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 positive electrode active material, the first positive electrode active material comprises lithium manganese iron phosphate and a coating layer arranged on the outer surface of the lithium manganese iron phosphate, and the coating layer comprises phosphate; a groove is formed in the surface of the first positive electrode active material. The first positive electrode active material (LMFP) is of a core-shell structure, the surface of the first positive electrode active material is provided with a coating layer, and the surface of the first positive electrode active material is provided with a groove, so that the stability of lithium manganese iron phosphate can be effectively improved, the dissolution of Mn < 2 + > can be effectively inhibited, and the conductivity of the first positive electrode active material is improved; the reaction activity of the first positive electrode active material is effectively improved, the structure of the positive electrode active material is prevented from being damaged in the charging and discharging process, the stability of the positive electrode active material in the processing process is improved, and the cycle performance and the safety performance of the secondary battery are effectively 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, as electrochemical energy storage devices, have advantages such as large capacity, no memory effect, and wide application window, and have been widely used in various fields.

[0003] As a key component of secondary batteries, the performance of the positive electrode active material largely determines the performance of the secondary battery. The market has increasingly higher requirements for the energy density, cycle performance, and safety performance of secondary batteries.

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

[0005] The purpose of this application is to overcome the shortcomings of the prior art and provide a positive electrode active material, a secondary battery and an electrical device. The positive electrode active material of this application can effectively improve the cycle performance of the secondary battery.

[0006] To achieve the above objectives, a first aspect of this application provides a positive electrode active material, comprising a first positive electrode active material, the first positive electrode active material comprising lithium manganese iron phosphate and a coating layer disposed on the outer surface of the lithium manganese iron phosphate, the coating layer comprising phosphate; the surface of the first positive electrode active material has grooves.

[0007] As an embodiment of this application, the average depth of the grooves on the surface of the first positive electrode active material is 59-88 nm.

[0008] As an embodiment of this application, the area ratio of the grooves on the surface of the first positive electrode active material is 15.2% to 37.8%.

[0009] As an embodiment of this application, the thickness of the coating layer is 5–30 nm; and / or The phosphate includes at least one of lithium phosphate, iron phosphate, and manganese phosphate.

[0010] As an embodiment of this application, the first positive electrode active material has a mass percentage content of 30% to 100% in the positive electrode active material.

[0011] As an embodiment of this application, the first positive electrode active material satisfies: 7≤(D90-D10) / D50≤11; D90μm is the particle size corresponding to the first positive electrode active material when the cumulative volume distribution percentage reaches 90%; D50μm is the particle size corresponding to the volume cumulative distribution percentage of the first positive electrode active material reaching 50%. D10 μm is the particle size corresponding to when the cumulative volume distribution percentage of the first positive electrode active material reaches 10%.

[0012] As an embodiment of the present application, it satisfies: 0.1 ≤ D10 ≤ 0.4.

[0013] As an embodiment of the present application, it satisfies: 0.5 ≤ D50 ≤ 2.

[0014] As an embodiment of the present application, it satisfies: 5 ≤ D90 ≤ 10.

[0015] As an embodiment of the present application, the positive electrode active material further includes lithium iron phosphate, and the mass percentage content of lithium iron phosphate in the positive electrode active material is greater than 0 and less than or equal to 70%.

[0016] As an embodiment of the present application, the lithium iron phosphate satisfies: 1 ≤ (D90' - D10') / D50' ≤ 5; D90' μm is the particle size corresponding to when the cumulative volume distribution percentage of the lithium iron phosphate reaches 90%; D50' μm is the particle size corresponding to when the cumulative volume distribution percentage of the lithium iron phosphate reaches 50%; D10' μm is the particle size corresponding to when the cumulative volume distribution percentage of the lithium iron phosphate reaches 10%.

[0017] As an embodiment of the present application, it satisfies: 0.1 ≤ D10' ≤ 0.3.

[0018] As an embodiment of the present application, it satisfies: 0.3 ≤ D50' ≤ 1.

[0019] As an embodiment of the present application, it satisfies: 3 ≤ D90' ≤ 5.

[0020] As an embodiment of the present application, the lithium iron manganese phosphate includes a compound with the general formula Li , 1-b , z , y , <000009I>, b , a , 1-y , x Mn y Fe 1-y M1 z PO4, where 0.9 ≤ x ≤ 1.1, 0 < y < 1, and 0 ≤ z ≤ 0.05, and M1 includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, and Cr; and / or The lithium iron phosphate includes a compound with the general formula Li a Fe 1-b M2 b PO4, where 0.9 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, and M2 is selected from at least one of Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Ti, V, Mg, and Al.

[0021] As an embodiment of this application, the lithium manganese iron phosphate includes lithium with the general formula Li x Mn y Fe 1-y Compounds of PO4; and / or the lithium iron phosphate comprising compounds of the general formula Li a Fe 1-b The compound shown in PO4.

