A secondary battery and an electrical device

Abstract

This application discloses a secondary battery and an electrical device, belonging to the field of battery technology. The negative electrode active material in the negative electrode film layer of the secondary battery provided by this application includes a negative electrode active material and a coating layer disposed on at least a portion of the surface of the negative electrode active material. The coating layer has a mesh structure containing polymer fibers, and an inorganic solid electrolyte is disposed in at least a portion of the pores of the mesh structure. Furthermore, the negative electrode binder includes polytetrafluoroethylene (PTFE). This effectively improves the initial coulombic efficiency and cycle performance of the secondary battery, and effectively reduces the self-discharge rate of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0002] Secondary batteries are widely used in electric vehicles due to their high energy density. However, the manufacturing process involves energy consumption and environmental issues. Traditional slurry-based electrode manufacturing requires significant energy consumption and toxic solvents. To address these problems, a solvent-free dry electrode manufacturing process has been proposed, eliminating the drying and solvent recovery processes, thus reducing manufacturing costs and environmental impact. However, using polytetrafluoroethylene (PTFE) as a binder in the dry process on the negative electrode can reduce the initial efficiency (ED) because PTFE is electrochemically unstable at low potentials and readily undergoes a defluorination reaction, reacting with lithium ions to form LiF, consuming lithium and reducing ED. If the ED is improved by reducing the PTFE content, electrode powder shedding is likely to occur, leading to deterioration of cycle performance. Summary of the Invention

[0003] The purpose of this application is to improve the technical problems of high self-discharge rate, low initial coulombic efficiency and poor cycle performance of secondary batteries that occur during the preparation of dry electrode sheets. It proposes a secondary battery and power device with high initial coulombic efficiency, good cycle performance and low self-discharge rate.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material, a negative electrode binder and a negative electrode conductive agent, the negative electrode active material including a negative electrode active material, and a coating layer disposed on at least a portion of the surface of the negative electrode active material; The coating layer has a mesh structure containing polymer fibers, and an inorganic solid electrolyte is disposed in at least a portion of the pores of the mesh structure. The negative electrode binder includes polytetrafluoroethylene.

[0005] As an embodiment of this application, the thickness of the coating layer is 50nm~800nm.

[0006] As an embodiment of this application, the average diameter of the polymer fiber is 20nm~500nm.

[0007] As an embodiment of this application, the polymer fiber includes at least one of polyethylene oxide, polyvinylidene fluoride, carboxymethyl cellulose, polyvinylpyrrolidone, and polymethyl methacrylate.

[0008] As an embodiment of this application, the inorganic solid electrolyte has a mass percentage of 0.5% to 3% based on the total mass of the negative electrode film.

[0009] As an embodiment of this application, the inorganic solid electrolyte includes at least one of oxide solid electrolyte and sulfide solid electrolyte.

[0010] As an embodiment of this application, the average particle size of the inorganic solid electrolyte is <500nm.

[0011] As an embodiment of this application, the oxide solid electrolyte includes any one of LLZO, LATP, and LLTO; As an embodiment of this application, the sulfide solid electrolyte includes LiPS5Cl, Li 10 GeP2S 12 Any one of Li2S-P2S5; As an embodiment of this application, the average particle size of the inorganic solid electrolyte is 10 nm to 400 nm.

[0012] As an embodiment of this application, the polytetrafluoroethylene is polytetrafluoroethylene fiber, and the average diameter of the polytetrafluoroethylene fiber is 50nm~500nm.

[0013] As an embodiment of this application, the polytetrafluoroethylene is polytetrafluoroethylene fiber, and the average length of the polytetrafluoroethylene fiber is 10μm~500μm.

[0014] As an embodiment of this application, the mass percentage of the negative electrode binder is 0.5% to 5% based on the total mass of the negative electrode film layer.

[0015] As an embodiment of this application, the secondary battery further includes an electrolyte, which includes fluoroethylene carbonate.

[0016] As an embodiment of this application, the mass percentage of the fluoroethylene carbonate is 0.5% to 5% based on the total mass of the electrolyte.

[0017] A second aspect of this application provides an electrical device including the secondary battery described in this application.

