A positive electrode active material, a secondary battery, and an electric device

Abstract

The application provides a positive electrode active material, a secondary battery and an electric device. The positive electrode active material of the application comprises an aluminum-doped lithium iron phosphate core and a carbon layer arranged on the surface of the core; the concentration of aluminum in the core decreases from the surface to the geometric center; and 5≤K≤20, K=(C1 / C2)×(I D / I G )×1000 / (Dv50×T) is satisfied; C1 is the average atomic concentration of aluminum in the surface region of the core; C2 is the average atomic concentration of aluminum in the central region of the core; I D is the intensity of the D peak of the positive electrode active material in the Raman spectrum; I G is the intensity of the G peak of the positive electrode active material in the Raman spectrum; Dv50 is the particle size corresponding to the volume accumulation of 50% of the positive electrode active material; and T is the thickness of the carbon layer. The positive electrode active material of the application can achieve higher rate performance, longer cycle life and excellent high-temperature stability when applied to a secondary battery.

Description

Technical Field

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

[0002] The high-value utilization of retired lithium iron phosphate (LFP) batteries has become a key issue in the industry. Current technological approaches face challenges such as: the core electrochemical performance of the obtained recycled LFP materials differs significantly from that of traditional commercially available new products, limiting their application in high-performance scenarios. Conventional recycled materials struggle to construct in-situ grown, strongly coupled, highly efficient conductive networks on their surfaces; physically mixed carbon materials exhibit high contact resistance with particles and fail to provide sufficient ion exchange active sites, resulting in low electronic conductivity and lithium-ion diffusion coefficients. Furthermore, unavoidable trace metal impurities (such as aluminum) in retired raw materials often exist in inactive or inert forms during traditional recycling processes. These impurities not only fail to contribute to performance but also become localized stress concentration points or catalytic centers for side reactions, disrupting the material's uniformity and mechanical integrity. This leads to poor long-term cycle performance of the secondary battery, and under conditions such as high temperature and high rate, the capacity of the secondary battery decays rapidly.

[0003] Therefore, there is an urgent need to develop recycled lithium iron phosphate cathode materials to overcome the dual bottlenecks of traditional recycled materials in charge transport dynamics and structural stability. Summary of the Invention

[0004] The purpose of this application is to improve the long-term cycle performance, high-temperature stability, and high-rate performance of secondary batteries using recycled lithium iron phosphate cathode active materials. This involves improving the cathode active material by providing an aluminum-doped lithium iron phosphate core with a carbon-coated surface. This material can improve the fast-charging performance of secondary batteries, reduce side reactions at the electrode (cathode)-electrolyte interface and the dissolution of active metal ions, significantly improve the structural stability of the battery during long-term cycling, and enhance the cycle life of secondary batteries under high-temperature and high-rate conditions.

[0005] To achieve the above objectives, a first aspect of this application provides a positive electrode active material, comprising an aluminum-doped lithium iron phosphate core and a carbon layer disposed on the surface of the core; wherein the aluminum concentration in the core decreases from the surface to the geometric center; the positive electrode active material satisfies: 5≤K≤20, K=(C1 / C2)×(I D / I G )×1000 / (Dv50×T); C1 represents the average atomic concentration of aluminum in the surface region of the core, expressed in %; the surface region is the area extending from the core surface toward the center to 50% of the radius of the core. C2 represents the average atomic concentration of aluminum in the central region of the core, expressed in %. The central region is a spherical region with the geometric center of the core as its center and a radius of 10% of the core's radius. I D This indicates the intensity of the D peak in the Raman spectrum of the positive electrode active material; I G This indicates the intensity of the G peak in the Raman spectrum of the positive electrode active material; Dv50 represents the particle size when the cumulative volume of the positive electrode active material reaches 50%, in nm. T represents the thickness of the carbon layer, measured in nm.

[0006] As an embodiment of this application, the value of C1 in the positive electrode active material satisfies: 0.3%≤C1%≤2.0%.

[0007] As an embodiment of this application, the value of C2 in the positive electrode active material satisfies: C2%≤0.5%.

[0008] As an embodiment of this application, the positive electrode active material satisfies: C1 / C2≥2.

[0009] As an embodiment of this application, the X-ray photoelectron spectrum of the positive electrode active material satisfies the following: the full width at half maximum (FWHM) of the 2p peak of aluminum is 1.9-2.3 eV.

[0010] As an embodiment of this application, the X-ray photoelectron spectrum of the positive electrode active material satisfies the following: the 1s peak binding energy of lithium is 55-55.3 eV.

[0011] As an embodiment of this application, the thickness T of the carbon layer satisfies the following condition: 1nm ≤ T nm ≤ 10nm.

