Secondary battery and electric device

Abstract

The invention discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries, the secondary battery comprises a positive pole piece, the positive pole piece comprises a positive current collector, and a first positive active material layer, a second positive active material layer and a ceramic layer which are sequentially arranged on at least one surface of the positive current collector; the ceramic layer comprises ceramic particles; the porosity of the first positive electrode active material layer is A, the porosity of the second positive electrode active material layer is B, the porosity of the ceramic layer is C, C is larger than or equal to 1.3 * B, and B is larger than or equal to A. The safety performance and the dynamic performance of the secondary battery can be effectively improved, and the cycle performance of the secondary battery is improved.

Description

Technical Field

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

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in electric vehicles, power tools, and 3C consumer electronics products due to their advantages such as high energy density, long cycle life, and environmental friendliness.

[0003] The separator in existing secondary batteries is prone to deformation during the charging and discharging process, which can lead to lithium plating and pose a safety risk.

[0004] Therefore, this application is submitted. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery and power device that can effectively improve the safety performance and dynamic performance of the secondary battery, and improve the cycle performance of the secondary battery.

[0006] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a first positive active material layer, a second positive active material layer and a ceramic layer sequentially disposed on at least one surface of the positive current collector; the ceramic layer includes ceramic particles; The porosity of the first positive electrode active material layer is A, the porosity of the second positive electrode active material layer is B, and the porosity of the ceramic layer is C, satisfying: C≥1.3×B, B≥A.

[0007] As an implementation scheme of this application, at least one of the following (1) to (3) is satisfied: (1) 20%≤A≤50%; (2) 22.5%≤B≤50%; (3) 30%≤C≤65%.

[0008] As an embodiment of this application, the ceramic layer covers an area of ​​10-99% of the surface of the second positive electrode active material layer.

[0009] As an embodiment of this application, the ceramic particles in the ceramic layer have a mass percentage content of 50-95%; and / or The ceramic particles include Li7La3Zr2O 12 Li 1+x Al x Ti 2-x (PO4)3, La 2 / 3-y Li 3y TiO3, Naz At least one of M12 and M23; wherein, 0.2 ≤ x ≤ 0.5; 0 < y < 0.67; 1 ≤ z ≤ 4; M1 includes at least one of Li, Na, K, Ni, Fe, Ca, Ti, Cr, Zn, Ag, Mo, Mg, and Mn, and M2 includes at least one of SiO4, PO4, and SO4.

[0010] As an embodiment of the present application, the ceramic layer further includes polymer particles, and the mass percentage content of the polymer particles in the ceramic layer is 5-50%.

[0011] The first positive electrode active material layer includes a first positive electrode active material; the first positive electrode active material includes a lithium-containing phosphate and a lithium nickel cobalt manganese oxide; the mass ratio of the lithium-containing phosphate to the lithium nickel cobalt manganese oxide is (10-90):(10-90); and / or The second positive electrode active material layer includes a second positive electrode active material; the second positive electrode active material includes a lithium-containing phosphate and a lithium nickel cobalt manganese oxide, and the mass ratio of the lithium-containing phosphate to the lithium nickel cobalt manganese oxide is (10-90):(10-90).

[0012] As an embodiment of the present application, the first positive electrode active material layer further includes a first binder, and the mass percentage content of the first binder in the first positive electrode active material layer is D1; The second positive electrode active material layer further includes a second binder, and the mass percentage content of the second binder in the second positive electrode active material layer is D2; Satisfying: D1 - D2 ≥ 0.2%.

[0013] As an embodiment of the present application, the mass percentage content of the first positive electrode active material in the first positive electrode active material layer is E1; The mass percentage content of the second positive electrode active material in the second positive electrode active material layer is E2; Satisfying: E2 - E1 ≥ 0.2%.

[0014] As an embodiment of the present application, at least one of the following (4)-(7) is satisfied: (4) 1.0% ≤ D1 ≤ 5%; (5) 0.8% ≤ D2 ≤ 4.8%; (6) 95% ≤ E1 ≤ 97.5% (7) 95.5% ≤ E2 ≤ 97.8%.

[0015] As an embodiment of this application, the thickness of the first positive electrode active material layer is t1, the thickness of the second positive electrode active material layer is t2, and the thickness of the ceramic layer is t3, satisfying: t2≥t1≥3t3.

