Lithium iron phosphate composite material and preparation method and application thereof

Abstract

The invention belongs to the technical field of new energy, and particularly relates to a lithium iron phosphate composite material and a preparation method and application thereof. The lithium iron phosphate composite material provided by the invention is composed of a lithium iron phosphate core and a coating layer, an anion and cation doping strategy is adopted, different doping strategies are adopted for particles with different sizes, high-cation and low-anion doping is adopted for large particles, and through anion and cation co-doping in a specific proportion, the lithium iron phosphate composite material is prepared. Lattice stress compensation is achieved through anion compensation, and the negative effect on the compaction density is reduced. And meanwhile, the oxidation-reduction activity can be enhanced by anion doping, the electronic conductivity is improved, and the rate capability is improved. A doping strategy of reducing cations and increasing the doping amount of anions is adopted for small particles, the local coordination environment of the FeO6 octahedron stabilized by anion doping is exerted, structural distortion is reduced, and the structural stability of the small particles is improved; the coating layer can reduce interface contact resistance, improve interface electron conductance and improve electrochemical reaction kinetics.

Description

Technical Field

[0001] This application belongs to the field of new energy technology, specifically relating to a lithium iron phosphate composite material, its preparation method, and its application. Background Technology

[0002] Currently, high-compaction lithium iron phosphate (LFP) materials struggle to simultaneously increase compaction density and maintain high-rate performance, failing to meet the demands of high-end power applications. This is because high compaction typically requires filling voids through particle size distribution (e.g., mixing micron-sized and nano-sized particles), but micron-sized particles affect lithium-ion diffusion rates, reducing electronic / ionic conductivity and consequently impacting rate performance. While doping modification can be achieved through Al... 3+ Ti 4+ Equal doping can broaden lithium-ion channels, but excessive doping can damage the stability of the olivine structure and affect cycle performance. Summary of the Invention

[0003] This application provides a lithium iron phosphate composite material, its preparation method, and its application, to solve the problems in the prior art where high-compact lithium iron phosphate composite materials cannot simultaneously achieve rate performance or cycle stability.

[0004] In a first aspect, this application provides a lithium iron phosphate composite material, comprising a first particle, a second particle, and a third particle; The first particle comprises a first core and a first coating layer located on at least a portion of the surface of the first core, and the first particle has a Ferete diameter of FDn50. 1 nm, the chemical composition of the first core includes: LiFe a1 M b1 (PO4) c1 Y d1 Wherein, 0.005≤b1≤0.020, a1+b1≤1, 0.002≤d1≤0.004, c1+d1≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. The second particle includes a second core and a second coating layer located on at least a portion of the surface of the second core, and the second particle has a Ferete diameter of FDn50. 2 nm, the chemical formula composition of the second core includes: LiFe a2 M b2 (PO4) c2 Y d2Wherein, 0.005≤b2≤0.020, a2+b2≤1, 0.002≤d2≤0.004, c2+d2≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. The third particle comprises a third core and a third coating layer located on at least a portion of the surface of the third core, and the third particle has a Ferete diameter of FDn50. 3 nm, the chemical formula composition of the third core includes: LiFe a3 M b3 (PO4) c3 Y d3 Wherein, 0.008≤b3≤0.025, a3+b3≤1, 0.001≤d3≤0.003, c3+d3≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. And it satisfies: FDn50 1 <FDn50 2 <FDn50 3 ;b3>b1; b3>b2; d1>d3; d2>d3.

[0005] In an alternative implementation, the FDn50 1 FDn50 2 FDn50 3 Satisfy: FDn50 1 =xFDn50 3 FDn50 2 =yFDn50 3 1000≤FDn50 3 ≤3000, where x ranges from 0.04 to 0.20, y ranges from 0.12 to 0.35, and x < y; Optionally, the value of x ranges from 0.06 to 0.10, and the value of y ranges from 0.18 to 0.22.

[0006] In one optional embodiment, the mass ratio of the first particle, the second particle, and the third particle is (0.5-3.0):(0.5-3.0):(5.5-8.0); alternatively, it is (0.6-1.4):(1.4-2.4):(6.5-7.5). And / or, 1500≤FDn50 3 ≤2000; And / or, 100≤FDn50 1 ≤160; And / or, 200≤FDn50 2 ≤400.

[0007] In one alternative embodiment, the first coating layer, the second coating layer, and the third coating layer each independently include a carbon coating layer; In the first particle, the mass percentage of the first coating layer is 1.0-1.5%; in the second particle, the mass percentage of the second coating layer is 1.0-1.5%; and in the third particle, the mass percentage of the third coating layer is 1.0-1.5%.

