Positive electrode active material, secondary battery, and electric device

Abstract

The invention discloses a positive electrode active material, a secondary battery and a power utilization device, and belongs to the technical field of batteries, the positive electrode active material comprises a first lithium-containing phosphate, a carbon coating layer and a lithium manganese iron phosphate coating layer which are sequentially arranged, and S (100) / S (002) is more than 1 and less than 4.5 by controlling S (100) / S (002) to be more than 1 and less than 4.5; the carbon coating layer enhances the bonding effect between the lithium-containing phosphate and the lithium-manganese-iron phosphate, is beneficial to charge transfer, improves the structural stability of the whole material and promotes the intercalation and deintercalation of lithium ions, and the first lithium-containing phosphate can effectively promote the intercalation of active lithium and effectively improve the gram volume of the lithium-manganese-iron phosphate, so that the service life of the lithium-manganese-iron phosphate is prolonged. The lithium death phenomenon in the charging and discharging process is effectively improved, the cycle life and the rate capability of the secondary battery are effectively improved, and the gas production phenomenon of the secondary battery is improved.

Description

Technical Field

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

[0002] Secondary batteries have been widely used in many industries due to their advantages such as high energy / power density, long cycle life, and no pollution.

[0003] With the development of rechargeable batteries, people have placed higher demands on them, especially power batteries. Long driving range has become an important factor affecting the application of electric vehicles.

[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 positive electrode active material, a secondary battery and an electrical device. The positive electrode active material described in this application can effectively improve the cycle life and rate performance of the secondary battery and improve the gas generation phenomenon of the secondary battery.

[0006] To achieve the above objectives, a first aspect of this application provides a positive electrode active material, including a core, a first coating layer disposed on at least a portion of the surface of the core, and a second coating layer disposed on at least a portion of the surface of the first coating layer;

[0007] The core includes a first lithium-containing phosphate;

[0008] The first coating layer includes a carbon coating layer;

[0009] The second coating layer comprises lithium manganese iron phosphate;

[0010] The positive electrode active material satisfies: 1 < S (100) / S (002) <4.5, where S (100) S represents the peak intensity of the (100) diffraction peak in the XRD pattern of the positive electrode active material. (002) The peak intensity of the (002) diffraction peak in the XRD pattern of the positive electrode active material is given.

[0011] As an embodiment of this application, the positive electrode active material satisfies: 1.30 ≤ S (100) / S (002) ≤4.33.

[0012] As an embodiment of this application, the molecular formula of the first lithium-containing phosphate is Li x1 FePO4, where 0 < X1 < 1.

[0013] As an embodiment of this application, the iron-manganese ratio in the lithium manganese iron phosphate is 1 to 9.

[0014] As an embodiment of this application, the thickness of the first coating layer is 5–20 nm; and / or

[0015] The thickness of the second coating layer is 30–120 nm.

[0016] As an embodiment of this application, the first coating layer further includes an M1 element, wherein the M1 element includes at least one of Li, N, O, F, Cu, Ti, Co, and Ni.

[0017] As an embodiment of this application, the mass percentage of element M1 in the first coating layer is 0.01-8%.

[0018] As an embodiment of this application, a third coating layer is also included, which is disposed on at least a portion of the surface of the second coating layer, and the third coating layer includes a carbon coating layer.

[0019] As an embodiment of this application, it also includes a fourth coating layer, which is disposed on at least a portion of the surface of the third coating layer, and the fourth coating layer includes a second lithium phosphate.

[0020] As an embodiment of this application, the molecular formula of the second lithium-containing phosphate is Li x2 FePO4, where 0 < X2 < 1.

[0021] As an embodiment of this application, the thickness of the third coating layer is 20–50 nm; and / or

[0022] The thickness of the fourth coating layer is 65–120 nm.

[0023] A second aspect of this application provides a secondary battery comprising the aforementioned positive electrode active material.