[0016] As an implementation scheme of this application, at least one of the following (8) to (10) is satisfied: (8) 3μm≤t1≤80μm; (9) 3μm≤t2≤100μm; (10) 1μm≤t3≤10μm.

[0017] A second aspect of this application provides an electrical device comprising the aforementioned secondary battery.

[0018] The beneficial effects of this application are as follows: By sequentially setting a first positive electrode active material layer, a second positive electrode active material layer, and a ceramic layer on the surface of the positive electrode current collector, and controlling their porosity to satisfy: C≥1.3×B, B≥A. The porosity of the first positive electrode active material layer, the second positive electrode active material layer, and the ceramic layer gradually increases, which can effectively improve the liquid absorption capacity of the positive electrode sheet, improve the wettability of the electrolyte to the positive electrode sheet, improve the lithium ion transport rate, improve the structural stability of the positive electrode sheet, reduce the lithium deposition potential, thereby reducing lithium ion deposition on the surface of the negative electrode sheet, effectively improving the safety performance and kinetic performance of the secondary battery, and improving the cycle performance of the secondary battery. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet of this application.

[0020] Figure 2 This is another structural schematic diagram of the positive electrode sheet of this application. Detailed Implementation

[0021] 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.

[0022] 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.

[0023] 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.

[0024] like Figure 1 As shown, this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive current collector 1 and a first positive active material layer 2, a second positive active material layer 3 and a ceramic layer 4 sequentially disposed on at least one surface of the positive current collector; the ceramic layer includes ceramic particles; The porosity of the first positive electrode active material layer is A, the porosity of the second positive electrode active material layer is B, and the porosity of the ceramic layer is C, satisfying: C≥1.3×B, B≥A.

[0025] This application, by sequentially depositing a first positive electrode active material layer, a second positive electrode active material layer, and a ceramic layer on the surface of the positive electrode current collector, and controlling their porosity to satisfy: C≥1.3×B, B≥A, with the porosity of the first positive electrode active material layer, the second positive electrode active material layer, and the ceramic layer gradually increasing, can effectively improve the liquid absorption capacity of the positive electrode sheet, improve the wettability of the electrolyte on the positive electrode sheet, improve the lithium ion transport rate, improve the structural stability of the positive electrode sheet, reduce the lithium deposition potential, thereby reducing lithium ion deposition on the surface of the negative electrode sheet, effectively improving the safety performance and kinetic performance of the secondary battery, and improving the cycle performance of the secondary battery.

[0026] The porosity testing method mentioned in this application is as follows: The secondary battery is charged to 3.65V at a constant current and voltage of 0.5C, with a cutoff current of 0.05C, and left to stand for 10 minutes. The secondary battery is then disassembled in a drying room to obtain the positive electrode sheet. Argon ion polishing technology is used to bombard the cross-section of the positive electrode sheet sample to obtain a flat cross-section. The sample is placed in the sample stage of an SEM, and the upper and lower electrode areas in the cross-section are selected respectively. Utilizing the color differences at the pores and active material locations, the area ratio of the darker regions (pores) is statistically analyzed using image processing software to calculate the porosity.

[0027] In some embodiments, 20% ≤ A ≤ 50%, for example, can be a range of 20%, 25%, 28%, 30%, 35%, 40%, 45%, 50%, or any two of these values. By controlling A within this range, the first active material layer near the positive current collector has a lower porosity, which is beneficial to improving the energy density of the positive electrode.

[0028] In some embodiments, 22.5%≤B≤50%, for example, can be a range of 22.5%, 25%, 30%, 35%, 40%, 45%, 50%, or any two of these values, so that the positive electrode absorbs more electrolyte, thereby promoting lithium-ion transport and diffusion on the surface electrode, reducing the lithium plating potential, thereby reducing lithium ion plating on the surface of the negative electrode, and improving the fast charging performance and battery life of the lithium-ion battery.

[0029] In some embodiments, 30%≤C≤65%, for example, can be a range of 30%, 35%, 39.5%, 44.5%, 49.5%, 54.5%, 59.5%, 60%, 62%, 65%, or any two of these values. By controlling C within this range, the ceramic layer away from the positive electrode current collector has a higher porosity, allowing the positive electrode to absorb more electrolyte, ensuring rapid transport and diffusion of lithium ions, i.e., increasing the lithium ion diffusion rate, effectively reducing impedance, effectively improving the safety and kinetic performance of the secondary battery, and ultimately improving the storage and cycle performance of the secondary battery.

