A secondary battery and a preparation method and application thereof

Abstract

The application relates to a secondary battery and a preparation method and application thereof, the secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte; the electrolyte comprises a first additive; the first additive comprises a negative electrode film-forming additive, a positive electrode film-forming additive and a high-temperature film-forming additive; the negative electrode film-forming additive comprises vinylene carbonate; the positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer located on at least one side of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, the positive electrode active material comprising lithium iron manganese phosphate and lithium manganate; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; at least one of the positive electrode sheet and the negative electrode sheet further comprises a solid electrolyte layer, the solid electrolyte layer being located on a side of the positive electrode active material layer away from the positive electrode current collector and / or on a side of the negative electrode active material layer away from the negative electrode current collector. The obtained secondary battery has excellent fast charging, safety, cycle performance and low-temperature performance.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to a secondary battery, its preparation method, and its application. Background Technology

[0002] With the development of new energy sources, people have placed increasing demands on the performance of lithium-ion batteries, such as operating performance under low-temperature conditions, higher safety performance, longer lifespan, and faster recharging speed. Currently, the mainstream cathode active material systems are ternary materials and lithium iron phosphate (LFP). Ternary materials offer superior low-temperature performance, short cycle life, and high energy density, but extremely poor safety performance. Lithium iron phosphate (LFP) materials offer good safety and long lifespan, but poor rate performance and extremely poor low-temperature performance. Therefore, the mainstream material system cannot simultaneously meet the requirements of fast charging, long lifespan, high safety performance, and operation under low-temperature conditions. In the development of cathode active material systems, manganese-based cathode active materials have also been discovered. Lithium manganese iron phosphate (LMFP), a olivine-like material, has high safety and theoretically has an energy density about 20% higher than LFP. Layered lithium manganese oxide (LMO) materials possess excellent fast charging and low-temperature charge / discharge capabilities. However, both Mn-based materials suffer from severe Mn leaching problems during use, with LMO being more severe. The leached Mn deposits at the negative electrode and deteriorates the SEI film (solid electrolyte interface film), and the structure of the positive electrode also collapses due to Mn leaching. These reasons have led to a serious lag in the industrial application of Mn-based materials. Summary of the Invention

[0003] One objective of this invention is to provide a secondary battery that solves the problems of existing manganese-based positive electrode active materials being unable to simultaneously achieve fast charging, high safety, long lifespan, and low-temperature performance; another objective is to provide a method for preparing the aforementioned secondary battery; and a third objective is to provide an electrical device.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte; The electrolyte includes a first additive; The first additive includes negative electrode film-forming additives, positive electrode film-forming additives, and high-temperature film-forming additives; The negative electrode film-forming additive includes vinylene carbonate (VC). The positive electrode sheet includes a positive current collector and a positive active material layer located on at least one side of the positive current collector. The positive active material layer includes a positive active material, which includes lithium iron manganese phosphate and lithium manganese oxide. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; At least one of the positive electrode and the negative electrode further includes a solid electrolyte layer, wherein the solid electrolyte layer is located on the side of the positive active material layer away from the positive current collector and / or on the side of the negative active material layer away from the negative current collector.

[0005] In one alternative implementation, the secondary battery conforms to the following relationship: 7.3≤W×100×(H1+H2) / S≤8.3, Wherein, W is the mass percentage of vinylene carbonate in the electrolyte; H1 is the numerical value of the thickness of the solid electrolyte layer on the negative electrode side, and the unit of the thickness of the solid electrolyte layer on the negative electrode side is μm; when there are negative electrode active material layers and solid electrolyte layers on both sides of the negative electrode, the thickness of the solid electrolyte layer on the negative electrode side is the sum of the thicknesses of the solid electrolyte layers on both sides of the negative electrode. H2 is the numerical value of the solid electrolyte layer thickness on the positive electrode side, and the unit of the solid electrolyte layer thickness on the positive electrode side is μm; when there are positive electrode active material layers and solid electrolyte layers on both sides of the positive electrode, the thickness of the solid electrolyte layer on the positive electrode side is the sum of the thicknesses of the solid electrolyte layers on both sides of the positive electrode. S is the key factor, S = (molar amount of Mn element in the positive electrode active material / molar amount of Fe element in the positive electrode active material). 1 / 2 .

[0006] In one optional embodiment, the electrolyte contains 0.1% to 15% by mass of the first additive.

[0007] In one optional embodiment, the mass content of the positive electrode film-forming additive in the first additive is 8% to 22%.

[0008] In one optional embodiment, the mass content of the negative electrode film-forming additive in the first additive is 60% to 80%.

[0009] In one optional embodiment, the mass content of the high-temperature film-forming additive in the first additive is 10% to 22%.

[0010] In one optional embodiment, the mass ratio of lithium iron manganese phosphate to lithium manganese oxide in the positive electrode active material is 5:5 to 9:1.

[0011] In one optional embodiment, the lithium iron manganese phosphate has the chemical formula LiMn. x Fe y PO4, where x+y=1, 0.1≤x≤0.9.

[0012] In one optional embodiment, the negative electrode film-forming additive further includes at least one of 1,3-propanesulfonic acid lactone (PS), fluoroethylene carbonate (FEC), and vinyl ethylene carbonate (VEC).

[0013] In one optional embodiment, the positive electrode film-forming additive includes at least one of tris(trimethylsilane) phosphate, lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium difluorooxalato)borate (LiDFOB), and lithium difluorobis(oxalato)borate (LiDFOP).

[0014] In one optional embodiment, the high-temperature film-forming additive includes at least one of vinyl sulfate (DTD), hexamethylene diisocyanate, methylene disulfonate, hexanetrionitrile, and propylene sulfonate lactone.

[0015] In one alternative embodiment, the electrolyte further includes lithium salt and organic solvent.

[0016] In one optional embodiment, the mass content of the vinylene carbonate in the first additive is 30% to 70%.

[0017] In one optional embodiment, the concentration of lithium salt in the electrolyte is 0.8~1.5 mol / L.

[0018] In one optional embodiment, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bisfluorosulfonamide, lithium difluoroborate, and lithium tetrafluoroborate.

[0019] In one optional embodiment, the organic solvent includes any two or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propylene carbonate.

[0020] In one alternative embodiment, lithium bisfluorosulfonamide accounts for 10% to 40% of the electrolyte, based on the total concentration of all lithium salts.

