Lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. The lithium iron phosphate positive electrode material comprises a substrate and a coating layer coating at least part of the surface of the substrate, the matrix comprises a plurality of lithium iron phosphate particles doped with metal elements; the coating layer comprises a carbon layer and a three-dimensional conductive network inserted in the carbon layer; and the three-dimensional conductive network is formed by conductive carbon and is embedded into the matrix. Compared with the prior art, high-valence metal is uniformly dispersed in lithium iron phosphate through liquid-phase in-situ doping, and a de-intercalation channel and diffusion kinetics of lithium ions are optimized; and a three-dimensional continuous conductive network is formed inside the particles and among the particles through synergistic introduction of carbon sources in stages, so that double guarantee is provided for high-rate charge-discharge performance and long cycle life of the material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become an important cathode material in the field of lithium-ion batteries due to its stable olivine structure, excellent thermal stability, long cycle life and cost advantages. However, its low electronic conductivity, small lithium-ion diffusion coefficient and low compaction density limit its application in power batteries and energy storage systems.

[0003] To improve the electrochemical performance of lithium iron phosphate (LFP), the traditional solid-state method typically involves mixing solid titanium dioxide with iron phosphate and lithium carbonate before the wet grinding process. However, this method often leads to uneven titanium doping, and excessively high titanium doping levels can inhibit the normal growth of LFP particles, thereby reducing the material's compaction density. To compensate for this deficiency, it is often necessary to extend the sintering time, increase the sintering temperature, or even employ a secondary sintering process to improve the compaction density, but these measures significantly increase the material's production cost. Summary of the Invention

[0004] In view of this, the purpose of this invention is at least to some extent to solve one of the technical problems in the related art. To this end, this invention provides a lithium iron phosphate cathode material, its preparation method, and its application. Through in-situ liquid-phase doping, high-valence metals are uniformly dispersed in lithium iron phosphate, optimizing the lithium-ion insertion / extraction channels and diffusion kinetics, and improving the compaction density. By introducing carbon sources in stages, a three-dimensional conductive network is formed inside and between particles, providing dual protection for the high-rate charge / discharge performance and long cycle life of the cathode material, thus solving the problem of the difficulty in simultaneously achieving high compaction density and capacity ratio in existing lithium iron phosphate materials.

[0005] To solve the above-mentioned technical problems, the present invention is implemented as follows: According to one aspect of the present invention, a lithium iron phosphate cathode material is provided, comprising a matrix and a coating layer covering at least a portion of the surface of the matrix; The matrix comprises several lithium iron phosphate particles doped with metal elements; The coating layer includes a carbon layer and a three-dimensional conductive network interspersed within the carbon layer; the three-dimensional conductive network is formed of conductive carbon and embedded in the matrix.

[0006] In some embodiments, the metal element in the lithium iron phosphate particles doped with metal elements includes one or more of titanium, niobium, vanadium, magnesium, and aluminum.

[0007] In some embodiments, the molar ratio of metal element:Li:Fe:P in the lithium iron phosphate particles doped with metal elements is (0.05~0.1):(1~2):1:1.

[0008] In some of these embodiments, the thickness of the carbon layer is 2 nm to 4 nm.

[0009] In some embodiments, the conductive carbon comprises carbon nanotubes and / or conductive graphene.

[0010] In some embodiments, the particle size of the lithium iron phosphate cathode material is 200 nm to 1 μm.

[0011] In some embodiments, the compaction density of the lithium iron phosphate cathode material is >2.6 g / cm³. 3 .

[0012] According to another aspect of the present invention, the present invention provides a method for preparing the lithium iron phosphate cathode material described in the above technical solution, comprising the following steps: S1. Mix the metal source, the first carbon source, the lithium source, the iron source, the phosphorus source and water to obtain a slurry; S2. The slurry obtained in step S1 is subjected to a hydrothermal reaction with a surfactant to obtain a lithium iron phosphate precursor. S3. The lithium iron phosphate precursor obtained in step S2 is mixed with the second carbon source and then calcined under an inert atmosphere to obtain the lithium iron phosphate cathode material.

[0013] In some embodiments, in step S1: the metal source includes one or more of titanium source, niobium source, vanadium source, magnesium source, and aluminum source; the titanium source includes one or more of organotitanium chelate and organotitanium coupling agent, preferably titanium methoxide and / or tetraethyl titanate.

[0014] In some of these embodiments, in step S1: the first carbon source includes one or more of glucose, polyvinyl alcohol, sucrose, citric acid, polyethylene glycol, and phenolic resin.

[0015] In some of these embodiments, in step S1: the amount of the first carbon source added is 0.2wt% to 1wt% of the slurry.

[0016] In some of these embodiments, in step S1: the lithium source includes one or more of LiOH, LiH2PO4, Li2HPO4, and CH3COOLi.

[0017] In some of these embodiments, in step S1, the iron source includes one or more of Fe(NO3)2, FeCl2, Fe(SO4)2, and FeC2O4.