[0030] like Figure 2 As shown, the coverage area of ​​the ceramic layer on the surface of the second positive electrode active material layer is 10-99%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, or any two of these values. By controlling the coverage area of ​​the ceramic layer on the surface of the second positive electrode active material layer within this range, the adhesion between the ceramic layer and the separator can be improved, making it less prone to deformation during charging and discharging that could lead to interface adhesion failure. At the same time, it effectively improves the diffusion rate of lithium ions, effectively improves the safety and kinetic performance of the secondary battery, and improves the cycle performance of the secondary battery.

[0031] In some of these embodiments, the mass percentage of the ceramic particles in the ceramic layer is 50% to 95%, for example, it can be 50%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or the range composed of any two of these values. By controlling the mass percentage of the ceramic particles in the ceramic layer within this range, the adhesion force between the positive electrode sheet and the separator can be improved, the deformation of the positive electrode sheet during charge and discharge can be avoided, the lithium ion transmission rate can be increased, the safety performance and kinetic performance of the secondary battery can be improved, and the cycle performance of the secondary battery can be improved.

[0032] The ceramic layer further includes polymer particles, and the mass percentage of the polymer particles in the ceramic layer is 5% to 50%, for example, it can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or the range composed of any two of these values.

[0033] In some of these embodiments, the ceramic particles include at least one of Li7La3Zr2O 12 , Li 1+x Al x Ti 2-x (PO4)3, La 2 / 3-y Li 3y TiO3, Na z M12M23; where, 0.2 ≤ x ≤ 0.5; 0 < y < 0.67; 1 ≤ z ≤ 4; M1 includes at least one of Li, Na, K, Ni, Fe, Ca, Ti, Cr, Zn, Ag, Mo, Mg, Mn, and M2 includes at least one of SiO4, PO4, SO4.

[0034] In some of these embodiments, the glass transition temperature of the polymer particles is -105°C to 25°C, for example, it can be -105°C, -100°C, -90°C, -80°C, -70°C, -60°C, -50°C, -40°C, -30°C, -20°C, -10°C, -0°C, 10°C, 20°C, 25°C or the range composed of any two of these values. By controlling the glass transition temperature of the polymer particles within this range, the flexibility of the positive electrode sheet can be improved, and the cracking phenomenon during the coating process can be avoided.

[0035] In some of these embodiments, the softening temperature of the polymer particles ≥ 45°C, which is beneficial to improving the stability of the positive electrode sheet.

[0036] In some embodiments, the Dv50 particle size of the polymer particles is 5~50μm, for example, it can be 5μm, 7μm, 8μm, 10μm, 12μm, 15μm, 20μm, 40μm, 50μm or any two of these values. By controlling the Dv50 particle size of the polymer particles within this range, it is beneficial to improve the adhesion between the polymer particles and the membrane, improve the dispersion effect, and promote its uniform dispersion in the ceramic layer.

[0037] In some embodiments, the Dv50 particle size of the polymer particles is 7~15μm, for example, it can be 7μm, 8μm, 10μm, 12μm, 15μm or any two of these values.

[0038] The glass transition temperature of the polymer particles can be measured using methods known in the art. For example, it can be determined using the Dynamic Mechanical Loss (DMA) method.

[0039] The softening point of the polymer particles can be measured using methods known in the art. For example, it can be obtained using differential scanning calorimetry (DSC).

[0040] In some embodiments, the polymer particles include at least one of polyimide, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, polyamide, polyurethane, polymethyl methacrylate, ethylene-vinyl acetate copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

[0041] The method for testing the type and content of polymer particles in the ceramic layer is as follows: The type of polymer is determined by infrared testing, that is, by detecting the type and intensity of characteristic peaks in infrared radiation, thus determining the molecular structure of the polymer. The polymer content is tested by TG-DSC. After determining the thermal decomposition temperature of the polymer by TG-DSC, the thermogravimetric loss at that decomposition temperature is calculated. Polymer content = thermogravimetric loss at polymer decomposition temperature / total weight of the test sample.