[0021] In one alternative embodiment, the electrolyte contains 50% to 80% of the total volume of all organic solvents, comprising ethyl methyl carbonate and dimethyl carbonate.

[0022] In one alternative embodiment, dimethyl carbonate comprises 15% to 60% of the total volume of all organic solvents in the electrolyte.

[0023] In one optional embodiment, the thickness of the solid electrolyte layer is 1~6 μm; in a further optional embodiment, the thickness of the solid electrolyte layer is 2~4 μm.

[0024] In one alternative embodiment, the solid electrolyte layer includes a solid electrolyte and a first binder.

[0025] In one optional embodiment, the positive electrode active material layer further includes a second binder and a conductive agent. In this invention, the proportions of each component in the positive electrode active material layer conform to conventional proportions in the art, and the particle size of the positive electrode active material particles does not need to be specifically specified, as long as it conforms to the size of commonly used positive electrode active material particles; typically, and not limited to, the mass ratio of the positive electrode active material, the second binder, and the conductive agent in the positive electrode active material layer is 90~95:0.5~3:0.5~2.

[0026] In one optional embodiment, the negative electrode active material layer includes a negative electrode active material, a third binder, and a conductive agent. In this invention, the proportions of each component in the negative electrode active material layer conform to conventional proportions in the art, and the particle size of the negative electrode active material particles does not need to be specifically specified, as long as it conforms to the size of commonly used negative electrode active material particles; typically, and not limited to, the mass ratio of the negative electrode active material, the third binder, and the conductive agent in the negative electrode active material layer is 90~95:0.5~3:0.5~2.

[0027] In one optional embodiment, the conductive agent used in this invention does not need to be specifically specified, and any common conductive agent in the art can be used. Typically, without limitation, the conductive agent includes at least one of conductive carbon black, conductive graphite, acetylene black, graphene, carbon nanotubes, Ketjen black, polyethylene dioxythiophene, sodium polystyrene sulfonate, carbon nanofibers, and polystyrene sulfonic acid.

[0028] In one optional embodiment, the solid electrolyte layer includes a second additive, the structure of which is shown in Formula I. Formula I, Wherein, Y includes H, or a chelating group; The chelating agent corresponding to the chelating group includes , , , , , , , , , At least one of them.

[0029] In one optional embodiment, the mass content of the second additive is 15% to 25% of the mass content of the first binder, based on the total mass of the solid electrolyte layer.

[0030] In one alternative embodiment, the first binder comprises 1% to 10% by mass of the total mass of the solid electrolyte layer.

[0031] In one optional embodiment, the solid electrolyte includes at least one of lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, lithium lanthanum zirconium oxide, lithium lanthanum zirconium tantalum oxide, and lithium lanthanum titanate.

[0032] In one optional embodiment, the negative electrode active material includes at least one of silicon-based materials and carbon-based materials.

[0033] In one alternative embodiment, the first adhesive, the second adhesive, and the third adhesive are each independently selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, lithium polyacrylate, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

[0034] Secondly, the present invention provides a method for preparing the above-mentioned secondary battery, comprising the following steps: S1: Prepare the electrolyte; S2: Mix the solid electrolyte and the first binder, add the first solvent, and obtain the solid electrolyte layer slurry; S3: Prepare a positive electrode active material layer on at least one side of the surface of the positive electrode current collector, prepare a negative electrode active material layer on at least one side of the surface of the negative electrode current collector, and coat the solid electrolyte layer slurry on the side of the positive electrode active material layer away from the positive electrode current collector and / or the side of the negative electrode active material layer away from the negative electrode current collector to obtain a positive electrode sheet and / or a negative electrode sheet. S4: Assemble the positive electrode, negative electrode, and electrolyte to obtain a secondary battery.

[0035] In the preparation method provided by this invention, the non-solid electrolyte layer portion of the positive electrode sheet and negative electrode sheet can be prepared using conventional methods in the art. Typically, but not limited to, the preparation method of the non-solid electrolyte layer portion of the positive electrode sheet includes the following steps: taking positive active material, second binder, and conductive agent in a mass ratio of 90~95:0.5~3:0.5~2, mixing them, adding a solvent (such as N-methylpyrrolidone), and preparing a positive electrode slurry with a solid content of 50%~70%; using an aluminum foil with a thickness of 10~14μm for the positive electrode current collector, coating the positive electrode slurry onto at least one side of the positive electrode current collector, and drying it after coating to obtain a positive electrode current collector coated with a positive active material layer. The preparation method of the non-solid electrolyte layer in the negative electrode sheet includes the following steps: taking negative electrode active material, third binder, and conductive agent in a mass ratio of 90~95:0.5~3:0.5~2, mixing them, adding solvent (such as deionized water), and preparing a negative electrode slurry with a solid content of 40%~60%; using a copper foil with a thickness of 4~8μm for the negative electrode current collector, coating the negative electrode slurry onto at least one side of the negative electrode current collector, and drying it after coating to obtain a negative electrode current collector coated with a negative electrode active material layer.

[0036] In an alternative embodiment, step S2 further includes the addition of a second additive.

[0037] In one optional embodiment, in step S2, the solid content of the solid electrolyte layer slurry is 60% to 70%.

[0038] In an optional embodiment, in S2, the first adhesive includes at least one of polyvinylidene fluoride and polyvinylidene fluoride-hexafluoropropylene copolymer.

[0039] In an optional embodiment, in S2, the first solvent includes at least one of N,N-dimethylformamide, N-methylpyrrolidone, ethyl acetate, methyl formate, and 1,1,2,2-tetrafluoroethyl-1,1,2,2-tetrafluoropropyl ether.

[0040] Thirdly, the present invention also provides an electrical device, including the above-mentioned secondary battery or the secondary battery prepared by the above-mentioned preparation method, wherein the secondary battery serves as the power supply for the electrical device.

[0041] The beneficial effects of this invention are: (1) The secondary battery provided by the present invention includes a positive electrode, a negative electrode, and an electrolyte; the electrolyte includes a first additive; the first additive includes a negative electrode film-forming additive, a positive electrode film-forming additive, and a high-temperature film-forming additive; the negative electrode film-forming additive includes vinylene carbonate; the positive electrode includes a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, the positive electrode active material including lithium iron manganese phosphate and lithium manganese oxide; the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; at least one of the positive electrode and the negative electrode further includes a solid electrolyte layer, the solid electrolyte layer being located on the side of the positive electrode active material layer away from the positive electrode current collector and / or on the side of the negative electrode active material layer away from the negative electrode current collector. The secondary battery provided by the present invention has excellent fast charging performance, safety performance, cycle performance, and low-temperature performance.