[0018] In some of these embodiments, in step S1: the phosphorus source includes one or more of H3PO4, NH4H2PO4, LiH2PO4, and Li2HPO4.

[0019] In some of these embodiments, in step S2: the surfactant includes one or more of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide, and sodium dodecyl sulfate.

[0020] In some embodiments, in step S2: the pressure of the hydrothermal reaction is 1 MPa to 5 MPa, the temperature of the hydrothermal reaction is 120°C to 160°C, and the time of the hydrothermal reaction is 4 h to 6 h.

[0021] In some embodiments, step S2 further includes filtering the obtained reaction mixture and vacuum drying the filter cake to obtain a lithium iron phosphate precursor.

[0022] In some of these embodiments, in step S3: the second carbon source includes one or more of conductive graphene and carbon nanotubes.

[0023] In some of these embodiments, in step S3: the mass ratio of the first carbon source to the second carbon source is (1~1.5):1.

[0024] In some embodiments, step S3 includes the following steps: adding water, wet grinding to adjust to the target particle size, and then spray drying; the target particle size is 500nm~700nm.

[0025] In some of these embodiments, in step S3: the inert atmosphere includes one or more of nitrogen, argon, and helium.

[0026] In some embodiments, in step S3: the calcination temperature is 750℃~800℃, and the calcination time is 6h~10h.

[0027] According to another aspect of the present invention, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material described in the above technical solution or the lithium iron phosphate cathode material prepared by the preparation method described in the above technical solution.

[0028] Implementing the technical solution of the present invention has at least the following beneficial effects: 1. This invention uses organic carbon as a "planar conductive substrate" that adheres tightly to the surface of lithium iron phosphate particles, constructing a continuous conductive interface to ensure efficient contact between electrons and the particle surface; it also uses conductive carbon as a "linear conductive bridge" that runs through the particles, connecting isolated particles through a carbon network to form a three-dimensional conductive pathway across particles, effectively shortening the electron transport path; the synergistic effect of the dual carbon sources of organic carbon and conductive carbon optimizes the material's pore structure, providing ample channels for lithium-ion migration, achieving a synergistic improvement in electron transport and ion diffusion, ultimately giving the battery superior rate discharge performance and long-cycle stability, especially suitable for the application requirements of high energy density and high-power power batteries.

[0029] 2. This invention uses liquid-phase in-situ doping of high-valence metal elements, which effectively avoids the composition segregation problem caused by insufficient element diffusion in the solid-phase method. At the same time, the hydrothermal method is used to synthesize nanoscale lithium iron phosphate particles doped with metal elements, which not only achieves uniform distribution of lithium, iron, phosphorus and high-valence metal elements in the system, but also makes the particles more rounded and the packing more compact, thereby significantly reducing the porosity of the material and increasing the compaction density. This allows the particle structure to be better maintained during cycling, thereby enhancing the cycle stability of the battery.

[0030] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0031] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0032] Figure 1 This is a SEM image of the lithium iron phosphate cathode material provided in Embodiment 1 of the present invention.

[0033] Figure 2 This is a schematic diagram of the conversion of lithium iron phosphate precursor into lithium iron phosphate cathode material according to Embodiment 1 of the present invention.

[0034] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0035] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0036] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0037] In the description of this application, "same chemical composition" should be interpreted broadly, that is, the main components of the two have the same chemical composition, or the two have substantially the same chemical composition, but may have errors or impurities within the acceptable range that can be understood by those skilled in the art.

[0038] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0039] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0040] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0041] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0042] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0043] Currently, lithium iron phosphate (LiFePO4) has become an important cathode material in the lithium-ion battery field due to its stable olivine structure, excellent thermal stability, long cycle life, and cost advantages. However, its low electronic conductivity, small lithium-ion diffusion coefficient, and low compaction density limit its application in power batteries and energy storage systems. To improve the electrochemical performance of LiFePO4, the traditional solid-state method typically involves mixing solid titanium dioxide with iron phosphate and lithium carbonate before adding a wet grinding process. However, this method easily leads to uneven titanium doping, and excessively high titanium doping can inhibit the normal growth of lithium iron phosphate particles, thereby reducing the compaction density of the material. To compensate for this defect, it is often necessary to extend the sintering time, increase the sintering temperature, or even adopt a secondary sintering process to improve the compaction density, but these measures significantly increase the production cost of the material.

[0044] Based on this, the present invention achieves uniform dispersion of high-valence metals in lithium iron phosphate through in-situ liquid-phase doping, precisely controls the cell parameters of lithium iron phosphate particles, and optimizes the lithium-ion insertion / extraction channels and diffusion kinetics. By introducing carbon sources in stages, a continuous conductive interface is constructed on the surface of lithium iron phosphate particles, forming a three-dimensional continuous conductive carbon network inside and between particles. This not only optimizes the material's pore structure and provides sufficient channels for lithium-ion migration, but also provides dual protection for the material's high-rate charge / discharge performance and long cycle life.