[0022] A second aspect of this application provides a secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including the positive active material described above.

[0023] 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.

[0024] The beneficial effects of this application are as follows: The first positive electrode active material (LMFP) described in this application has a core-shell structure and a coating layer on its surface. Furthermore, the surface of the first positive electrode active material has grooves. It is understood that the grooves on the surface of the first positive electrode active material can increase the roughness and surface area of ​​the material. Increased roughness helps to improve the peeling force of the first positive electrode active material in the electrode sheet, because the rougher particles can better lock together after compaction, increasing their respective sliding friction. Increased specific surface area helps to improve the migration channels of lithium ions, thereby improving the migration efficiency of lithium ions. Increased specific surface area also helps to improve… The amount of coating on the surface of high-manganese iron phosphate (LMFP) particles can effectively improve the stability of LMFP, isolate direct contact between LMFP and electrolyte, reduce the erosion of Mn-O bonds by corrosive substances such as HF in the electrolyte, inhibit the acid dissolution of Mn, and improve the conductivity of the first positive electrode active material. This forms a continuous lithium-ion pathway, increases the lithium-ion transport rate, enhances the reactivity of the first positive electrode active material, prevents the structure of the first positive electrode active material from being damaged during charging and discharging, improves the stability and thermal stability of the positive electrode active material during processing, and effectively improves the cycle performance and safety performance of the secondary battery. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the first positive electrode active material of this application.

[0026] Figure label: 1. Lithium manganese iron phosphate; 2. Coating layer disposed on the outer surface of lithium manganese iron phosphate; 3. Groove on the surface of the first positive electrode active material. Detailed Implementation

[0027] 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.

[0028] 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.

[0029] 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.

[0030] The inventors of this application have discovered that, compared to ternary cathode active materials (lithium nickel cobalt manganese oxide), lithium manganese iron phosphate cathode active material (LMFP) has significant advantages in terms of safety and cost. LMFP, as a novel cathode active material, combines the safety of lithium iron phosphate (LFP) with a higher operating voltage; however, it is prone to Mn oxidation during cycling. 2+ Leaching leads to capacity decay, severely impacting applications in scenarios with specific requirements such as long battery life and fast charging.

[0031] Therefore, based on the above-mentioned problems, embodiments of this application provide a positive electrode active material, including a first positive electrode active material, such as... Figure 1 As shown, the first positive electrode active material includes lithium manganese iron phosphate 1 and a coating layer 2 disposed on the outer surface of the lithium manganese iron phosphate, the coating layer including phosphate; the surface of the first positive electrode active material has grooves 3.

[0032] The first positive electrode active material (LMFP) described in this application has a core-shell structure and a coating layer on its surface. The surface of the first positive electrode active material also has grooves. It is understood that the grooves on the surface of the first positive electrode active material increase the material's roughness and surface area. Increased roughness helps improve the peeling force of the first positive electrode active material in the electrode sheet because the rougher particles can better lock together after compaction, increasing their sliding friction. Increased specific surface area helps improve the migration channels for lithium ions, thereby improving lithium ion migration efficiency. Increased specific surface area also helps improve the efficiency of lithium ion migration. The amount of coating on the surface of lithium particles can effectively improve the stability of lithium manganese iron phosphate (LMFP), isolate LMFP from direct contact with the electrolyte, reduce the erosion of Mn-O bonds by corrosive substances such as HF in the electrolyte, inhibit the acid dissolution of Mn, improve the conductivity of the first positive electrode active material, form a continuous lithium-ion pathway, increase the lithium-ion transport rate, improve the reactivity of the first positive electrode active material, avoid the structural damage of the first positive electrode active material during charging and discharging, improve the stability and thermal stability of the positive electrode active material during processing, and effectively improve the cycle performance and safety performance of the secondary battery.

[0033] In some embodiments, the average depth of the grooves on the surface of the first positive electrode active material is 59-88 nm, for example, it can be a range of 59 nm, 60 nm, 62 nm, 65 nm, 70 nm, 72 nm, 75 nm, 76 nm, 78 nm, 80 nm, 82 nm, 85 nm, 88 nm or any two of these values. By controlling the average groove depth on the surface of the first positive electrode active material to be within this range, the specific surface area of ​​the first positive electrode active material can be increased, the interfacial bonding strength can be improved, and the cycle performance and safety performance of the secondary battery can be effectively improved.