[0042] Using a hybrid positive electrode active material of lithium iron manganese phosphate and lithium manganese oxide can comprehensively leverage the high safety and high energy density of lithium iron manganese phosphate, and the fast charging and low-temperature charge-discharge capabilities of lithium manganese oxide. By adding negative electrode film-forming additives, positive electrode film-forming additives, and high-temperature film-forming additives to the electrolyte, with specific negative electrode film-forming additives including the limitation of vinylene carbonate, and the setting of a solid electrolyte layer on the positive and / or negative electrode plates, the dissolution and migration of Mn elements from the positive electrode to the negative electrode can be physically prevented, thus eliminating its catalytic and destructive effects on the SEI film. The negative electrode film-forming additives added to the electrolyte form a high-quality SEI and prevent the reduction and deposition of Mn elements at the negative electrode. In particular, vinylene carbonate has a low film-forming potential and is more likely to form an SEI film. The positive electrode film-forming additives can make the CEI film (positive electrode electrolyte interface film) formed at the positive electrode interface more stable and help suppress the dissolution of Mn elements in the positive electrode sheet. High-temperature film-forming additives can improve the high-temperature storage stability of the battery. The setting of solid electrolyte layers on the positive electrode sheet and / or negative electrode sheet can directly reduce the contact between the electrolyte and the positive and negative electrode active material layers, and reduce the reaction area between the electrode liquid and the two active material layers. The solid electrolyte layer set at the positive electrode sheet can reduce the dissolution of Mn elements in the positive electrode active material layer and also suppress the excessive formation of CEI film. The solid electrolyte layer set at the negative electrode sheet can reduce the enrichment of Mn elements at the negative electrode active material layer and also suppress the excessive formation of SEI film, thereby avoiding the negative impact of excessive CEI film and SEI film formation on the electrical performance of the secondary battery. The combination of various additives and the solid electrolyte layer provides multiple layers of protection for both the positive and negative electrodes at both physical and chemical levels, improving the structural stability of the positive and negative electrode sheets. It also, to a certain extent, prevents gas generation in the electrolyte due to Mn catalysis at high temperatures. These two aspects work together to improve the cycle performance and high-temperature performance of the secondary battery. Furthermore, the highly ion-conducting solid electrolyte layer at the electrode level allows lithium ions to transfer rapidly from the positive to the negative electrode, reducing cell polarization. Simultaneously, it reduces the charging time at the high-voltage region at the charging cutoff point of the positive electrode, suppressing structural distortion under high voltage, thus beneficially improving the cell's cycle performance and fast-charging capability.

[0043] (2) The secondary battery conforms to the following relationship: 7.3≤W×100×(H1+H2) / S≤8.3, where W is the mass percentage of vinylene carbonate in the electrolyte; H1 is the thickness of the solid electrolyte layer on the negative electrode side, and the unit of the thickness of the solid electrolyte layer on the negative electrode side is μm; H2 is the thickness of the solid electrolyte layer on the positive electrode side, and the unit of the thickness of the solid electrolyte layer on the positive electrode side is μm; S is the key factor, S=(molar amount of Mn element in the positive electrode active material / molar amount of Fe element in the positive electrode active material) 1 / 2By optimizing the total amount of vinylene carbonate, solid electrolyte layer thickness, manganese content, and iron content in the positive electrode active material, the high-temperature cycle performance, fast-charging capability, and safety of the secondary battery can be further improved.

[0044] (3) In the secondary battery provided by the present invention, the mass content of the first additive in the electrolyte is 0.1% to 15%; the mass content of the positive electrode film-forming additive in the first additive is 10% to 20%, which can further improve the high-temperature cycle performance of the battery cell; the mass content of the negative electrode film-forming additive in the first additive is 60% to 80%, which can further improve the long-term cycle performance of the battery cell; the mass content of the high-temperature film-forming additive in the first additive is 10% to 20%, which can further suppress the risk of high-temperature gas generation and improve the safety performance of the battery cell.

[0045] (4) In the secondary battery provided by the present invention, the mass ratio of lithium iron manganese phosphate to lithium manganese oxide in the positive electrode active material is 5:5 to 9:1. By controlling a reasonable ratio of lithium iron manganese phosphate to lithium manganese oxide in the positive electrode active material, the high-temperature cycle performance of the secondary battery can be further improved.

[0046] (5) In the secondary battery provided by the present invention, the negative electrode film-forming additive further includes at least one of 1,3-propanesulfonate lactone, fluoroethylene carbonate, and vinyl ethylene carbonate; the positive electrode film-forming additive includes at least one of tris(trimethylsilane) phosphate, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and lithium difluoro(oxalato)phosphate; the high-temperature film-forming additive includes at least one of vinyl sulfate, hexamethylene diisocyanate, methane disulfonate, hexanetrionitrile, and propylene sulfonate lactone. Preferably, non-carboxylic acid ester film-forming additives are used to avoid the decomposition of carboxylic acid ester compounds at high temperatures, indirectly improving the high-temperature cycle performance of the secondary battery.

[0047] (6) In the secondary battery provided by the present invention, the mass content of the first additive, the vinylene carbonate, is 30% to 60%, which can enable the negative electrode to form a better film and prevent the continuous consumption of Mn dissolved from the positive electrode. The SEI can improve the long-cycle performance.

[0048] (7) In the secondary battery provided by the present invention, the concentration of lithium salt in the electrolyte is 0.8~1.5 mol / L. A reasonable concentration of lithium salt in the electrolyte can fully utilize the conductivity of the electrolyte.

