Composite positive electrode material precursor, preparation method therefor and use thereof

Abstract

A composite positive electrode material precursor, a preparation method therefor and a use thereof. The composite positive electrode material precursor comprises a porous iron phosphate core, a carbon-coated inner layer covering the surface of the core and doped with metal phosphide, and an iron phosphate-coated outer layer covering the surface of the carbon-coated inner layer; and at least part of the carbon-coated inner layer is arranged in pores of the porous iron phosphate core. The surface and pores of the porous iron phosphate core are provided with the carbon-coated inner layer doped with the metal phosphide, so that the electrical conductivity of the material can be significantly improved, the use of conductive carbon is reduced, and the tap density can be prevented from being greatly reduced while improving the rate performance and low-temperature performance of the material. In addition, forming the iron phosphate-coated outer layer on the surface of the carbon-coated inner layer effectively avoids direct contact between the metal phosphide and an electrolyte and reduces side reactions, thereby improving the cycle performance of lithium ion batteries.

Description

A composite positive electrode material precursor and its preparation method and application Technical Field

[0001] The present disclosure belongs to the field of battery technology, and particularly relates to a composite positive electrode material precursor and a preparation method and application thereof. Background Art

[0002] Currently, lithium battery positive electrode materials mainly include lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide and ternary materials. Compared with other lithium battery positive electrode materials, lithium iron phosphate positive electrode material has the advantages of high safety, long cycle life and low manufacturing cost. It is one of the most promising lithium-ion battery positive electrode materials. However, the defects of the material itself, such as low electronic conductivity and lithium ion diffusion coefficient, need to be overcome so as to maximize the performance of the material and meet market needs.

[0003] As a precursor to lithium iron phosphate, its structure, morphology, conductivity, and dispersion directly impact its performance. Currently, conductivity is typically enhanced by coating the material with a conductive substance, typically by coating the surface with a layer of carbon to create a lithium iron phosphate / carbon composite. However, this method only increases the surface conductivity of the lithium iron phosphate, while maintaining its internal conductivity. Furthermore, the carbon coating reduces the compaction density of the material.

[0004] Metal phosphides are often used as negative electrode materials for supercapacitors, lithium-ion batteries, or in hydrogen evolution applications. They have some metallic properties and possess electronic conductivity far higher than that of oxides. Therefore, coating lithium iron phosphate with metal phosphides can effectively improve the electrochemical properties of lithium iron phosphate. However, the coating on the surface of lithium iron phosphate is prone to contact with the electrolyte, causing corrosion.

[0005] Therefore, how to avoid direct contact between metal phosphides and electrolytes, reduce the occurrence of side reactions, and thus improve the cycle performance of lithium-ion batteries is a problem that needs to be solved urgently.

[0006] Summary of the Invention

[0007] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0008] In view of the deficiencies in the prior art, the purpose of the present disclosure is to provide a composite positive electrode material precursor, a preparation method thereof, and an application thereof. The present disclosure provides a carbon-coated inner layer doped with metal phosphide on the surface and in the pores of the porous iron phosphate core, which can significantly improve the conductivity of the material, reduce the use of conductive carbon, and avoid a significant decrease in the tap density while improving the rate performance and low-temperature performance of the material. In addition, the present disclosure forms an iron phosphate-coated outer layer on the surface of the carbon-coated inner layer, which effectively avoids direct contact between the metal phosphide and the electrolyte, reduces the occurrence of side reactions, and thus improves the cycle performance of the lithium-ion battery.

[0009] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a composite cathode material precursor, comprising a porous iron phosphate core, a metal phosphide-doped carbon coating inner layer coated on the surface of the core, and an iron phosphate coating outer layer coated on the surface of the carbon coating inner layer;

[0011] At least a portion of the carbon coating inner layer is disposed in the pores of the porous ferric phosphate inner core.

[0012] The present invention provides a metal phosphide-doped carbon coating layer on the surface and in the pores of a porous iron phosphate core, significantly improving the material's conductivity and reducing the use of conductive carbon. This improves the material's rate capability and low-temperature performance while avoiding a significant decrease in tap density. Furthermore, the present invention forms an iron phosphate coating layer on the surface of the carbon coating layer, effectively preventing direct contact between the metal phosphide and the electrolyte, reducing the occurrence of side reactions and thereby improving the cycling performance of lithium-ion batteries.

[0013] As an optional technical solution of the present disclosure, the particle size D50 of the porous iron phosphate core is 0.5-1.0 μm, for example, it can be 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm or 1.0 μm.

[0014] In one embodiment, the average pore size of the porous iron phosphate core is 10-20 nm, such as 10 nm, 15 nm or 20 nm, and the porosity is 5-20%, such as 5%, 10%, 15% or 20%.

[0015] In the present disclosure, the average pore size of the porous iron phosphate core is 10-20 nm, and the porosity is 5-20%, which helps to control the doping content of metal phosphide and carbon in the iron phosphate pores, improve the conductivity of iron phosphate while maintaining the stability of the iron phosphate structure.

[0016] As an optional technical solution of the present disclosure, the metal phosphide is M2P or M3P, wherein M is a transition metal.

[0017] In one embodiment, the transition metal includes any one of Ni, Co, or Ti, or a combination of at least two thereof.

[0018] In one embodiment, based on the mass of the carbon-coated inner layer, the doping amount of the metal phosphide is 0.5-2wt.%, for example, it can be 0.5wt.%, 0.6wt.%, 0.7wt.%, 0.8wt.%, 0.9wt.%, 1.0wt.%, 1.2wt.%, 1.5wt.% or 2wt.%, etc.