[0045] Specifically, the present invention adopts the following technical solution: According to one aspect of the present invention, a lithium iron phosphate cathode material is provided, comprising a matrix and a coating layer covering at least a portion of the surface of the matrix; The matrix comprises several lithium iron phosphate particles doped with metal elements; The coating layer includes a carbon layer and a three-dimensional conductive network interspersed within the carbon layer; the three-dimensional conductive network is formed of conductive carbon and embedded in the matrix.

[0046] In a specific embodiment of the present invention, the matrix comprises a plurality of lithium iron phosphate particles doped with metal elements. The metal elements in the doped lithium iron phosphate particles are high-valence metal elements, preferably including one or more of titanium, niobium, vanadium, magnesium, and aluminum, and more preferably titanium. Based on this, the molar ratio of metal element:Li:Fe:P in the doped lithium iron phosphate particles is preferably (0.05~0.1):(1~2):1:1. Doping with high-valence metal elements can regulate the cell parameters of lithium iron phosphate. The present invention uses the above molar ratio to optimize the lithium-ion insertion / extraction channels and diffusion kinetics by changing the lattice spacing and crystal morphology, thereby significantly improving the electrochemical activity and cycle stability of the cathode material.

[0047] In a specific embodiment of the present invention, the coating layer covers at least a portion of the surface of the substrate. The term "at least a portion of the surface" means that the coating layer can completely cover the substrate, or the coating layer can only cover a portion of the outer surface of the substrate, preferably completely covering it; this can better achieve the effect of conductivity.

[0048] In a specific embodiment of the present invention, the coating layer includes a carbon layer and a three-dimensional conductive network interspersed within the carbon layer. The thickness of the carbon layer is preferably 2nm to 4nm, specifically 2nm, 3nm, 4nm, or any value between these two. If the carbon layer thickness is too small, it can only cover a portion of the particle surface, leading to a break in the electron transport path and an increase in the contact resistance between particles; if the carbon layer thickness is too large, lithium ions need to traverse a longer carbon layer to reach the surface of the active material, making it impossible to complete the insertion / extraction process in a timely manner.

[0049] In a specific embodiment of the present invention, the three-dimensional conductive network is formed by conductive carbon and embedded in the matrix. The conductive carbon preferably includes carbon nanotubes and / or conductive graphene. The present invention selects the aforementioned conductive carbon, which can crosslink with the carbon layer and interlock to form a three-dimensional through-type conductive network within the matrix. This connects isolated lithium iron phosphate particles doped with metal elements, shortens the electron transport distance, and reduces charge transfer resistance. Furthermore, due to the extremely high tensile strength and elastic modulus of the aforementioned conductive carbon, its three-dimensional conductive network can serve as a mechanical support framework.

[0050] In a specific embodiment of the present invention, the particle size of the lithium iron phosphate cathode material is preferably 200 nm to 1 μm, specifically 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1 μm, or any value between the above two. In this invention, the particle size of the lithium iron phosphate cathode material has a gradient distribution structure, where small-diameter particles can fully fill the gaps between large and medium-diameter particles, effectively reducing porosity during particle packing, significantly improving the packing density of the material, and thus achieving a significant increase in the compaction density of the lithium iron phosphate material. Based on this, the compaction density of the lithium iron phosphate cathode material is >2.6 g / cm³. 3 This provides crucial structural support for the fabrication of high-energy-density battery cells.

[0051] According to another aspect of the present invention, the present invention provides a method for preparing the lithium iron phosphate cathode material described in the above technical solution, comprising the following steps: S1. Mix the metal source, the first carbon source, the lithium source, the iron source, the phosphorus source and water to obtain a slurry; S2. The slurry obtained in step S1 is subjected to a hydrothermal reaction with a surfactant to obtain a lithium iron phosphate precursor. S3. The lithium iron phosphate precursor obtained in step S2 is mixed with the second carbon source and then calcined under an inert atmosphere to obtain the lithium iron phosphate cathode material.

[0052] In this invention, the raw materials used in the above preparation method can be from commercially available sources or self-made sources that are well known to those skilled in the art, and this invention does not have any special restrictions on this.

[0053] This invention first mixes a metal source, a first carbon source, a lithium source, an iron source, a phosphorus source, and water to obtain a slurry. By utilizing the molecular-level dispersion characteristics of the liquid phase system, high-valence metal dopants can be uniformly dispersed at the atomic level in lithium iron phosphate, fundamentally avoiding problems such as element agglomeration and segregation that are prone to occur during solid-phase doping, and ensuring that the dopants can fully exert their lattice regulation role.