[0008] Secondly, this application provides a method for preparing the above-mentioned lithium iron phosphate composite material, comprising the following steps: S1, the lithium source, the first carbon source, the iron source, the phosphorus source, the anion dopant, the cation dopant and the solvent are sand-milled and spray-dried, and then sintered in a protective atmosphere to obtain the first intermediate; S2, the lithium source, the second carbon source, the iron source, the phosphorus source, the anion dopant, the cation dopant and the solvent are sand-milled and spray-dried, and then sintered in a protective atmosphere to obtain the second intermediate; S3 involves grinding, spray drying, and sintering a lithium source, a third carbon source, an iron source, a phosphorus source, an anion dopant, a cationic dopant, and a solvent in a protective atmosphere to obtain a third intermediate. Wherein, the temperature of the first sintering is less than the temperature of the second sintering and the temperature of the third sintering; S4, the first intermediate, the second intermediate, the third intermediate and the fourth carbon source are mixed and sintered in a protective atmosphere to obtain the lithium iron phosphate cathode material.

[0009] In one alternative embodiment, the temperature of the first sintering, the second sintering, and the third sintering are independently 600-800°C, and the sintering time is independently 4-10h. And / or, the fourth sintering temperature is 500-700℃, and the sintering time is 4-10h.

[0010] In one optional embodiment, the temperature of the first sintering is 650-700°C, and the sintering time is 8-10 hours; And / or, the second sintering temperature is 700-750℃, and the sintering time is 8-10h; And / or, the third sintering temperature is 750-800℃, and the sintering time is 8-10h; And / or, the fourth sintering temperature is 600-650℃, and the sintering time is 4-10h.

[0011] Thirdly, this application provides a positive electrode sheet, comprising: Positive current collector, and A positive electrode active material layer disposed on at least one side of the positive electrode current collector, the positive electrode active material layer comprising the above-mentioned lithium iron phosphate composite material or the lithium iron phosphate composite material prepared by the above-mentioned preparation method.

[0012] Fourthly, this application provides a secondary battery, including the aforementioned positive electrode.

[0013] Fifthly, this application provides an electrical device including the aforementioned secondary battery.

[0014] The technical solution of this application has the following advantages: The lithium iron phosphate composite material provided in this application consists of a lithium iron phosphate core and a coating layer. It employs a cationic and anionic doping strategy, with different doping strategies applied to particles of different sizes. For large particles, a high-cation, low-anionic doping method is used. Through co-doping of cationic and anionic particles in a specific ratio, anionic compensation is utilized to achieve lattice stress compensation, reducing the negative impact on compaction density. Simultaneously, anionic doping enhances redox activity, improves electronic conductivity, and increases rate performance. For smaller particles, a doping strategy of reducing cations and increasing anionic doping is adopted. Anionic doping stabilizes the local coordination environment of the FeO6 octahedron, reducing structural distortion and increasing the structural stability of small particles. The coating layer reduces interfacial contact resistance, improves interfacial electronic conductivity, and enhances electrochemical reaction kinetics.

[0015] Additional aspects and advantages of this application 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 this application. Detailed Implementation

[0016] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0018] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "at least one" means one or more, unless otherwise explicitly defined.

[0019] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0020] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0021] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0022] Unless otherwise specified, all steps in this application 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.

[0023] As described in the background section, current high-compaction lithium iron phosphate (LFP) materials struggle to simultaneously improve compaction density and high-rate performance, failing to meet the demands of high-end power applications. Existing patent literature discloses a method for preparing high-compaction LFP materials using vanadium, titanium, and boron doping. Vanadium and boron doping are used to improve rate performance. However, in this patent, the boron content is significantly higher than the titanium and vanadium content, causing an imbalance of positive and negative charges. Furthermore, excessive boron... 3+ (27pm) Replaces P 5+ At 17 pm, lattice distortion occurs due to differences in ionic radii, resulting in excess B. 3+ With Li + The reaction generates an insulating phase, Li3BO3, which affects the material's discharge voltage and discharge efficiency, and consequently its rate performance, cycle stability, and energy density.