[0042] In some embodiments, the first positive electrode active material layer comprises a first positive electrode active material; the first positive electrode active material comprises lithium phosphate and lithium nickel cobalt manganese oxide; the mass ratio of the lithium phosphate and lithium nickel cobalt manganese oxide is (10~90):(10~90), and / or The second positive electrode active material layer includes a second positive electrode active material; the second positive electrode active material includes lithium phosphate and lithium nickel cobalt manganese oxide, wherein the mass ratio of the lithium phosphate and lithium nickel cobalt manganese oxide is (10~90):(10~90).

[0043] In some embodiments, the mass percentage of the first positive electrode active material in the first positive electrode active material layer is E1; The mass percentage of the second positive electrode active material in the second positive electrode active material layer is E2; Satisfy: E2-E1≥0.2%.

[0044] In some embodiments, the following condition is satisfied: 95% ≤ E1 ≤ 97.5%, for example, it can be a range of 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, or any two of these values. Since the first positive electrode active material layer is close to the current collector, too much positive electrode active material (i.e., too little binder) results in low adhesion, causing the material to detach at the interface during charging and discharging due to expansion. Too little active material affects energy density. By controlling the mass percentage of the first positive electrode material in the first positive electrode active material layer within this range, both kinetic performance and adhesion between the first active layer and the current collector can be balanced, ensuring that the active material does not detach. The mass percentage of active material in the first active layer needs to balance the adhesion between the positive electrode active material and the current collector, as well as energy density.

[0045] In some embodiments, the following condition is met: 95.5% ≤ E2 ≤ 97.8%. The second active layer has a higher mass percentage of active material because it is relatively less prone to detachment, thus reducing the amount of adhesive used. Furthermore, the increase in binder has a positive impact on the battery's kinetics, further improving the battery's energy density and kinetics, and reducing the impedance of the positive electrode.

[0046] In some embodiments, the Dv50 particle size of the first positive electrode active material is 6~15μm, for example, it can be 6μm, 7μm, 8μm, 8μm, 10μm, 12μm, 15μm or any two of these values. The Dv50 particle size of the second positive electrode active material is 4~20μm, for example, it can be 4μm, 5μm, 6μm, 8μm, 10μm, 15μm, 20μm or any two of these values. By controlling the Dv50 particle size of the first and second positive electrode active materials within this range, the kinetics of the second positive electrode active material layer are superior to those of the first positive electrode active material layer. This is beneficial for further improving the lithium ion diffusion rate, reducing side reactions between the positive electrode and the electrolyte, reducing gas generation, and improving the safety and kinetic performance of the secondary battery, as well as its cycle performance.

[0047] In some embodiments, the first positive electrode active material layer further includes a first binder, wherein the mass percentage of the first binder in the first positive electrode active material layer is D1. The second positive electrode active material layer also includes a second binder, the second binder having a mass percentage of D2 in the second positive electrode active material layer; satisfying: D1-D2≥0.2%. By controlling D1-D2≥0.2%, the adhesion between the positive electrode current collector and the first positive electrode active material layer can be effectively improved, the film layer of the positive electrode sheet can be prevented from falling off, the lithium ion transport efficiency can be improved, and the dynamic performance of the secondary battery can be enhanced.

[0048] In some embodiments, 1.0% ≤ D1 ≤ 5%, for example, can be a range consisting of 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any two of these values; 0.8% ≤ D2 ≤ 4.8%, for example, can be a range consisting of 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 4.8%, or any two of these values. In some embodiments, 0.8% ≤ D2 ≤ 3%. In some embodiments, the first positive electrode active material layer further includes a first conductive agent, and the second positive electrode active material layer further includes a second conductive agent.

[0049] In some embodiments, the thickness of the first positive electrode active material layer is t1, the thickness of the second positive electrode active material layer is t2, and the thickness of the ceramic layer is t3, satisfying: t1≥t2≥3t3. By controlling t1≥t2≥3t3, it is beneficial to improve the adhesion between the positive electrode sheet and the separator, avoid short circuit between the positive and negative electrodes, avoid the risk of thermal diffusion, and optimize the lithium-ion transport efficiency, taking into account the kinetics, safety performance and cycle performance of the secondary battery.