[0012] As an embodiment of this application, in the Raman spectrum of the positive electrode active material, the ratio of the D peak intensity to the G peak intensity is I. D / I G It ranges from 1.0 to 1.8.

[0013] As an embodiment of this application, the Raman spectrum of the positive electrode active material satisfies: 50 cm⁻¹ -1 ≤ D Fw≤80cm -1 , D Fw represents the full width at half maximum (FW) of the D peak in the Raman spectrum.

[0014] As an embodiment of this application, the Raman spectrum of the positive electrode active material satisfies: 50 cm⁻¹ -1 ≤ G Fw≤80cm-1 , G Fw represents the full width at half maximum (FW) of the G peak in the Raman spectrum.

[0015] As an embodiment of this application, the particle size Dv50 of the primary particles of the positive electrode active material satisfies: 80nm≤Dv50 nm≤300nm.

[0016] A second aspect of this application also provides a secondary battery, including a positive electrode sheet, said positive electrode sheet comprising the positive electrode active material as described in the first aspect of this application.

[0017] As an embodiment of this application, the positive electrode includes a current collector and a positive electrode active material layer disposed on at least one surface of the current collector and containing the positive electrode active material as described in the first aspect of this application.

[0018] As an embodiment of this application, the secondary battery further includes a negative electrode, a separator, and an electrolyte.

[0019] A third aspect of this application also provides an electrical device that includes the secondary battery described in the second aspect of this application.

[0020] Compared with the prior art, the beneficial effects of this application are: This application, by employing a specific core and carbon layer of the positive electrode active material and optimizing empirical parameters including the particle size of the positive electrode active material, the thickness of the carbon layer, the Raman spectrum, and the concentration distribution of aluminum in the core, can alleviate the problem of low intrinsic electronic conductivity and ion diffusion rate of recycled lithium iron phosphate materials, and significantly improve the fast-charging performance of the prepared secondary battery. Through gradient doping of a specific aluminum concentration in the lithium iron phosphate core, it effectively reduces side reactions at the electrode-electrolyte interface and the dissolution of active metal ions. By adopting a "gradient enhancement-core retention" mechanism, which works synergistically with the carbon layer, it greatly improves the structural stability of the material during long-term battery cycling. This application provides a positive electrode active material with excellent comprehensive electrochemical performance and realizes the closed-loop and upgraded remanufacturing of retired battery materials. Detailed Implementation

[0021] To better illustrate the purpose, technical solution, and advantages of this application, specific embodiments will be used to further describe this application below. However, these embodiments do not limit this application in any way. Unless otherwise specified, the reagents, methods, and equipment used in this application are conventional reagents, methods, and equipment in this technical field. Unless otherwise specified, all reagents and materials used in this application are commercially available.

[0022] In a first aspect of this application, an embodiment provides a positive electrode active material comprising an aluminum-doped lithium iron phosphate core and a carbon layer disposed on the surface of the core; wherein the aluminum concentration in the core decreases from the surface to the geometric center; the positive electrode active material satisfies: 5≤K≤20, K=(C1 / C2)×(I D / I G )×1000 / (Dv50×T); C1 represents the average atomic concentration of aluminum in the surface region of the core, expressed in %; the surface region is the area extending from the core surface toward the center to 50% of the radius of the core. C2 represents the average atomic concentration of aluminum in the central region of the core, expressed in %. The central region is a spherical region with the geometric center of the core as its center and a radius of 10% of the core's radius. I D This indicates the intensity of the D peak in the Raman spectrum of the positive electrode active material; I G This indicates the intensity of the G peak in the Raman spectrum of the positive electrode active material; Dv50 represents the particle size when the cumulative volume of the positive electrode active material reaches 50%, in nm. T represents the thickness of the carbon layer, measured in nm.

[0023] This application provides a cathode active material with an aluminum-doped lithium iron phosphate core and a carbon layer coated on the surface. The aluminum concentration in the core of the cathode active material decreases from the surface to the geometric center; for example, the core can be made of decommissioned lithium iron phosphate with a decreasing aluminum concentration from the surface to the geometric center. This gradient distribution of aluminum concentration (high on the outside, low on the inside) allows the aluminum dopant to effectively stabilize the crystal structure, reduce iron ion dissolution, and strengthen the interface on the particle surface, thereby greatly improving the cycle stability of the secondary battery. Inside the particle, the low aluminum concentration largely maintains the intrinsic high lithium-ion diffusion coefficient of the lithium iron phosphate cathode active material, reducing capacity and rate losses that may result from uniform doping.