[0024] A third aspect of this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0025] The beneficial effects of this application are as follows: The positive electrode active material described in this application includes a first lithium-containing phosphate, a carbon coating layer, and a lithium manganese iron phosphate coating layer sequentially arranged, and by controlling 1 < S (100) / S (002)<4.5, the carbon coating layer strengthens the bonding between lithium phosphate and lithium manganese iron phosphate, which is beneficial for charge transfer and improves the overall structural stability of the material. It promotes the insertion and extraction of lithium ions. The first lithium phosphate can effectively promote the insertion of active lithium, effectively improve the specific capacity of lithium manganese iron phosphate, effectively improve the phenomenon of dead lithium during charging and discharging, effectively improve the cycle life and rate performance of the secondary battery, and improve the gas generation phenomenon of the secondary battery. Detailed Implementation

[0026] 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.

[0027] 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.

[0028] 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.

[0029] The inventors of this application have discovered that lithium manganese iron phosphate (LFP) is a derivative material of lithium iron phosphate. By incorporating manganese into LFP, the charging voltage can be increased by more than 15%, while maintaining a cost roughly equivalent to LFP. A significant drawback of LFP is that manganese leaching leads to gas production in the secondary battery, causing crystal structure collapse and rapid cycle degradation. Furthermore, dead lithium is prone to occur during charging and discharging, resulting in insufficient specific capacity and low capacity in the secondary battery, severely impacting its electrochemical performance.

[0030] Therefore, based on the above problems, this application provides a positive electrode active material, including a core, a first coating layer disposed on at least a portion of the surface of the core, and a second coating layer disposed on at least a portion of the surface of the first coating layer;

[0031] The core includes a first lithium-containing phosphate;

[0032] The first coating layer includes a carbon coating layer;

[0033] The second coating layer comprises lithium manganese iron phosphate;

[0034] The positive electrode active material satisfies: 1 < S (100) / S (002) <4.5, where S (100) S represents the peak intensity of the (100) diffraction peak in the XRD pattern of the positive electrode active material. (002) The peak intensity of the (002) diffraction peak in the XRD pattern of the positive electrode active material is given.

[0035] The positive electrode active material described in this application comprises a first lithium-containing phosphate, a carbon coating layer, and a lithium manganese iron phosphate coating layer sequentially disposed therefrom, by controlling 1 < S (100) / S (002) <4.5, the carbon coating layer strengthens the bonding between lithium phosphate and lithium manganese iron phosphate, which is beneficial for charge transfer and improves the overall structural stability of the material. It promotes the insertion and extraction of lithium ions. The first lithium phosphate can effectively promote the insertion of active lithium, effectively improve the specific capacity of lithium manganese iron phosphate, effectively improve the phenomenon of dead lithium during charging and discharging, effectively improve the cycle life and rate performance of the secondary battery, and improve the gas generation phenomenon of the secondary battery.

[0036] The method for obtaining the XRD pattern of the positive electrode active material is as follows: the secondary battery is discharged to 2.5V at a constant current of 0.33C, the positive electrode and negative electrode are separated, the positive electrode powder is scraped off with a ceramic knife, 0.5g of positive electrode powder is ground in a mortar for 5min, and dried in a vacuum drying oven for 1h; after drying, it is taken out onto a special glass slide for XRD, a layer of powder is spread out, the XRD scanning angle is set to 10-80 degrees, the XRD pattern is obtained after scanning, and the diffraction peaks of (100) and (002) are obtained by comparing with the standard diffraction card (PDF card).

[0037] In some embodiments, the positive electrode active material satisfies: 1.30 ≤ S (100) / S (002) ≤4.33, especially S (100) / S (002) Within this range, the cycle life and rate performance of the secondary battery can be further improved, and the gas generation phenomenon of the secondary battery can be reduced.

[0038] In some embodiments, the molecular formula of the first lithium-containing phosphate is Li. x1FePO4, wherein 0 < X1 < 1, for example, can be a range of 0.01, 0.05, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9, 0.99 or any combination thereof. By controlling 0 < X1 < 1, the first lithium-containing phosphate is a lithium-poor lithium-containing phosphate with lithium vacancies. During the discharge process of the secondary battery, it can fully accept lithium ions, effectively improving the specific capacity of the secondary battery and effectively improving the first-stage efficiency of the secondary battery.

[0039] In some of these implementations, 0.1 ≤ X1 ≤ 0.9.