[0050] In some embodiments, the lithium-containing phosphates mentioned in this application include those with the general chemical formula Li. a1 Fe 1- b1 M1 b1 Compounds of PO4, wherein 0.8 ≤ a1 ≤ 1.2, 0 ≤ b1 ≤ 0.9, and M1 is selected from at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Ti, V, Mg, and Al.

[0051] In some embodiments, the lithium nickel cobalt manganese oxide mentioned in this application includes the chemical formula Li d Ni a2 Co b2 Mn c M3 (1-a2-b2-c1)The compound represented by O2, wherein 0.85≤d≤1.1, 0.6≤a2≤0.96, 0.03≤b2≤0.08, 0.01≤c≤0.32, a1+b1+c≤1, wherein M3 includes at least one of Zr, B, Al, Ba, Sr, Y, Ti, Mg, Ca, Mo, Sn, Nb, Ce, Sb or W.

[0052] In some implementations, 3μm≤t1≤80μm, for example, can be a range of 3μm, 7μm, 15μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm or any combination thereof. By controlling t1 within this range, the energy density and kinetics of the lithium-ion battery can be balanced.

[0053] In some embodiments, 3μm≤t2≤100μm, for example, can be a range of 3μm, 7μm, 15μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm or any combination thereof; the second positive electrode active material layer has better kinetic performance due to the reduced amount of binder and increased porosity. By controlling t2 within this range, both kinetics and energy density can be taken into account.

[0054] In some implementations, 1μm≤t3≤10μm, for example, can be a range of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or any combination thereof. By controlling t3 within this range, both energy density and the safety performance of the secondary battery can be taken into account.

[0055] In this application, there are no particular limitations on the positive electrode current collector, which can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper. In one embodiment, the positive electrode current collector is a metallic material. In one embodiment, the positive electrode current collector is aluminum.

[0056] There are no particular restrictions on the form of the positive electrode current collector. When the positive electrode current collector is a metallic material, it can be in the form of metal foil, metal cylinder, metal strip, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, it can be in the form of carbon plate, carbon film, carbon cylinder, etc.

[0057] In some embodiments, the secondary battery further includes a negative electrode.

[0058] In some embodiments, the negative electrode sheet includes a negative current collector and a layer of negative active material disposed on at least one surface of the negative current collector.

[0059] In this application, there are no particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collector, etc.

[0060] In some embodiments, the negative electrode active material layer further includes a third conductive agent and a third binder.

[0061] In some embodiments, the negative electrode active material may be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, or spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloys and metallic lithium.

[0062] In some embodiments, the types of the first conductive agent, second conductive agent, and third conductive agent mentioned in this application are not limited, and for example, they can be at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0063] In some embodiments, the types of the first, second, and third adhesives mentioned in this application are not limited. For example, they may be at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydrogenation, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer.

[0064] In some embodiments, the secondary battery includes an electrolyte, and the type of electrolyte is not specifically limited. The electrolyte comprises an electrolyte salt and an organic solvent, and the specific types of the electrolyte salt and organic solvent are not specifically limited and can be selected according to actual needs. The electrolyte may also include additives, and the type of additive is not particularly limited. These additives can be film-forming additives for the positive electrode and / or the positive electrode, or additives that can improve certain battery performance, such as additives that improve the battery's high or low temperature performance.

[0065] In the secondary battery mentioned in this application, a separator is usually provided between the positive electrodes to prevent short circuits. There are no particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effectiveness of this application.

[0066] In some embodiments, the diaphragm comprises a porous sheet-like or nonwoven material with excellent liquid retention properties. Materials for resin or glass fiber diaphragms include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone.

[0067] In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the diaphragm described above can be used alone or in any combination.

[0068] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0069] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0070] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0071] One embodiment of this application provides an electrical device including the secondary battery described above.

[0072] 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.

[0073] Example 1 A method for preparing a secondary battery includes the following steps: (1) Preparation of the positive electrode sheet: Preparation of the first positive electrode active material layer slurry: ternary cathode LiNi 0.68 Co 0.02 Mn 0.3 O2 (NCM) and lithium manganese iron phosphate (LiFe) 0.4 Mn 0.6PO4 (LMFP) is mixed evenly to obtain the first positive electrode active material. The first positive electrode active material is then mixed evenly with conductive carbon black SP, carbon nanotubes CNT, and polyvinylidene fluoride PVDF at a speed of 1500 rpm for 30 min. Then, deionized water is added and the mixture is stirred rapidly at a speed of 2500 rpm for 180 min to obtain the first positive electrode active material slurry.