[0024] The particle size distribution, carbon layer thickness, Raman spectrum, and aluminum concentration distribution in the core of the positive electrode active material, among other key parameters, synergistically influence each other by adjusting the range of the aforementioned empirical relationships, jointly determining the final comprehensive performance of the positive electrode active material. Specifically, C1 / C2 reflects the aluminum concentration gradient in the core and is used to regulate the material's surface and interface structure and stability. D / I GThe ratio of Raman spectroscopic characteristics is used to characterize the defects and activity of the material's surface and interface; Dv50 reflects the material's particle size, and T reflects the carbon layer thickness, both of which jointly affect the bulk ion transport resistance. It is understood that the empirical relationship K value defined in this application quantitatively considers both the stability and activity factors (C1 / C2 and I) of the material's surface and interface. D / I G The material achieves effective synergy between interface strengthening and fast bulk transport in its structural design when the K value meets the above-mentioned limits, as well as the geometric resistance factors (Dv50 and T) of bulk transport. This enables the material to simultaneously achieve excellent high-rate performance and high-temperature cycling stability.

[0025] The positive electrode active material of this application has a specific core-carbon layer coating structure, with the core being aluminum-doped lithium iron phosphate. The aluminum doping concentration decreases from the surface to the geometric center, providing a short-range diffusion path for lithium ions and constructing a rapid electron conduction and surface ion storage interface, significantly improving the rate performance of the secondary battery. Furthermore, by jointly controlling the material's particle size distribution, carbon layer thickness, Raman spectroscopy, and aluminum concentration distribution parameters in the core using empirical relationships, the material balances interface strengthening and rapid bulk transport. This mitigates technical problems such as side reactions at the electrode-electrolyte interface and the dissolution of active metal ions, improving the material's fast-charging performance and structural stability during long-term cycling.

[0026] In this application, the value of K satisfies: 5 ≤ K ≤ 20. More specifically, the value of K can be any one of the following: 5, 5.05, 5.10, 5.31, 5.76, 5.94, 6.76, 7.21, 7.57, 7.63, 8.10, 8.71, 9.26, 9.54, 9.61, 9.91, 10.82, 12.72, 12.79, 14.42, 14.90, 16.60, 18.03, 19.47, 19.66, 19.67, 19.92, 20.0, or an interval formed by any two values.

[0027] In some embodiments of this application, the value of C1 in the positive electrode active material satisfies: 0.3% ≤ C1% ≤ 2.0%; In some embodiments of this application, the value of C2 in the positive electrode active material satisfies: C2%≤0.5%.

[0028] In some embodiments of this application, the positive electrode active material satisfies: C1 / C2≥2.

[0029] Specifically, the average atomic concentration C1 of aluminum in the surface region of the core and the average atomic concentration C2 in the central region of the core can be obtained by quantitative analysis using STEM-EDS.

[0030] The cathode active material used in this application employs aluminum-doped lithium iron phosphate as its core. The aluminum distribution exhibits a concentration gradient from the particle surface to the geometric center, and its aluminum doping concentration conforms to the aforementioned ranges of average aluminum atomic concentration C1 in the core surface region and average aluminum atomic concentration C2 in the aforementioned core center region. Aluminum elements derived from waste are effectively doped into the crystal lattice in a concentration gradient manner, which helps stabilize the interface structure on the cathode material surface, effectively reducing iron dissolution and side reactions. Meanwhile, the low aluminum content in the core maintains the inherent high lithium-ion diffusion capacity of lithium iron phosphate materials.

[0031] In this application, C1 / C2 further satisfies: 2≤C1 / C2≤8. More specifically, the value of C1 / C2 can be any one of the following: 2, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2, 4.4, 4.6, 4.8, 5.0, 5.2, 5.4, 5.6, 5.8, 6.0, 6.2, 6.4, 6.6, 6.8, 7.0, 7.2, 7.4, 7.6, 7.8, 8.0, or any interval formed by any two values.

[0032] In some embodiments of this application, the mass content of aluminum in the core is 0.5-1.2%.

[0033] In some embodiments of this application, the molar ratio of lithium to iron in the core is (0.85-0.91):1.

[0034] In some embodiments of this application, the X-ray photoelectron spectrum of the positive electrode active material satisfies the following: the full width at half maximum (FWHM) of the 2p peak of aluminum is 1.9-2.3 eV.

[0035] In some embodiments of this application, the X-ray photoelectron spectrum of the positive electrode active material satisfies the following: the 1s peak binding energy of lithium is 55-55.3 eV.