[0018] Compared with the prior art, the beneficial effects of this application are: The negative electrode active material in the negative electrode film layer of the secondary battery provided in this application includes a negative electrode active material and a coating layer disposed on at least a portion of the surface of the negative electrode active material. The coating layer has a mesh structure containing polymer fibers, and an inorganic solid electrolyte is disposed in at least a portion of the pores of the mesh structure. Furthermore, the negative electrode binder includes polytetrafluoroethylene (PTFE). This effectively provides electronic insulation between the negative electrode active material and PTFE, reducing the risk of PTFE reduction. In addition, the mesh fiber structure of the coating layer can effectively encapsulate and bond the main material while minimizing the amount of PTFE added, further improving the initial coulombic efficiency of the secondary battery, while also improving cycle performance, and effectively reducing the self-discharge rate of the secondary battery. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0021] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0022] In one embodiment of this application, a secondary battery is provided, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material, a negative electrode binder and a negative electrode conductive agent, the negative electrode active material including a negative electrode active material, and a coating layer disposed on at least a portion of the surface of the negative electrode active material. The coating layer has a mesh structure containing polymer fibers, and an inorganic solid electrolyte is disposed in at least a portion of the pores of the mesh structure. The negative electrode binder includes polytetrafluoroethylene (PTFE).

[0023] This application research found that the negative electrode active material in the negative electrode film layer of the secondary battery provided by this application includes a negative electrode active material and a coating layer disposed on at least a portion of the surface of the negative electrode active material. The coating layer has a mesh structure containing polymer fibers, and an inorganic solid electrolyte is disposed in at least a portion of the pores of the mesh structure. Furthermore, the negative electrode binder includes polytetrafluoroethylene (PTFE). This effectively provides electronic insulation between the negative electrode active material and PTFE, reducing the risk of PTFE reduction. In addition, the mesh fiber structure of the coating layer can effectively encapsulate and bond the main material while minimizing the amount of PTFE added, further improving the initial coulombic efficiency of the secondary battery, while also improving cycle performance, and effectively reducing the self-discharge rate of the secondary battery.

[0024] Specifically, the coating layer of this application includes a network structure formed by polymer fibers. On the one hand, the network structure can achieve a certain degree of adhesion to the negative electrode active material and the negative electrode film, thereby reducing the amount of polytetrafluoroethylene (PTFE) added and improving the initial coulombic efficiency of the secondary battery. On the other hand, by encapsulating the negative electrode active material and leveraging the electron-insulating properties of polymer fibers, it can reduce the reaction between the negative electrode active material and PTFE, reducing PTFE reduction, thereby further improving the initial coulombic efficiency of the secondary battery and reducing its self-discharge rate. Furthermore, this application fills the pores of the network structure with an inorganic solid electrolyte. Introducing the inorganic solid electrolyte in this form achieves good ion conduction and electronic insulation. Since the negative electrode active material is jointly coated by the polymer fiber network structure and the binder, it is less prone to pulverization and detachment, thus effectively improving the cycle performance of the secondary battery.

[0025] In one embodiment, the thickness of the coating layer is 50 nm to 800 nm.

[0026] It should be noted that the thickness of the coating layer is tested by using TEM (transmission electron microscopy) to test the negative electrode active material. Due to the difference in atomic number / density between the coating material and the matrix material, a contrast difference will be generated in the image, thus revealing the boundary of the coating layer.

[0027] For example, the thickness of the coating layer can be any point value between 50nm and 800nm ​​or a range between any two points, such as 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm, 800nm, etc.

[0028] In one embodiment, the thickness of the coating layer is 350 nm to 500 nm. For example, it can be 350 nm, 380 nm, 400 nm, 420 nm, 450 nm, 480 nm, 500 nm, etc.

[0029] This study found that the thickness of the coating layer affects its protective effect on the negative electrode active material. It not only affects the degree of reaction between the negative electrode active material and polytetrafluoroethylene, but also affects the buffering capacity against volume expansion during subsequent cycles and the degree of side reactions with the electrolyte. At the same time, the thickness of the coating layer also affects the ion transport path, thereby affecting the ion transport efficiency. When the thickness of the coating layer is further selected within the above-mentioned range, especially within the further preferred range, the resulting secondary battery has better cycle performance, higher initial coulombic efficiency, and lower self-discharge rate.

[0030] In one embodiment, the average diameter of the polymer fiber is 20 nm to 500 nm.

[0031] It should be noted that the average diameter testing method for the polymer fibers is as follows: TEM and mapping tests are performed on the polymer fibers. Mapping is first used to distinguish between polymer fibers and binder fibers, and then TEM is used to measure the diameter of the polymer fibers. The average diameter is calculated as follows: five sample areas are randomly selected, and the diameter is measured at two points in each area, for a total of 10 fiber diameters. The average value is then taken.

[0032] For example, the diameter of the polymer fiber can be any point value between 20nm and 500nm or a range between any two points, such as 20nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, or 500nm.