[0019] In the present disclosure, if the doping amount of the metal phosphide is too small, the electronic conductivity of the iron phosphate will be too small; if the doping amount of the metal phosphide is too large, the ionic conductivity will be too small, affecting the diffusion of lithium ions.

[0020] In one embodiment, the thickness of the carbon coating inner layer is 5-20 nm, for example, 5 nm, 10 nm, 15 nm or 20 nm.

[0021] In one embodiment, the thickness of the iron phosphate coating outer layer is 100-600 nm, for example, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm or 600 nm.

[0022] In one embodiment, the thickness ratio of the carbon coating inner layer to the iron phosphate coating outer layer is 1:(20-120), for example, it can be 1:20, 1:40, 1:60, 1:80, 1:100 or 1:120.

[0023] In the present disclosure, if the thickness ratio of the carbon-coated inner layer and the iron phosphate-coated outer layer is too small, that is, the thickness of the iron phosphate-coated outer layer is too large, the conductivity of the outside of the entire iron phosphate particle is poor; if the thickness ratio of the carbon-coated inner layer and the iron phosphate-coated outer layer is too large, that is, the thickness of the iron phosphate-coated outer layer is too small, it is easy to cause uneven coating layers, and the metal phosphide doped in the carbon-coated inner layer is easily exposed to contact with the electrolyte, affecting the electrochemical performance of lithium iron phosphate.

[0024] In a second aspect, the present disclosure provides a method for preparing the composite cathode material precursor as described in the first aspect, the preparation method comprising the following steps:

[0025] (1) mixing a porous iron phosphate material, a phosphorus source, a polymer monomer, an initiator, a metal salt, and a solvent, and reacting the mixture to obtain a porous iron phosphate coated with a polymer coating layer doped with phosphorus and metal;

[0026] (2) using the porous iron phosphate coated with the polymer coating layer doped with phosphorus and metal as a seed crystal, mixing it with an iron source, a phosphorus source and a catalyst, and performing a hydrothermal reaction to obtain a hydrothermal product;

[0027] (3) Sintering the hydrothermal product to obtain the composite positive electrode material precursor.

[0028] In the present disclosure, a phosphorus source and metal ions in a metal salt will produce a coordination effect. The addition of an initiator can cause the polymerized monomers after the coordination of the metal ions in the reaction system to undergo a polymerization reaction, thereby being deposited in the pores and surface of the porous iron phosphate material to form a polymer coating layer doped with phosphorus and metal. The polymer coating layer is then used as a seed crystal to synthesize iron phosphate. After undergoing a hydrothermal reaction treatment, the polymer coating layer doped with phosphorus and metal is carbonized, and a metal phosphide is generated at the same time. Since the electronic conductivity of the metal phosphide is relatively high, the conductivity of the material can be significantly improved, thereby reducing the use of conductive carbon, improving the rate performance and low-temperature performance of the material while avoiding a significant decrease in the tap density.

[0029] Moreover, since the metal phosphide is in direct contact with the electrolyte, it is very easy to corrode, thereby reducing the service life of the electrode material. Therefore, an iron phosphate layer continues to form on the surface of the seed crystal, avoiding direct contact between the metal phosphide and the electrolyte, reducing the occurrence of side reactions, and thus improving the cycle performance of the lithium-ion battery.

[0030] As an optional technical solution of the present disclosure, the method for preparing the porous iron phosphate material in step (1) includes:

[0031] The iron-based metal organic framework material and phosphate buffer are mixed and subjected to a synthesis reaction to obtain a porous iron phosphate material.

[0032] In one embodiment, the concentration of the phosphate buffer is 0.1-1 mol / L, for example, 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 0.7 mol / L or 0.9 mol / L.

[0033] In one embodiment, the pH of the solution after the iron-based metal-organic framework material and the phosphate buffer solution are mixed is 4-8, for example, 4, 5, 6, 7 or 8.

[0034] In one embodiment, the synthesis reaction temperature is 70-90°C, for example, 70°C, 75°C, 80°C, 85°C or 90°C, and the reaction time is 12-24h, for example, 12h, 15h, 20h or 24h.

[0035] In one embodiment, the phosphate buffer comprises any one or a combination of at least two of disodium hydrogen phosphate-citrate buffer, disodium hydrogen phosphate-sodium dihydrogen phosphate buffer solution, disodium hydrogen phosphate-potassium dihydrogen phosphate buffer or PBS buffer.

[0036] In one embodiment, the preparation method of the iron-based metal organic framework material comprises:

[0037] An iron source, an organic acid and a solvent are mixed and subjected to a hydrothermal reaction to obtain the iron-based metal organic framework material.

[0038] In one embodiment, the iron source comprises ferric chloride hexahydrate.

[0039] In one embodiment, the organic acid comprises trimesic acid.

[0040] In one embodiment, the solvent comprises water.

[0041] In one embodiment, the temperature of the hydrothermal reaction is 120-150°C, for example, 120°C, 130°C, 140°C or 150°C, and the time is 36-48h, for example, 36h, 38h, 40h, 42h, 44h, 46h or 48h.

[0042] In one embodiment, after the hydrothermal reaction, washing and drying steps are further performed.

[0043] In one embodiment, the solid-liquid ratio of the iron source, organic acid and solvent is 1g:(3-6)g:(1-3)mL, wherein the selection range of the organic acid "(3-6)g" can be, for example, 3g, 4g, 5g or 6g, and the selection range of the solvent "(1-3)mL" can be, for example, 1mL, 2mL or 3mL.