[0036] The positive electrode active material of this application can use retired lithium iron phosphate as the core, and the doped element Al can be derived from the aluminum current collector in the retired positive electrode sheet rather than introducing new materials. It is understood that both waste-to-aluminum conversion and exogenously doped aluminum are based on Al. 3+ While the form exists, the Al 2p peak half-width at half-maximum (FWHM) of waste-converted aluminum is wider (1.9-2.3 eV), while the FWHM of exogenously doped aluminum in X-ray photoelectron spectroscopy (XPS) is narrower (1.8 eV). On the other hand, the doping energy of waste-converted aluminum more significantly changes the electron cloud density of Li in the lattice, weakens the Li-O bond, and leads to a more significant reduction in its binding energy: specifically, the binding energy of Li 1s in waste-converted aluminum is 55-55.3 eV, while the binding energy of Li 1s in exogenously doped aluminum is 55.4 eV.

[0037] In some embodiments of this application, the thickness T of the carbon layer satisfies: 1nm ≤ T nm ≤ 10nm.

[0038] Specifically, the thickness T of the carbon layer can be obtained by transmission electron microscopy.

[0039] In this application, the thickness T of the carbon layer further satisfies: 3nm ≤ T nm ≤ 5nm. More specifically, the value of T can be any one of 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, and 5.0, or a range formed by any two values.

[0040] In some embodiments of this application, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the positive electrode active material is I. D / I G The value is 1.0-1.8. It should be noted that the Raman spectroscopy test yielded the structural characteristics of the carbon in the carbon layer on the surface of the positive electrode active material of this application.

[0041] In some embodiments of this application, the Raman spectrum of the positive electrode active material satisfies: 50 cm⁻¹ -1 ≤ D Fw≤80cm -1 , D Fw represents the full width at half maximum (FW) of the D peak in the Raman spectrum.

[0042] In some embodiments of this application, the Raman spectrum of the positive electrode active material satisfies: 50 cm⁻¹ -1 ≤ G Fw≤80cm -1 , G Fw represents the full width at half maximum (FW) of the G peak in the Raman spectrum. The surface of the positive electrode active material described in this application is coated with a carbon layer, which can originate from the in-situ pyrolysis of citric acid. It is understood that the carbon layer on the surface of the positive electrode active material is rich in structural defects that can serve as active sites for rapid lithium-ion storage / extraction. The intensity ratio of the D peak to the G peak in its Raman spectrum is between 1.0 and 1.8, significantly higher than that of carbon coatings formed by the pyrolysis of conventional sugar carbon sources. Furthermore, this outer carbon layer is amorphous carbon and is strongly coupled to the surface of the positive electrode active material matrix, rather than a physically mixed conductive agent (such as carbon black) or highly graphitized carbon. The FW of both the D peak and the G peak in the Raman spectrum are greater than 50 cm⁻¹. -1 less than 80cm -1This unique structure ensures that the positive electrode active material has excellent interfacial electronic conductivity and chemical stability, which can further improve the rate performance and cycle life of the secondary battery.

[0043] In some embodiments of this application, the particle size Dv50 of the primary particles of the positive electrode active material satisfies: 80nm≤Dv50 nm≤300nm.

[0044] Specifically, the particle size Dv50 of the primary particles of the positive electrode active material can be obtained by laser particle size analysis.

[0045] It is understood that the particle size of the positive electrode active material in this application meets the above range, which helps to reduce the excessively high surface energy of the particles, reduce particle agglomeration, and reduce interfacial side reactions; at the same time, it reduces the diffusion path of lithium ions in the solid phase, which helps to improve rate performance.

[0046] In this application, the particle size Dv50 of the primary particles of the positive electrode active material further satisfies: 100nm ≤ Dv50nm ≤ 200nm. More specifically, the value of Dv50 can be any one of 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, or a range formed by any two values.

[0047] In some embodiments of this application, the preparation method of the positive electrode active material includes the following steps: S1. Oxalic acid and citric acid are added sequentially to the retired lithium iron phosphate cathode material powder and dispersed evenly to obtain a suspension slurry. S2. The suspension slurry is reacted at 150-170°C, and the solid is separated and dried to obtain a precursor powder; S3. The precursor powder is reacted under argon protection at 620-670°C, cooled in a continuous argon flow, pulverized and sieved to obtain the positive electrode active material.

[0048] In some embodiments of this application, the method for preparing the decommissioned lithium iron phosphate cathode material powder includes the following steps: (1) Immerse the retired lithium iron phosphate power battery in sodium chloride aqueous solution for discharge, take it out, and disassemble and separate the positive electrode under argon protection; (2) The positive electrode sheet is treated with N-methylpyrrolidone at 70-90℃ to remove the binder, and the resulting powder is washed until neutral, dried and sieved to obtain the retired lithium iron phosphate positive electrode material powder.