[0054] In a specific embodiment of the present invention, the metal source preferably includes one or more of titanium, niobium, vanadium, magnesium, and aluminum sources, more preferably titanium; the titanium source preferably includes one or more of organotitanium chelates and organotitanium coupling agents, more preferably titanium methoxide and / or tetraethyl titanate. By selecting the above-mentioned metal sources, the present invention introduces high-valence metal elements while precisely controlling the cell parameters of lithium iron phosphate. By changing the lattice spacing and crystal morphology, the lithium-ion insertion / extraction channels and diffusion kinetics are optimized, thereby significantly improving the electrochemical activity and cycle stability of the material.

[0055] In a specific embodiment of the present invention, the first carbon source preferably includes one or more of glucose, polyvinyl alcohol, sucrose, citric acid, polyethylene glycol, and phenolic resin; the amount of the first carbon source added is preferably 0.2wt% to 1wt% of the slurry, specifically 0.2wt%, 0.4wt%, 0.6wt%, 0.8wt%, 1wt%, or any value between the above two. By selecting the above-mentioned first carbon source and its addition amount, the present invention can ensure that the subsequently obtained carbon layer coats the surface of the lithium iron phosphate particles doped with metal elements, reducing the contact resistance between particles, thereby further obtaining a lithium iron phosphate cathode material that meets the performance requirements of the present invention.

[0056] In specific embodiments of the present invention, the specific selection of the lithium source is not particularly limited, and can be any lithium source well known in the art, including but not limited to at least one of LiOH, LiH2PO4, Li2HPO4, and CH3COOLi; the specific selection of the iron source is not particularly limited, and can be any iron source well known in the art, including but not limited to at least one of Fe(NO3)2, FeCl2, Fe(SO4)2, and FeC2O4; the specific selection of the phosphorus source is not particularly limited, and can be any phosphorus source well known in the art, including but not limited to at least one of H3PO4, NH4H2PO4, LiH2PO4, and Li2HPO4; the preferred molar ratio of the lithium source, the iron source, and the phosphorus source is (1~2):1:1, specifically it can be 1:1:1, 1.2:1:1, 1.4:1:1, 1.6:1:1, 1.8:1:1, 2:1:1, or any value between the above two. The present invention selects the above-mentioned lithium source, iron source, phosphorus source and dosage ratio to ensure that the subsequently obtained lithium iron phosphate precursor has good performance, thereby further obtaining a lithium iron phosphate cathode material that meets the performance requirements of the present invention.

[0057] After obtaining the slurry, the present invention performs a hydrothermal reaction between the slurry and a surfactant to obtain a lithium iron phosphate precursor. The lithium iron phosphate particles, consisting of a first carbon source and a doped metal source, are fully bonded together. The lithium iron phosphate precursor formed by the hydrothermal reaction includes a matrix precursor and a carbon layer precursor coated on the surface of the matrix precursor.

[0058] In a specific embodiment of the present invention, the surfactant preferably includes one or more of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide, and sodium dodecyl sulfate; under the regulation of the above surfactant, the present invention generates spherical nanoscale lithium iron phosphate precursors in situ, which is beneficial to improving particle morphology and conductivity.

[0059] In a specific embodiment of the present invention, the pressure of the hydrothermal reaction is preferably 1 MPa to 5 MPa, specifically 1 MPa, 2 MPa, 3 MPa, 4 MPa, 5 MPa, or any value between the two; the temperature of the hydrothermal reaction is preferably 120℃ to 160℃, specifically 120℃, 140℃, 160℃, or any value between the two; the time of the hydrothermal reaction is preferably 4h to 6h, specifically 4h, 5h, 6h, or any value between the two. By selecting the above hydrothermal reaction conditions, the present invention can achieve a uniform distribution of lithium, iron, and phosphorus elements within the system, resulting in a tightly packed lithium iron phosphate precursor. This not only significantly reduces material porosity and increases compaction density but also better maintains the integrity of the particle structure during cycling, thereby enhancing the cycle stability of the battery.

[0060] In a specific embodiment of the present invention, the hydrothermal reaction further includes: filtering the obtained reaction mixture and vacuum drying the filter cake to obtain a lithium iron phosphate precursor. Through the above-mentioned filtration and vacuum drying operations, the present invention ensures that the subsequently obtained lithium iron phosphate precursor has good performance, thereby further obtaining a lithium iron phosphate cathode material that meets the performance requirements of the present invention.

[0061] After obtaining the lithium iron phosphate precursor, the present invention mixes the obtained lithium iron phosphate precursor with a second carbon source and then calcines it under an inert atmosphere to obtain lithium iron phosphate cathode material.

[0062] In a specific embodiment of the present invention, the second carbon source includes one or more of conductive graphene and carbon nanotubes; the mass ratio of the first carbon source to the second carbon source is preferably (1~1.5):1, specifically it can be 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, or any value between the above two. Figure 2 As shown, in this invention, the amorphous carbon generated after the pyrolysis of the first carbon source is mostly granular or thin-layered, easily forming a discontinuous carbon coating layer on the surface of lithium iron phosphate particles, resulting in dispersed electron transport paths and high resistance. In contrast, the second carbon source can interlock within the lithium iron phosphate precursor to form a three-dimensional through-type conductive network. This network connects isolated lithium iron phosphate precursors, shortening the electron transport distance and reducing charge transfer resistance. Furthermore, the amorphous carbon layer formed by the pyrolysis of the first carbon source has low mechanical strength and is prone to breakage during cycling due to the volume expansion of the lithium iron phosphate particles, leading to conductive network failure and capacity decay. The second carbon source, however, possesses extremely high tensile strength and elastic modulus, and its three-dimensional conductive network can serve as a mechanical support framework.