[0049] (8) In the secondary battery provided by the present invention, the lithium bisfluorosulfonyl imide accounts for 10% to 40% of the total concentration of all lithium salts in the electrolyte; and the volume of dimethyl carbonate accounts for 15% to 60% of the total mass of all organic solvents in the electrolyte. The use of high content lithium bisfluorosulfonyl imide (LiFSI) and dimethyl carbonate (DMC) can reduce the viscosity of the electrolyte and increase the degree of lithium salt dissociation. Increasing the degree of lithium salt dissociation increases the concentration of dissociated lithium salts, further improving the conductivity of the electrolyte. This ensures that even with a high quality of negative electrode film-forming additives in the electrolyte, the impedance will not deteriorate, thus improving the rate charging and low-temperature charge-discharge capabilities of the secondary battery. Furthermore, the use of a certain amount of lithium bisfluorosulfonyl imide as a lithium salt can also improve the stability of the secondary battery at high temperatures. Controlling the lithium bisfluorosulfonamide content, i.e. the presence of multiple lithium salts in the electrolyte, reduces the highly corrosive HF and highly oxidizing PF5 produced by the electrolyte in high-temperature environments compared to conventional electrolytes. This reduces the harm of HF and PF5 to the positive electrode and electrolyte, thus improving the high-temperature safety performance of secondary batteries.

[0050] (9) In the secondary battery provided by the present invention, the thickness of the solid electrolyte layer is 1~6μm. A reasonable solid electrolyte membrane thickness can give full play to its effect of reducing the dissolution and enrichment of Mn element, and can also avoid the excessive formation of CEI membrane and SEI membrane to a certain extent, while not having a negative impact on the electrical performance of the secondary battery.

[0051] (10) In the secondary battery provided by the present invention, at least one of the solid electrolyte layer, the positive electrode active material layer, and the negative electrode active material layer includes a second additive, the structure of which is shown in Formula I. Formula I; Wherein, Y includes H, or a chelating group; the chelating agent corresponding to the chelating group includes , , , , , , , , , At least one of the following. The second additive can capture the dissolved Mn element, reduce the probability of Mn element migrating to the negative electrode, thereby inhibiting the deposition and reduction of Mn element on the negative electrode side, which would damage the negative electrode SEI film, further improve the structural stability of the secondary battery, and thus improve its cycle life.

[0052] (11) In the secondary battery provided by the present invention, the mass content of the second additive is 15% to 25% of the mass content of the first binder, based on the total mass of the solid electrolyte layer. A reasonable mass content ratio of the second additive to the first binder can control the reasonable content of the second additive in the solid electrolyte layer, so that it can give full play to the capture effect of dissolved Mn elements, and will not affect the stability of the internal physical structure of the solid electrolyte layer.

[0053] (12) The present invention provides a method for preparing the above-mentioned secondary battery, comprising the following steps: S1: preparing an electrolyte; S2: mixing a solid electrolyte and a first binder, adding a first solvent to obtain a solid electrolyte layer slurry; S3: preparing a positive electrode active material layer on at least one side of the surface of the positive electrode current collector, preparing a negative electrode active material layer on at least one side of the surface of the negative electrode current collector, and coating the solid electrolyte layer slurry on the side of the positive electrode active material layer away from the positive electrode current collector and / or the side of the negative electrode active material layer away from the negative electrode current collector to obtain a positive electrode sheet and / or a negative electrode sheet; S4: assembling the positive electrode sheet, the negative electrode sheet, and the electrolyte to obtain a secondary battery. The method for preparing the above-mentioned secondary battery provided by the present invention does not require special processes, is simple and convenient, and is suitable for large-scale production. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the structure of the secondary battery obtained in Embodiment 1 of the present invention after charging and discharging.

[0055] Among them, 1-negative electrode current collector; 2-positive electrode current collector; 3-negative electrode active material layer; 4-positive electrode active material layer; 401-manganese ion; 5-SEI membrane; 6-CEI membrane; 7-solid electrolyte layer; 701-solid electrolyte; 702-second additive; 8-separator. Detailed Implementation

[0056] The embodiments of the present invention will be described below with reference to the accompanying drawings and preferred embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention and not for limiting the scope of protection of the present invention.

[0057] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0058] Example 1 This embodiment provides a secondary battery and its preparation method, including the following steps: (1) Preparation of electrolyte: In an argon-filled glove box (H2O content ≤ 1ppm), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC) were mixed as an organic solvent, and lithium electrolyte salt LiPF6 and the first additive were added and mixed to obtain the electrolyte. The first additive included: anode film-forming additives ethylene carbonate (VC) and fluoroethylene carbonate (FEC); cathode film-forming additives lithium difluorooxalate borate (LiODFB), tris(trimethylsilane phosphate) (TMSB), and lithium difluorophosphate (LiPO2F2); and high-temperature film-forming additives hexamethylene diisocyanate (HDI) and hexanetrionitrile (HTCN).

[0059] In the organic solvent, the volume ratio of DMC, EMC, EC, and PC is 40:10:25:25; in the electrolyte, the concentration of LiPF6 is 1 mol / L; based on the total mass percentage of the electrolyte, the mass content of the first additive is 4.60%; in the first additive, the mass content of the negative electrode film-forming additive is 77.17%, the mass content of the positive electrode film-forming additive is 9.78%, and the mass content of the high-temperature film-forming additive is 13.05%; in the negative electrode film-forming additive, the mass ratio of VC and FEC is 85:15; in the positive electrode film-forming additive, the mass ratio of LiODFB, TMSB, and LiPO2F2 is 15:25:60; in the high-temperature film-forming additive, the mass ratio of HDI and HTCN is 10:90.

[0060] (2) Preparation of positive electrode: The positive electrode active material is LiMn with a mass ratio of 7:3. 0.9 Fe 0.1 PO4 and LiMnO2 were mixed in a mass ratio of 96:1.4:0.4:2.2, along with positive electrode active material, conductive agent SP (conductive carbon black), conductive agent CNT (carbon nanotubes), and secondary binder PVDF (polyvinylidene fluoride). NMP (N-methylpyrrolidone) solvent was added to prepare the positive electrode slurry, which had a solid content of 60%. A 12 μm thick aluminum foil was used as the positive electrode current collector, and the positive electrode slurry was coated onto both sides of the current collector, resulting in a double-sided coating density of 20.2 mg / cm². 2 After coating, drying yields a positive current collector coated with positive active material layers on both sides.

[0061] The first binder, the second additive, and the solid electrolyte lithium lanthanum zirconium tantalum oxide (LLZTO) were used in a mass ratio of 1.5:0.3:98.2. The first binder was polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) in a mass ratio of 2:25. The structural formula of the second additive is shown in Formula I, where Y is a chelating agent. The corresponding chelating group.