[0044] In one embodiment, the phosphorus source in step (1) comprises an organic phosphorus source, and the organic phosphorus source comprises hexachlorocyclotriphosphazene and / or triisopropyl phosphite.

[0045] The polymer monomer in step (1) comprises any one of 4,4-dihydroxybiphenyl, 4,4-dihydroxydiphenyl sulfone or 4,4-diaminobiphenyl or a combination of at least two thereof;

[0046] The initiator in step (1) includes any one of benzylamine, 4-nitro-N-methylamine or 4-dinitrosoaniline, or a combination of at least two thereof.

[0047] The metal salt in step (1) is a transition metal salt, and the transition metal salt includes any one or a combination of at least two of nickel nitrate, nickel sulfate, nickel chloride, cobalt nitrate, cobalt sulfate, cobalt chloride, titanium nitrate, titanium sulfate or titanium chloride.

[0048] In the present disclosure, hexachlorocyclotriphosphazene is used as a phosphorus source to produce a coordination effect with the metal ions in the transition metal salt. An initiator is added to cause the polymerized monomers after the coordination of the metal ions in the reaction system to undergo a polymerization reaction, thereby depositing in the pores and surface of the porous iron phosphate to form a polymer coating layer doped with phosphorus and metal, which can effectively improve the electrochemical performance of lithium iron phosphate.

[0049] In one embodiment, the solid-liquid ratio of the porous iron phosphate material and the solvent in step (1) is (3-5) mg:1 mL, for example, 3 mg:1 mL, 3.5 mg:1 mL, 4 mg:1 mL, 4.5 mg:1 mL or 5 mg:1 mL.

[0050] In the present disclosure, the solid-liquid ratio of the porous ferric phosphate material and the solvent is (3-5) mg:1 mL, which can effectively control the concentration of the reactants during the coating process and make the coating layer more uniform.

[0051] In one embodiment, the mass ratio of the porous iron phosphate material, phosphorus source, polymer monomer and metal salt in step (1) is 1: (2-3): (3-6): (8-10), wherein the selection range of the phosphorus source "2-3" can be, for example, 2.2, 2.4, 2.6 or 2.8, the selection range of the polymer monomer "3-6" can be, for example, 3, 4, 5 or 6, and the selection range of the metal salt "8-10" can be, for example, 8, 8.5, 9, 9.5 or 10, etc.

[0052] In the present disclosure, if the mass ratio of the phosphorus source and the metal salt is too small, excessive metal phosphide will be generated subsequently, affecting the ionic conductivity of the iron phosphate; if the mass ratio of the phosphorus source and the metal salt is too large, insufficient metal phosphide will be generated subsequently, affecting the electronic conductivity of the iron phosphate.

[0053] In one embodiment, the mixing method in step (1) includes: adding the porous ferric phosphate material, the phosphorus source, the polymer monomer and the metal salt into a solvent and mixing to obtain a mixed solution, and then adding an initiator into the mixed solution and continuing to mix.

[0054] In the present disclosure, the above-mentioned mixing method is adopted to help increase the polymerization reaction rate and make the coating layer more uniform.

[0055] In one embodiment, the volume ratio of the initiator to the mixed solution is (1-2):40, for example, 1:40, 1.2:40, 1.4:40, 1.6:40, 1.8:40 or 2:40.

[0056] In the present disclosure, if the volume ratio of the initiator to the mixed solution is too small, the polymerization reaction will be incomplete, the formed coating layer will be too thin, and the equipment utilization rate will be reduced; if the volume ratio of the initiator to the mixed solution is too large, the polymerization reaction will be too fast, the polymerization time will be too short, and the formed coating layer will be uneven.

[0057] In one embodiment, the reaction temperature of step (1) is 150-250°C, for example, 150°C, 175°C, 200°C, 225°C or 250°C, and the reaction time is 12-24h, for example, 12h, 15h, 20h or 24h.

[0058] In the present disclosure, the reaction in step (1) is carried out at a temperature of 150-250° C. for 12-24 hours, which can control the polymerization reaction to form a more uniform coating layer and reduce the occurrence of side reactions.

[0059] As an optional technical solution of the present disclosure, the iron source in step (2) is a trivalent iron salt.

[0060] In one embodiment, the phosphorus source in step (2) includes any one of phosphoric acid, ammonium dihydrogen phosphate or diammonium hydrogen phosphate, or a combination of at least two thereof.

[0061] In one embodiment, the catalyst comprises any one of ethylenediamine, 2-hydroxyethylamine or malonamide, or a combination of at least two thereof.

[0062] In the present disclosure, the purpose of adding a catalyst is to regulate the growth direction of the ferric phosphate so that the product has a more uniform morphology, a more complete structure and a higher crystallinity.

[0063] In one embodiment, the ratio of the amount of iron atoms in the iron source, the amount of phosphorus atoms in the phosphorus source, and the amount of ethylenediamine in step (2) is 1:(1-4):(15-20), wherein the selection range of phosphorus atoms in the phosphorus source "1-4" can be, for example, 1, 2, 3 or 4, and the selection range of ethylenediamine "15-20" can be, for example, 15, 16, 17, 18, 19 or 20, etc.

[0064] In the present disclosure, the purpose of adding a catalyst during the growth of ferric phosphate on the surface of the seed crystal is to regulate the growth direction of the ferric phosphate crystals, so that the product morphology is more uniform and the crystallinity is higher.

[0065] In the present disclosure, if the ratio of the amount of iron atoms in the iron source to the amount of the catalyst is too small, the generation rate of iron phosphate will be reduced, side reactions will be easily generated, and the purity of the iron phosphate coating will be reduced; if the ratio of the amount of iron atoms in the iron source to the amount of the catalyst is too large, the iron phosphate coating will be unevenly generated.