[0033] In one embodiment, the diameter of the polymer fiber is 150 nm to 300 nm. For example, it can be 150 nm, 170 nm, 190 nm, 210 nm, 230 nm, 250 nm, 270 nm, 300 nm, etc.

[0034] This study found that the diameter of polymer fibers affects the mechanical strength of the network structure and the size of the pores formed, thus influencing the uniformity and continuity of the distribution of the inorganic solid electrolyte filling the pores. Furthermore, the diameter of the polymer fibers also affects their isolation effect between the negative electrode active material, polytetrafluoroethylene (PTFE), and the electrolyte, thereby affecting the degree of side reactions and lithium dendrite penetration. Simultaneously, the diameter of the polymer fibers also affects the specific surface area, thus influencing interfacial side reactions. Further selecting an average polymer fiber diameter within the aforementioned range helps improve the cycle performance and initial coulombic efficiency of the secondary battery, while reducing the self-discharge rate.

[0035] In one embodiment, the polymer fiber includes at least one of polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP), and polymethyl methacrylate (PMMA).

[0036] This study found that selecting the above-mentioned types of polymer fibers helps to improve the overall performance of secondary batteries.

[0037] In one embodiment, the inorganic solid electrolyte has a mass percentage of 0.5% to 3% based on the total mass of the negative electrode film.

[0038] It should be noted that the test method for the mass percentage of the inorganic solid electrolyte, based on the total mass of the negative electrode film, is as follows: thermogravimetric analysis is used for testing. Specifically, a secondary battery is disassembled to obtain the negative electrode sheet. The negative electrode film layer on the surface of the electrode sheet is scraped off, and 10 mg of powder sample is placed in an alumina crucible for testing. Taking graphite as the negative electrode active material and oxide electrolyte as the inorganic solid electrolyte as an example, the test temperature is 30-1000℃, the heating rate is 10℃ / min, and the test atmosphere is air. Within this temperature range, except for the oxide electrolyte, which remains stable, all other substances are oxidized and decomposed, and the remaining mass is the content of oxide solid electrolyte.

[0039] For example, the mass percentage of the inorganic solid electrolyte, based on the total mass of the negative electrode film layer, can be any point value or a range between any two points between 0.5% and 3%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc.

[0040] This study found that the mass percentage of inorganic solid electrolyte in the negative electrode film layer affects ionic conductivity, thereby influencing the uniformity of lithium deposition in the subsequent secondary battery; it also affects the degree of side reactions between the inorganic solid electrolyte and lithium metal. Further selecting a mass percentage of the inorganic solid electrolyte within the aforementioned range, based on the total mass of the negative electrode film layer, helps improve the cycle performance and initial coulombic efficiency of the secondary battery, while reducing the self-discharge rate.

[0041] In one embodiment, the inorganic solid electrolyte includes at least one of an oxide solid electrolyte and a sulfide solid electrolyte.

[0042] For example, the oxide solid electrolyte includes LLZO, LATP, LLTO, etc.; the sulfide solid electrolyte includes LiPS5Cl, Li 10 GeP2S 12 Li2S-P2S5, etc.

[0043] In one embodiment, the average particle size of the inorganic solid electrolyte is <500 nm.

[0044] It should be noted that the test method for the average particle size of the inorganic solid electrolyte is as follows: disassemble the secondary battery to obtain the negative electrode sheet, scrape off the negative electrode film layer on the surface of the electrode sheet, and then use SEM to test it. It can be observed that the inorganic solid electrolyte particles are distributed in the gaps of the polymer fiber filaments. Take 10 points to measure the particle size and calculate the average particle size.

[0045] In one embodiment, the average particle size of the inorganic solid electrolyte is 10 nm to 400 nm.

[0046] For example, the average particle size of the inorganic solid electrolyte can be any point value between 10nm and 500nm or a range between any two points, such as 10nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, etc.

[0047] This study found that the average particle size of inorganic solid electrolytes affects their distribution uniformity in the network structure, thereby influencing ionic conductivity and the integrity of the formed ionic pathways. Furthermore, the average particle size of inorganic solid electrolytes also affects their specific surface area, thus impacting the degree of side reactions with lithium. Further selecting an average particle size of the inorganic solid electrolyte within the aforementioned range helps improve the overall performance of the secondary battery.

[0048] In one embodiment, the polytetrafluoroethylene is polytetrafluoroethylene fiber, and the average diameter of the polytetrafluoroethylene fiber is 50nm~500nm.