[0034] The average groove depth was obtained using atomic force microscopy (AFM) three-dimensional profile scanning. AFM offers high resolution (nanometer level) and can accurately assess changes in surface microstructure, making it particularly suitable for evaluating etching depth distribution. The sample preparation steps are as follows: the positive electrode active material sample is evenly sprinkled onto a clean silicon wafer or mica sheet, excess powder is removed to reduce stacking, isopropanol is dropped onto the sample surface to help the particles spread and dry and fix them, and AFM is used for scanning to obtain a three-dimensional morphology image of the particle surface in non-contact mode. Height difference analysis is performed on single or multiple particles using imaging software to calculate the height difference between the recessed and raised areas, obtaining depth information at multiple points. The average groove depth is obtained by averaging the groove depth values ​​from multiple particles (10 particles).

[0035] In some embodiments, the area ratio of the grooves on the surface of the first positive electrode active material is 15.2% to 37.8%, for example, it can be 15.2%, 15.5%, 16%, 18%, 20%, 22%, 25%, 26%, 28%, 30%, 32%, 34%, 35%, 36%, 37%, 37.5%, 37.8%, or any two of these values. By controlling the area ratio of the grooves on the surface of the first positive electrode active material within this range, the adhesion of the phosphate coating layer can be effectively promoted, forming a more uniform and dense phosphate coating layer, effectively increasing the nucleation density of phosphate, and more effectively inhibiting Mn. 2+ Dissolution improves the stability of the first positive electrode active material and enhances its structural stability. It forms a continuous electron / ion transport path, avoids excessive exposure of the crystal surface of lithium manganese iron phosphate, which would affect the structural stability of the material and the continuity of the electron / ion transport path, and effectively improves the cycle performance and safety performance of the secondary battery.

[0036] The area ratio of the grooves on the surface of the first positive electrode active material was obtained by the specific surface area method (BET adsorption method combined with image analysis correction). The sample preparation steps were as follows: The positive electrode active material samples before and after etching were dried (120℃, vacuum for 6 hours) to remove moisture and adsorbed impurities. Nitrogen adsorption was measured using a specific surface area analyzer (such as the Micromeritics ASAP series) to obtain the BET specific surface area (m² / g), and the change in BET specific surface area before and after etching was recorded (ΔS = S_etched – S_unetched). Since the surface area per unit mass of sample is positively correlated with the change in particle morphology, the ΔS part can be considered as the "new surface area" caused by the increase in surface grooves. The groove area on the surface of the first positive electrode active material = (ΔS / S) unetched )*100%. Alternatively, X-ray nano-CT can be used to construct a three-dimensional volume to calculate the proportion of the groove area on the surface of the first positive electrode active material. Specifically, the surface of the first positive electrode active material before etching is simulated, and the protrusions on the surface of the first positive electrode active material particles are taken as points on the surface before etching. Then, all the protrusions are fitted to form a surface, that is, all the protrusions on the surface of the first positive electrode active material particles are fitted as points on a curved surface. After fitting, the surface area S1 before etching is obtained; X-ray nano-CT is used to construct a three-dimensional volume to calculate the actual surface area S2 after etching. Then, the proportion of the groove area on the surface of the first positive electrode active material is (S2-S1) / S1*100%.

[0037] In some embodiments, the thickness of the coating layer is 5–30 nm, for example, it can be a range of 5 nm, 6 nm, 8 nm, 10 nm, 12 nm, 15 nm, 16 nm, 20 nm, 22 nm, 25 nm, 26 nm, 28 nm, 30 nm, or any two of these values. By controlling the thickness of the coating layer within this range, Mn can be effectively suppressed. 2+ Dissolution prevents corrosion by the electrolyte and induces the formation of more stable surface crystal planes, achieving a balance between ion conduction and interfacial stability. This allows for electrochemical passivation while maintaining good electron / ion transport performance and suppressing Fe... 2+ / Mn 2+ Migrate into the electrolyte.

[0038] The method for detecting the thickness of the coating layer is as follows: using TEM cross-sectional characterization, 20 first positive electrode active materials are randomly selected from the same sample, and the coating layer thickness is measured at 3 points on each of the first positive electrode active materials. The average value is then calculated to obtain the thickness of the coating layer.

[0039] In some embodiments, the phosphate includes at least one of lithium phosphate, iron phosphate, and manganese phosphate.

[0040] In some embodiments, the first positive electrode active material has a mass percentage content of 30% to 100% in the positive electrode active material, for example, it can be a range of 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, or any two of these values.

[0041] In some embodiments, the first positive electrode active material satisfies: 7≤(D90-D10) / D50≤11, for example, it can be a range of 7, 8, 9, 10, 11 or any two of these values; D90μm is the particle size corresponding to the volume cumulative distribution percentage of the first positive electrode active material reaching 90%. D50μm is the particle size corresponding to the volume cumulative distribution percentage of the first positive electrode active material reaching 50%. D10μm is the particle size corresponding to the cumulative volume distribution percentage of the first positive electrode active material reaching 10%.