[0066] In one embodiment, the mass of the seed crystals in step (2) accounts for 10-20% of the mass of the composite positive electrode material precursor, for example, it can be 10%, 12%, 14%, 16%, 18% or 20%.

[0067] In the present disclosure, if the ratio of the mass of the seed crystal to the mass of the composite positive electrode material precursor is too small, the crystal growth will be slow, and the generation rate of the iron phosphate coating layer will be reduced; if the ratio of the mass of the seed crystal to the mass of the composite positive electrode material precursor is too large, the uniformity of the iron phosphate coating layer will be reduced.

[0068] In one embodiment, the temperature of the hydrothermal reaction in step (2) is 150-190°C, for example, 150°C, 160°C, 170°C, 180°C or 190°C, and the time is 1-5h, for example, 1h, 2h, 3h, 4h or 5h, etc.

[0069] In the present disclosure, if the temperature of the hydrothermal reaction is too low, the reaction rate will be too low and the content of the outer layer coated iron phosphate will be too little; if the temperature of the hydrothermal reaction is too high, side reactions will easily occur, reducing the purity of the iron phosphate.

[0070] As an optional technical solution of the present disclosure, the sintering treatment in step (3) is carried out in a vacuum atmosphere;

[0071] In one embodiment, the sintering treatment temperature in step (3) is 550-750°C, for example, 550°C, 600°C, 650°C, 700°C or 750°C, and the sintering time is 3-6h, for example, 3h, 4h, 5h or 6h.

[0072] In the present disclosure, if the sintering temperature is too low, the iron phosphate coating layer will not be completely carbonized, resulting in reduced conductivity and stability; if the sintering temperature is too high, it will easily lead to excessive defects in the iron phosphate crystals and the appearance of impurities, resulting in reduced structural stability.

[0073] As an optional technical solution of the present disclosure, the preparation method includes the following steps:

[0074] (I) dissolving an iron source and an organic acid in water and mixing them uniformly, then performing a hydrothermal reaction at 120-150° C. for 36-48 hours. After the reaction, the resulting precipitate is washed and centrifuged, and then dried at 130-140° C. for 12-24 hours to obtain an iron-based metal-organic framework material;

[0075] The solid-liquid ratio of the iron source, organic acid, and solvent is 1 g:(3-6) g:(1-3) mL;

[0076] (II) crushing the iron-based metal-organic framework material, soaking it in a phosphate buffer solution with a concentration of 0.1-1 mol / L, controlling the pH of the solution to 4-8 and stirring, and then performing a synthesis reaction at 70-90° C. for 12-24 hours. After the reaction, washing and drying the product to obtain a porous iron phosphate material;

[0077] (III) adding a porous iron phosphate material, an organic phosphorus source, a polymer monomer, and a transition metal salt to a solvent and stirring and mixing them uniformly to obtain a mixed solution, then adding an initiator to the mixed solution and continuing to stir and mix, and then reacting at 150-250° C. for 12-24 hours. After the reaction is completed, filtering and washing are performed to obtain a porous iron phosphate coated with a polymer coating layer doped with phosphorus and metal;

[0078] The solid-liquid ratio of the porous iron phosphate material and the solvent is (3-5) mg:1 mL, the mass ratio of the porous iron phosphate material, the phosphorus source, the polymer monomer and the metal salt is 1:(2-3):(3-6):(8-10), and the volume ratio of the initiator and the mixed solution is (1-2):40;

[0079] (IV) using the porous iron phosphate coated with the polymer coating layer doped with phosphorus and metal as a seed crystal, mixing it with a trivalent iron salt, phosphoric acid and a catalyst, and performing a hydrothermal reaction at 150-190° C. for 1-5 hours to obtain a hydrothermal product;

[0080] The ratio of the amount of iron atoms in the iron source, the amount of phosphorus atoms in the phosphoric acid, and the amount of the catalyst is 1:(1-4):(15-20), and the mass of the seed crystal accounts for 10-20% of the mass of the composite cathode material precursor;

[0081] (V) sintering the hydrothermal product in a vacuum atmosphere to obtain the composite positive electrode material precursor;

[0082] The sintering temperature is 550-750°C and the sintering time is 3-6 hours.

[0083] In a third aspect, the present disclosure provides a positive electrode material, which is obtained by mixing and sintering the composite positive electrode material precursor described in the first aspect and a lithium source.

[0084] In a fourth aspect, the present disclosure provides a method for preparing the positive electrode material according to the third aspect, the preparation method comprising the following steps:

[0085] The composite cathode material precursor and the lithium source are mixed, and the molar ratio of lithium, iron and phosphorus in the reaction system is adjusted to (1-1.2):1:1. Then, an organic carbon source is added and ball milled, and the mixture is calcined in a nitrogen atmosphere at 750-850° C. for 12-24 hours to obtain the cathode material.

[0086] In one embodiment, the positive electrode material is lithium iron phosphate.

[0087] In one embodiment, the organic carbon source comprises starch.

[0088] In a fifth aspect, the present disclosure provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery includes the positive electrode material as described in the third aspect.

[0089] The numerical range described in the present disclosure includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present disclosure no longer exhaustively lists the specific point values ​​included in the range.

[0090] Compared with the prior art, the present disclosure has the following beneficial effects:

[0091] The present invention provides a metal phosphide-doped carbon coating layer on the surface and in the pores of a porous iron phosphate core, significantly improving the material's conductivity and reducing the use of conductive carbon. This improves the material's rate capability and low-temperature performance while avoiding a significant decrease in tap density. Furthermore, the present invention forms an iron phosphate coating layer on the surface of the carbon coating layer, effectively preventing direct contact between the metal phosphide and the electrolyte, reducing the occurrence of side reactions and thereby improving the cycling performance of lithium-ion batteries.