[0024] In order to solve the problems existing in the above-mentioned related technologies, according to the first aspect of this application, a lithium iron phosphate composite material is provided, comprising a first particle, a second particle and a third particle; The first particle comprises a first core and a first coating layer located on at least a portion of the surface of the first core, and the first particle has a Ferete diameter of FDn50. 1 nm, the chemical composition of the first core includes: LiFe a1 M b1 (PO4) c1 Y d1 Wherein, 0.005≤b1≤0.020, a1+b1≤1, 0.002≤d1≤0.004, c1+d1≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. The second particle includes a second core and a second coating layer located on at least a portion of the surface of the second core, and the second particle has a Ferete diameter of FDn50. 2 nm, the chemical formula composition of the second core includes: LiFe a2 M b2(PO4) c2 Y d2 Wherein, 0.005≤b2≤0.020, a2+b2≤1, 0.002≤d2≤0.004, c2+d2≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. The third particle comprises a third core and a third coating layer located on at least a portion of the surface of the third core, and the third particle has a Ferete diameter of FDn50. 3 nm, the chemical formula composition of the third core includes: LiFe a3 M b3 (PO4) c3 Y d3 Wherein, 0.008≤b3≤0.025, a3+b3≤1, 0.001≤d3≤0.003, c3+d3≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. And it satisfies: FDn50 1 <FDn50 2 <FDn50 3 ;b3>b1; b3>b2; d1>d3; d2>d3.

[0025] In some alternative implementations, 0.008≤b1≤0.014, 0.002≤d1≤0.003; 0.008≤b2≤0.014, 0.002≤d2≤0.003; 0.014≤b3≤0.018, 0.001≤d3≤0.002; For example, the value of b1 can be 0.005, 0.008, 0.010, 0.012, 0.015, 0.018, 0.020, or any value within the range of the above; the value of d1 can be 0.002, 0.0025, 0.003, 0.0035, 0.004, or any value within the range of the above; the value of b2 can be 0.005, 0.008, 0.010, 0.012, 0.015, 0.018, 0.020, or any value within the range of the above. Within the range of; d2 can take values ​​of 0.002, 0.0025, 0.003, 0.0035, 0.004, or any combination of the above values; b3 can take values ​​of 0.008, 0.010, 0.012, 0.015, 0.017, 0.020, 0.023, 0.025, or any combination of the above values; d3 can take values ​​of 0.001, 0.0015, 0.002, 0.0025, 0.003, or any combination of the above values. In this application, the composition of the first particle and the second particle can be the same or different.

[0026] To address the poor rate performance and cycle stability of high-pressure lithium iron phosphate (LFP) materials, this application provides a LFP composite material consisting of a LFP core and a coating layer, employing an anion and cation doping strategies, with different doping strategies applied to different particles. For large particles (the third particle), a high-cation, low-anion doping method is used. The cation doping element M improves electronic conductivity, but Fe... 2+ Reducing doping capacity sacrifices yield, while excessive doping causes FeO6 octahedral shrinkage, leading to a decrease in compaction density. By using a specific ratio of cation and anion co-doping, lattice stress compensation is achieved through anion compensation, reducing the negative impact on compaction density. Simultaneously, anions enhance Fe by adjusting the covalent bonds of the Fe-O bonds. 2+ / Fe 3+ This enhances redox activity, improves electronic conductivity, and boosts rate performance. For smaller particles (the first and second particles), a doping strategy of reducing cations and increasing anion doping is employed. This leverages the stabilizing effect of anion doping on the local coordination environment of the FeO6 octahedron, reducing Fe... 2+ / Fe 3+ Structural distortion during redox processes reduces Fe 2+ Dissolution increases the structural stability of small particles. The coating layer can reduce interfacial contact resistance and improve interfacial electronic conductivity.

[0027] In some alternative implementations, the FDn50 1 FDn50 2 FDn50 3 Satisfy: FDn501 =xFDn50 3 FDn50 2 =yFDn50 3 1000≤FDn50 3 ≤3000, where x ranges from 0.04 to 0.20, y ranges from 0.12 to 0.35, and x < y; Optionally, the value of x ranges from 0.06 to 0.10, and the value of y ranges from 0.18 to 0.22.

[0028] For example, the value of x can be 0.04, 0.06, 0.08, 0.10, 0.12, 0.15, 0.17, 0.20, or any of the above values; the value of y can be 0.12, 0.15, 0.18, 0.20, 0.23, 0.25, 0.27, 0.3, 0.33, 0.35, or any of the above values.

[0029] In some optional embodiments, the mass ratio of the first particle, the second particle, and the third particle is (0.5-3.0):(0.5-3.0):(5.5-8.0); alternatively, it is (0.6-1.4):(1.4-2.4):(6.5-7.5).

[0030] In this application, by limiting different particle sizes and optimizing the ratio between different particles, lithium iron phosphate composite materials can be more tightly packed, thereby further improving the compaction density.