[0040] In some embodiments, the iron-manganese ratio in the lithium manganese iron phosphate is 0.01 to 9, for example, it can be a range of 0.01, 0.05, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or any combination thereof. By controlling the iron-manganese ratio in the lithium manganese iron phosphate within this range, the structural stability of the lithium manganese iron phosphate can be effectively improved, the spontaneous dissolution tendency of manganese can be effectively reduced, the cycle life and rate performance of the secondary battery can be effectively improved, and the gas generation phenomenon of the secondary battery can be improved.

[0041] In some embodiments, the thickness of the first coating layer is 5-20 nm, for example, it can be a range of 5 nm, 6 nm, 8 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm or any combination thereof. By controlling the thickness of the first coating layer within this range, the diffusion path of lithium ions can be effectively shortened, the lithium ion transport rate can be improved, the bonding between lithium phosphate and lithium manganese iron phosphate can be strengthened more effectively, and the structural stability can be further improved, thereby effectively improving the cycle life and rate performance of the secondary battery and improving the gas generation phenomenon of the secondary battery.

[0042] In some embodiments, the thickness of the second coating layer is 30-120 nm, for example, it can be a range of 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm or any combination thereof. By controlling the thickness of the second coating layer within this range, the structural stability of lithium manganese iron phosphate can be improved, the stability and thermal stability of the positive electrode active material during processing can be improved, crystal structure collapse can be avoided, and the cycle performance and rate performance of the secondary battery can be effectively improved.

[0043] In some embodiments, the first coating layer further includes an M1 element, which includes at least one of Li, N, O, F, Cu, Ti, Co, and Ni. By doping the first coating layer with an M1 element, the doping element can stabilize the first lithium-containing phosphate, improve the conductivity of the first coating layer, better strengthen the bonding between the lithium-containing phosphate and lithium manganese iron phosphate, and effectively improve the cycle life and rate performance of the secondary battery.

[0044] In some embodiments, the mass percentage of the M1 element in the first coating layer is 0.01 to 8%, for example, it can be a range of 0.01%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or any combination thereof.

[0045] The method for detecting the M1 element in the first coating layer is as follows: the secondary battery is discharged at a constant current of 0.33C to 2.5V, the positive electrode and negative electrode are separated, the positive electrode powder is scraped off with a ceramic knife, 0.01g of the positive electrode powder is ground in a mortar for 5min, and dried in a vacuum drying oven for 1h; after drying, it is taken out and dispersed in an ethanol solvent, and the morphology of the positive electrode material is photographed by HRTEM. The content and distribution of the M1 element can be obtained by EDS at the first coating layer position.

[0046] In some embodiments, a third coating layer is further included, which is disposed on at least a portion of the surface of the second coating layer. The third coating layer includes a carbon coating layer. By using the second coating layer (lithium manganese iron phosphate), the structural stability of the positive electrode active material can be further improved, preventing lithium manganese iron phosphate from directly contacting the electrolyte, reducing the erosion of the Mn-O bond of lithium manganese iron phosphate by the electrolyte, effectively inhibiting the dissolution of manganese in lithium manganese iron phosphate, effectively improving the gas generation phenomenon of the secondary battery, and improving conductivity, thereby effectively improving the cycle life and rate performance of the secondary battery.

[0047] In some embodiments, the third coating layer further includes an M2 element, which includes at least one of Li, N, O, F, Cu, Ti, Co, and Ni. By incorporating the M2 element into the third coating layer, the structural stability of the positive electrode active material can be further improved, and the dissolution of Mn element can be more effectively suppressed, thereby effectively improving the cycle life and rate performance of the secondary battery.

[0048] In some embodiments, the mass percentage of the M2 element in the third coating layer is 0.01 to 8%, for example, it can be a range of 0.01%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or any combination thereof.

[0049] The method for detecting the M2 element in the third coating layer is as follows: the secondary battery is discharged at a constant current of 0.33C to 2.5V, the positive and negative electrode plates are separated, the positive electrode powder is scraped off with a ceramic knife, 0.01g of the positive electrode powder is ground in a mortar for 5 minutes, and dried in a vacuum drying oven for 1 hour; after drying, it is taken out and dispersed in an ethanol solvent, and the morphology of the positive electrode material is photographed by HRTEM. The content and distribution of the M2 element can be obtained by EDS at the location of the third coating layer. In some embodiments, a fourth coating layer is also included, which is disposed in at least a portion of the surface of the third coating layer. The fourth coating layer includes a second lithium phosphate. By setting the fourth coating layer on the surface of the third coating layer, the structural stability of lithium manganese iron phosphate can be further improved, the charge balance of the positive electrode active material can be ensured, the compaction density of the positive electrode active material can be increased, and the cycle life and rate performance of the secondary battery can be effectively improved.