[0092] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0094] FIG1 is a SEM image of the composite cathode material precursor prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0095] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0096] Example 1

[0097] This embodiment provides a composite cathode material precursor, comprising a porous iron phosphate core, a metal phosphide-doped carbon coating inner layer coated on the surface of the core, and an iron phosphate coating outer layer coated on the surface of the carbon coating inner layer;

[0098] At least a portion of the carbon coating inner layer is disposed in the pores of the porous iron phosphate inner core;

[0099] The particle size D50 of the porous iron phosphate core is 0.7 μm, the average pore size of the porous iron phosphate core is 15 nm, and the porosity is 15%;

[0100] The metal phosphide is nickel phosphide, and the doping amount of the metal phosphide is 1.2 wt.% based on the mass of the carbon-coated inner layer;

[0101] The thickness ratio of the carbon coating inner layer to the iron phosphate coating outer layer is 1:70.

[0102] This embodiment also provides a method for preparing the composite positive electrode material precursor, the preparation method comprising the following steps:

[0103] (1) dissolving an iron source and an organic acid in water and mixing them uniformly, then placing them in a hydrothermal reactor and performing a hydrothermal reaction at 135° C. for 42 hours. After the reaction, the obtained precipitate is washed with deionized water and centrifuged, and then dried at 135° C. for 18 hours to obtain an iron-based metal-organic framework material;

[0104] The iron source is ferric chloride hexahydrate, the organic acid is pyromellitic acid, and the solid-liquid ratio of the iron source, the organic acid, and the solvent is 1 g:4 g:2 mL;

[0105] (2) crushing the iron-based metal-organic framework material, soaking it in a phosphate buffer solution with a concentration of 0.5 mol / L, controlling the pH of the solution to 6 and stirring it, and then performing a synthesis reaction at 80° C. for 18 hours. After the reaction is completed, the product is washed with deionized water and vacuum-dried to obtain a porous iron phosphate material;

[0106] Wherein, the phosphate buffer is a disodium hydrogen phosphate-sodium dihydrogen phosphate buffer solution;

[0107] (3) adding the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the transition metal salt into methanol and stirring and mixing them uniformly to obtain a mixed solution, then adding an initiator thereto and continuing to stir and mix, and then placing it in a reactor and reacting it at 200° C. for 18 hours. After the reaction is completed, filtering and washing are performed to obtain a porous iron phosphate coated with a polymer coating layer doped with phosphorus and metal;

[0108] The organic phosphorus source is hexachlorocyclotriphosphazene, the polymer monomer is 4,4-dihydroxybiphenyl, the transition metal salt is nickel nitrate, the initiator is benzylamine, the solid-liquid ratio of the porous iron phosphate material and methanol is 4 mg:1 mL, the mass ratio of the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the metal salt is 1:2.5:4.5:9, and the volume ratio of the initiator to the mixed solution is 1.5:40;

[0109] (4) using the porous iron phosphate coated with the polymer coating layer doped with phosphorus and metal as a seed crystal, uniformly mixing it with trivalent iron salt, phosphoric acid and ethylenediamine, and then performing a hydrothermal reaction at 170° C. for 3 hours to obtain a hydrothermal product;

[0110] The iron source is ferric sulfate, the ratio of the amount of iron atoms in the iron source, the phosphorus atoms in the phosphoric acid and the amount of ethylenediamine is 1:2:17, and the mass of the seed crystal accounts for 15% of the mass of the composite positive electrode material precursor;

[0111] (5) placing the hydrothermal product in a vacuum tube furnace in a vacuum atmosphere and sintering it at 650° C. for 4 h to remove crystallization water and carbonize the polymer coating layer, thereby obtaining the composite positive electrode material precursor.

[0112] This embodiment also provides a method for preparing a positive electrode material, the preparation method comprising the following steps:

[0113] The above-mentioned composite positive electrode material precursor and lithium carbonate were mixed, and the molar ratio of lithium, iron and phosphorus in the reaction system was adjusted to 1.1:1:1. Then, starch with a mass ratio of 10% of the lithium iron phosphate product was added and ball-milled with ethanol as a medium. After drying, the mixture was placed in a muffle furnace and calcined in a nitrogen atmosphere at 800°C for 18 hours to obtain the positive electrode material, namely lithium iron phosphate.

[0114] FIG1 shows that the iron phosphate prepared in this embodiment has a small particle size and a relatively uniform particle distribution.

[0115] Example 2

[0116] This embodiment provides a composite cathode material precursor, comprising a porous iron phosphate core, a metal phosphide-doped carbon coating inner layer coated on the surface of the core, and an iron phosphate coating outer layer coated on the surface of the carbon coating inner layer;

[0117] At least a portion of the carbon coating inner layer is disposed in the pores of the porous iron phosphate inner core;

[0118] The particle size D50 of the porous iron phosphate core is 0.5 μm, the average pore size of the porous iron phosphate core is 10 nm, and the porosity is 5%;

[0119] The metal phosphide is cobalt phosphide, and the doping amount of the metal phosphide is 0.5 wt.% based on the mass of the carbon-coated inner layer;

[0120] The thickness ratio of the carbon coating inner layer to the iron phosphate coating outer layer is 1:20.