[0049] It should be noted that the method for testing the diameter of the polytetrafluoroethylene (PTFE) fibers is as follows: The secondary battery is disassembled to obtain the negative electrode sheet. The negative electrode film layer on the surface of the electrode sheet is scraped off, and then TEM and mapping tests are performed. Mapping is first used to distinguish between polymer fibers and PTFE fibers, and then TEM is used to measure the diameter of the PTFE fibers. The average diameter is calculated as follows: Five sample areas are randomly selected, and the diameter is measured at two points in each area, for a total of 10 fiber diameters. The average value is then taken.

[0050] For example, the average diameter of the polytetrafluoroethylene fiber can be any point value between 50nm and 500nm or a range between any two points, such as 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, etc.

[0051] In one embodiment, the average diameter of the polytetrafluoroethylene fiber is 200 nm to 300 nm. For example, it can be 200 nm, 220 nm, 240 nm, 260 nm, 280 nm, 300 nm, etc.

[0052] This study found that polytetrafluoroethylene exists in the form of fibers with diameters within the aforementioned range, which can effectively improve the uniformity of the negative electrode film and better achieve the bonding effect of the negative electrode active material and the negative electrode conductive agent. This reduces the pulverization and shedding of the negative electrode active material in the negative electrode active material during cycling, thereby improving the cycle performance of the secondary battery, reducing the self-discharge rate, and improving the first coulombic efficiency.

[0053] In one embodiment, the polytetrafluoroethylene is polytetrafluoroethylene fiber, and the average length of the polytetrafluoroethylene fiber is 10 μm to 500 μm.

[0054] It should be noted that the test method for the length of the polytetrafluoroethylene fiber is the same as the test method for the average diameter of polytetrafluoroethylene fiber.

[0055] For example, the average length of the polytetrafluoroethylene fiber can be any point value or a range between any two points between 10μm and 500μm, such as 10μm, 50μm, 100μm, 150μm, 200μm, 250μm, 300μm, 350μm, 400μm, 450μm, 500μm, etc.

[0056] In one embodiment, the average length of the polytetrafluoroethylene fiber is 80 μm to 100 μm. For example, it can be 80 μm, 82 μm, 85 μm, 88 μm, 90 μm, 92 μm, 95 μm, 98 μm, 100 μm, etc.

[0057] This study found that the length of polytetrafluoroethylene (PTFE) fibers affects the structural stability of the negative electrode film and its interaction with the negative electrode active material. Further selection of the length of PTFE fibers within the above range helps to improve the overall performance of the secondary battery.

[0058] In one embodiment, the mass percentage of the negative electrode binder is 0.5% to 5% based on the total mass of the negative electrode film layer.

[0059] It should be noted that the test method for the mass percentage of the negative electrode binder, based on the total mass of the negative electrode film layer, is as follows: thermogravimetric analysis is used. Specifically, a secondary battery is disassembled to obtain the negative electrode sheet. The negative electrode film layer on the surface of the electrode sheet is scraped off, and 10 mg of powder sample is placed in an alumina crucible for testing. The test temperature is 30-600℃, the heating rate is 10℃ / min, and the test atmosphere is nitrogen. The negative electrode binder decomposes at 500-600℃, while the polymer fibers (taking PEO as an example) in the coating layer decompose at 350-450℃. The weight loss steps of the two are significantly different, and the mass percentage of the negative electrode binder can be calculated from the mass loss within the corresponding temperature range.

[0060] For example, the mass percentage of the negative electrode binder, based on the total mass of the negative electrode film layer, can be any point value or a range between any two points between 0.5% and 5%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc.

[0061] In one embodiment, the mass percentage of the negative electrode binder is 1% to 3% based on the total mass of the negative electrode film layer. For example, it can be 1%, 1.2%, 1.5%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, etc.

[0062] This study found that the mass percentage of the negative electrode binder in the negative electrode film layer affects the structural stability of the negative electrode film layer and also affects the proportion of the negative electrode active material. When the mass percentage of the negative electrode binder in the negative electrode film layer is further selected within the above range, it helps to improve the cycle performance, reduce the self-discharge rate, and improve the first coulombic efficiency while increasing the energy density of the secondary battery.

[0063] It should be noted that the negative electrode binder may also include at least one of polyamic acid and fluorinated ethylene propylene copolymer.

[0064] In one embodiment, the secondary battery further includes an electrolyte comprising fluoroethylene carbonate.

[0065] This study found that adding fluoroethylene carbonate to the electrolyte results in a higher reduction potential, enabling it to be reduced before polytetrafluoroethylene (PTFE) is reduced, forming an SEI film on the negative electrode surface. This slows down the reduction of PTFE and effectively improves the initial coulombic efficiency of the secondary battery.