[0050] In some embodiments, the molecular formula of the second lithium-containing phosphate is Li. x2 FePO4, wherein 0 < x2 < 1, for example, can be a range of 0.01, 0.05, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9, 0.99 or any combination thereof. By controlling 0 < x2 < 1, the second lithium-containing phosphate is a lithium-poor lithium-containing phosphate with lithium vacancies. During the discharge process of the secondary battery, it can fully accept lithium ions, effectively improving the disadvantage of low specific capacity of lithium manganese iron phosphate, effectively increasing the specific capacity of the secondary battery, and effectively improving the first-time efficiency of the secondary battery.

[0051] In some of these implementations, 0.1 ≤ X2 ≤ 0.9.

[0052] In some of these implementations, 0.5 ≤ X1 + X2 ≤ 1.

[0053] In some embodiments, the thickness of the third coating layer is 20-50 nm, for example, it can be a range of 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm or any combination thereof. By controlling the thickness of the third coating layer within this range, the bonding between lithium manganese iron phosphate and the second lithium-containing phosphate can be effectively improved, the structural stability of lithium manganese iron phosphate can be improved, the dissolution of manganese can be suppressed, the gas generation phenomenon of the secondary battery can be effectively improved, and the cycle performance and rate performance of the secondary battery can be improved.

[0054] In some embodiments, the thickness of the fourth coating layer is 65 to 120 nm, for example, it can be a range of 65 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm or any combination thereof.

[0055] In some embodiments, the Dv50 particle size of the first lithium phosphate is 1 to 50 nm, for example, it can be a range of 1 nm, 2 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm or any combination thereof.

[0056] In some embodiments, the Dv50 particle size of the lithium manganese iron phosphate is 50 to 250 nm, for example, it can be a range of 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, 220 nm, 250 nm or any combination thereof.

[0057] In some embodiments, the Dv50 particle size of the second lithium phosphate is 250 to 1000 nm, for example, it can be a range of 250 nm, 300 nm, 350 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm or any combination thereof.

[0058] The positive electrode active material described in this application has an continuously increasing Dv50 particle size from the inside out, which can effectively improve the coating effect, enhance the stability of the positive electrode active material, prevent the dissolution of Mn elements, effectively improve the gas generation phenomenon of secondary batteries, and improve the cycle performance and rate performance of secondary batteries.

[0059] In some embodiments, the ratio of the intensity of the D peak to the intensity of the G peak in the Raman spectrum of the positive electrode active material is >1.

[0060] The Raman spectrum D peak of the positive electrode active material is located in the range of 1300–1400 cm⁻¹. -1between, the Raman G peak of the positive electrode active material is located at 1530-1630 cm -1 between.

[0061] The intensity of the Raman D peak and the G peak of the positive electrode active material can be tested by any known method in the art. In this application, the intensity of the D peak and the G peak are analyzed and obtained according to GB / T 40219-2021.

[0062] In some embodiments, the ratio of the intensity of the Raman D peak to the G peak of the positive electrode active material is 1.3-2.8, for example, it can be 1.3, 1.5, 1.6, 1.8, 2, 2.5, 2.8 or the range composed of any two of them.

[0063] In some embodiments, the lithium manganese iron phosphate includes a compound with the general formula Li x Mn y Fe 1-y M z PO4, where 0.7≤x≤1.3, 0.1<y<0.9, 0≤z≤0.09, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

[0064] In some embodiments, 0.8≤x≤1.2.

[0065] One embodiment of this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the above positive electrode active material.

[0066] In this application, the type of the positive electrode current collector is not particularly limited, and it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc. and carbon materials such as carbon cloth and carbon paper.