[0031] It should be noted that the Feret diameter is a term used in mathematics and particle size analysis to describe particle size, defined as the distance between the parallel lines of the two boundaries of the particle's projected profile measured along a certain direction. The Feret diameter is a commonly used parameter for describing the size of irregular particles; here, it refers to the longest diameter of the particle. In this application, FDn50 refers to the Feret diameter corresponding to a cumulative quantity distribution percentage of 50%. In this application, FDn50 can be obtained by argon ion polishing of the particles to obtain a cross-section, acquiring a cross-sectional electron microscope, and measuring the Feret diameter.

[0032] In this application, the method obtains the primary diameter of the particles, which can more accurately reflect the primary particle size of the material than the Dv50 (including particle agglomerates) obtained by a laser particle size analyzer.

[0033] In some alternative implementations, 1500 ≤ FDn50 3 ≤2000; In some alternative implementations, 100 ≤ FDn50 1 ≤160; In some alternative implementations, 200 ≤ FDn50 2 ≤400.

[0034] As an example, the Freette diameter of the first particle can be 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, or within any range of the above values; the Freette diameter of the second particle can be 200nm, 220nm, 240nm, 250nm, 260nm, 280nm, 300nm, 330nm, 350nm, 370nm, 400nm, or within any range of the above values; the Freette diameter of the third particle can be 1000nm, 1200nm, 1500nm, 1700nm, 1900nm, 2000nm, 2200nm, 2500nm, 2800nm, 3000nm, or within any range of the above values.

[0035] In some alternative embodiments, the first coating layer, the second coating layer, and the third coating layer independently include a carbon coating layer; in this application, the coating layer is a uniform composite carbon layer, which can reduce interfacial contact resistance, improve interfacial electronic conductivity, and further enhance electrochemical reaction kinetics.

[0036] In some alternative embodiments, the first coating layer accounts for 1.0-1.5% of the mass of the first particle; the second coating layer accounts for 1.0-1.5% of the mass of the second particle; and the third coating layer accounts for 1.0-1.5% of the mass of the third particle.

[0037] This application limits the proportion of carbon coating to maintain a high level of interfacial electronic conductivity, thus avoiding the impact of low interfacial electronic conductivity on the product's rate performance.

[0038] According to a second aspect of this application, a method for preparing the above-mentioned lithium iron phosphate composite material is provided, characterized by comprising the following steps: S1, the lithium source, the first carbon source, the iron source, the phosphorus source, the anion dopant, the cation dopant and the solvent are sand-milled and spray-dried, and then sintered in a protective atmosphere to obtain the first intermediate; S2, the lithium source, the second carbon source, the iron source, the phosphorus source, the anion dopant, the cation dopant and the solvent are sand-milled and spray-dried, and then sintered in a protective atmosphere to obtain the second intermediate; S3 involves grinding, spray drying, and sintering a lithium source, a third carbon source, an iron source, a phosphorus source, an anion dopant, a cationic dopant, and a solvent in a protective atmosphere to obtain a third intermediate. Wherein, the temperature of the first sintering is less than the temperature of the second sintering and the temperature of the third sintering; S4, the first intermediate, the second intermediate, the third intermediate and the fourth carbon source are mixed and sintered in a protective atmosphere to obtain the lithium iron phosphate cathode material.

[0039] In some alternative embodiments, the temperatures of the first, second, and third sintering processes are independently 600-800°C, and the sintering times are independently 4-10 hours. As an example, the temperatures of the first, second, and third sintering processes are independently 600°C, 630°C, 650°C, 680°C, 700°C, 720°C, 750°C, 77°C, 800°C, or within any range of the above values; the times of the first, second, and third sintering processes are independently 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or within any range of the above values.

[0040] And / or, the fourth sintering temperature is 500-700℃, and the sintering time is 4-10h. As an example, the fourth sintering temperature can be 500℃, 550℃, 600℃, 650℃, 700℃, or within any range of the above values; the sintering time can be 4h, 5h, 6h, 7h, 8h, 9h, 10h, or within any range of the above values.

[0041] In some alternative embodiments, the temperature of the first sintering is 650-700°C and the sintering time is 8-10 hours; And / or, the second sintering temperature is 700-750℃, and the sintering time is 8-10h; And / or, the third sintering temperature is 750-800℃, and the sintering time is 8-10h; And / or, the fourth sintering temperature is 600-650℃, and the sintering time is 4-10h.

[0042] In this application, the particle size of each group of particles can be controlled within the above-mentioned limited range by controlling the sintering temperature.