[0121] This embodiment also provides a method for preparing the composite positive electrode material precursor, the preparation method comprising the following steps:

[0122] (1) dissolving an iron source and an organic acid in water and mixing them uniformly, then placing them in a hydrothermal reactor and performing a hydrothermal reaction at 150° C. for 36 hours. After the reaction, the obtained precipitate is washed with deionized water and centrifuged, and then dried at 130° C. for 24 hours to obtain an iron-based metal-organic framework material;

[0123] The iron source is ferric chloride hexahydrate, the organic acid is pyromellitic acid, and the solid-liquid ratio of the iron source, the organic acid, and the solvent is 1 g:3 g:1 mL;

[0124] (2) crushing the iron-based metal-organic framework material, soaking it in a phosphate buffer solution with a concentration of 0.1 mol / L, controlling the pH of the solution to 8 and stirring it, and then conducting a synthesis reaction at 70° C. for 24 hours. After the reaction is completed, the product is washed with deionized water and vacuum-dried to obtain a porous iron phosphate material;

[0125] Wherein, the phosphate buffer is a disodium hydrogen phosphate-sodium dihydrogen phosphate buffer solution;

[0126] (3) adding the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the transition metal salt into methanol and stirring and mixing them uniformly to obtain a mixed solution, then adding an initiator thereto and continuing to stir and mix, and then placing it in a reactor and reacting it at 250° C. for 12 hours. After the reaction is completed, filtering and washing are performed to obtain a porous iron phosphate coated with a polymer coating layer doped with phosphorus and metal;

[0127] The organic phosphorus source is hexachlorocyclotriphosphazene, the polymer monomer is 4,4-dihydroxybiphenyl, the transition metal salt is cobalt nitrate, the initiator is benzylamine, the solid-liquid ratio of the porous iron phosphate material and methanol is 5 mg:1 mL, the mass ratio of the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the metal salt is 1:3:6:10, and the volume ratio of the initiator to the mixed solution is 2:40;

[0128] (4) using the porous iron phosphate coated with the polymer coating layer doped with phosphorus and metal as a seed crystal, uniformly mixing it with trivalent iron salt, phosphoric acid and ethylenediamine, and then performing a hydrothermal reaction at 190° C. for 1 hour to obtain a hydrothermal product;

[0129] The iron source is ferric sulfate, the ratio of the amount of iron atoms in the iron source, the phosphorus atoms in the phosphoric acid and the amount of ethylenediamine is 1:4:15, and the mass of the seed crystal accounts for 20% of the mass of the composite positive electrode material precursor;

[0130] (5) placing the hydrothermal product in a vacuum tube furnace in a vacuum atmosphere and sintering it at 550° C. for 6 h to remove crystal water and carbonize the polymer coating layer, thereby obtaining the composite positive electrode material precursor.

[0131] This embodiment also provides a method for preparing a positive electrode material, the preparation method comprising the following steps:

[0132] The above-mentioned composite positive electrode material precursor and lithium carbonate are mixed, and the molar ratio of lithium, iron and phosphorus in the reaction system is adjusted to 1:1:1. Then, starch with a mass of 10% of the lithium iron phosphate product is added and ball milled with ethanol as a medium. After drying, the mixture is placed in a muffle furnace and calcined in a nitrogen atmosphere at 750°C for 24 hours to obtain the positive electrode material, namely lithium iron phosphate.

[0133] Example 3

[0134] This embodiment provides a composite cathode material precursor, comprising a porous iron phosphate core, a metal phosphide-doped carbon coating inner layer coated on the surface of the core, and an iron phosphate coating outer layer coated on the surface of the carbon coating inner layer;

[0135] At least a portion of the carbon coating inner layer is disposed in the pores of the porous iron phosphate inner core;

[0136] The particle size D50 of the porous iron phosphate core is 1.0 μm, the average pore size of the porous iron phosphate core is 20 nm, and the porosity is 20%;

[0137] The metal phosphide is iron phosphide, and the doping amount of the metal phosphide is 2 wt.% based on the mass of the carbon-coated inner layer;

[0138] The thickness ratio of the carbon coating inner layer to the iron phosphate coating outer layer is 1:120.

[0139] This embodiment also provides a method for preparing the composite positive electrode material precursor, the preparation method comprising the following steps:

[0140] (1) dissolving an iron source and an organic acid in water and mixing them uniformly, then placing them in a hydrothermal reactor and performing a hydrothermal reaction at 120° C. for 48 hours. After the reaction, the obtained precipitate is washed with deionized water and centrifuged, and then dried at 140° C. for 12 hours to obtain an iron-based metal-organic framework material;

[0141] The iron source is ferric chloride hexahydrate, the organic acid is pyromellitic acid, and the solid-liquid ratio of the iron source, the organic acid, and the solvent is 1 g:3 g:1 mL;

[0142] (2) crushing the iron-based metal-organic framework material, soaking it in a phosphate buffer solution with a concentration of 1 mol / L, controlling the pH of the solution to 4 and stirring it, and then performing a synthesis reaction at 90° C. for 12 hours. After the reaction is completed, the product is washed with deionized water and vacuum-dried to obtain a porous iron phosphate material;

[0143] Wherein, the phosphate buffer is a disodium hydrogen phosphate-sodium dihydrogen phosphate buffer solution;

[0144] (3) adding the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the transition metal salt into methanol and stirring and mixing them uniformly to obtain a mixed solution, then adding an initiator thereto and continuing to stir and mix, and then placing the mixture in a reactor and reacting it at 150° C. for 24 hours. After the reaction is completed, filtering and washing are performed to obtain a porous iron phosphate coated with a polymer coating layer doped with phosphorus and metal;