[0049] The method for preparing the positive electrode active material provided in this application can be carried out using retired lithium iron phosphate positive electrode material as raw material, wherein the aluminum comes from the positive electrode aluminum foil current collector in retired lithium iron phosphate batteries. During the above-mentioned disassembly, immersion discharge, NMP treatment and powder stripping process, a small amount of aluminum foil debris will be mixed into the positive electrode material powder, and a small amount of aluminum foil debris will be mixed into the stripped powder. Then, through hydrothermal treatment, sintering, drying and grinding processes, aluminum-doped lithium iron phosphate positive electrode material is obtained.

[0050] In a second aspect of this application, an embodiment also provides a secondary battery, including a positive electrode sheet, the positive electrode sheet comprising the positive electrode active material as described in the first aspect of this application.

[0051] Specifically, in some embodiments of this application, the positive electrode includes a current collector and a positive electrode active material layer disposed on at least one surface of the current collector and containing the positive electrode active material as described in the first aspect of this application.

[0052] In some embodiments of this application, the positive electrode active material layer further includes a conductive agent and a binder.

[0053] This application does not limit the types of conductive agents and binders. The conductive agents include, but are not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers. The binders include, but are not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0054] In some embodiments of this application, the secondary battery further includes a negative electrode, a separator, and an electrolyte.

[0055] In some embodiments of this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector and containing a negative electrode active material.

[0056] This application does not specifically limit the type of negative electrode active material. Any negative electrode active material commonly used in the art for preparing secondary batteries can be used in this application. For example, the negative electrode active material includes, but is not limited to, at least one of natural graphite, artificial graphite, soft carbon, hard carbon, nano-carbon, and silicon-carbon composites.

[0057] In some embodiments of this application, the negative electrode active material layer further includes a conductive agent, a thickener, and a binder. This application does not limit the types of conductive agents, thickeners, and binders in the negative electrode active layer. The conductive agents include, but are not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene-reduced graphene oxide, and carbon nanofibers. The thickeners include, but are not limited to, sodium carboxymethyl cellulose (CMC). The binders include, but are not limited to, polyacrylic acid (PAA).

[0058] In some embodiments of this application, the positive current collector is aluminum foil; the negative current collector may include, but is not limited to, metal foil, such as electrolytic copper foil, rolled copper foil and its surface modified products, and may also include copper alloy foil, nickel foil, stainless steel foil or polymer composite current collector with copper coating.

[0059] In some embodiments of this application, the diaphragm may include, but is not limited to, polyolefin microporous membranes, such as polyethylene or polypropylene single-layer or multi-layer diaphragms; surface-modified diaphragms, such as composite diaphragms coated with ceramic coatings such as alumina or silica, or polyvinylidene fluoride or aramid polymer coatings; and special substrate diaphragms, such as nonwoven fabric, electrospun fiber membranes, or gel polymer diaphragms.

[0060] In some embodiments of this application, the electrolyte may include, but is not limited to, a system composed of lithium salt, organic solvent, and functional additives; the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium difluorooxalate borate; the organic solvent includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; the functional additives include at least one of film-forming additives such as vinylene carbonate and 1,3-propanesulfonate lactone, and flame-retardant additives such as phosphate esters.

[0061] In some embodiments of this application, the preparation of the secondary battery includes: stacking the positive electrode, separator, and negative electrode in sequence, so that the separator is positioned between the positive and negative electrode to act as an separator, then winding it into a square bare cell, hot pressing and ultrasonic welding, then installing it into a battery casing, then baking it at 65-95°C to remove water, then injecting electrolyte, and then undergoing high-temperature wetting, formation, capacity testing, sealing sheet welding, helium detection and other processes to obtain the secondary battery.

[0062] In some embodiments of this application, the secondary battery may include an outer packaging, which 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 may also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, or an aluminum-plastic film, such as an aluminum-plastic film formed by a composite of a PA layer, an aluminum layer, and a PP layer. The shape of the secondary battery is not particularly limited; it can be cylindrical, square, or any other arbitrary shape.

[0063] In a third aspect of this application, an electrical device is also provided, which includes the secondary battery described in the second aspect of this application.

[0064] The aforementioned secondary batteries can be used in electrical devices, including the secondary batteries described in the first aspect of this application. These electrical devices can be vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools, among other applications. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. This application does not impose any special limitations on the aforementioned devices.

[0065] The following are specific embodiments of this application, and the technical solutions of this application are further described in conjunction with the embodiments. However, this application is not limited to these embodiments. Unless otherwise specified, the reagents, methods, and equipment used in this application are all conventional reagents, methods, and equipment in this technical field.