[0043] In this application, the raw materials used in the preparation of lithium iron phosphate composite materials are all conventional and typically non-limiting in the art. The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, or lithium hydrogen phosphate; the iron source is selected from at least one of iron phosphate, iron oxalate, iron oxide, or ammonium iron phosphate; the phosphorus source is selected from at least one of iron phosphate, phosphoric acid, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, or ammonium iron phosphate; the anionic dopant is selected from at least one of lithium fluoride, ammonium fluoride, urea, melamine, boric acid, ammonium borate, boron oxide, sodium silicate, and ammonium thiosulfate; the cationic dopant is selected from at least one of titanium dioxide, metatitanic acid, tetrabutyl titanate, ammonium metavanadate, vanadium pentoxide, vanadium oxalate, magnesium oxide, or basic magnesium carbonate. Preferably, the lithium source is lithium carbonate, the iron source is iron phosphate, the phosphorus source is iron phosphate, the anionic dopant is boric acid, the titanium source is titanium dioxide, the vanadium source is vanadium pentoxide, and the magnesium source is magnesium oxide.

[0044] In some optional embodiments, the first carbon source, and / or the second carbon source, and / or the third carbon source, and / or the fourth carbon source are each independently selected from one or more of glucose, sucrose, starch, phenolic resin, epoxy resin, polyvinylpyrrolidone, polyethylene glycol, N,N-bishydroxyethyldodecylamide, and polysorbate. The first carbon source, the second carbon source, and the third carbon source are added at 1% to 12% of the theoretical mass of the first particle, the second particle, and the third particle, respectively; the fourth carbon source is added at 0.2% to 5% of the total mass of the first intermediate, the second intermediate, and the third intermediate.

[0045] In this application, the operating parameters of the spray drying are conventional and typically non-limiting in the field, namely, the atomizer pressure is 0.3~0.5MPa, the inlet air temperature is 230~260℃, and the outlet air temperature is 100~110℃.

[0046] In some alternative implementations, the mixing equipment used in S4 is one of a high-speed mixer, a coating mixer, or a ball mill.

[0047] According to a third aspect of this application, a positive electrode sheet is provided, comprising: Positive current collector, and A positive electrode active material layer disposed on at least one side of the positive electrode current collector, the positive electrode active material layer comprising the above-mentioned lithium iron phosphate composite material or the lithium iron phosphate composite material prepared by the above-mentioned preparation method.

[0048] According to a fourth aspect of this application, a secondary battery is provided, comprising the aforementioned positive electrode.

[0049] According to the fifth aspect of the present application, there is provided an electrical device including the above secondary battery.

[0050] The secondary battery and the electrical device of the present application will be described below.

[0051] Normally, a secondary battery includes a positive electrode plate, a negative electrode plate, an electrolyte, and a separator. During the charging and discharging process of the battery, active ions intercalate and deintercalate between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The separator is disposed between the positive electrode plate and the negative electrode plate, mainly to prevent short circuit between the positive and negative electrodes, and at the same time allows ions to pass through.

[0052] [Positive electrode plate] The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the positive electrode active material of the first aspect of the present application.

[0053] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0054] The positive electrode active material layer includes a positive electrode active material. The positive electrode active material can be selected from materials capable of absorbing and releasing lithium.

[0055] In some embodiments, the positive electrode active material layer in the positive electrode plate of the present application does not exclude other positive electrode active materials other than the lithium iron phosphate composite material provided by the present application. For example, other positive electrode active materials can adopt positive electrode active materials well-known in the art for batteries. The specific type of the positive electrode active material is not specifically limited and can be selected according to requirements. As an example, the positive electrode active material can include, but is not limited to, lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), spinel lithium manganate (LiMn2O4), spinel lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), layered lithium manganate (LiMnO2), lithium nickel oxide (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium manganate (LiMgO2), lithium calcium oxide (LiCaO2), lithium copper oxide (LiCuO2), lithium zinc oxide (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi0.8 Co 0.15 Al 0.05 O2), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, etc.), lithium-rich materials (such as lithium-rich nickel cobalt manganese oxide), manganese oxide (MnO2), vanadium oxide, sulfur oxide, silicate oxide, and at least one of their respective modified compounds. These materials can be used alone or in combination of two or more.

[0056] The modified compounds of the above positive electrode active materials can be doping modification, surface coating modification or doping and coating modification of the positive electrode active materials simultaneously.

[0057] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. 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 (such as 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.).

[0058] In some embodiments, the positive electrode active material layer may further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0059] In some embodiments, the positive electrode active material layer may further optionally include a conductive agent. As an example, the conductive agent can include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0060] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0061] [Negative electrode plate] The negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer comprising a negative electrode active material. In some optional embodiments, the negative electrode can be directly made of a lithium-containing metal sheet. The lithium-containing metal sheet can be lithium metal or an alloy formed of lithium metal with other metals or non-metallic elements.