[0062] LLZTO and N,N-dimethylformamide (DMF) were mixed at a mass ratio of 1:2 to prepare an LLZTO-DMF dispersion. A binder and solvent DMF were mixed at a mass ratio of 1:30. Specifically, the binder was poured into the DMF at a stirring speed of 800 rpm / min, and then stirred at 500 rpm / min for 2 hours until the first binder was completely dissolved in the DMF. The prepared LLZTO-DMF dispersion was added to adjust the solid content to 65%, and then a second additive was added. The mixture was then stirred at 500 rpm / min for 10 hours. After stirring, the mixture was allowed to stand under vacuum for 0.5 hours to remove air bubbles, resulting in a solid electrolyte layer slurry. This solid electrolyte layer slurry was coated onto the positive electrode active material layer on the side away from the positive electrode current collector and dried to obtain the positive electrode sheet. The total thickness of the solid electrolyte layer on both sides of the positive electrode sheet was 4 μm (unless otherwise specified, the thickness of the solid electrolyte layer on both sides of the electrode sheet is the same, hereinafter the same).

[0063] (3) Preparation of negative electrode sheet: A negative electrode active material Gr (Shanshan Technology, artificial graphite G02D), conductive agent SP (conductive carbon black), conductive agent CNT (carbon nanotubes), third binder CMC (sodium carboxymethyl cellulose), and third binder SBR (styrene-butadiene rubber) were mixed in a mass ratio of 96:0.5:0.4:1.3:1.8, and deionized water was added to obtain the negative electrode slurry. The solid content of the negative electrode slurry was 50%. A 6µm thick copper foil was used as the negative electrode current collector. The negative electrode slurry was coated onto both sides of the negative electrode current collector, with a double-sided coating density of 7.5 mg / cm². 2 After coating, drying yields a negative electrode current collector coated with negative electrode active material layers on both sides.

[0064] Using the solid electrolyte layer slurry obtained in step (2), the solid electrolyte layer slurry is coated onto the side of the negative electrode active material layer away from the negative electrode current collector, and dried to obtain a negative electrode sheet. The total thickness of the solid electrolyte layer on both sides of the negative electrode sheet is 4 μm.

[0065] (4) The diaphragm is made of PE (polyethylene). In an argon-filled glove box (H2O content ≤1ppm), the positive electrode, diaphragm, negative electrode and electrolyte are assembled into a soft-pack secondary battery. The electrolyte injection volume is 3.9g / Ah.

[0066] W represents the percentage of vinylene carbonate by mass in the electrolyte; H1 represents the thickness of the solid electrolyte layer on the negative electrode side, in μm; H2 represents the thickness of the solid electrolyte layer on the positive electrode side, in μm; S is the key factor, S = (molar amount of Mn in the positive electrode active material / molar amount of Fe in the positive electrode active material) 1 / 2 Calculate W×100×(H1+H2) / S=7.8.

[0067] Figure 1 This is a schematic diagram of the secondary battery obtained in this embodiment after charging and discharging. The following is a description of its structure in conjunction with... Figure 1 The secondary battery fabricated in this embodiment includes a positive electrode, a negative electrode, and a separator 8. The negative electrode comprises a negative current collector 1, a negative active material layer 3, and a solid electrolyte layer 7. The positive electrode comprises a positive current collector 2, a positive active material layer 4, and a solid electrolyte layer 7. The solid electrolyte layer 7 includes a solid electrolyte 701 and a second additive 702. During the charging and discharging process of the secondary battery, an SEI film 5 is formed between the negative active material layer 3 and the solid electrolyte layer 7 in the negative electrode; and an CEI film 6 is formed between the positive active material layer 4 and the solid electrolyte layer 7 in the positive electrode. Furthermore, during the charging and discharging process, manganese ions 401 dissolve from the positive active material in the positive active material layer 4. The solid electrolyte layer 7 prevents the manganese ions 401 from dissolving out of the positive electrode. Simultaneously, the second additive 702 also captures the manganese ions 401.

[0068] Example 2 This embodiment provides a secondary battery and its preparation method. Compared with Embodiment 1, the difference is that in step (1), the mass content of the first additive is 4.41% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 67.12%, the mass content of the positive electrode film-forming additive is 11.34%, and the mass content of the high-temperature film-forming additive is 21.54%; in step (2), an equal mass of LiMn is used. 0.1 Fe 0.9 PO4 as a substitute for LiMn 0.9 Fe 0.1 PO4, the total thickness of the solid electrolyte layer on both sides of the positive electrode is 1 μm; in step (3), the total thickness of the solid electrolyte layer on both sides of the negative electrode is 1 μm; calculate W×100×(H1+H2) / S=7.7.

[0069] Example 3 This embodiment provides a secondary battery and its preparation method. Compared with Embodiment 1, the difference is that in step (1), the mass content of the first additive is 4.94% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 63.16%, the mass content of the positive electrode film-forming additive is 18.83%, and the mass content of the high-temperature film-forming additive is 18.02%; in step (2), an equal mass of LiMn is used. 0.6 Fe 0.4 PO4 as a substitute for LiMn 0.9 Fe 0.1 PO4, the total thickness of the solid electrolyte layer on both sides of the positive electrode is 2μm; in step (3), the total thickness of the solid electrolyte layer on both sides of the negative electrode is 2μm; calculate W×100×(H1+H2) / S=7.8.

[0070] Example 4 This embodiment provides a secondary battery and its preparation method. Compared with embodiment 3, the difference is that in step (1), the mass content of the first additive is 4.72% based on the total mass percentage of the electrolyte; the mass content of the negative electrode film-forming additive in the first additive is 66.10%, the mass content of the positive electrode film-forming additive is 15.89%, and the mass content of the high temperature film-forming additive is 18.01%; LiFSI is also added to the electrolyte, the concentration of LiPF6 is 0.9mol / L, and the concentration of LiFSI is 0.1mol / L; calculate W×100×(H1+H2) / S=7.8.

[0071] Example 5 This embodiment provides a secondary battery and its preparation method. Compared with Example 4, the difference is that in step (1), the concentration of LiPF6 in the electrolyte is 0.7 mol / L and the concentration of LiFSI is 0.3 mol / L.

[0072] Example 6 This embodiment provides a secondary battery and its preparation method. Compared with embodiment 5, the difference is that in step (2), the first binder, the second additive, and the solid electrolyte are taken in a mass ratio of 1.5:0.23:98.27.