[0067] [[ID=第31]]Among them, the form of the positive electrode current collector is not particularly limited. When the positive electrode current collector is a metal material, the form of the positive electrode current collector can be a metal foil, a metal cylinder, a metal strip roll, a metal plate, a metal foil, a metal plate mesh, a stamped metal, a foamed metal, etc. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector can include but is not limited to a carbon plate, a carbon film, a carbon cylinder, etc.

[0068] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0069] In one embodiment, the secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material.

[0070] 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.

[0071] In one embodiment, the negative electrode active material can 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.

[0072] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0073] In one embodiment, there is no limitation on the type of conductive agent mentioned in this application, and any known conductive agent may be used.

[0074] In one embodiment, the conductive agent includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0075] In one embodiment, there is no limitation on the type of adhesive mentioned in this application, and any known positive electrode adhesive can be used.

[0076] In one embodiment, the adhesive includes 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, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer.

[0077] In the secondary battery mentioned in this application, a separator is usually provided between the positive and negative 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.

[0078] In one embodiment, 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.

[0079] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is at least one of polypropylene and polyethylene. The materials of the diaphragm described above can be used alone or in any combination.

[0080] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0081] 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.

[0082] In some embodiments, the type of electrolyte is not specifically limited. The electrolyte includes 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 additives is not particularly limited. These additives can be film-forming additives for the positive and / or negative electrodes, or additives that can improve certain battery performance, such as additives that improve the battery's high or low temperature performance.

[0083] 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.

[0084] One embodiment of this application provides an electrical device including the secondary battery described above, wherein the secondary battery serves as the power supply for the electrical device.

[0085] 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.

[0086] The present application is further illustrated below with specific embodiments:

[0087] Example 1

[0088] A method for preparing a secondary battery includes the following steps:

[0089] (1) Preparation of positive electrode active material:

[0090] S1, Li 0.5 FePO4 (core), glucose, and deionized water were mixed at a mass ratio of 1:10:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0091] S2, Li 0.5 FePO4@C and LiFe 0.5 Mn 0.5 PO4 and deionized water were mixed at a mass ratio of 1:1:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and then placed in a tube furnace and calcined at 900°C for 6 hours under a nitrogen atmosphere to form LiFe on the surface of the first carbon coating layer. 0.5 Mn 0.5 PO4 coating (Li for short) 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4).

[0092] The parameter table of the positive electrode active material in this embodiment is shown in Table 1.

[0093] (2) Preparation of positive electrode sheet: The positive active material prepared above is mixed with the binder polyvinylidene fluoride and the conductive agent acetylene black at a mass ratio of 85:7.5:7.5. N-methylpyrrolidone (NMP) is added and stirred evenly under the action of a vacuum stirrer to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil with a thickness of 12μm. The electrode sheet coated with the positive electrode slurry is dried, cold pressed and cut to obtain a positive electrode sheet with a size of 700mm*120mm.

[0094] (3) Preparation of negative electrode sheet: The negative electrode active material hard carbon, thickener sodium carboxymethyl cellulose, binder styrene-butadiene rubber and conductive agent acetylene black are mixed in a mass ratio of 95.7:0.8:2.5:1, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on a copper foil with a thickness of 4.5μm; the coated electrode sheet is dried, cold pressed and slit to obtain the negative electrode sheet.

[0095] (4) Separator: The diaphragm is a 10μm PE diaphragm.

[0096] (5) Preparation of electrolyte: At room temperature, in a glove box filled with argon (H2O<1ppm, O2<1ppm), DEC:EC are mixed evenly to obtain a mixed solvent. Then, lithium salt LiPF6 is added to the mixed solvent and stirred evenly to obtain the electrolyte.

[0097] The mass ratio of LiPF6, DEC, and EC is 1:10:10.

[0098] (6) Assembly of secondary battery: The prepared positive electrode, separator and negative electrode are stacked in sequence, with the separator in the middle of the positive electrode and negative electrode. 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. After standing, formation and capacity testing, the preparation of secondary battery is completed.

[0099] Examples 2-4, Comparative Example 3

[0100] The difference between Examples 2-4 and Example 1 is that Li is changed. 0.5 FePO4 (kernel), as shown in Table 1.

[0101] Example 2 uses Li 0.3 FePO4 (kernel).

[0102] Example 3 uses Li 0.1 FePO4 (kernel).