[0042] This application controls the first positive electrode active material to satisfy: 7≤(D90-D10) / D50≤11, which can effectively improve the structural stability, compaction density, and packing density of the first positive electrode active material, effectively shorten the lithium ion diffusion path, and the coexistence of multiple particle sizes makes it easier for the conductive agent / electrolyte to fully penetrate the gaps between particles, improving the dual transport channels of electrons and ions; the small particles form an "encircling" coverage on the surface of the large particles, which is conducive to increasing the contact area between particles and improving the overall reaction uniformity.

[0043] In some implementations, the following condition is satisfied: 0.1≤D10≤0.4, for example, it can be a range of 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 or any two of these values.

[0044] In some implementations, the condition 0.5 ≤ D50 ≤ 2 is satisfied, for example, it can be a range of 0.5, 0.6, 0.8, 1, 1.2, 1.5, 1.6, 1.8, 2 or any two of these values.

[0045] In some implementations, the following condition is satisfied: 5≤D90≤10, for example, it can be 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10 or a range of any two of these values.

[0046] In some embodiments, the positive electrode active material further includes lithium iron phosphate, wherein the mass percentage of lithium iron phosphate in the positive electrode active material is greater than 0 and less than or equal to 70%, for example, it can be 0.1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any two of these values. By mixing the above-mentioned specific first positive electrode active material with lithium iron phosphate, the continuity of the conductive path of the first positive electrode active material can be effectively improved, the electrochemical stability can be improved, and the structural stability can be maintained more effectively during cycling. This avoids the structural damage of the positive electrode active material during charging and discharging, avoids the structural damage of the positive electrode active material during rolling, and reduces the reactivity of Mn on the surface of the first positive electrode active material with the electrolyte, reducing the occurrence of side reactions and effectively improving the cycle performance and safety performance of the secondary battery.

[0047] In some embodiments, the lithium iron phosphate satisfies: 1≤(D90'-D10') / D50'≤5, for example, it can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or a range of any two of these values; D90'μm is the particle size corresponding to the cumulative volume distribution percentage of the lithium iron phosphate reaching 90%; D50'μm is the particle size corresponding to the cumulative volume distribution percentage of the lithium iron phosphate reaching 50%; D10'μm is the particle size corresponding to a cumulative volume distribution percentage of 10% for the lithium iron phosphate.

[0048] This application controls lithium iron phosphate to satisfy 1≤(D90'-D10') / D50'≤5, which can improve the structural stability of lithium iron phosphate, keep its discharge platform stable and the charge-discharge process uniform. By designing the particle size distributions of the first positive electrode active material and lithium iron phosphate, a framework structure and a hierarchical pore network are constructed to improve the filling efficiency and stability, and the synergistic construction of the two materials in terms of physical structure, electrochemical performance, and conductive network is achieved, thereby enhancing the cycle performance, rate capability, volumetric energy density, and safety of the overall electrode.

[0049] Particle size test of lithium iron phosphate: Take 0.1 g of the positive electrode active material, evenly coat it on the conductive tape, blow off the excess part, and then use EDS (energy dispersive spectrometer) technology to distinguish LMFP and LFP (distinguished by the signal of Mn element) after magnifying 300 times with a scanning electron microscope. Calibrate the particle size and count the particle size distribution of LFP in the electron microscope image magnified 300 times, and calculate D90'μm, D50'μm, and D10'μm, so as to obtain (D90'-D10') / D50'.

[0050] In some embodiments, it satisfies 0.1≤D10'≤0.3, for example, it can be 0.1, 0.15, 0.2, 0.25, 0.3 or the range composed of any two of these values.

[0051] In some embodiments, it satisfies 0.3≤D50'≤1, for example, it can be 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or the range composed of any two of these values.

[0052] In some embodiments, it satisfies 3≤D90'≤5, for example, it can be 3, 3.5, 4, 4.5, 5 or the range composed of any two of these values.

[0053] In some embodiments, the lithium manganese iron phosphate includes a compound with the general formula Li x Mn y Fe 1-y M1 z PO4, where 0.9≤x≤1.1, 0<y<1, 0≤z≤0.05, and M1 includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

[0054] In some embodiments, the lithium manganese iron phosphate includes a compound with the general formula Li x Mn y Fe 1-yCompounds of PO4. In this scheme, the outer surface of lithium manganese iron phosphate has a coating layer, which improves the stability of lithium manganese iron phosphate. Lithium manganese iron phosphate includes compounds with the general formula Li. x Mn y Fe 1-y Compounds of PO4, without the addition of other elements, help to improve the specific capacity of positive electrode active materials.

[0055] In some embodiments, the lithium iron phosphate comprises lithium with the general formula Li a Fe 1-b M2 b The compound shown in PO4, wherein 0.9 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, and M2 is selected from at least one of Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Ti, V, Mg, and Al.

[0056] In some embodiments, the lithium iron phosphate comprises lithium with the general formula Li a Fe 1-b The compound shown in PO4, without the addition of other elements, helps to improve the specific capacity of the positive electrode active material.