[0074] In the preparation of the first positive electrode active material layer slurry, by mass parts, (NCM+LMFP):SP:CNT:PVDF=95:2:0.5:2.5, NCM:LMFP=3:7.

[0075] Preparation of the slurry for the second positive electrode active material layer: LiNi 0.68 Co 0.02 Mn 0.3 O2 (NCM) and lithium manganese iron phosphate (LiFe) 0.4 Mn 0.6 PO4 (LMFP) is mixed evenly to obtain the second positive electrode active material. The second positive electrode active material is then mixed evenly with conductive carbon black SP, carbon nanotubes CNT, and polyvinylidene fluoride PVDF at a speed of 1500 rpm for 30 min. Then, deionized water is added and the mixture is stirred rapidly at a speed of 2500 rpm for 180 min to obtain the second positive electrode active material slurry.

[0076] In the preparation of the second positive electrode active material layer slurry, by mass parts, (NCM+LMFP):SP:CNT:PVDF=97.5:0.2:0.8:1.5, NCM:LMFP=7:3.

[0077] Preparation of ceramic layer slurry: Li 1.3 Al 0.3 Ti 1.7 (PO4)3 and polyvinylidene fluoride (PVDF) were mixed uniformly at a mass ratio of 85:15, at a speed of 1500 rpm for 30 min, and then NMP was added and the mixture was stirred rapidly at a speed of 2500 rpm for 180 min to obtain a ceramic slurry.

[0078] The first and second positive electrode active material layer slurries are respectively fed into a double-layer die head. The two slurries are coated onto a 12μm thick positive electrode current collector aluminum foil by a coating machine. The ceramic layer slurry is fed into a dispensing tank and then conveyed to a dispensing head by a screw. The dispensing head extrudes the ceramic layer slurry onto the surface of the second positive electrode active layer. The resulting positive electrode sheet with a three-layer structure is dried in a coating machine oven to prepare a fully dried positive electrode sheet. It is then rolled by a roller press to obtain the positive electrode sheet.

[0079] The coating area of ​​the ceramic layer is achieved by adjusting the gap between the dispensing heads, the opening size of the dispensing heads, and the interval time of intermittent dispensing. In this embodiment, the gap between the dispensing heads is 40mm, the opening is 29mm, the intermittent dispensing time is 8s, the coating area is 40%, and the porosity of the ceramic layer is 55%.

[0080] (2) Preparation of negative electrode sheet: The negative electrode active material (artificial graphite), binder (styrene-butadiene rubber, SBR), conductive agent (conductive carbon black), and thickener (sodium carboxymethyl cellulose, CMC) were dissolved in an aqueous solvent at a mass ratio of 96:2:1:1. The negative electrode slurry was prepared by high-speed stirring in a continuous disperser at 2500 rpm for 20 cycles. The negative electrode slurry was coated onto a 4.5 μm negative electrode copper foil using a coating machine and dried. The negative electrode sheet was then rolled by a roller press. (3) Separator: 9μm PE membrane; (4) Preparation of electrolyte: At room temperature, in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethylene carbonate EC and dimethyl carbonate DMC are mixed evenly at a mass ratio of 50:50, and then 1mol / L lithium salt LiPF6 is added and stirred evenly to obtain the electrolyte.

[0081] (5) Assembly of secondary battery: The prepared positive electrode, separator and negative electrode are stacked in the order of negative electrode / separator / positive electrode / separator, so that the separator is in the middle of the negative electrode and positive electrode. Finally, the negative electrode and separator are finished off, with half a turn / one turn more respectively to ensure that each layer of positive electrode can be aligned with the negative electrode and each layer of negative electrode is wrapped by the separator. After winding, hot pressing and shaping, the tabs are welded to obtain the bare cell. The bare cell is placed in the outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24h. The electrolyte prepared above is injected into the dried battery, and the battery is allowed to stand, form and be capacity tested to obtain the secondary battery.

[0082] The detailed parameters of the positive electrode are shown in Tables 1 to 3.