[0145] The organic phosphorus source is hexachlorocyclotriphosphazene, the polymer monomer is 4,4-dihydroxybiphenyl, the transition metal salt is ferric chloride, the initiator is benzylamine, the solid-liquid ratio of the porous iron phosphate material and methanol is 3 mg:1 mL, the mass ratio of the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the metal salt is 1:2:3:8, and the volume ratio of the initiator to the mixed solution is 1:40;

[0146] (4) using the porous iron phosphate coated with the polymer coating layer doped with phosphorus and metal as a seed crystal, uniformly mixing it with trivalent iron salt, phosphoric acid and ethylenediamine, and then performing a hydrothermal reaction at 150° C. for 5 hours to obtain a hydrothermal product;

[0147] The iron source is ferric sulfate, the ratio of the amount of iron atoms in the iron source, the phosphorus atoms in the phosphoric acid and the amount of ethylenediamine is 1:1:20, and the mass of the seed crystal accounts for 20% of the mass of the composite positive electrode material precursor;

[0148] (5) placing the hydrothermal product in a vacuum tube furnace in a vacuum atmosphere and sintering it at 750° C. for 3 h to remove crystallization water and carbonize the polymer coating layer, thereby obtaining the composite positive electrode material precursor.

[0149] This embodiment also provides a method for preparing a positive electrode material, the preparation method comprising the following steps:

[0150] The above-mentioned composite positive electrode material precursor and lithium carbonate are mixed, and the molar ratio of lithium, iron and phosphorus in the reaction system is adjusted to 1.2:1:1. Then, starch with a mass ratio of 10% of the lithium iron phosphate product is added and ball milled with ethanol as a medium. After drying, the mixture is placed in a muffle furnace and calcined in a nitrogen atmosphere at 850°C for 12 hours to obtain the positive electrode material, namely lithium iron phosphate.

[0151] Example 4

[0152] The difference between this embodiment and embodiment 1 is that the amount of the organic phosphorus source and the metal salt in step (3) is adjusted so that the doping amount of the metal phosphide in the carbon coating inner layer is 0.1%. The rest of the preparation method and parameters are the same as those in embodiment 1.

[0153] Example 5

[0154] The difference between this embodiment and embodiment 1 is that the amount of the organic phosphorus source and the metal salt in step (3) is adjusted so that the doping amount of the metal phosphide in the carbon coating inner layer is 3%. The rest of the preparation method and parameters are the same as those in embodiment 1.

[0155] Example 6

[0156] The difference between this embodiment and embodiment 1 is that the amount of each parameter in step (4) is adjusted so that the thickness ratio of the carbon coating inner layer to the iron phosphate coating outer layer is 1:1. The remaining preparation methods and parameters are consistent with those in embodiment 1.

[0157] Example 7

[0158] The difference between this embodiment and embodiment 1 is that the amount of each parameter in step (4) is adjusted so that the thickness ratio of the carbon coating inner layer to the iron phosphate coating outer layer is 1:220. The remaining preparation methods and parameters are consistent with those in embodiment 1.

[0159] Example 8

[0160] The difference between this embodiment and embodiment 1 is that the amount of the initiator in step (3) is adjusted so that the volume ratio of the initiator to the mixed solution is 0.5:40.

[0161] The rest of the preparation methods and parameters remained the same as in Example 1.

[0162] Example 9

[0163] The difference between this embodiment and embodiment 1 is that the amount of the initiator in step (3) is adjusted so that the volume ratio of the initiator to the mixed solution is 5:40.

[0164] The rest of the preparation methods and parameters remained the same as in Example 1.

[0165] Example 10

[0166] The difference between this embodiment and embodiment 1 is that the amount of ethylenediamine in step (4) is adjusted so that the ratio of the amount of iron atoms in the iron source to the amount of ethylenediamine is 1:25.

[0167] The rest of the preparation methods and parameters remained the same as in Example 1.

[0168] Example 11

[0169] The difference between this embodiment and embodiment 1 is that the amount of ethylenediamine in step (4) is adjusted so that the ratio of the amount of iron atoms in the iron source to the amount of ethylenediamine is 1:10.

[0170] The rest of the preparation methods and parameters remained the same as in Example 1.

[0171] Example 12

[0172] The difference between this embodiment and embodiment 1 is that the mass of the seed crystal in step (4) accounts for 5% of the mass of the composite positive electrode material precursor.

[0173] The rest of the preparation methods and parameters remained the same as in Example 1.

[0174] Example 13

[0175] The difference between this embodiment and embodiment 1 is that the mass of the seed crystal in step (4) accounts for 25% of the mass of the composite positive electrode material precursor.

[0176] The rest of the preparation methods and parameters remained the same as in Example 1.

[0177] Example 14

[0178] The difference between this embodiment and embodiment 1 is that the temperature of the sintering treatment in step (5) is 500°C.

[0179] The rest of the preparation methods and parameters remained the same as in Example 1.

[0180] Example 15

[0181] The difference between this embodiment and embodiment 1 is that the temperature of the sintering treatment in step (5) is 800°C.

[0182] The rest of the preparation methods and parameters remained the same as in Example 1.

[0183] Comparative Example 1

[0184] The difference between this comparative example and Example 1 is that step (3) is not performed, so that the surface of the porous iron phosphate core does not contain the carbon coating inner layer doped with metal phosphide.

[0185] The rest of the preparation methods and parameters remained the same as in Example 1.

[0186] Comparative Example 2

[0187] The difference between this comparative example and Example 1 is that step (4) is not performed, so that the composite positive electrode material precursor does not contain an iron phosphate coating outer layer.

[0188] The rest of the preparation methods and parameters remained the same as in Example 1.