[0103] Example 4 uses Li 0.9 FePO4 (kernel).

[0104] Comparative Example 3 uses LiFePO4 (core).

[0105] Examples 5-7

[0106] The difference between Examples 5-7 and Example 1 is that the thickness of the first coating layer (carbon coating layer) is changed by changing the stirring speed and time in step S1.

[0107] The stirring speed in Example 5 was 400 rpm.

[0108] The stirring speed in Example 6 was 500 rpm.

[0109] The stirring speed in Example 7 was 200 rpm.

[0110] Examples 8-12

[0111] The difference between Examples 8-12 and Example 1 is that the preparation method of step S1 is changed, and elements are doped into the first coating layer (carbon coating layer).

[0112] Step S1 of Example 8 specifically involves: placing Li 0.5 FePO4 (core), NTA, and deionized water were mixed at a mass ratio of 1:10:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0113] Step S1 of Example 9 specifically involves: placing Li 0.5 FePO4 (core), NTA, and deionized water were mixed at a mass ratio of 1:2:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0114] Step S1 of Example 10 specifically involves: placing Li 0.5 FePO4 (core), NTA, and deionized water were mixed at a mass ratio of 1:40:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0115] Step S1 of Example 11 specifically involves: placing Li 0.5 FePO4 (core), NTA, and deionized water were mixed at a mass ratio of 1:80:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0116] Step S1 of Example 12 specifically involves: placing Li 0.5 FePO4 (core), NTA, and deionized water were mixed at a mass ratio of 1:0.2:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0117] Example 13

[0118] Example 13 differs from Example 1 in that the preparation method of step S1 is changed, and elements are doped into the first coating layer (carbon coating layer).

[0119] Step S1 of Example 13 specifically involves: placing Li 0.5 FePO4 (core), glucose, titanium dioxide, and deionized water were mixed at a mass ratio of 1:9.95:0.05:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0120] Examples 14-18

[0121] Examples 14-18 differ from Example 1 in that the LiFe in step S2 is changed. 0.5 Mn 0.5 The chemical formula of PO4, which in turn changes the iron-manganese ratio, is shown in Table 1.

[0122] Examples 19-21

[0123] The difference between Examples 19-21 and Example 1 is that the stirring speed and time in step S2 are changed, thereby changing the thickness of the second coating layer (carbon coating layer).

[0124] The stirring speed in Example 19 was 400 rpm.

[0125] The stirring speed in Example 20 was 200 rpm.

[0126] The stirring speed in Example 21 was 100 rpm.

[0127] Example 22

[0128] The difference between Example 22 and Example 1 is that the preparation method of the positive electrode active material is different.

[0129] The preparation method of the positive electrode active material in this embodiment is as follows:

[0130] (1) Preparation of positive electrode active material:

[0131] S1, Li 0.5FePO4 (core), glucose, and deionized water were mixed at a mass ratio of 1:10:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0132] S2, Li 0.5 FePO4@C and LiFe 0.5 Mn 0.5 PO4 and deionized water were mixed at a mass ratio of 1:1:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and then placed in a tube furnace and calcined at 900°C for 6 hours under a nitrogen atmosphere to form LiFe on the surface of the first carbon coating layer. 0.5 Mn 0.5 PO4 coating (Li for short) 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4).

[0133] S3, Li 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4, glucose (NTA), and deionized water were mixed at a mass ratio of 1:30:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. (LiFe) 0.5 Mn 0.5 A second carbon coating (Li) forms on the surface of the PO4 coating layer. 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4@C).

[0134] The parameter table of the positive electrode active material in this embodiment is shown in Table 2.

[0135] Examples 23-25

[0136] The difference between Examples 23-25 ​​and Example 22 is that the thickness of the third coating layer (carbon coating layer) is changed by changing the rotation speed in step S3.

[0137] The stirring speed in Example 23 was 400 rpm.

[0138] The stirring speed in Example 24 was 500 rpm.

[0139] The stirring speed in Example 25 was 200 rpm.

[0140] Example 26

[0141] The difference between Example 26 and Example 1 is that the preparation method of the positive electrode active material is different.