[0066] In one embodiment, the mass percentage of the fluoroethylene carbonate is 0.5% to 5% based on the total mass of the electrolyte.

[0067] It should be noted that the mass percentage of fluoroethylene carbonate, based on the total mass of the electrolyte, is determined using gas chromatography. Fluoroethylene carbonate has sufficient volatility to be separated and detected in a gas chromatograph. The secondary battery is disassembled to obtain the electrolyte. The electrolyte is diluted with a low-boiling-point solvent such as dimethyl carbonate to reduce viscosity and corrosiveness. Fluorobenzene (a known amount of an internal standard with similar properties but not present in the original sample) is added to the sample. Quantification is performed by comparing the peak area ratio of the target additive and the internal standard.

[0068] For example, the mass percentage of the fluoroethylene carbonate, based on the total mass of the electrolyte, can be any point value or a range between any two points between 0.5% and 5%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc.

[0069] In one embodiment, the mass percentage of the fluoroethylene carbonate is 2% to 3% based on the total mass of the electrolyte. For example, it can be 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, etc.

[0070] This study found that the mass percentage of fluoroethylene carbonate affects its ability to form an SEI film, as well as the stability of the formed SEI film and its ability to isolate the electrolyte from the negative electrode side reactions. Further selection of the mass percentage of fluoroethylene carbonate in the electrolyte within the above range helps to improve the overall performance of the secondary battery.

[0071] This application does not impose any particular limitation on the selection of the negative electrode active material, and any negative electrode active material conventionally available in the art can be used. For example, the negative electrode active material can be any one of artificial graphite, natural graphite, silicon carbide, and silicon oxide.

[0072] This application does not impose any particular limitation on the selection of the negative electrode conductive agent, and any negative electrode conductive agent conventionally available in the art can be used. For example, the negative electrode conductive agent can be any one of carbon black (Super P), graphite, carbon nanotubes (CNT), graphene, carbon fiber, and acetylene black.

[0073] In one embodiment, the secondary battery further includes a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active material, a positive binder and a positive conductive agent.

[0074] This application does not impose any particular limitation on the selection of the positive electrode active material, and any positive electrode active material conventionally available in the art can be used. For example, the positive electrode active material can be any one of lithium iron phosphate, lithium manganese iron phosphate, or ternary positive electrode materials.

[0075] This application does not impose any particular limitation on the selection of the positive electrode binder; any positive electrode binder conventionally available in the art can be used. For example, the positive electrode binder may be any one of polytetrafluoroethylene, polyvinylidene fluoride, polyacrylic acid, carboxymethyl cellulose, polyimide, and sodium alginate.

[0076] This application does not impose any particular limitation on the selection of the positive electrode conductive agent, and any positive electrode conductive agent conventionally available in the art can be used. For example, the positive electrode conductive agent can be any one of carbon black (Super P), graphite, carbon nanotubes (CNT), and graphene.

[0077] In one embodiment, the electrolyte further includes an organic solvent and a lithium salt.

[0078] This application does not impose any particular limitation on the selection of organic solvents, and organic solvents conventionally used in the art can be used. For example, the organic solvent may be ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethyl acetate (EA), propyl propionate (PP), ethyl propionate (EP), dimethyl carbonate (DMC), etc.

[0079] This application does not impose any particular limitation on the selection of lithium salt, and lithium salts conventionally available in the art can be used. For example, the lithium salt may be lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), etc.

[0080] In one embodiment, the method for preparing the secondary battery includes the following steps: (1) Preparation of negative electrode sheet S1. After dissolving the negative electrode active material in the polymer matrix, it is subjected to high-voltage electrostatic drawing and coating treatment. Then, the electrospun film is dispersed into powder at high speed. Subsequently, an inorganic solid electrolyte is added and ball-milled to obtain the negative electrode active material. S2. Add negative electrode conductive agent to negative electrode active material for the first mixing, then add negative electrode binder for the second mixing, roll press to form a film after the second mixing, and combine with negative electrode current collector to obtain negative electrode sheet; (2) Preparation of positive electrode sheet S3. The positive electrode active material and the positive electrode conductive agent are mixed for the first time, and then the positive electrode binder is added for the second mixing. After the second mixing, the mixture is rolled into a film and then combined with the positive electrode current collector to obtain the positive electrode sheet. (3) Preparation of electrolyte S4. Add lithium salt and additives to the organic solvent to obtain the electrolyte; (4) Preparation of secondary batteries The positive electrode, negative electrode, separator and other components are assembled and then processed through winding, hot pressing, super welding, core bonding, casing, baking, liquid injection, high-temperature wetting, formation, aging, sealing and capacity testing to obtain a secondary battery.