[0062] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0063] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0064] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0065] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of 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).

[0066] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0067] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0068] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0069] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0070] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0071] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0072] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0073] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0074] [Isolation membrane] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0075] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0076] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0077] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0078] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0079] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0080] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0081] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0082] In some embodiments, the electrical device provided in this application may also include a battery module or battery pack assembled from the aforementioned secondary batteries. The secondary batteries, battery modules, or battery packs can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0083] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements. An example electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0084] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0085] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0086] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0087] Example 1 This embodiment provides a lithium iron phosphate composite material, which consists of a first particle, a second particle, and a third phosphorus particle. The first particle includes a first core and a first carbon coating layer located on at least a portion of the surface of the first core. The second particle includes a second core and a second carbon coating layer located on at least a portion of the surface of the second core. The third particle includes a third core and a third carbon coating layer located on at least a portion of the surface of the third core. The Freette diameters of the first, second, and third particles satisfy: FDn50. 1 <FDn50 2 <FDn50 3 ; The specific preparation method of the lithium iron phosphate composite material provided in this embodiment is as follows: Preparation of the first intermediate: Lithium carbonate, iron phosphate, titanium dioxide, and boron oxide were mixed in a molar ratio of Li:Fe:Ti:P:B = 1:0.97:0.01:0.99:0.002. The amount of glucose added was 8% of the theoretical mass of the first particle. The mixture was wet-milled to 120 nm, spray-dried, and sintered at 700 °C for 8 h to obtain the first intermediate. Preparation of the second lithium iron phosphate intermediate: Lithium carbonate, iron phosphate, titanium dioxide, and boron oxide were mixed in a molar ratio of Li:Fe:Ti:P:B = 1:0.97:0.01:0.99:0.002. Glucose was added at 8% of the theoretical mass of the second particle. The mixture was wet-milled to 300 nm, spray-dried, and then sintered for the first time at 750℃ for 8 hours to obtain the second intermediate. Preparation of the third intermediate: Lithium carbonate, iron phosphate, titanium dioxide, and boron oxide were mixed in a molar ratio of Li:Fe:Ti:P:B = 1:0.97:0.014:0.99:0.0012. The amount of glucose added was 9% of the theoretical mass of the third particle. The mixture was wet-milled to 500 nm, spray-dried, and sintered for the first time at 800 °C for 8 h to obtain the third intermediate. The first, second, and third intermediates were each subjected to argon ion polishing to obtain cross-sections. Cross-sections were then obtained using electron microscopy and the Ferrette diameter was measured to obtain FDn50. Testing showed that FDn50... 1 =128nm, FDn50 2 =320nm, FDn50 3 =1600nm (x=0.08, y=0.20). Since the particle size of the intermediate does not change much after subsequent sintering, for ease of measurement, this application uses the FDn50 of the first, second and third intermediates to represent the FDn50 of the first, second and third particles in the composite material.

[0088] The first, second, and third intermediates, polyethylene glycol, and N,N-bishydroxyethyldodecylamide were mixed in a mass ratio of 1.2:2:7 and sintered at 650°C for 8 hours. The mixture was then subjected to air jet milling to prepare a lithium iron phosphate composite cathode material. The amounts of polyethylene glycol and N,N-bishydroxyethyldodecylamide added were 2% and 3% of the total mass of the intermediates, respectively.

[0089] The third core has the chemical formula LiFe. a3 Ti b3 (PO4) c3 Y d3Where a3=0.97, b3=0.017, c3=0.99, and d3=0.0012. The first and second kernels have the same composition, with the chemical formula LiFe. a1 Ti b1 (PO4) c1 Y d1 Where a1=0.97, b1=0.012, c1=0.99, d1=0.002; The carbon coating layer accounts for 1.1% of the mass of the first and second particles, and the third carbon coating layer accounts for 1.2% of the mass of the third particle.

[0090] Examples 2-6 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the composition of the first core, the second core, and the third core are different. The specific composition of the core is adjusted by adjusting the feed ratio of raw materials during the intermediate preparation process in the following embodiments, as shown in the table below.

[0091] Table 1

[0092] Examples 7-10 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the ratio of the first particle, the second particle, and the third particle is different. In the following embodiments, the ratio of the first particle, the second particle, and the third particle in the final product is controlled by the ratio of intermediates, as shown in the table below.