[0073] Example 7 This embodiment provides a secondary battery and its preparation method. Compared with embodiment 5, the difference is that in step (2), the first binder, the second additive, and the solid electrolyte are taken in a mass ratio of 1.5:0.38:98.12.

[0074] Example 8 This embodiment provides a secondary battery and its preparation method. Compared with Embodiment 5, the difference is that in step (1), the volume ratio of DMC, EMC, EC, and PC in the organic solvent is 65:15:10:10; the mass content of the first additive is 6.03% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 64.68%, the mass content of the positive electrode film-forming additive is 20.73%, and the mass content of the high-temperature film-forming additive is 14.59%; in step (2), the positive electrode active material is LiMn in a mass ratio of 5:5. 0.6 Fe 0.4 PO4 and LiMnO2; calculate W×100×(H1+H2) / S=7.8.

[0075] Example 9 This embodiment provides a secondary battery and its preparation method. Compared with Embodiment 8, the difference is that in step (1), the mass content of the first additive is 4.86% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 70.99%, the mass content of the positive electrode film-forming additive is 15.43%, and the mass content of the high-temperature film-forming additive is 13.58%; in step (2), the positive electrode active material is LiMn with a mass ratio of 6:4. 0.6 Fe 0.4 PO4 and LiMnO2; calculate W×100×(H1+H2) / S=7.8.

[0076] Example 10 This embodiment provides a secondary battery and its preparation method. Compared with embodiment 9, the difference is that in step (1), the mass content of the first additive is 5.09% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 72.30%, the mass content of the positive electrode film-forming additive is 14.73%, and the mass content of the high temperature film-forming additive is 12.97%; calculate W×100×(H1+H2) / S=8.3.

[0077] Example 11 This embodiment provides a secondary battery and its preparation method. Compared with embodiment 9, the difference is that in step (1), the mass content of the first additive is 4.64% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 69.61%, the mass content of the positive electrode film-forming additive is 16.16%, and the mass content of the high temperature film-forming additive is 14.22%; calculate W×100×(H1+H2) / S=7.3.

[0078] Example 12 This embodiment provides a secondary battery and its preparation method. Compared with Example 11, the difference is that in step (1), the concentration of LiPF6 in the electrolyte is 0.9 mol / L and the concentration of LiFSI is 0.6 mol / L; the mass content of the first additive is 2% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 80%, the mass content of the positive electrode film-forming additive is 10%, and the mass content of the high-temperature film-forming additive is 10%; in step (2), the first binder, the second additive, and the solid electrolyte lithium lanthanum zirconium tantalum oxide (LLZTO) are taken in a mass ratio of 10:2.5:87.5, and the total thickness of the solid electrolyte layer on both sides of the positive electrode is 6 μm; in step (3), the thickness of the solid electrolyte layer on both sides of the negative electrode is 4 μm. Calculate W×100×(H1+H2) / S=7.7.

[0079] Example 13 This embodiment provides a secondary battery and its preparation method. Compared with Example 11, the difference is that in step (1), the concentration of LiPF6 in the electrolyte is 0.7 mol / L and the concentration of LiFSI is 0.1 mol / L; the mass content of the first additive is 15% based on the total mass percentage of the electrolyte; in the first additive, the mass content of the negative electrode film-forming additive is 60%, the mass content of the positive electrode film-forming additive is 20%, and the mass content of the high-temperature film-forming additive is 20%; in step (2), the total thickness of the solid electrolyte layer on both sides of the positive electrode is 1.8 μm; in step (3), the negative electrode does not contain a solid electrolyte layer. Calculate W×100×(H1+H2) / S=7.8.

[0080] Comparative Example 1 This comparative example provides a secondary battery and its preparation method. Compared with Example 1, the difference is that in step (1), there is no negative electrode film-forming additive. In the first additive, the mass content of the positive electrode film-forming additive is 48.36% and the mass content of the high temperature film-forming additive is 51.64%.

[0081] Comparative Example 2 This comparative example provides a secondary battery and its preparation method. Compared with Example 1, the difference is that in step (1), there is no positive electrode film-forming additive. In the first additive, the mass content of the negative electrode film-forming additive is 82.06%, and the mass content of the high temperature film-forming additive is 17.94%.

[0082] Comparative Example 3 This comparative example provides a secondary battery and its preparation method. Compared with Example 1, the difference is that in step (1), there is no high-temperature film-forming additive. In the first additive, the mass content of the negative electrode film-forming additive is 83.70% and the mass content of the positive electrode film-forming additive is 16.30%.

[0083] Comparative Example 4 This comparative example provides a secondary battery and its preparation method. Compared with Example 1, the difference is that in step (1), FEC of equal mass is used instead of VC.

[0084] Comparative Example 5 This comparative example provides a secondary battery and its preparation method. Compared with Example 1, the difference is that in step (2), no solid electrolyte layer slurry is prepared and no solid electrolyte layer is prepared on the positive electrode sheet; in step (3), no solid electrolyte layer is prepared on the negative electrode sheet.

[0085] Comparative Example 6 This comparative example provides a secondary battery and its preparation method. Compared with Example 2, the difference is that in step (2), an equal mass of LiMn is used. 0.1 Fe 0.9 PO4 can replace LiMnO2.

[0086] Experimental Example 1 The cycle capacity retention rate of the secondary batteries prepared in the test examples and comparative examples after 2500 cycles at 25°C, the cycle capacity retention rate after 1200 cycles at 45°C, the residual capacity and recoverable capacity after 60 days of storage at 60°C, the trigger temperature of thermal runaway, the 4C constant current charging ratio, the low temperature discharge capacity at -20°C, and the Mn dissolution content of the negative electrode are all tested. The specific test methods are as follows, and the data obtained are shown in Table 1.

[0087] Cyclic capacity retention rate after 2500 cycles at 25℃ (25℃ retention rate @ 2500cls): The secondary battery was connected to the Xinwei Battery Testing System and charged at 25℃ with a constant current density of 1C to the cutoff voltage of 4.2V. Then, it was charged with a constant voltage to the cutoff current of 0.05C. After standing for 60 minutes, it was discharged with a constant current density of 1C to the cutoff voltage of 2.8V. The above steps were repeated for 2500 cycles. The discharge specific capacity of the first cycle was measured as Q1, and the discharge specific capacity of the 2500th cycle was measured as Q2. The cycle capacity retention rate was calculated as Q2 / Q1×100%.