[0063] In a specific embodiment of the present invention, the mixing process includes: adding water, wet grinding to adjust to the target particle size, and then spray drying; the target particle size is preferably 500nm~700nm, specifically 500nm, 600nm, 700nm, or any value between the above two. The present invention employs the above mixing operation on the lithium iron phosphate precursor and the second carbon source to precisely control the particle size, ensuring uniform mixing and guaranteeing that the subsequently obtained lithium iron phosphate cathode material has good performance.

[0064] In a specific embodiment of the present invention, the inert atmosphere preferably includes one or more of nitrogen, argon, and helium; the present invention selects the above-mentioned inert atmosphere to prevent the first carbon source, the second carbon source, and the metal source from oxidation during the calcination process, thereby ensuring that the subsequently obtained lithium iron phosphate cathode material has good performance.

[0065] In a specific embodiment of the present invention, the calcination temperature is preferably 750℃~800℃, specifically 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, or any value between the two; the calcination time is 6h~10h, specifically 6h, 7h, 8h, 9h, 10h, or any value between the two. The amorphous carbon generated after the pyrolysis of the first carbon source forms a discontinuous carbon layer precursor on the surface of the lithium iron phosphate particles. The second carbon source undergoes cracking and carbonization during calcination, cross-linking and overlapping with the carbon layer precursor, ultimately forming a carbon layer and constructing a three-dimensional conductive network penetrating the interior and inter-particle spaces of the material. This three-dimensional conductive network not only reduces the contact resistance between lithium iron phosphate particles but also effectively improves the electron transport rate within the material, while simultaneously suppressing particle agglomeration and pulverization during charging and discharging, providing dual protection for the material's high-rate charge-discharge performance and long cycle life.

[0066] This invention employs a liquid-phase in-situ doping process to achieve uniform dispersion of high-valence metals in lithium iron phosphate, precisely controlling the cell parameters of lithium iron phosphate particles and optimizing lithium-ion insertion / extraction channels and diffusion kinetics. A hydrothermal method is used to synthesize lithium iron phosphate precursors doped with metal elements, resulting in tightly packed particles that reduce material porosity and increase compaction density. A staged carbon source synergistic introduction strategy is adopted: a first carbon source is added during the liquid-phase synthesis stage to generate lithium iron phosphate particles coated with a carbon layer precursor; a second carbon source is added during the high-temperature calcination stage to crosslink and overlap with the carbon layer precursor, forming a carbon layer and constructing a three-dimensional continuous conductive network penetrating the interior and interparticles. This provides dual assurance for the high-rate charge / discharge performance and long cycle life of the cathode material.

[0067] According to another aspect of the present invention, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material described in the above technical solution or the lithium iron phosphate cathode material prepared by the preparation method described in the above technical solution.

[0068] Specifically, the lithium-ion battery is preferably assembled in the order of negative electrode, separator, and positive electrode. In a preferred embodiment of the present invention, the battery preparation process includes: sequentially assembling the negative electrode, separator, and positive electrode, then impregnating them with an electrolyte, and finally processing them through formation and other processes to form a lithium-ion battery. The specific conditions and parameters for each step in the above preparation process can be achieved using battery preparation techniques well-known to those skilled in the art, and the present invention does not impose any special limitations on them.

[0069] In a specific embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive coating disposed on at least one surface of the positive current collector along its thickness direction; wherein, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive coating is disposed on either or both of the two opposite surfaces of the positive current collector. The positive current collector can be a metal foil or a composite current collector; for example, aluminum foil can be used as a metal foil; the composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer; the composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). The present invention does not have any special restrictions on the source of the positive current collector; commercially available products or self-made products well known to those skilled in the art can be used.

[0070] In a specific embodiment of the present invention, the positive electrode coating comprises the lithium iron phosphate positive electrode material described in the above-described technical solution or the lithium iron phosphate positive electrode material prepared by the preparation method described in the above-described technical solution. Therefore, the positive electrode coating possesses all the features and advantages of the lithium iron phosphate positive electrode material described in the above-described technical solution, which will not be repeated here.

[0071] In a specific embodiment of the present invention, the positive electrode coating further includes a conductive agent and a binder; wherein, the conductive agent includes, but is not limited to, one or more of conductive carbon, conductive carbon black (SP), carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, preferably SP; the binder includes, but is not limited to, one or more of polyvinylidene fluoride (PVDF), hydroxymethyl cellulose (CMC), 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), preferably PVDF.