[0142] The preparation method of the positive electrode active material in this embodiment is as follows:

[0143] (1) Preparation of positive electrode active material:

[0144] S1, Li 0.5 FePO4 (core), glucose, and deionized water were mixed at a mass ratio of 1:10:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0145] S2, Li 0.5 FePO4@C and LiFe 0.5 Mn 0.5 PO4 and deionized water were mixed at a mass ratio of 1:1:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and then placed in a tube furnace and calcined at 900°C for 6 hours under a nitrogen atmosphere to form LiFe on the surface of the first carbon coating layer. 0.5 Mn 0.5 PO4 coating (Li for short) 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4).

[0146] S3, Li 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4, glucose, and deionized water were mixed at a mass ratio of 1:30:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. (LiFe) 0.5 Mn 0.5 A second carbon coating (Li) forms on the surface of the PO4 coating layer. 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4@C).

[0147] S4, Li 0.5 FePO4@C@LiFe 0.5 Mn 0.5PO4@C and Li 0.5 FePO4 and deionized water were mixed at a mass ratio of 1:3:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere to form Li on the surface of the second carbon coating layer. 0.5 FePO4 coating (Li for short) 0.5 FePO4@C@LiFe 0.5 Mn 0.5 PO4@C@Li 0.5 FePO4).

[0148] The parameter table of the positive electrode active material in this embodiment is shown in Table 2.

[0149] Examples 27-29

[0150] Examples 27-29 differ from Example 26 in that the LiFe in step S4 is changed. 0.5 Mn 0.5 The chemical formula of PO4 is shown in Table 2.

[0151] Example 27 uses Li 0.3 FePO4.

[0152] Example 28 uses Li 0.1 FePO4.

[0153] Example 29 uses Li 0.9 FePO4.

[0154] Examples 30-32

[0155] The difference between Examples 30-32 and Example 26 is that the thickness of the fourth coating layer (carbon coating layer) is changed by changing the rotation speed of S4.

[0156] The rotational speed in Example 30 was 350 rpm.

[0157] The rotational speed in Example 31 was 150 rpm.

[0158] The rotational speed in Example 32 was 500 rpm.

[0159] Comparative Example 1

[0160] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not contain a first carbon coating layer, but all other aspects are the same.

[0161] A method for preparing a secondary battery includes the following steps:

[0162] (1) Preparation of positive electrode active material:

[0163] S1, Li 0.5 FePO4 (core) and LiFe 0.5 Mn 0.5 PO4 and deionized water were mixed at a mass ratio of 1:1:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 LiFe forms on the FePO4 (core) surface 0.5 Mn 0.5 PO4 coating (Li for short) 0.5 FePO4@LiFe 0.5 Mn 0.5 PO4).

[0164] Comparative Example 2

[0165] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not contain LiFe. 0.5 Mn 0.5 PO4 coating layer.

[0166] The preparation method of the positive electrode active material in this embodiment is as follows:

[0167] (1) Preparation of positive electrode active material:

[0168] S1, Li 0.5 FePO4 (core), glucose, and deionized water were mixed at a mass ratio of 1:10:100 and stirred at 300 rpm for 3 hours. The mixture was then dried in a 60°C oven to constant weight, and subsequently calcined in a tube furnace at 900°C for 6 hours under a nitrogen atmosphere. 0.5 A first carbon coating layer (Li) forms on the surface of FePO4. 0.5 FePO4@C).

[0169] Comparative Example 4

[0170] The difference between Comparative Example 4 and Example 1 is that the preparation method of step S1 is changed, thereby adjusting the value of S(100) / S(002) to be greater than 4.6.

[0171] The S1 step of Comparative Example 4 is as follows: FePO4 (core), glucose, and deionized water are stirred at 300 rpm for 3 hours in a mass ratio of 1:10:100. The mixture is then dried in a 60°C oven until constant weight, and then placed in a tube furnace and calcined at 900°C for 6 hours under a nitrogen atmosphere to form a first carbon coating layer (FePO4@C) on the surface of FePO4.

[0172] Table 1

[0173]

[0174]

[0175] Table 2

[0176] Performance testing

[0177] Gas production performance test: The secondary battery was placed in a charge / discharge test chamber at a constant temperature of 25°C, with a voltage range of 2.5–4.25V, a charge rate of 1C, and a discharge rate of 1C, and cyclically tested. The test was stopped after 100 cycles. The gas production volume of the secondary battery was tested using the water displacement method.