[0088] Cycle retention rate after 1200 cycles at 45℃ (45℃ retention rate @ 1200cls): The secondary battery was connected to the Xinwei Battery Test System and charged at 45℃ with a constant current density of 1C to the cutoff voltage of 4.2V. Then it was charged with a constant voltage to the cutoff current of 0.05C, left to stand for 60 minutes, and discharged with a constant current density of 1C to the cutoff voltage of 2.8V. The above steps were repeated for 1200 cycles. The discharge specific capacity of the first cycle was measured as Q3, and the discharge specific capacity of the 2500th cycle was measured as Q4. The cycle capacity retention rate was calculated as Q4 / Q3×100%.

[0089] Residual and recoverable capacity after 60 days of storage at 60℃: Under normal temperature (25℃), the secondary battery was connected to the Xinwei Battery testing system and cycled 3 times with a charge / discharge cutoff voltage of 4.2~2.8V and a charge / discharge current density of 0.33C0 (C0 is the nominal capacity of the secondary battery cell, the same below). The average capacity was recorded as C1. Then, the secondary battery stored at 60℃ for 60 days was discharged to 2.8V at 0.33C1, and the resulting discharge capacity was recorded as C2. After standing for 60 minutes, the secondary battery was charged to 4.2V at a constant current of 0.33C1, and then charged at a constant voltage to the cutoff current of 0.05C. After standing for 60 minutes, the cell was discharged to 2.8V at a constant current of 0.33C1, and this cycle was repeated 3 times. The average discharge capacity was recorded as C3. Residual capacity = C2 / C1 × 100%, recoverable capacity = C3 / C1 × 100%.

[0090] 4C constant current charging ratio: Under normal temperature (25℃) conditions, the secondary battery is connected to the Xinwei battery testing system and charged and discharged for 3 cycles at a charge / discharge cutoff voltage of 4.2~2.8V and a charge / discharge current density of 0.33C. The average capacity is recorded as C12. The secondary battery is then discharged to 2.8V at 0.33C12, left to stand for 60 minutes, charged to 4.2V at 4.0C11, left to stand for 60 minutes, and then discharged to 2.8V at 0.33C11. The discharge capacity is recorded as C22. The 4C constant current charging ratio = C22 / C12 × 100%.

[0091] -20℃ Low Temperature Discharge Capacity Ratio: Under ambient temperature (25℃), the secondary battery was connected to the Xinwei Battery Testing System and cycled 3 times with a charge / discharge cutoff voltage of 4.2~2.8V and a charge / discharge current density of 0.33C. The average capacity was recorded as C13. The secondary battery was placed in a high and low temperature chamber. Under ambient temperature (25℃), it was discharged to 2.8V at 0.33C13, allowed to stand for 60 minutes, charged to 4.2V at a constant current of 1C13, and then charged at a constant voltage to the cutoff current of 0.05C13, allowed to stand for 60 minutes. The ambient temperature was adjusted to -20℃, allowed to stand for 30 minutes, and then discharged to 2.8V at 1C13. The discharge capacity was obtained as C23. The -20℃ low temperature discharge capacity ratio = C23 / C13 × 100%.

[0092] Trigger temperature of thermal runaway (stepped thermal chamber test): Under normal temperature of 25℃, the secondary battery is connected to the Xinwei battery test system and charged and discharged for 3 cycles at a charge and discharge cutoff voltage of 4.2~2.8V and a charge and discharge current density of 0.33C. The average capacity is recorded as C14. At room temperature (25℃), the battery was discharged to 2.8V with 0.33C14, allowed to stand for 60 minutes, charged to 4.2V with a constant current of 0.33C14, and charged to the cutoff current of 0.05C14 with a constant voltage. The secondary battery was then placed in a heating oven, and the oven was heated to 130℃ at a rate of 5℃ / min, and then allowed to stand at this temperature for 30 minutes. Subsequently, the secondary battery was heated to different temperature gradients at a rate of 5℃ / min and allowed to stand for 30 minutes. The temperature point at which the secondary battery experienced thermal runaway was defined as the thermal runaway trigger temperature point. The specific temperature gradient was in the range of 130℃~250℃, with each gradient being 5℃. For example, starting from 130℃, the temperature was increased to 135℃ and allowed to stand for 30 minutes, then increased to 140℃ and allowed to stand for 30 minutes, and so on.

[0093] Table 1

[0094] As shown in Table 1, the secondary batteries provided in the embodiments of this application possess excellent room temperature cycling performance, high temperature cycling performance, high temperature recoverable capacity, thermal runaway temperature, fast charging performance, and low temperature discharge capacity. The secondary battery provided in Comparative Example 1 does not contain negative electrode film-forming additives in its electrolyte; Comparative Example 2 does not contain positive electrode film-forming additives in its electrolyte; Comparative Example 3 does not contain high-temperature film-forming additives in its electrolyte; Comparative Example 4 does not use vinylene carbonate as a negative electrode film-forming additive; Comparative Example 5 does not contain a solid electrolyte layer in either the positive or negative electrode; and Comparative Example 6 uses only lithium manganese iron phosphate as the positive electrode active material. Compared with the secondary batteries provided in the embodiments, the secondary batteries provided in Comparative Examples 1 to 5 show significantly worse cycle retention rates at 25°C and 45°C, performance after 60 days of storage at 60°C, thermal runaway temperature, 4C constant current charging ratio, and -20°C low-temperature discharge capacity. The secondary battery provided in Comparative Example 6, although showing no significant change in cycle retention and high-temperature performance compared to the examples, exhibits a marked decrease in fast-charging performance and low-temperature discharge capacity, failing to simultaneously possess cycle performance, low-temperature performance, safety performance, and fast-charging performance. In contrast, the secondary battery provided in this application, through the use of negative electrode film-forming additives, positive electrode film-forming additives, and high-temperature film-forming additives in the electrolyte (the negative electrode film-forming additives include vinylene carbonate), positive electrode active materials including lithium iron manganese phosphate and lithium manganese oxide, and the combination of at least one of the positive and negative electrode plates including a solid electrolyte layer, achieves a synergistic effect of the beneficial properties of lithium iron manganese phosphate and lithium manganese oxide, simultaneously possessing excellent cycle performance, low-temperature performance, safety performance, and fast-charging performance.