[0057] In some embodiments, the preparation method of the first positive electrode active material is as follows: Lithium manganese iron phosphate with a D50 particle size of 10 μm was ball-milled, etched by immersion in etching solution, washed, and dried to obtain a precursor. The precursor was added to an ammonium phosphate solution, and ammonia was added to bring the pH to 7.8–8.2. The mixture was then calcined to obtain the first positive electrode active material.

[0058] In some embodiments, the milling medium is an ethanol solution, and the milling balls are zirconia balls.

[0059] In some embodiments, the ball mill speed is 100 to 300 rpm, for example, it can be 100 rpm, 120 rpm, 150 rpm, 180 rpm, 200 rpm, 220 rpm, 250 rpm, 280 rpm, 300 rpm or any two of these values.

[0060] In some embodiments, the ball milling time is 20 to 60 minutes, for example, it can be 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes or any two of these values.

[0061] In some embodiments, the volume concentration of the ethanol solution is 40-80%, for example, it can be 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, or any two of these values.

[0062] In some embodiments, during ball milling, the ratio of lithium manganese iron phosphate, milling media, and milling balls is 1g:(2-5)mL:(10-30)g.

[0063] In some embodiments, the etching solution includes citric acid, a complexing agent, and water.

[0064] In some embodiments, the molar concentration of citric acid in the etching solution is 0.01 to 0.1 mol / L, for example, it can be 0.01 mol / L, 0.02 mol / L, 0.03 mol / L, 0.05 mol / L, 0.06 mol / L, 0.08 mol / L, 0.1 mol / L, or any two of these values.

[0065] In some embodiments, the immersion time in the etching solution is 5 to 30 minutes, for example, it can be 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes or any two of these values.

[0066] In some embodiments, the molar concentration of the ammonium phosphate solution is 0.01 to 0.1 mol / L, for example, it can be 0.01 mol / L, 0.02 mol / L, 0.03 mol / L, 0.05 mol / L, 0.06 mol / L, 0.08 mol / L, 0.1 mol / L, or any two of these values.

[0067] In some embodiments, the ammonium phosphate solution comprises ammonium phosphate and an ethanol solution, wherein the volume concentration of the ethanol solution is 40-80%, for example, it may be 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, or any combination of two of these values.

[0068] In some embodiments, the calcination temperature is 350–450°C, for example, it can be 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, or a range of any two of these values.

[0069] In some embodiments, the calcination time is 1 to 4 hours, for example, it can be 1 hour, 2 hours, 3 hours, 4 hours or any two of these values.

[0070] A second aspect of this application provides a secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including the positive active material described above.

[0071] In this application, there is no particular limitation on the type of 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.

[0072] 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.

[0073] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0078] 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.

[0079] In one embodiment, the conductive agent includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] The present application is further illustrated below with specific embodiments: Example 1 A method for preparing a secondary battery includes the following steps: (1) Preparation of the first positive electrode active material Prepare lithium manganese iron phosphate (LiFe) with a D50 particle size of 10 μm. 0.5 Mn 0.5 Lithium manganese iron phosphate (LMFP) was ball-milled at 200 rpm for 30 min. After ball milling, the LMFP material was pre-dried under vacuum (120℃, 6 h) to remove adsorbed water. The ball milling medium was a 50% ethanol solution, and the grinding balls were zirconia balls. The ratio of lithium manganese iron phosphate, ball milling medium, and grinding balls during ball milling was 1 g: 3 mL: 20 g.

[0092] An aqueous solution (etching solution) containing 0.05 mol / L citric acid and 0.025 mol / L complexing agent (EDTA) was prepared. Lithium manganese iron phosphate was dispersed in the etching solution and stirred at 70°C for 10 min. The mixture was then subjected to ultrasonic-assisted stirring for 10 min. The reaction residue was neutralized with ammonia buffer solution at pH 7.5, washed, and vacuum dried (80°C, 12 h) to obtain the precursor. Prepare an ethanol solution with a volume concentration of 50 wt%. Disperse ammonium phosphate with a concentration of 0.02 mol / L in the ethanol solution and stir until homogeneous to obtain an ammonium phosphate solution (i.e., coating solution). Add the precursor to the ammonium phosphate solution (solid-liquid ratio of 1 kg: 10 L). Add ammonia solution dropwise until the pH of the solution is 8 to promote uniform deposition of phosphate in the surface grooves. Remove the solution and calcine it at 400 °C for 2 h in an oxygen atmosphere to form a phosphate coating layer (a mixed coating layer of iron phosphate, lithium phosphate and manganese phosphate), thus obtaining the first positive electrode active material.

[0093] The first positive electrode active material obtained through the above steps includes lithium manganese iron phosphate and a coating layer disposed on the outer surface of lithium manganese iron phosphate; the surface of the first positive electrode active material has grooves.