[0178] Rate performance test: Secondary batteries that have not undergone electrochemical testing are placed on a charge / discharge tester at room temperature. They are charged at 0.33C to 4.25V and discharged at 0.33C to 2.5V, yielding a capacity of Q2. They are then charged at 0.33C to 4.25V and discharged at 5C to 2.5V, yielding a capacity of Q3. Rate performance = Q3 / Q2*100%.

[0179] Cyclic performance test: The secondary battery is placed in a charge / discharge test chamber at a constant temperature of 25°C, with a voltage range of 2.5–4.35V, a charge rate of 1C, and a discharge rate of 1C for cyclic testing. The test is stopped when the capacity drops to 80% of the initial capacity, and the number of cycles is recorded.

[0180] Table 3

[0181]

[0182]

[0183] As can be seen from Table 3, the positive electrode active material described in this application includes a first lithium-containing phosphate, a carbon coating layer, and a lithium manganese iron phosphate coating layer arranged sequentially, by controlling 1 < S (100) / S (002) <4.5, the carbon coating layer strengthens the bonding between lithium phosphate and lithium manganese iron phosphate, which is beneficial for charge transfer and improves the overall structural stability of the material. It promotes the insertion and extraction of lithium ions. The first lithium phosphate can effectively promote the insertion of active lithium, effectively improve the specific capacity of lithium manganese iron phosphate, effectively improve the phenomenon of dead lithium during charging and discharging, effectively improve the cycle life and rate performance of the secondary battery, and improve the gas generation phenomenon of the secondary battery.

[0184] 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 positive electrode active material, characterized by, The core, a first coating layer arranged on at least part of the surface of the core, and a second coating layer arranged on at least part of the surface of the first coating layer; The core comprises a first lithium-containing phosphate; The first coating layer comprises a carbon coating layer; The second coating layer comprises a lithium-manganese-iron phosphate; The positive electrode active material satisfies: 1 < S (100) / S (002) < 4.5, wherein S (100) is the peak intensity of the (100) diffraction peak in the XRD pattern of the positive electrode active material, and S (002) is the peak intensity of the (002) diffraction peak in the XRD pattern of the positive electrode active material.

2. The positive electrode active material according to claim 1, characterized by The positive electrode active material satisfies: 1.30 ≤ S (100) / S (002) ≤ 4.

33.

3. The positive electrode active material according to claim 1, characterized by The first lithium-containing phosphate has a molecular formula of Li x1 FePO4, wherein 0 < X1 < 1 ; and / or The iron-manganese element ratio in the lithium-manganese-iron phosphate is 1-9.

4. The positive electrode active material according to claim 1, characterized by The thickness of the first coating layer is 5-20 nm; and / or The thickness of the second coating layer is 30-120 nm.

5. The positive electrode active material according to claim 1, characterized by The first coating layer further comprises an M1 element, and the M1 element comprises at least one of Li, N, O, F, Cu, Ti, Co, and Ni.

6. The positive electrode active material according to claim 5, characterized by The mass percentage of the M1 element in the first coating layer is 0.01-8%.

7. The positive electrode active material according to claim 1, characterized by A third coating layer is further included, and the third coating layer is arranged on at least part of the surface of the second coating layer, and the third coating layer comprises a carbon coating layer.

8. The positive electrode active material according to claim 7, characterized by A fourth coating layer is further included, and the fourth coating layer is arranged on at least part of the surface of the third coating layer, and the fourth coating layer comprises a second lithium-containing phosphate.

9. The positive electrode active material according to claim 8, characterized by The second lithium-containing phosphate has a molecular formula of Li x2 FePO4, wherein 0 < X2< 1.

10. The positive electrode active material according to any one of claims 7 to 9, characterized by The thickness of the third coating layer is 20-50 nm; and / or The thickness of the fourth coating layer is 65-120 nm.

11. A secondary battery characterized by comprising: The positive electrode active material as claimed in any one of claims 1-10.

12. An electrical device, characterized by The secondary battery as claimed in claim 11 is used as a power supply for the power consumption device.

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