[0093] Table 2

[0094] Examples 11-14 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the Ferrette diameters of the first intermediate, the second intermediate, and the third intermediate are different. In the following embodiments, the Ferrette diameter is controlled by sintering temperature and sintering time. Generally speaking, the higher the sintering temperature and the longer the sintering time, the larger the Ferrette diameter. See the table below for details.

[0095] Table 3

[0096] Examples 15-17 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the parameters of the first sintering, the second sintering, and the third sintering are different, as shown in the table below.

[0097] Table 4

[0098] Examples 18-19 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the proportion of carbon coating layer in particles of different sizes is different. In the following embodiments, the proportion of coating layer is controlled by the amount of carbon source added, as shown in the table below.

[0099] Table 5

[0100] Example 20 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the anionic dopant is different, and ammonium fluoride is used instead of boric acid in the preparation of each intermediate.

[0101] Example 21 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the anionic dopant is different, and sodium silicate is used instead of boric acid in the preparation of each intermediate.

[0102] Example 22 This embodiment provides a lithium iron phosphate composite material and its preparation method. The difference from Embodiment 1 is that the cationic dopant is different, and an equimolar amount of ammonium metavanadate is used instead of titanium dioxide in the preparation of each intermediate.

[0103] Comparative Examples 1-3 This comparative example provides a lithium iron phosphate composite material and its preparation method. The difference between this example and Example 14 is that the composition of the first core, the second core, and the third core are different, as shown in the table below.

[0104] Table 6

[0105] Comparative Example 4 This comparative example provides a lithium iron phosphate composite material and its preparation method. The difference between this example and Example 14 is that no anion doping is performed, and no anion dopant is added during the preparation of each intermediate.

[0106] Comparative Example 5 This comparative example provides a lithium iron phosphate composite material and its preparation method. The difference between this example and Example 14 is that no cation doping is performed, and no titanium dioxide is added during the preparation of each intermediate.

[0107] Test case The lithium iron phosphate composite materials provided in each embodiment and comparative example were subjected to various performance tests. The specific test methods are as follows: 1. Powder compaction test: Powder compaction tester (pressure 3t) / GB / T41232.2.

[0108] 2. Electrical performance testing: Battery Assembly: 1) Dissolve lithium iron phosphate composite material / acetylene black / polyvinylidene fluoride in N-methylpyrrolidone at a weight ratio of 90:5:5, stir evenly, coat it onto aluminum foil, and dry it in a forced-air drying oven at 100℃. Cut the dried electrode into small round pieces with a diameter of 12mm as positive electrode sheets. 2) Use lithium metal sheets as negative electrode sheets, polypropylene microporous membranes as separators, and 1 mol / L LiPF6 dissolved in a solvent of ethylene carbonate (EC): dimethyl carbonate (DMC) = 1:1. Assemble CR2025 coin cells in an argon-filled glove box.

[0109] The coin cell capacity was tested using a coin cell tester and a BTS-5V / 5mA battery testing system to test the charge and discharge performance of the battery, with a voltage range of 2.0-3.75V. 1) Charge / discharge specific capacity: In a 25℃ constant temperature chamber, the assembled battery was charged at a constant current rate of 0.1C to 3.75V, then charged at a constant voltage of 3.75V until the cutoff current was 0.05C, and then discharged at a rate of 0.1C to 2.0V. The charge / discharge capacity was obtained, and the first-time efficiency was calculated as: First-time efficiency = 0.1C discharge capacity / 0.1C charge capacity × 100%. The battery was then charged at a constant current rate of 1C to 3.75V, then charged at a constant voltage of 3.75V until the cutoff current was 0.5C, and then discharged at a rate of 1C to 2.0V to obtain the 1C discharge capacity, reflecting the rate performance. The 3.2V percentage is the ratio of the discharge capacity corresponding to 1C discharge to 3.2V to the discharge capacity corresponding to discharge to 2.0V. The ratio of the discharge capacity after 50 cycles at 1C to the initial discharge capacity (cycle capacity retention rate) reflects the cycle performance.

[0110] The specific test results are shown in the table below.

[0111] Table 7

[0112] As can be seen from the test results in the table above, the embodiments of this application, by adopting anion and cation doping strategies and using different doping strategies for particles of different sizes, can achieve both rate performance and cycle stability while realizing high compaction of lithium iron phosphate composite materials.