[0094] The average depth of the grooves on the surface of the first positive electrode active material is 69 nm, and the area of ​​the grooves on the surface of the first positive electrode active material accounts for 25.3%.

[0095] (2) Preparation of the positive electrode sheet: The first positive electrode active material, conductive agent Super P, and PVDF binder were mixed uniformly at a mass ratio of 97.25:2:0.75, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to prepare a slurry with a solid content of 60%. The slurry was then coated onto both sides of an aluminum foil (coating thickness 100 μm), baked at 120℃ for 2 hours, and rolled until the compaction density reached 2.3 g / cm³. 3 After slitting and cutting, positive electrode sheets are obtained.

[0096] (3) Preparation of negative electrode sheet: The negative electrode sheet was prepared using 95% graphite + 2% conductive carbon black + 1% CMC (sodium carboxymethyl cellulose) + 2% SBR (styrene-butadiene rubber). Graphite with a D50 particle size of 15 μm was divided into two batches: the first batch was 50%, and the second batch was 45%. After adding 50% graphite to a mixing tank, 2% conductive carbon black + 1% CMC (sodium carboxymethyl cellulose) + 2% SBR (styrene-butadiene rubber) were added, followed by the remaining 45% graphite. Deionized water (solid:water = 6:4 mass ratio) was poured in, and the mixture was stirred (300 rpm) for 4 hours. The slurry was coated onto copper foil (5 μm) to a single-sided coating thickness of 60 μm (double-sided coating), baked at 80°C for 2 hours, and then rolled until the compaction density reached 1.4 g / cm³. 3 After slitting and cutting, negative electrode sheets are obtained.

[0097] (4) Preparation of electrolyte: Prepare 1 L of solvent with a solvent volume ratio of ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 1:1:1. Add 151.91 g of LiPF6 (1 mol / L) and pre-stir at 25 °C for 1 h (magnetic rotor speed 100 rpm). Add 19.28 g of 1.5% wt vinylene carbonate (VC) and stir at 25 °C for 2 h (magnetic rotor speed 100 rpm). Set aside.

[0098] (5) Separator: 8μm polypropylene diaphragm.

[0099] (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 and negative electrode. After winding, hot pressing and shaping, Hi-pot test, and electrode tab welding, a bare cell is obtained. The bare cell is packaged in an outer aluminum-plastic film and baked in an oven at 100±5℃ for 48h. The electrolyte prepared above is injected into the dried battery. After standing, formation and capacity testing, a secondary battery is obtained.

[0100] The parameters of the positive electrode active material in Example 1 are shown in Table 1 and Table 2.

[0101] Examples 2-11 The difference between Examples 2-11 and Example 1 is that the ball milling speed, time, and etching solution immersion time are changed, thereby changing the average groove depth and the percentage of the recessed area.

[0102] In Example 2, the ball milling speed was 100 rpm, the ball milling time was 30 min, and the etching solution immersion time was 5 min.

[0103] In Example 3, the ball milling speed was 100 rpm, the ball milling time was 30 min, and the etching solution immersion time was 10 min.

[0104] In Example 4, the ball milling speed was 100 rpm, the ball milling time was 30 min, and the etching solution immersion time was 15 min.

[0105] In Example 5, the ball milling speed was 200 rpm, the ball milling time was 30 min, and the etching solution immersion time was 5 min.

[0106] In Example 6, the ball milling speed was 200 rpm, the ball milling time was 30 min, and the etching solution immersion time was 15 min.

[0107] In Example 7, the ball milling speed was 300 rpm, the ball milling time was 30 min, and the etching solution immersion time was 5 min.

[0108] In Example 8, the ball milling speed was 300 rpm, the ball milling time was 30 min, and the etching solution immersion time was 10 min.

[0109] In Example 9, the ball milling speed was 300 rpm, the ball milling time was 30 min, and the etching solution immersion time was 15 min.

[0110] In Example 10, the ball milling speed was 200 rpm, the ball milling time was 15 min, and the etching solution immersion time was 5 min.

[0111] In Example 11, the ball milling speed was 200 rpm, the ball milling time was 45 min, and the etching solution immersion time was 30 min.

[0112] Examples 12-14 The difference between Examples 12-14 and Example 1 is that the concentration of ammonium phosphate in the coating solution is different, thereby changing the thickness of the coating layer.

[0113] The ammonium phosphate concentration in Example 12 was 0.025 mol / L.

[0114] The concentration of ammonium phosphate in Example 13 was 0.01 mol / L.

[0115] The ammonium phosphate concentration in Example 14 was 0.05 mol / L.

[0116] Example 15 The difference between Example 15 and Example 1 is that the preparation method of the positive electrode sheet is different.