[0081] In some embodiments, in step S1, during the high-voltage electrostatic drawing and coating process, the voltage is 10 kV~20 kV, the receiving distance is 10 cm~20 cm, the polymer matrix concentration is 3 wt%~8 wt%, and the polymer content is 0.5%~5%.

[0082] In some embodiments, in step S1, the dispersion speed of high-speed dispersion is 15 m / s to 30 m / s, and the dispersion time is 10 min to 20 min.

[0083] In some embodiments, in step S1, an inorganic solid electrolyte is added to a ball mill and mixed, with a ball milling speed of 1000 rpm to 1500 rpm and a time of 10 min to 20 min.

[0084] In some embodiments, in step S2, the dispersion rate of the first mixing is 15 m / s to 20 m / s, and the dispersion time is 5 min to 15 min.

[0085] In some embodiments, in step S2, the second mixing is first dispersed at a temperature of 0℃ to 5℃ at a dispersion speed of 15m / s to 20m / s for 5min to 10min, and then dispersed at a temperature of 80℃ to 120℃ at a dispersion speed of 30m / s to 40m / s for 10min to 20min.

[0086] In some embodiments, in step S2, the pressure of the roll forming process is 2T~10T, and the temperature is 80℃~120℃.

[0087] In some embodiments, in step S2, the pressure of the composite is 2T~10T and the temperature is 80℃~120℃.

[0088] In some embodiments, in step S3, the dispersion rate of the first mixing is 15 m / s to 20 m / s, and the dispersion time is 5 min to 15 min.

[0089] In some embodiments, in step S3, the second mixing is first dispersed at a temperature of 0℃ to 5℃ at a dispersion speed of 15m / s to 20m / s for 5min to 10min, and then dispersed at a temperature of 80℃ to 120℃ at a dispersion speed of 30m / s to 40m / s for 10min to 20min.

[0090] In some embodiments, the pressure of the roll forming process in step S3 is 2T to 10T.

[0091] In some embodiments, the pressure of the composite process in step S3 is 2T to 10T.

[0092] It should be noted that the average diameter of the polymer fibers can be changed by adjusting the polymer matrix concentration and the parameters of the high-voltage electrostatic drawing coating process in step S1.

[0093] It should be noted that the thickness of the coating layer can be changed by adjusting the amount of polymer matrix added in step S1.

[0094] It should be noted that the mass percentage of inorganic solid electrolyte in the negative electrode film can be changed by adjusting the amount of inorganic solid electrolyte added in step S1.

[0095] It should be noted that the average particle size of the inorganic solid electrolyte can be changed by adjusting the particle size of the inorganic solid electrolyte raw material and the mixing parameters in step S1.

[0096] It should be noted that the average diameter and average length of polytetrafluoroethylene fibers can be changed by adjusting the dispersion rate, temperature and dispersion time of the second mixing in step S2.

[0097] In one embodiment of this application, an electrical device is provided, including the secondary battery described in this application.

[0098] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0099] Example 1 This application provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of negative electrode sheet S1. The negative electrode active material (graphite) is dissolved in a polymer matrix (PEO) and then subjected to high-voltage electrostatic drawing and coating treatment (specific parameters are: PEO concentration of 5wt%, voltage of 15kV, receiving distance of 15cm, and PEO addition of 2% based on the total mass of negative electrode active material, negative electrode binder and negative electrode conductive agent) to obtain an electrospun film. The electrospun film is then dispersed at high speed (dispersion speed of 20m / s, dispersion time of 10min) to form powder. Then, an inorganic solid electrolyte (LLZO, average particle size of 200nm) is added and ball-milled (specific parameters: inorganic solid electrolyte addition of 1.5% based on the total mass of negative electrode active material, negative electrode binder and negative electrode conductive agent, ball milling speed of 1200rpm, ball milling time of 10min) to obtain the negative electrode active material. S2. Add negative electrode conductive agent (acetylene black) to the negative electrode active material and mix for the first time (using a high-speed disperser, dispersion speed of 20m / s, dispersion time of 10min). Then add negative electrode binder (PTFE) and mix for the second time (first disperse at 0℃ at a dispersion speed of 18m / s for 8min, then disperse at 100℃ at a dispersion speed of 40m / s for 15min). After the second mixing, roll press to form a film (temperature of 100℃, pressure of 5T) and composite with negative electrode current collector (temperature of 100℃, pressure of 5T) to obtain the negative electrode sheet. The mass ratio of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder is 97.5:1:1.5. (2) Preparation of positive electrode sheet S3. The positive electrode active material (lithium iron phosphate) and the positive electrode conductive agent (acetylene black) are mixed for the first time using a high-speed disperser at a dispersion speed of 20 m / s for 10 min. Then, the positive electrode binder PTFE is added for the second mixing (first dispersed at 0℃ at a dispersion speed of 18 m / s for 8 min, then dispersed at 100℃ at a dispersion speed of 40 m / s for 15 min). After the second mixing, the mixture is rolled into a film (temperature 100℃, pressure 5T) and then combined with the positive electrode current collector (temperature 100℃, pressure 5T) to obtain the positive electrode sheet. The mass ratio of the positive electrode active material, the positive electrode conductive agent and the positive electrode binder is 96:2:2; (3) Preparation of electrolyte S4. Add lithium salt (lithium hexafluorophosphate) and additive (fluoroethylene carbonate) to an organic solvent (a mixture of EC, EMC and DEC in a volume ratio of 1:1:1) to obtain an electrolyte. The electrolyte contains 10% lithium salt and 2.5% fluoroethylene carbonate by mass. (4) Preparation of secondary batteries The positive electrode, negative electrode, separator and other components are assembled and then processed through winding, hot pressing, super welding, core bonding, casing, baking, liquid injection, high-temperature wetting, formation, aging, sealing and capacity testing to obtain a secondary battery.