[0083] Examples 2-6, Comparative Examples 1-4 The difference between Examples 2-6 and Comparative Examples 1-4 and Example 1 is that the porosity is changed by altering the composition or Dv50 particle size of the first positive electrode active material, the composition or Dv50 particle size of the second positive electrode active material layer, and the Dv50 particle size of the ceramic particles, as shown in Tables 2-3.

[0084] Examples 7-10 The difference between Examples 7-10 and Example 1 is that the coverage area of ​​the ceramic layer on the surface of the second positive electrode active material layer is adjusted by adjusting the gap between the dispensing heads, the opening width of the dispensing head, and the dispensing interval.

[0085] The gaps between the dispensing heads in Examples 7-10 are 35mm, 30mm, 45mm, and 1mm, respectively.

[0086] The opening widths of the dispensing heads in Examples 7-10 are 32mm, 37mm, 10mm, and 99mm, respectively.

[0087] The dispensing intervals for Examples 7-10 were 6s, 4s, 15s, and 0s, respectively.

[0088] The detailed parameters of Examples 7 to 10 are shown in Tables 1 to 3.

[0089] Examples 11-12 The difference between Examples 11 and 12 and Example 1 is that the type of ceramic particles is changed.

[0090] Example 11: The ceramic particles are of the Li type. 0.33 La 0.56 TiO3.

[0091] Example 12: The ceramic particles are Na3Zr2Si2PO 12 .

[0092] The detailed parameters of Examples 11-12 are shown in Tables 1-3.

[0093] Examples 13-18 The difference between Examples 13-18 and Example 1 is that the thicknesses of the first positive electrode active material layer, the second positive electrode active material layer, and the ceramic layer are changed.

[0094] Examples 19-21 The difference between Examples 19-21 and Example 1 is that the composition ratio of the ceramic layer is changed, as shown in Table 2.

[0095] Examples 22-24 The difference between Examples 22-24 and Example 1 is that the proportions of the first positive electrode active material layer and the second positive electrode active material layer are changed.

[0096] In Example 22, in the preparation of the first positive electrode active material layer slurry, the mass ratio of (NCM+LMFP):SP:CNT:PVDF is 97.5:1.2:0.3:1.

[0097] In Example 22, in the preparation of the second positive electrode active material layer slurry, the mass fraction (NCM+LMFP):SP:CNT:PVDF=97.8:0.4:1.2:0.8.

[0098] In Example 23, in the preparation of the first positive electrode active material layer slurry, the mass fraction (NCM+LMFP):SP:CNT:PVDF=94:0.8:0.2:5.

[0099] In Example 23, in the preparation of the second positive electrode active material layer slurry, the mass fraction (NCM+LMFP):SP:CNT:PVDF=96:0.2:0.8:3.

[0100] In Example 24, in the preparation of the first positive electrode active material layer slurry, the mass fraction (NCM+LMFP):SP:CNT:PVDF=95:2:0.5:2.5.

[0101] In Example 24, in the preparation of the second positive electrode active material layer slurry, the mass fraction (NCM+LMFP):SP:CNT:PVDF=95.5:0.6:2.4:1.5.

[0102] Examples 25-27 The difference between Examples 25-27 and Example 1 is that the proportion of positive electrode active material in the first positive electrode active material layer and the second positive electrode active material layer is changed.

[0103] Table 1 Table 2 Table 3 Performance testing 1. Secondary battery DCR test: The test is conducted using a high-precision Newway or Arbin machine, and the test procedure is as follows: The capacity of the secondary battery was tested by placing it in a constant temperature chamber (temperature 25℃) at a 1C rate. The tested capacity is C0. Discharge the battery at a 1C rate for 30 minutes to reduce the secondary battery charge to 50% SOC, and let it stand for 30 minutes. Charge for 10 seconds at a 4C rate; Read the voltage V0 0 seconds after the secondary battery finishes charging and the voltage V10 10 seconds after it is placed. DCR = (V0 - V10) / discharge current.

[0104] 2. Cyclic performance test: Capacity test ①: The secondary battery capacity was tested by placing it in a constant temperature chamber (temperature is 25℃) at a 1C rate. The start-stop voltage was 2.8~4.4V. The test was performed for 3 cycles, and the average discharge capacity of the 3 cycles was taken as D0. Battery cycling: The secondary battery was cycled in a constant temperature room at 25℃. The charging process was: 1C charging to 4.4V, and the discharging process was: 1C discharging to 2.8V. After repeating the above steps 1000 times, test the capacity of the secondary battery according to capacity test ① to obtain D1000.