[0117] The first positive electrode active material and lithium iron phosphate (LiFePO4) were mixed uniformly at a mass ratio of 8:2 to obtain the positive electrode active material. The positive electrode active material, conductive agent Super P, and PVDF binder were mixed uniformly at a mass ratio of 97.25:2:0.75, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to prepare a slurry with a solid content of 60%. The mixed slurry was coated on both sides of aluminum foil (coating thickness of 100 μm), baked at 120℃ for 2 hours, and rolled until the compaction density was 2.3 g / cm³. 3 After slitting and cutting, positive electrode sheets are obtained.

[0118] Examples 16-19 Examples 16-19 differ from Example 15 in that the mass ratio of the first positive electrode active material and lithium iron phosphate is changed, as shown in Table 1.

[0119] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the positive electrode active material of Comparative Example 1 is lithium manganese iron phosphate with a D50 particle size of 10 μm.

[0120] Preparation of the positive electrode sheet in this comparative example: Lithium manganese iron phosphate with a D50 particle size of 10 μm, conductive agent Super P, and PVDF binder were mixed uniformly at a mass ratio of 97.25:2:0.75, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to prepare a slurry with a solid content of 60%. The slurry was then coated on both sides of an aluminum foil (coating thickness of 100 μm), baked at 120℃ for 2 hours, and rolled until the compaction density reached 2.3 g / cm³. 3 After slitting and cutting, positive electrode sheets are obtained.

[0121] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the first positive electrode active material is different (no grooves on the surface).

[0122] Preparation of the first positive electrode active material in this comparative example (1) Preparation of the first positive electrode active material Prepare lithium iron phosphate with a D50 particle size of 10 μm; Prepare an ethanol solution with a volume concentration of 50 wt%. Disperse ammonium phosphate with a final concentration of 0.02 mol / L in the ethanol solution and stir until homogeneous to obtain an ammonium phosphate solution (i.e., coating solution). Add lithium manganese iron phosphate to the ammonium phosphate solution (solid-liquid ratio of 1 kg: 10 L). Add ammonia solution dropwise until the pH of the solution is 8 to promote uniform deposition of phosphate in the surface grooves. Remove the solution and calcine it at 400 °C for 2 h in an oxygen atmosphere to form a phosphate coating layer, thus obtaining the first positive electrode active material.

[0123] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the first positive electrode active material is different (no coating layer on the surface).

[0124] (1) Preparation of the first positive electrode active material Lithium manganese iron phosphate (LiFe) with a D50 particle size of 10 μm was used. 0.5 Mn 0.5 Lithium manganese iron phosphate (LMFP) was ball-milled at 200 rpm for 30 min. After ball milling, the LMFP material was pre-dried under vacuum (120℃, 6 h) to remove adsorbed water. The ball milling medium was a 50% ethanol solution, and the grinding balls were zirconia balls. The ratio of lithium manganese iron phosphate, ball milling medium, and grinding balls during ball milling was 1 g: 3 mL: 20 g.

[0125] An aqueous solution (etching solution) containing 0.05 mol / L citric acid and 0.025 mol / L complexing agent (EDTA) was prepared. Lithium manganese iron phosphate was dispersed in the etching solution and stirred at 70°C for 10 min. The mixture was then subjected to ultrasonic-assisted stirring for 10 min. The reaction residue was neutralized with ammonia buffer solution at pH 7.5, washed, and vacuum dried (80°C, 12 h) to obtain the first positive electrode active material.

[0126] Table 1 Table 2 Performance testing 1. Peeling force: Select electrodes of the same area, fix the electrodes on the test stage, and use an instrument to test the peel force three times and take the average value. Specifically, attach the release tape to the surface of the electrode and then test the peel force between the release tape and the electrode.

[0127] Release tape: Nitto double-sided tape; roll 5 times.

[0128] Electrode sample length: 150mm; Electrode sample width: 40mm.

[0129] Peeling speed: 50 mm / min.

[0130] Three parallel samples were tested.

[0131] Data processing: Take the average peel force from an initial stroke of 40mm to an end stroke of 120mm.

[0132] 2. Charge / discharge efficiency: Using the examples and comparative examples, perform 1C charging and 5C discharging, and record the 10th charge / discharge efficiency. (Average value of 10 batteries); Specifically, 1. Discharge the batteries at a uniform 1C rate to 2.5V; 2. Then charge at a constant current and constant voltage of 1C to 4.2V, with a cutoff current of 0.05C; 3. Let stand for 10 minutes after charging; 4. Then discharge at a constant current of 5C to 2.5V; 5. Let stand for 10 minutes; Repeat steps 2-5 for a total of 10 times, and record the 10th charge / discharge efficiency; The formula for calculating charge / discharge efficiency is: 100% * (10th 5C discharge capacity) / (10th 1C charge capacity); 3. Mn dissolution amount: The amount of Mn dissolution was analyzed in the electrolyte composition of the secondary battery after the 5C capacity retention test.