[0113] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A lithium iron phosphate composite material, characterized in that, Including the first particle, the second particle, and the third particle; The first particle comprises a first core and a first coating layer located on at least a portion of the surface of the first core, and the first particle has a Ferete diameter of FDn50. 1 nm, the chemical composition of the first core includes: LiFe a1 M b1 (PO4) c1 Y d1 Wherein, 0.005≤b1≤0.020, a1+b1≤1, 0.002≤d1≤0.004, c1+d1≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. The second particle includes a second core and a second coating layer located on at least a portion of the surface of the second core, and the second particle has a Ferete diameter of FDn50. 2 nm, the chemical formula composition of the second core includes: LiFe a2 M b2 (PO4) c2 Y d2 Wherein, 0.005≤b2≤0.020, a2+b2≤1, 0.002≤d2≤0.004, c2+d2≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. The third particle comprises a third core and a third coating layer located on at least a portion of the surface of the third core, and the third particle has a Ferete diameter of FDn50. 3 nm, the chemical formula composition of the third core includes: LiFe a3 M b3 (PO4) c3 Y d3 Wherein, 0.008≤b3≤0.025, a3+b3≤1, 0.001≤d3≤0.003, c3+d3≤1; M includes at least one of Ti, V, and Mg; optionally Ti; Y includes at least one of boron, fluorine, sulfur, silicon, and nitrogen; optionally at least one of boron or fluorine. And it satisfies: FDn50 1 <FDn50 2 <FDn50 3 ;b3>b1; b3>b2; d1>d3; d2>d3.

2. The lithium iron phosphate composite material according to claim 1, characterized in that, The FDn50 1 FDn50 2 FDn50 3 Satisfy: FDn50 1 =xFDn50 3 FDn50 2 =yFDn50 3 1000≤FDn50 3 ≤3000, where x ranges from 0.04 to 0.20, y ranges from 0.12 to 0.35, and x < y; Optionally, the value of x ranges from 0.06 to 0.10, and the value of y ranges from 0.18 to 0.

22.

3. The lithium iron phosphate composite material according to claim 2, characterized in that, The mass ratio of the first particle, the second particle, and the third particle is (0.5-3.0):(0.5-3.0):(5.5-8.0); optionally, it is (0.6-1.4):(1.4-2.4):(6.5-7.5). And / or, 1500≤FDn50 3 ≤2000; And / or, 100≤FDn50 1 ≤160; And / or, 200≤FDn50 2 ≤400.

4. The lithium iron phosphate composite material according to any one of claims 1-3, characterized in that, The first coating layer, the second coating layer, and the third coating layer each independently include a carbon coating layer; In the first particle, the mass percentage of the first coating layer is 1.0-1.5%; in the second particle, the mass percentage of the second coating layer is 1.0-1.5%; and in the third particle, the mass percentage of the third coating layer is 1.0-1.5%.

5. A method for preparing the lithium iron phosphate composite material according to any one of claims 1-4, characterized in that, Includes the following steps: S1, the lithium source, the first carbon source, the iron source, the phosphorus source, the anion dopant, the cation dopant and the solvent are sand-milled and spray-dried, and then sintered in a protective atmosphere to obtain the first intermediate; S2, the lithium source, the second carbon source, the iron source, the phosphorus source, the anion dopant, the cation dopant and the solvent are sand-milled and spray-dried, and then sintered in a protective atmosphere to obtain the second intermediate; S3 involves grinding, spray drying, and sintering a lithium source, a third carbon source, an iron source, a phosphorus source, an anion dopant, a cationic dopant, and a solvent in a protective atmosphere to obtain a third intermediate. Wherein, the temperature of the first sintering is less than the temperature of the second sintering and the temperature of the third sintering; S4, the first intermediate, the second intermediate, the third intermediate and the fourth carbon source are mixed and sintered in a protective atmosphere to obtain the lithium iron phosphate cathode material.

6. The method for preparing the lithium iron phosphate composite material according to claim 5, characterized in that, The temperatures for the first, second, and third sintering processes are independently 600-800℃, and the sintering times are independently 4-10h. And / or, the fourth sintering temperature is 500-700℃, and the sintering time is 4-10h.

7. The method for preparing the lithium iron phosphate composite material according to claim 6, characterized in that, The first sintering temperature is 650-700℃, and the sintering time is 8-10h; And / or, the second sintering temperature is 700-750℃, and the sintering time is 8-10h; And / or, the third sintering temperature is 750-800℃, and the sintering time is 8-10h; And / or, the fourth sintering temperature is 600-650℃, and the sintering time is 4-10h.

8. A positive electrode sheet, characterized in that, include: Positive current collector, and A positive electrode active material layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the lithium iron phosphate composite material according to any one of claims 1-4 or the lithium iron phosphate composite material prepared by the preparation method according to any one of claims 5-7.

9. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 8.

10. An electrical device, characterized in that, Includes the secondary battery as described in claim 9.

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