[0066] Examples 1-30, Comparative Examples 1-9 The embodiments and comparative examples of this application provide a series of positive electrode active materials and secondary batteries, the preparation of which all include the following steps: Preparation of positive electrode sheet The positive electrode active material, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 96.5:1.5:2.0, and then thoroughly dispersed in N-methylpyrrolidone (NMP) solvent to form a uniform slurry. The slurry was then coated onto both surfaces of a 15μm aluminum foil, with the coating density controlled at 180g / m². 2 The electrode sheets were then dried and rolled (compacted density 2.4 g / cm³). 3The positive electrode sheet is obtained by slitting and die-cutting.

[0067] The preparation of the positive electrode active material includes the following steps: First, the retired lithium iron phosphate cathode material powder is prepared: (1) The retired lithium iron phosphate power battery was immersed in a 5wt% sodium chloride aqueous solution for 48 hours until the open circuit voltage dropped to below 0.5 V. Then the battery was disassembled in an argon-protected glove box and the positive electrode was separated. (2) The polyvinylidene fluoride (PVDF) binder was removed by stirring with N-methylpyrrolidone (NMP) at 80°C for 4 hours. The obtained positive electrode powder was repeatedly washed with deionized water until neutral, and then vacuum dried at 80°C for 12 hours and passed through a 200-mesh sieve to obtain the retired lithium iron phosphate positive electrode material powder (black powder). The Li / Fe molar ratio was 0.88 and the aluminum (Al) content was 0.82 wt% as determined by ICP.

[0068] Then, the above-mentioned decommissioned lithium iron phosphate cathode material powder is used to prepare the cathode active material: S1. Prepare 100 mL each of 1.0 mol / L oxalic acid aqueous solution and 1.0 mol / L citric acid aqueous solution in a fume hood; weigh 50.0 g of the above-mentioned retired lithium iron phosphate cathode material powder, place it in a 500 mL polytetrafluoroethylene beaker, add 60 mL of the prepared oxalic acid solution and 60 mL of the citric acid solution in sequence, and then add deionized water to adjust the total volume to 300 mL, so that the solid content is about 15 wt%, and stir at 500 rpm for 2 hours to form a uniform suspension slurry; S2. Transfer the suspension slurry to a 500mL polytetrafluoroethylene-lined hydrothermal reactor, seal it, and place the reactor in a forced-air drying oven. Increase the temperature to 160℃ at a rate of 2℃ / min and maintain the temperature at this level for 8 hours. Allow it to cool naturally to room temperature. Transfer the product to a centrifuge tube and centrifuge at 8000rpm for 5 minutes. Discard the supernatant, wash and centrifuge three times with deionized water, and wash twice with anhydrous ethanol under the same centrifugation conditions. Place the resulting precipitate in a petri dish and dry it in a vacuum drying oven at 80℃ for 24 hours to obtain a dark gray precursor powder. S3. Weigh 10.0g of the obtained precursor powder and spread it evenly in a boat, with the powder layer thickness controlled within 3mm. Place the boat in the constant temperature zone of a tube furnace, seal the furnace, and introduce high-purity argon gas (purity >99.999%) at a flow rate of 200mL / min for 60 minutes to completely remove air from the furnace. Under argon protection, heat the furnace to 650℃ at a heating rate of 3℃ / min and maintain the temperature for 6 hours. After the holding time is completed, stop heating and allow the furnace to cool naturally to room temperature in a continuous argon flow (flow rate 100mL / min). Remove the product and grind it in an agate mortar for 5 minutes, then pass it through a 300-mesh sieve to finally obtain the positive electrode active material.

[0069] The relevant parameters of the positive electrode active materials in the following embodiments and comparative examples are detailed in Table 1, including: the average atomic concentration of aluminum in the surface region of the core, C1; the average atomic concentration of aluminum in the central region of the core, C2; and the intensity of the D peak I in the Raman spectrum of the positive electrode active material. D The intensity of the G peak I in the Raman spectrum of the positive electrode active material G ; Particle size Dv50nm when the cumulative volume of positive electrode active material reaches 50%; Carbon layer thickness T nm; K value, K=(C1 / C2)×(I D / I G )×1000 / (Dv50×T).

[0070] The average atomic concentrations of aluminum in the surface region of the core (C1%) and in the central region of the core (C2%) were obtained by quantitative analysis using STEM-EDS. Specifically, the sample was dispersed in anhydrous ethanol and then dropped onto a copper grid. Linear scanning with a step size of 2-5 nm was performed on at least 10 randomly selected particles at an accelerating voltage of 200 kV. EDS data were collected from ≥5 points in the surface region and ≥3 points in the central region for each particle. The average values ​​were taken as the surface concentration and the central concentration of the particle, respectively. The energy spectrum was quantitatively calculated using the Cliff-Lorimer method. The atomic concentrations were normalized to 100% for all detected elements (except Li) to obtain the final average values ​​C1 and C2. The thickness T nm of the carbon layer was obtained by transmission electron microscopy. The particle size corresponding to a cumulative volume of positive electrode active material reaching 50% is Dv50nm, which was obtained through laser particle size analysis.