[0100] Examples 2-4 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of polymer matrix added in step S1 and the parameters of the high voltage electrostatic drawing coating treatment are adjusted to achieve the parameters in Table 1.

[0101] Examples 5-7 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the high-voltage electrostatic drawing and coating process in step S1 are adjusted to achieve the parameters in Table 1.

[0102] Examples 8-9 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of inorganic solid electrolyte added in step S1 is adjusted to achieve the parameters in Table 1.

[0103] Examples 10-12 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the average particle size of the inorganic solid electrolyte in step S1 is adjusted to achieve the parameters in Table 1.

[0104] Examples 13-15 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the second mixing in step S2 are adjusted to achieve the parameters in Table 1.

[0105] Examples 16-17 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of negative electrode binder added in step S2 is adjusted to achieve the parameters in Table 1.

[0106] Examples 18-19 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of fluoroethylene carbonate added in step S4 is adjusted to achieve the parameters in Table 1.

[0107] Example 20 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that fluoroethylene carbonate is not added in step S4 to achieve the parameters in Table 1.

[0108] Example 21 This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that silicon carbon is used instead of graphite to achieve the parameters in Table 1.

[0109] Example 22 This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that LATP is used instead of LLZO to achieve the parameters in Table 1.

[0110] Example 23 This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that PVDF is used instead of PEO to achieve the parameters in Table 1.

[0111] Comparative Example 1 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that there is no coating layer, that is, no step S1.

[0112] Comparative Example 2 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that no inorganic solid electrolyte is added in step S1.

[0113] Comparative Example 3 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that fluorinated ethylene propylene copolymer is used instead of PTFE in step S2.

[0114] The thickness h of the coating layer, the average diameter r of the polymer fiber, the mass percentage W1 of the inorganic solid electrolyte based on the total mass of the negative electrode film, the average particle size d of the inorganic solid electrolyte, the average diameter D of the polytetrafluoroethylene fiber, the average length L of the polytetrafluoroethylene fiber, the mass percentage W2 of the negative electrode binder based on the total mass of the negative electrode film, the mass percentage W3 of the fluoroethylene carbonate based on the total mass of the electrolyte, the type of negative electrode active material, the type of inorganic solid electrolyte, and the type of polymer fiber in the examples and comparative examples are shown in Table 1. Table 1 The performance of the secondary batteries prepared in the examples and comparative examples was tested, including the following aspects: 1. Cyclic performance test At room temperature (25±3℃), the voltage, internal resistance, thickness, and DC internal resistance of the secondary battery were tested. The secondary battery was placed in an environment of 25±3℃ and allowed to stand for 10 minutes. It was then discharged at a current of 0.1C to the cutoff voltage of 2.5V. After standing for 10 minutes, it was charged at a constant current of 0.5C to the upper limit voltage of 3.65V. After reaching the upper limit voltage, it was maintained at a constant voltage of 3.65V with a cutoff current of 0.02C. After standing for 10 minutes, it was discharged at 0.2C to the lower limit voltage of 2.5V (for initial capacity testing).

