenzene group-modified fluorinated acrylate copolymer help to produce 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 difference between Examples 1, 14 and 15 is that the thickness ratio of the first coating, the second coating and the third coating is different. It can be seen from the data results that the appropriate thickness ratio of the first coating, the second coating and the third coating helps to improve the mutual synergistic effect among the three, thereby helping to improve the first discharge gram capacity and cycle stability of the battery.

[0126] From the data results of Example 1 and Comparative Example 1, it can be seen that the addition of the azobenzene group-modified fluorinated acrylate copolymer is eliminated, and the viscosity of the third slurry cannot be reduced by ultraviolet light, which is not conducive to reducing the thickness of the edge of the positive electrode, resulting in severe bulging. In Comparative Example 2, the addition of nano-SiO2 and Ti3C2T x The addition of carbon nanotubes will not have a significant effect on the thickness of the edge of the positive electrode sheet, but it is not conducive to improving the conductivity and ion transmission efficiency of the positive electrode sheet, and thus is not conducive to improving the first discharge gram capacity and cycle stability of the battery; in Comparative Example 3, the addition of carbon nanotubes is cancelled, which will not have a significant effect on the thickness of the edge of the positive electrode sheet, but is not conducive to optimizing the wettability, ion transmission efficiency and surface interface stability of the positive electrode sheet, thereby reducing the first discharge gram 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, making it difficult to significantly reduce the viscosity of the positive electrode slurry by ultraviolet light, resulting in obvious bulging edges of the positive electrode sheet; in Comparative Example 5, the addition of nano-SiO2 is cancelled, and Ti3C2T x The defects such as micro cracks and holes between the layers and the contact surface between the first coating and the current collector are difficult to fill. x It is easy to agglomerate, which leads to a decrease in the first discharge capacity and cycle stability of the battery; in Comparative Example 6, Ti3C2T xThe addition of the first positive electrode slurry and the second positive electrode slurry will reduce the conductivity and lithium ion transmission efficiency of the positive electrode sheet. Although it does not affect the thickness of the edge of the positive electrode sheet, it is not conducive to improving the first discharge capacity and cycle stability of the battery. In Comparative Example 7, the first positive electrode slurry and the second positive electrode slurry are mixed and then coated 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, and thus is not conducive to improving the cycle stability of the battery. In Comparative Example 8, the second positive electrode slurry is not coated on the surface of the first coating layer. The second positive electrode slurry and the third positive electrode slurry are mixed and then coated on the first coating layer. The surface of the layer, the viscosity of the slurry is too high, the concentration of the azobenzene group-modified fluorinated acrylate copolymer is diluted, which is not conducive to ultraviolet light irradiation, and thus not conducive to reducing the thickness of the positive electrode edge, resulting in bulging edge; in Comparative Example 9, the edge UV curing treatment is cancelled, and the azobenzene group-modified fluorinated acrylate copolymer in the third positive electrode slurry does not respond to ultraviolet light, making it difficult to reduce the viscosity of the third positive electrode slurry, and the surface tension is high, resulting in serious bulging edge at the edge of the positive electrode sheet, thereby significantly reducing the first discharge gram capacity and capacity retention rate of the battery.

[0127] In this application, a first positive electrode active material, nano-SiO2, two-dimensional titanium carbide, a first conductive agent, a first binder and a first solvent are first 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. Nano-SiO2 and two-dimensional titanium carbide are bonded through Si-O-Ti, which helps to strengthen the bonding between the two. On the one hand, the addition of nano-SiO2 helps to fill the microcracks, holes 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 of the first coating to the current collector; on the other hand, nano-SiO2 can act 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 interface continuity. Then, a second positive electrode active material, carbon nanotubes, a second conductive agent, a second binder, and a 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 layer and dried to form a second coating layer. The addition of carbon nanotubes helps to improve the conductivity of the positive electrode sheet. Finally, a third positive electrode active material, an azobenzene-modified fluorinated acrylate copolymer, a third conductive agent, a third binder, and a 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 layer. The edge UV curing treatment and drying are sequentially performed to form a third coating layer. Under UV irradiation, the azobenzene groups in the azobenzene-modified fluorinated acrylate copolymer transform from a trans structure to a cis structure, increasing the molecular polarity, weakening the hydrogen bonds and van der Waals interactions between the polymer segments, and decreasing the viscosity of the slurry. During the coating process, the viscosity of the edge area is reduced by UV irradiation, so that the slurry flows toward the center area under the drive of 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-modified fluorinated acrylate copolymer tend to accumulate on the slurry surface, forming 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 traditional positive electrode slurries and coating methods, this application prepares a composite slurry and performs a 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 with the same shell size; on the other hand, the 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 merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection 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 cell with a housing and a top cover, and then 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; The viscosity of the first positive electrode slurry is 8000-12000 mPa·s, the viscosity of the second positive electrode slurry is 6000-8000 mPa·s, and the viscosity of the third positive electrode slurry is 4000-6000 mPa·s.

2. The method for preparing a secondary battery according to claim 1, wherein: 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, wherein: 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, wherein: 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 35,000-40,000 Da.

5. The method for preparing a secondary battery according to claim 1, wherein: 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 ultraviolet light irradiation area of ​​the edge ultraviolet light curing treatment is within a 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.

7. 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 materials of the first coating layer include 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; The viscosity of the slurry forming the first coating layer is 8000-12000 mPa·s, the viscosity of the slurry forming the second coating layer is 6000-8000 mPa·s, and the viscosity of the slurry forming the third coating layer is 4000-6000 mPa·s.

8. The secondary battery according to claim 7, wherein: 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).

9. The secondary battery according to claim 8, characterized in that The mass ratio of the nano-SiO2 to the two-dimensional titanium carbide is 1:(2-3).

10. The secondary battery according to claim 7, wherein 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 35,000-40,000 Da.

11. The secondary battery according to claim 7, wherein 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).

12. The secondary battery according to any one of claims 7 to 11, characterized in that: The total thickness of the first coating layer, the second coating layer, and the third coating layer is 160-200 μm; and / or the ratio of the thickness of the first coating layer, the second coating layer, and the third coating layer is (80-100):(30-40):(50-60).

13. 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 6 or a secondary battery according to any one of claims 7 to 12.

14. An electrical device, characterized in that: Including the energy storage system according to claim 13, the energy storage system is used to provide power for the electrical equipment.

Images