[0071] In the following examples and comparative examples, retired lithium iron phosphate power batteries with different cycle / service conditions were used to prepare retired lithium iron phosphate materials to achieve different C1% and C2% values ​​(the higher the service temperature, the gentler the gradient between C1% and C2%; the more cycles, the higher the C1% and C2% values). Simultaneously, the C1% and C2% values ​​were controlled by combining the deionized water washing time and number of cycles in preparation step (2). Different I values ​​were achieved by adjusting the reaction temperature in S2. D / I G Different Dv50 values ​​were obtained by using different sieving conditions; different carbon layer thicknesses T nm were obtained by adjusting the amount of oxalic acid solution and citric acid solution added.

[0072] Furthermore, the difference between Comparative Example 1 and Example 3 is that the same batch of black powder was regenerated using the traditional solid-state method to prepare the positive electrode active material, which was then directly mixed with lithium carbonate and glucose and sintered at 750°C for 10 hours. The difference between Comparative Example 2 and Example 3 is that the core material was replaced with lithium iron phosphate material obtained by uniformly doping aluminum salts using a liquid-phase method with chemical reagents, simulating the same aluminum content; The difference between Comparative Example 3 and Example 3 is that only oxalic acid was used in the hydrothermal repair stage, without the addition of citric acid; The difference between Comparative Example 4 and Example 3 is that the sintering process in S3 for the obtained precursor powder was changed to rapid heating to 850°C and short-time (1h) holding. The difference between Comparative Example 5 and Example 3 is that the S1 and S2 processes are not performed, and the decommissioned lithium iron phosphate cathode material is directly sintered to obtain a carbon-free cathode active material. The difference between Comparative Example 6 and Example 3 is that the aluminum concentration on the core surface was increased to 2.1% by reducing the number of water washes and the time during the preparation of the decommissioned lithium iron phosphate cathode material. The difference between Comparative Example 7 and Example 3 is that in S3, the material is passed through a 600-mesh sieve to obtain a positive electrode active material with a smaller particle size and a Dv50 of 80 nm. The difference between Comparative Example 8 and Example 3 is that a carbon layer thickness of 2 nm was obtained by reducing the amount of oxalic acid solution and citric acid solution added. The difference between Comparative Example 9 and Example 3 is that retired lithium iron phosphate cathode material was prepared using a power battery with a higher service temperature, and the number of water washings during processing was increased to achieve different gradient distributions of C1 and C2 in the core, and to make the aluminum concentration in the center higher than that on the surface.

[0073] Table 1 Preparation of negative electrode sheet The negative electrode active material graphite, conductive carbon black, sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:1:1.5:1.5, deionized water solvent is added, and the mixture is stirred at high speed to form a uniform slurry. The slurry is coated on copper foil, dried under vacuum at 120°C, rolled and slit to obtain the negative electrode sheet.

[0074] Separator A composite membrane is used, the structure of which consists of a 3μm PVDF coating, a 14μm PP base film, and a 3μm PVDF coating arranged sequentially.

[0075] Preparation of electrolyte At room temperature (25℃), in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed in a volume ratio of 1:1:1 as a solvent, and LiPF6 was added to prepare a 1.0 mol / L solution. Then, 1% by mass of vinylene carbonate and 2% by mass of fluoroethylene carbonate additives were added and mixed evenly.

[0076] Assembly of secondary battery The positive electrode, negative electrode, separator, and electrolyte prepared above are processed into a secondary battery through processes such as winding, hot pressing, welding, baking, electrolyte injection, formation, capacity testing, and settling.