[0105] Cyclic capacity retention rate = D1000 / D0 × 100%.

[0106] Table 4 As can be seen from Table 4, this application can effectively improve the safety and dynamic performance of the secondary battery, improve the cycle performance of the secondary battery, and reduce the impedance of the secondary battery by sequentially setting a first positive electrode active material layer, a second positive electrode active material layer and a ceramic layer on the surface of the positive electrode current collector, and controlling its porosity to meet the following conditions: C≥1.3×B, B≥A.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended 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, characterized in that, The positive electrode includes a positive current collector and a first positive active material layer, a second positive active material layer, and a ceramic layer sequentially disposed on at least one surface of the positive current collector; the ceramic layer includes ceramic particles. The porosity of the first positive electrode active material layer is A, the porosity of the second positive electrode active material layer is B, and the porosity of the ceramic layer is C, satisfying: C≥1.3×B, B≥A.

2. The secondary battery according to claim 1, characterized in that, Satisfying at least one of the following (1) to (3): （1）20%≤A≤50%； （2）22.5%≤B≤50%； （3）30%≤C≤65%。 3. The secondary battery according to claim 1, characterized in that, The ceramic layer covers 10-99% of the surface area of ​​the second positive electrode active material layer.

4. The secondary battery according to claim 1, characterized in that, The ceramic particles constitute 50-95% by mass in the ceramic layer; and / or The ceramic particles include Li7La3Zr2O 12 Li 1+x Al x Ti 2-x (PO4)3, La 2 / 3-y Li 3y TiO3, Na z At least one of M12 and M23; wherein 0.2≤x≤0.5; 0≤y<0.6; 1≤z≤3; M1 includes at least one of Li, Na, K, Ni, Fe, Ca, Ti, Cr, Zn, Ag, Mo, Mg, and Mn, and M2 includes at least one of SiO4, PO4, and SO4.

5. The secondary battery according to claim 4, characterized in that, The ceramic layer also includes polymer particles, and the polymer particles in the ceramic layer have a mass percentage of 5-50%.

6. The secondary battery according to claim 1, characterized in that, The first positive electrode active material layer includes a first positive electrode active material; the first positive electrode active material includes lithium phosphate and lithium nickel cobalt manganese oxide; the mass ratio of the lithium phosphate and lithium nickel cobalt manganese oxide is (10~90):(10~90); and / or The second positive electrode active material layer includes a second positive electrode active material; the second positive electrode active material includes lithium phosphate and lithium nickel cobalt manganese oxide, wherein the mass ratio of the lithium phosphate and lithium nickel cobalt manganese oxide is (10~90):(10~90).

7. The secondary battery according to claim 6, characterized in that, The first positive electrode active material layer further includes a first binder, wherein the mass percentage of the first binder in the first positive electrode active material layer is D1; The second positive electrode active material layer further includes a second binder, wherein the mass percentage of the second binder in the second positive electrode active material layer is D2; The condition is satisfied that D1-D2≥0.2%.

8. The secondary battery according to claim 6, characterized in that, The mass percentage of the first positive electrode active material in the first positive electrode active material layer is E1; The mass percentage of the second positive electrode active material in the second positive electrode active material layer is E2; Satisfy: E2-E1≥0.2%.

9. The secondary battery according to any one of claims 7 to 8, characterized in that, Satisfy at least one of the following (4) to (7): （4）1.0%≤D1≤5%； （5）0.8%≤D2≤4.8%； （6）95%≤E1≤97.5% （7）95.5%≤E2≤97.8%。 10. The secondary battery according to claim 1, characterized in that, The thickness of the first positive electrode active material layer is t1, the thickness of the second positive electrode active material layer is t2, and the thickness of the ceramic layer is t3, satisfying: t2≥t1≥3t3.

11. The secondary battery according to claim 10, characterized in that, Satisfy at least one of the following (8) to (10): (8) 3μm≤t1≤80μm; (9) 3μm≤t2≤100μm; (10) 1μm≤t3≤10μm.

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

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