[0072] In a specific embodiment of the present invention, the preparation method of the positive electrode sheet adopts a method well known to those skilled in the art: first, the raw materials are mixed in a solvent in a certain proportion to form a slurry, and then the positive electrode slurry is coated on a positive electrode current collector, and dried and rolled to prepare the positive electrode sheet. The solvent is preferably N-methylpyrrolidone (NMP).

[0073] In a specific embodiment of the present invention, the negative electrode sheet preferably includes a negative electrode current collector and a negative electrode coating disposed on at least one surface of the negative electrode current collector; wherein, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode coating is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0074] In a specific embodiment of the present invention, the negative electrode coating preferably comprises a negative electrode active material, a binder, and a conductive agent; wherein, the negative electrode active material preferably comprises graphite; the binder comprises one or more of polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), 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), preferably CMC; the conductive agent comprises one or more of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, preferably carbon black. The present invention does not impose any special restrictions on the source of the above-mentioned binder and conductive agent; commercially available products well known to those skilled in the art can be used.

[0075] In a specific embodiment of the present invention, the method for preparing the negative electrode sheet includes: thoroughly mixing a negative electrode active material, a binder, and a conductive agent to prepare a slurry; coating the slurry onto a negative electrode current collector; and drying, cold pressing, and slitting to obtain the negative electrode sheet. The negative electrode current collector can be a metal foil well-known to those skilled in the art, such as lithium foil.

[0076] In a specific embodiment of the present invention, the electrolyte comprises an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt may be LiPF6 and / or LiBOB used in high-temperature electrolytes; or one or more of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; or one or more of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-resistant electrolytes; or one or more of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent may be a cyclic carbonate, including propylene carbonate (PC) and / or ethylene carbonate (EC); or a chain carbonate, including at least one of diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC); or a carboxylic acid ester, including at least one of methyl formate, methyl acetate, ethyl acetate, and methyl propionate. The additives include, but are not limited to, at least one of the following: film-forming additives, conductive additives, flame-retardant additives, overcharge prevention additives, additives for controlling the H2O and HF content in the electrolyte, additives for improving low-temperature performance, and multifunctional additives. In a preferred embodiment of the present invention, the electrolyte is a LiPF6 electrolyte, and the solvent is a mixed solvent of EC and DMC.

[0077] The present application will be described in detail below with reference to the accompanying drawings and embodiments. However, the implementation and protection of the present invention are not limited thereto. The following embodiments are only some embodiments of the present application and are not intended to limit the present application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0078] Example 1 S1. Add 8.6g of titanium methoxide and 5g of glucose to 2000g of water and stir. Then add 41.84g of lithium acetate, 96.368g of ferrous sulfate and 62.132g of phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0079] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 140℃, filter after 4h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0080] S3. Add the lithium iron phosphate precursor obtained in step S2 to 180g of water, then add 5g of carbon nanotubes, mix evenly and grind to a particle size of 600nm, spray dry, and place the dried material in a muffle furnace filled with nitrogen and calcine at 780℃ for 7h to obtain lithium iron phosphate cathode material.

[0081] Example 2 S1. Add 11.4g tetraethyl titanate and 5g glucose to 2000g water and stir. Then add 41.84g lithium acetate, 91.2g ferrous oxalate and 62.132g phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0082] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 140℃, filter after 4h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0083] S3. Add the lithium iron phosphate precursor obtained in step S2 to 180g of water, then add 5g of carbon nanotubes, mix evenly and grind to a particle size of 600nm, spray dry, and place the dried material in a muffle furnace filled with nitrogen and calcine at 780℃ for 7h to obtain lithium iron phosphate cathode material.

[0084] Example 3 S1. Add 20.4g tetraethyl titanate and 5g glucose to 2000g water and stir. Then add 41.84g lithium acetate, 91.2g ferrous oxalate and 62.132g phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0085] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 120℃, filter after 4h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0086] S3. Add the lithium iron phosphate precursor obtained in step S2 to 180g of water, then add 5g of carbon nanotubes, mix evenly and grind to a particle size of 600nm, spray dry, and place the dried material in a muffle furnace filled with nitrogen and calcine at 780℃ for 7h to obtain lithium iron phosphate cathode material.

[0087] Example 4 S1. Add 11.4g tetraethyl titanate and 5g glucose to 2000g water and stir. Then add 41.84g lithium acetate, 91.2g ferrous oxalate and 62.132g phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0088] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 160℃, filter after 5h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0089] S3. Add the lithium iron phosphate precursor obtained in step S2 to 180g of water, then add 5g of carbon nanotubes, mix evenly and grind to a particle size of 600nm, spray dry, and place the dried material in a muffle furnace filled with nitrogen and calcine at 780℃ for 7h to obtain lithium iron phosphate cathode material.