[0095] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

Claims

1. A secondary battery, characterized in that, Includes positive electrode, negative electrode, and electrolyte; The electrolyte includes a first additive; The first additive includes negative electrode film-forming additives, positive electrode film-forming additives, and high-temperature film-forming additives; The negative electrode film-forming additive includes vinylene carbonate; The positive electrode sheet includes a positive current collector and a positive active material layer located on at least one side of the positive current collector. The positive active material layer includes a positive active material, which includes lithium iron manganese phosphate and lithium manganese oxide. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; At least one of the positive electrode and the negative electrode further includes a solid electrolyte layer, wherein the solid electrolyte layer is located on the side of the positive active material layer away from the positive current collector and / or on the side of the negative active material layer away from the negative current collector.

2. The secondary battery according to claim 1, characterized in that, The secondary battery conforms to the following relationship: 7.3≤W×100×(H1+H2) / S≤8.3, Wherein, W is the mass percentage of vinylene carbonate in the electrolyte; H1 is the numerical value of the solid electrolyte layer thickness on the negative electrode side, and the unit of the solid electrolyte layer thickness on the negative electrode side is μm; H2 is the numerical value of the solid electrolyte layer thickness on the positive electrode side, and the unit of the solid electrolyte layer thickness on the positive electrode side is μm; S is the key factor, S = (molar amount of Mn element in the positive electrode active material / molar amount of Fe element in the positive electrode active material). 1 / 2 .

3. The secondary battery according to claim 1 or 2, characterized in that, In the electrolyte, the mass content of the first additive is 0.1% to 15%; And / or, in the first additive, the mass content of the positive electrode film-forming additive is 8%~22%; And / or, in the first additive, the mass content of the negative electrode film-forming additive is 60%~80%; And / or, in the first additive, the mass content of the high-temperature film-forming additive is 10%~22%; And / or, in the first additive, the mass content of the vinylene carbonate is 30% to 70%; And / or, the mass ratio of lithium iron manganese phosphate to lithium manganese oxide in the positive electrode active material is 5:5 to 9:1; And / or, the chemical formula of the lithium iron manganese phosphate is LiMn x Fe y PO4, where x+y=1, 0.1≤x≤0.

9.

4. The secondary battery according to claim 1 or 2, characterized in that, The negative electrode film-forming additive also includes at least one of 1,3-propanesulfonate lactone, fluoroethylene carbonate, and vinyl ethylene carbonate. And / or, the positive electrode film-forming additive includes at least one of tris(trimethylsilane) phosphate, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and lithium difluoro(oxalato)phosphate. And / or, the high-temperature film-forming additive includes at least one of vinyl sulfate, hexamethylene diisocyanate, methylene disulfonate, hexanetrionitrile, and propylene sulfonate lactone; And / or, the electrolyte may also include lithium salts and organic solvents.

5. The secondary battery according to claim 4, characterized in that, The concentration of lithium salt in the electrolyte is 0.8~1.5 mol / L; And / or, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorosulfonamide, lithium difluoroborate, and lithium tetrafluoroborate; And / or, the organic solvent includes any two or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propylene carbonate.

6. The secondary battery according to claim 5, characterized in that, In the electrolyte, lithium bisfluorosulfonamide accounts for 10% to 40% of the total concentration of all lithium salts; And / or, in the electrolyte, the sum of the volumes of methyl ethyl carbonate and dimethyl carbonate accounts for 50% to 80% of the total volume of all organic solvents; And / or, in the electrolyte, dimethyl carbonate accounts for 15% to 60% of the total volume of all organic solvents.

7. The secondary battery according to claim 1 or 2, characterized in that, The thickness of the solid electrolyte layer is 1~6μm; And / or, the solid electrolyte layer includes a solid electrolyte and a first binder; And / or, the positive electrode active material layer further includes a second binder and a conductive agent; And / or, the negative electrode active material layer includes a negative electrode active material, a third binder, and a conductive agent; And / or, the solid electrolyte layer includes a second additive, the structure of which is shown in Formula I. Formula I; Wherein, Y includes H, or a chelating group; The chelating agent corresponding to the chelating group includes , , , , , , , , , At least one of them.

8. The secondary battery according to claim 7, characterized in that, The solid electrolyte includes at least one of lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, lithium lanthanum zirconium oxide, lithium lanthanum zirconium tantalum oxide, and lithium lanthanum titanate. And / or, the negative electrode active material includes at least one of silicon-based materials and carbon-based materials; And / or, the first adhesive, the second adhesive, and the third adhesive are each independently selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, lithium polyacrylate, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

9. The secondary battery according to claim 7, characterized in that, Based on the total mass of the solid electrolyte layer, the mass content of the second additive is 15% to 25% of the mass content of the first binder; And / or, based on the total mass of the solid electrolyte layer, the mass percentage of the first binder is 1% to 10%.

10. A method for preparing a secondary battery as described in any one of claims 1 to 9, characterized in that, The steps include the following: S1: Prepare the electrolyte; S2: Mix the solid electrolyte and the first binder, add the first solvent, and obtain the solid electrolyte layer slurry; S3: Prepare a positive electrode active material layer on at least one side of the surface of the positive electrode current collector, prepare a negative electrode active material layer on at least one side of the surface of the negative electrode current collector, and coat the solid electrolyte layer slurry on the side of the positive electrode active material layer away from the positive electrode current collector and / or the side of the negative electrode active material layer away from the negative electrode current collector to obtain a positive electrode sheet and / or a negative electrode sheet. S4: Assemble the positive electrode, negative electrode, and electrolyte to obtain a secondary battery.

11. The preparation method according to claim 10, characterized in that, The S2 process also includes the addition of a second additive; And / or, in S2, the solid content of the solid electrolyte layer slurry is 60%~70%; And / or, in S2, the first adhesive includes at least one of polyvinylidene fluoride and polyvinylidene fluoride-hexafluoropropylene copolymer; And / or, in S2, the first solvent includes at least one of N,N-dimethylformamide, N-methylpyrrolidone, ethyl acetate, methyl formate, and 1,1,2,2-tetrafluoroethyl-1,1,2,2-tetrafluoropropyl ether.

12. An electrical appliance, characterized in that, The secondary battery includes the secondary battery as described in any one of claims 1 to 9, or the secondary battery prepared by the preparation method as described in claim 10 or 11, wherein the secondary battery serves as the power supply for the electrical device.

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