[0077] Performance testing The performance of the secondary batteries obtained in the above embodiments and comparative examples was tested. The specific test items, test methods, and results are as follows: 1. 1C Rate Cycling Performance Test: At 25±2℃, the secondary batteries obtained in the above examples and comparative examples were subjected to charge-discharge cycle tests at a charge-discharge rate of 1C / 1C within the range of 2.5~3.65V. The discharge specific capacity of the battery in the first cycle and the discharge specific capacity after 1000 cycles were recorded. The capacity retention rate after 1000 cycles = discharge specific capacity of 1000 cycles / discharge specific capacity in the first cycle × 100%. The recorded results are shown in Table 2. 2. 5C Capacity Retention Rate Test: At 25±2℃, the secondary batteries obtained in the above examples and comparative examples were first charged to 3.65V at a constant current and constant voltage of 0.33C, and then discharged to 2.5V at constant current rates of 0.33C, 0.5C, 1C, 2C, and 5C, respectively. The discharge specific capacity at each rate was recorded. The 5C discharge rate capacity retention rate = 5C discharge specific capacity / 0.33C discharge specific capacity × 100%. The recorded results are shown in Table 2. 3. Iron dissolution concentration test: After the battery cycle was completed, the positive electrode was disassembled, cleaned by soaking in dimethyl carbonate (DMC), dried, and the positive electrode coating powder was scraped off and accurately weighed. The powder was placed in a digestion vessel, and high-purity concentrated hydrochloric acid and concentrated nitric acid (volume ratio 3:1) were added. After complete digestion by microwave digestion, the solution was transferred to a fixed volume. The iron concentration in the solution was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The iron dissolution content in the positive electrode material was calculated based on the sample mass and the fixed volume. The results are shown in Table 2.

[0078] Table 2 The results above show that: This application provides a short-range diffusion path for lithium ions by coating an aluminum-doped lithium iron phosphate core with a carbon layer, thus constructing a rapid electron conduction and surface ion storage interface and significantly improving the rate performance of the secondary battery. By using retired lithium iron phosphate cathode material powder, aluminum is doped into the lattice at a specific concentration gradient, stabilizing the surface interface structure of the cathode material and effectively reducing iron dissolution and side reactions. Meanwhile, the core maintains the intrinsic high lithium ion diffusion capacity of the material. This doping characteristic, together with the Raman spectral characteristics, thickness, and particle thickness of the carbon layer, achieves a gradient enhancement-core retention mechanism, which together endows the cathode active material of this application with an ultra-long cycle life and excellent high-temperature stability.

[0079] 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 in that, The cathode material comprises an aluminum-doped lithium iron phosphate core and a carbon layer disposed on the surface of the core; in the core, the aluminum concentration decreases from the surface to the geometric center; the positive electrode active material satisfies the following: 5≤K≤20,K=(C1 / C2)×(I D / I G )×1000 / (Dv50×T); C1 represents the average atomic concentration of aluminum in the surface region of the core, expressed in %; the surface region is the area extending from the core surface toward the center to 50% of the radius of the core. C2 represents the average atomic concentration of aluminum in the central region of the core, expressed in %. The central region is a spherical region with the geometric center of the core as its center and a radius of 10% of the core's radius. I D This indicates the intensity of the D peak in the Raman spectrum of the positive electrode active material; I G This indicates the intensity of the G peak in the Raman spectrum of the positive electrode active material; Dv50 represents the particle size when the cumulative volume of the positive electrode active material reaches 50%, in nm. T represents the thickness of the carbon layer, measured in nm.

2. The positive electrode active material according to claim 1, characterized in that, It satisfies at least one of the following characteristics: （Ⅰ）0.3%≤C1%≤2.0%； （Ⅱ）C2%≤0.5%。 3. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material satisfies: C1 / C2≥2.

4. The positive electrode active material according to claim 1, characterized in that, It satisfies at least one of the following characteristics: (a) In the X-ray photoelectron spectrum, the full width at half maximum (FWHM) of the 2p peak of aluminum is 1.9–2.3 eV; (b) In the X-ray photoelectron spectroscopy, the binding energy of the 1s peak of lithium is 55-55.3 eV.

5. The positive electrode active material according to claim 1, characterized in that, The thickness T of the carbon layer satisfies the following condition: 1 nm ≤ T nm ≤ 10 nm.

6. The positive electrode active material according to claim 1, characterized in that, In the Raman spectrum of the positive electrode active material, the ratio of the D peak intensity to the G peak intensity is I. D / I G It ranges from 1.0 to 1.

8.

7. The positive electrode active material according to claim 6, characterized in that, It satisfies at least one of the following characteristics: (1) 50cm -1 ≤ D Fw≤80cm -1 , D Fw represents the full width at half maximum (FW) of the D peak in the Raman spectrum of the positive electrode active material; (2) 50cm -1 ≤ G Fw≤80cm -1 , G Fw represents the full width at half maximum (FW) of the G peak in the Raman spectrum of the positive electrode active material.

8. The positive electrode active material according to claim 1, characterized in that, The particle size Dv50 of the primary particles of the positive electrode active material satisfies the following condition: 80nm ≤ Dv50 nm ≤ 300nm.

9. A secondary battery, characterized in that, It includes a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode active material as described in any one of claims 1-8.

10. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in claim 9.

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