[0090] Example 5 S1. Add 11.4g tetraethyl titanate and 7g glucose to 2000g water and stir. Then add 41.84g lithium acetate, 91.2g ferrous oxalate and 62.132g phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0091] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 160℃, filter after 6h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0092] S3. Add the lithium iron phosphate precursor obtained in step S2 to 180g of water, then add 5g of carbon nanotubes, mix evenly and grind to a particle size of 600nm, spray dry, and place the dried material in a muffle furnace filled with nitrogen and calcine at 780℃ for 7h to obtain lithium iron phosphate cathode material.

[0093] Comparative Example 1 S1. Add 8.6g of titanium methoxide and 5g of glucose to 2000g of water and stir. Then add 41.84g of lithium acetate, 96.368g of ferrous sulfate and 62.132g of phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0094] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 140℃, filter after 4h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0095] Comparative Example 2 S1. Add 20.4g tetraethyl titanate and 10g glucose to 2000g water and stir. Then add 41.84g lithium acetate, 91.2g ferrous oxalate and 62.132g phosphoric acid in sequence and stir thoroughly to obtain a slurry.

[0096] S2. Transfer the slurry obtained in step S1 to a high-pressure reactor, add 1g of sodium dodecylbenzenesulfonate for hydrothermal reaction at 120℃, filter after 4h, and then vacuum dry the filter cake to obtain lithium iron phosphate precursor.

[0097] Comparative Example 3 S1. Add 100g of ferric phosphate, 25g of lithium carbonate, and 10g of glucose to 2000g of water and stir thoroughly to obtain a slurry.

[0098] S2. The slurry obtained in step S1 is wet-milled until the particle size in the slurry is controlled to be 400nm, and then atomized and dried into dry powder by spray drying equipment.

[0099] S3. The dry powder obtained in step S2 is transferred to an atmosphere furnace, nitrogen is introduced as a protective gas, the temperature is raised to 750°C and sintered at a constant temperature for 8 hours. After the sintered product is cooled, it is crushed to obtain lithium iron phosphate cathode material.

[0100] Performance testing: The lithium iron phosphate cathode material prepared in Example 1 was subjected to SEM testing, and the results are as follows: Figure 1 As shown, the lithium iron phosphate cathode material prepared by the liquid-phase process of this invention possesses a controllable multi-level particle size distribution. The lithium iron phosphate precursor synthesized by the liquid-phase process has a particle size that can be precisely controlled within the range of 200nm to 350nm. After directional nucleus growth through a calcination process, the material can form a particle system with a three-level particle size distribution: small particle size (200nm~350nm), medium particle size (approximately 500nm), and large particle size (approximately 1μm). The particle size of each level is significantly smaller than that of the lithium iron phosphate cathode material particles (small particle size (400nm~500nm), medium particle size (approximately 700nm), and large particle size (2μm~3μm)) prepared by the traditional solid-phase synthesis method in Comparative Example 3. The above-mentioned gradient particle size distribution structure has significant packing advantages: small particle size particles can fully fill the gaps between large and medium particle size particles, effectively reducing the porosity during particle packing, significantly improving the packing density of the material, and thus achieving a significant increase in the compaction density of the lithium iron phosphate material.

[0101] The physicochemical properties of the products obtained in the above embodiments and comparative examples were tested, including powder compaction density and powder resistivity. The specific methods are as follows: Powder compaction density test method: Refer to national standard GB / T 44330-2024 "Determination of compaction density of lithium-ion battery cathode material powder".

[0102] Powder resistivity test method: Refer to national standard GB / T 45324-2025 "Determination of powder resistivity of positive electrode material for lithium-ion batteries".

[0103] Using polyvinylidene fluoride (PVDF) as a binder, conductive carbon black (SP) as a conductive agent, and N-methylpyrrolidone (NMP) as a solvent, the products obtained in the above examples and comparative examples were used as the positive electrode active material. The preparation process of the positive electrode sheet was as follows: a positive electrode slurry was prepared according to a product:PVDF:SP mass ratio of 90:5:5, coated on aluminum foil using an automatic coating machine, and the coated aluminum foil was dried in an oven and cut to obtain the positive electrode sheet. Using lithium foil as the negative electrode sheet, LiPF6 as the solute, and ethylene carbonate and dimethyl carbonate as solvents, an electrolyte was prepared. The positive electrode shell, negative electrode shell, positive electrode sheet, negative electrode sheet, glass fiber membrane, and electrolyte were assembled into a button cell. The button cell was hung on a battery testing system and left to stand before testing. Specific tests included: At 25°C, the button cell was charged at a constant current of 0.1C to 3.75V, then charged at a constant voltage until the current dropped to 0.05C, at which point charging was stopped, and the initial charge capacity was recorded. Next, it was discharged at a constant current of 0.1C to a cutoff voltage of 2.0V, and the initial discharge capacity was recorded. The 0.1C discharge specific capacity of the positive electrode material was calculated based on the initial discharge capacity, and the initial charge-discharge efficiency of the button cell was calculated using the following formula: Initial charge-discharge efficiency = (Initial discharge capacity / Initial charge capacity) × 100%.