[0133] Test method: After the battery had completed 800 cycles, it was disassembled, and 2 ml of electrolyte was squeezed out. The Mn content was detected using an elemental analysis instrument. The cycling steps were as follows: 1. Discharge the battery at a 1C rate to 2.5V; 2. Then charge it at a 1C rate with constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; 3. Let it stand for 10 minutes after charging; 4. Then discharge it at a 1C rate with constant current to 2.5V; 5. Let it stand for 10 minutes. Repeat steps 2-5 a total of 800 times.

[0134] 4. Cycle capability test: The batteries were charged and discharged at a rate of 1C in a constant temperature chamber at 25℃ for 800 cycles. The average value of 10 secondary batteries was taken. Specifically, 1. The batteries were discharged uniformly at a rate of 1C to 2.5V; 2. Then, they were charged at a rate of 1C with constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; 3. After charging, the batteries were allowed to stand for 10 minutes; 4. Then, they were discharged at a rate of 1C with constant current to 2.5V; 5. The batteries were allowed to stand for 10 minutes; Steps 2-5 were repeated for a total of 800 cycles, and the discharge capacity of the first and 800th cycles was recorded. Cycle capacity retention = 100% * (800th discharge capacity) / (1st discharge capacity) Table 3 As can be seen from Table 3, the first positive electrode active material (LMFP) described in this application has a core-shell structure with a coating layer on its surface. Furthermore, the surface of the first positive electrode active material has grooves, which can effectively improve the stability of lithium manganese iron phosphate and effectively suppress Mn. 2+Dissolution improves the conductivity of the first positive electrode active material, effectively enhances its reactivity, prevents structural damage during charging and discharging, improves its stability during processing, and effectively enhances the cycle performance and safety performance of the secondary battery.

[0135] 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 first positive electrode active material comprises lithium iron manganese phosphate and a coating layer arranged on the outer surface of the lithium iron manganese phosphate, the coating layer comprising a phosphate; the surface of the first positive electrode active material has grooves.

2. The positive electrode active material according to claim 1, characterized by The average depth of the grooves on the surface of the first positive electrode active material is 59-88 nm.

3. The positive electrode active material according to claim 1, characterized by The area ratio of the grooves on the surface of the first positive electrode active material is 15.2-37.8%.

4. The positive electrode active material according to claim 1, characterized by The thickness of the coating layer is 5-30 nm; and / or The phosphate comprises at least one of lithium phosphate, iron phosphate and manganese phosphate.

5. The positive electrode active material according to claim 1, characterized by The mass percentage of the first positive electrode active material in the positive electrode active material is 30-100%.

6. The positive electrode active material according to claim 1, characterized by The first positive electrode active material satisfies 7≤(D90-D10) / D50≤11; D90μm is the particle size corresponding to the cumulative volume percentage of 90% of the first positive electrode active material; D50μm is the particle size corresponding to the cumulative volume percentage of 50% of the first positive electrode active material; D10μm is the particle size corresponding to the cumulative volume percentage of 10% of the first positive electrode active material.

7. The positive electrode active material according to claim 1, characterized by The positive electrode active material further comprises lithium iron phosphate, the mass percentage of the lithium iron phosphate in the positive electrode active material being greater than 0 and less than or equal to 70%.

8. The positive electrode active material according to claim 7, characterized by The lithium iron phosphate satisfies 1≤(D90'-D10') / D50'≤5; D90'μm is the particle size corresponding to the cumulative volume percentage of 90% of the lithium iron phosphate; D50'μm is the particle size corresponding to the cumulative volume percentage of 50% of the lithium iron phosphate; D10'μm is the particle size corresponding to the cumulative volume percentage of 10% of the lithium iron phosphate.

9. The positive electrode active material according to any one of claims 1 to 8, characterized by The lithium iron manganese phosphate includes a compound of the general formula Li x Mn y Fe 1-y M1 z PO4, wherein 0.9≤x≤1.1, 0 y<1, 0≤z≤0.05, M1 includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr, and / or The lithium iron phosphate includes a compound represented by a general formula of Li a Fe 1-b M2 b PO4, wherein 0.9≤a≤1.1, 0≤b≤0.9, and M2 is selected from at least one of Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Ti, V, Mg, Al.

10. The positive electrode active material according to claim 9, characterized by The lithium manganese iron phosphate includes a compound of the general formula Li x Mn y Fe 1-y PO4; and / or The lithium iron phosphate includes a compound represented by a general formula of Li a Fe 1-b PO4.

11. A secondary battery characterized by comprising: The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising the positive electrode active material according to any one of claims 1-10.

12. An electrical device, characterized by The secondary battery according to claim 11 is used as a power supply for the electric device.

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