[0115] The cyclic test is as follows: 1) After resting for 10 minutes, charge at 0.5C to the upper limit voltage of 3.65V. After reaching the upper limit voltage, maintain a constant voltage of 3.65V during charging, with a cutoff current of 0.02C. 2) After resting for 10 minutes, discharge at 0.2C to the cutoff voltage of 2.5V. Repeat steps 1-2 400 times. Every 50 cycles, collect data during the full charge test, including voltage, internal resistance, thickness, and DC internal resistance. Every 100 cycles, repeat the capacity test at 25℃. After the cycle is complete, measure the full charge data: voltage, internal resistance, thickness, and DC internal resistance. Capacity retention rate = C2 / C1 × 100%, where C1 is the initial capacity test value and C2 is the capacity after the 300th discharge cycle.

[0116] 2. K-value test The K-value refers to the voltage drop per unit time, usually expressed in mV / h, and is used to measure the self-discharge rate of a secondary battery under full charge. OCV1 is measured at time t1, and OCV2 is measured at time t2; the interval between t1 and t2 is 48 hours.

[0117] Where K = OCV1 - OCV2 / t1 - t2.

[0118] 3. First Coulomb efficiency test First-cycle coulombic efficiency (CEE), or initial efficiency for short, is a performance indicator used to quantify the anode material of lithium-ion batteries. It is defined as the ratio of discharge capacity to charge capacity during the first charge-discharge cycle of a lithium-ion battery. There are two main ways CEE is lost: 1. Formation of an SEI film; 2. Irreversible lithium intercalation at the anode.

[0119] The first-efficiency test is as follows: 1) Charge at a constant current of 0.1C to the cutoff voltage of 3.65V, maintain a constant voltage of 3.65V during charging, and cut off the current at 0.02C. 2) Allow to stand for 10 minutes, then discharge at 0.2C to the cutoff voltage of 2.5V. First-efficiency = Discharge capacity / Charge capacity × 100%.

[0120] The results of the above tests are shown in Table 2. Table 2 As can be seen from Table 2, when the technical solution provided in this application is adopted, the obtained secondary battery has excellent comprehensive performance; specifically, the initial coulombic efficiency of the obtained secondary battery is above 81.8%, the K value is below 0.49mV / h, and the capacity retention rate is above 83.1%. As can be seen from Examples 1-23 and Comparative Examples 1-3, when the negative electrode film layer does not meet the requirements of this application, the resulting secondary battery cannot achieve the effect of this application.

[0121] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer comprising a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent, characterized in that, The negative electrode active material includes a negative electrode active material and a coating layer disposed on at least a portion of the surface of the negative electrode active material; The coating layer has a mesh structure containing polymer fibers, and an inorganic solid electrolyte is disposed in at least a portion of the pores of the mesh structure. The negative electrode binder includes polytetrafluoroethylene.

2. The secondary battery according to claim 1, characterized in that, The thickness of the coating layer is 50nm~800nm; And / or, the average diameter of the polymer fiber is 20 nm to 500 nm.

3. The secondary battery according to claim 1, characterized in that, The polymer fiber includes at least one of polyethylene oxide, polyvinylidene fluoride, carboxymethyl cellulose, polyvinylpyrrolidone, and polymethyl methacrylate.

4. In the secondary battery according to claim 1, the inorganic solid electrolyte has a mass percentage of 0.5% to 3% based on the total mass of the negative electrode film.

5. The secondary battery according to claim 1, characterized in that, The inorganic solid electrolyte includes at least one of oxide solid electrolyte and sulfide solid electrolyte; The average particle size of the inorganic solid electrolyte is <500 nm.

6. The secondary battery according to claim 5, characterized in that, Satisfy any of the following: The oxide solid electrolyte includes any one of LLZO, LATP, and LLTO. The sulfide solid electrolyte includes LiPS5Cl, Li 10 GeP2S 12 Any one of Li2S-P2S5; The average particle size of the inorganic solid electrolyte is 10 nm to 400 nm.

7. The secondary battery according to claim 1, characterized in that, The polytetrafluoroethylene is polytetrafluoroethylene fiber, and the average diameter of the polytetrafluoroethylene fiber is 50nm~500nm; And / or, the polytetrafluoroethylene is polytetrafluoroethylene fiber, and the average length of the polytetrafluoroethylene fiber is 10μm~500μm.

8. The secondary battery according to claim 1, characterized in that, The mass percentage of the negative electrode binder is 0.5% to 5% based on the total mass of the negative electrode film layer.

9. The secondary battery according to claim 1, characterized in that, The secondary battery also includes an electrolyte, which contains fluoroethylene carbonate.

10. The secondary battery according to claim 9, characterized in that, The mass percentage of the fluoroethylene carbonate is 0.5% to 5% based on the total mass of the electrolyte.

11. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 10.

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