[0104] The test results are shown in Table 1 below.

[0105] Table 1. Performance test results of button batteries As shown in Table 1, the lithium iron phosphate cathode material prepared using the method of this invention possesses excellent and stable electrochemical performance. Regarding core physical performance indicators, powder compaction density directly reflects the particle packing characteristics and processing adaptability of the material. The compaction density of the lithium iron phosphate cathode materials prepared in Examples 1-5 all consistently reached 2.6 g / cm³. 3 The above values ​​are significantly higher than the compaction density of the lithium iron phosphate precursor in Comparative Examples 1 and 2 (2.48 g / cm³). 3 ~2.5g / cm 3 This demonstrates that the lithium iron phosphate cathode materials prepared in Examples 1-5 can meet the requirements of electrode rolling processes for high-energy-density battery cells. Furthermore, their powder resistivity is controlled below 30 Ω·cm, laying a solid foundation for the electronic conductivity of the cathode material.

[0106] In terms of electrochemical activity indicators, the lithium iron phosphate cathode materials in Examples 1-5 all have a discharge specific capacity of no less than 159 mAh / g at a 0.1C rate, and the first charge-discharge efficiency is as high as 99% or more, which effectively reduces irreversible capacity loss during the first charge-discharge process and improves the energy utilization rate of the cell.

[0107] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0108] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0109] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, Includes a substrate and a covering layer covering at least a portion of the surface of the substrate; The matrix comprises several lithium iron phosphate particles doped with metal elements; The coating layer includes a carbon layer and a three-dimensional conductive network interspersed within the carbon layer; the three-dimensional conductive network is formed of conductive carbon and embedded in the matrix.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The metal elements in the lithium iron phosphate particles doped with metal elements include one or more of titanium, niobium, vanadium, magnesium, and aluminum. And / or, the molar ratio of metal element:Li:Fe:P in the lithium iron phosphate particles doped with metal elements is (0.05~0.1):(1~2):1:

1.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The thickness of the carbon layer is 2nm to 4nm.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that, The conductive carbon includes carbon nanotubes and / or conductive graphene.

5. The lithium iron phosphate cathode material according to claim 1, characterized in that, The particle size of the lithium iron phosphate cathode material is 200 nm to 1 μm. And / or, the compaction density of the lithium iron phosphate cathode material is >2.6 g / cm³. 3 .

6. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Mix the metal source, the first carbon source, the lithium source, the iron source, the phosphorus source and water to obtain a slurry; S2. The slurry obtained in step S1 is mixed with a surfactant and then subjected to a hydrothermal reaction to obtain a lithium iron phosphate precursor. S3. The lithium iron phosphate precursor obtained in step S2 is mixed with the second carbon source and then calcined under an inert atmosphere to obtain the lithium iron phosphate cathode material.

7. The preparation method according to claim 6, characterized in that, In step S1: The metal source includes one or more of titanium, niobium, vanadium, magnesium, and aluminum; the titanium source includes one or more of organotitanium chelates and organotitanium coupling agents, preferably titanium methoxide and / or tetraethyl titanate. And / or, the first carbon source includes one or more of glucose, polyvinyl alcohol, sucrose, citric acid, polyethylene glycol, and phenolic resin; And / or, the amount of the first carbon source added is 0.2wt% to 1wt% of the slurry; And / or, the lithium source includes one or more of LiOH, LiH2PO4, Li2HPO4, and CH3COOLi; And / or, the iron source includes one or more of Fe(NO3)2, FeCl2, Fe(SO4)2, and FeC2O4; And / or, the phosphorus source includes one or more of H3PO4, NH4H2PO4, LiH2PO4, and Li2HPO4.

8. The preparation method according to claim 6, characterized in that, In step S2: The surfactant includes one or more of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide, and sodium dodecyl sulfate. And / or, the pressure of the hydrothermal reaction is 1MPa~5MPa, the temperature of the hydrothermal reaction is 120℃~160℃, and the time of the hydrothermal reaction is 4h~6h; And / or, the hydrothermal reaction further includes: filtering the obtained reaction mixture, and vacuum drying the filter cake to obtain a lithium iron phosphate precursor.

9. The preparation method according to claim 6, characterized in that, In step S3: The second carbon source includes one or more of conductive graphene and carbon nanotubes; And / or, the mass ratio of the first carbon source to the second carbon source is (1~1.5):1; And / or, the mixing process includes: adding water, wet grinding to adjust to the target particle size, and then spray drying; the target particle size is 500nm~700nm; And / or, the inert atmosphere includes one or more of nitrogen, argon, and helium; And / or, the calcination temperature is 750℃~800℃, and the calcination time is 6h~10h.

10. A lithium-ion battery, characterized in that, The lithium iron phosphate cathode material includes the lithium iron phosphate cathode material according to any one of claims 1 to 5 or the lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 6 to 9.

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