[0189] Comparative Example 3

[0190] The difference between this comparative example and Example 1 is that the carbon coating inner layer is not doped with metal phosphide, that is, step (3) is replaced by the following steps:

[0191] The porous iron phosphate material and glucose were added to methanol and stirred for 2 hours to mix evenly, then dried and calcined at 400°C in a nitrogen atmosphere for 5 hours to form a carbon-coated porous iron phosphate material; the solid-liquid ratio of the porous iron phosphate material and methanol was 4 mg:1 mL, and the volume ratio of glucose to methanol was 1.5:40.

[0192] The rest of the preparation methods and parameters remained the same as in Example 1.

[0193] Performance Testing

[0194] The positive electrode materials prepared in the above examples and comparative examples were made into positive electrode slurries to prepare positive electrode sheets, which were then assembled with negative electrode sheets, electrolytes, and separators to obtain 2025 button batteries. The specific steps include:

[0195] Lithium iron phosphate positive electrode material, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5 and added to N-methylpyrrolidone. The mixture was stirred evenly to obtain a positive electrode slurry. This slurry was evenly coated on an aluminum foil positive electrode current collector and dried to obtain a positive electrode sheet. An electrolyte solution with a LiPF6 concentration of 1.0 mol / L was prepared by mixing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 1:1:1. A polyethylene film was used as the separator, and a lithium metal sheet was used as the negative electrode sheet to assemble a 2025 button-type battery.

[0196] The electrochemical performance test of the button cell was conducted: charge and discharge test was conducted at 2-3.75V at room temperature.

[0197] The test results are shown in Table 1.

[0198] Table 1

[0199] analyze:

[0200] As can be seen from the above table, the present disclosure effectively improves the poor conductivity of lithium iron phosphate materials by doping a highly conductive metal phosphide during the preparation process of iron phosphate. Under the premise of ensuring high gram capacity, the rate and cycle performance of the material are greatly improved, and the tap density is greatly reduced. At the same time, the provision of the iron phosphate coating outer layer avoids direct contact between the metal phosphide and the electrolyte, reduces the occurrence of side reactions, and thus improves the cycle performance of the lithium-ion battery.

[0201] From the comparison of the data results of Example 1 and Examples 4-5, it can be seen that if the doping amount of metal phosphide in the carbon-coated inner layer is too small, the electronic conductivity and compaction density of the material will be reduced, thereby reducing the discharge capacity of lithium iron phosphate; if the doping amount of metal phosphide in the carbon-coated inner layer is too large, the ionic conductivity of the material will be reduced, thereby reducing the rate performance of the material.

[0202] From the comparison of the data results of Example 1 and Examples 6-7, it can be seen that if the thickness ratio of the carbon-coated inner layer and the iron phosphate-coated outer layer is too small, the external conductivity of the lithium iron phosphate is reduced, and the battery rate performance is reduced; if the thickness ratio of the carbon-coated inner layer and the iron phosphate-coated outer layer is too large, the compaction density of the lithium iron phosphate is reduced, and the metal phosphide doped in the carbon-coated inner layer is easily exposed to contact with the electrolyte, thereby reducing the battery cycle performance.

[0203] From the comparison of the data results of Example 1 and Examples 8-9, it can be seen that if the volume ratio of the initiator to the mixed solution is too small, the polymerization reaction is insufficient and the carbon coating layer formed is too thin, thereby reducing the rate performance and cycle performance of the battery; if the volume ratio of the initiator to the mixed solution is too large, the carbon coating layer formed is uneven, thereby reducing the compaction density and cycle stability of the material.

[0204] From the comparison of the data results of Example 1 and Examples 10-11, it can be seen that if the ratio of the amount of iron atoms in the iron source to the amount of ethylenediamine is too small, side reactions are likely to occur during the generation of iron phosphate, thereby reducing the rate performance and cycle performance of the battery; if the ratio of the amount of iron atoms in the iron source to the amount of ethylenediamine is too large, the iron phosphate coating layer is unevenly generated, thereby reducing the rate performance and cycle performance of the battery.

[0205] From the comparison of the data results of Example 1 and Examples 12-13, it can be seen that if the ratio of the mass of the seed crystal to the mass of the composite positive electrode material precursor is too small, the generation rate of the iron phosphate coating layer will be reduced, thereby reducing the compaction density and cycle performance of the lithium iron phosphate; if the ratio of the mass of the seed crystal to the mass of the composite positive electrode material precursor is too large, it will easily lead to uneven coating of the iron phosphate outer layer, thereby reducing the rate performance and cycle performance of the battery material.

[0206] By comparing the data results of Example 1 with those of Examples 14-15, it can be seen that if the sintering temperature is too low, the carbonization of the iron phosphate coating layer is incomplete, thereby reducing the conductivity and stability of the lithium iron phosphate; if the sintering temperature is too high, it is easy to cause too many defects in the iron phosphate crystals, thereby reducing the cycle stability performance of the lithium iron phosphate.

[0207] From the comparison of the data results of Example 1 and Comparative Example 1, it can be seen that if the surface of the porous iron phosphate core does not contain a carbon-coated inner layer doped with metal phosphide, the conductivity inside the lithium iron phosphate will be reduced, thereby reducing the battery rate performance and cycle performance.

[0208] From the comparison of the data results of Example 1 and Comparative Example 2, it can be seen that if the composite positive electrode material precursor does not contain an iron phosphate coating outer layer, the metal phosphide is easily exposed to the electrolyte, thereby reducing the cycle performance of the battery.

[0209] From the comparison of the data results of Example 1 and Comparative Example 3, it can be seen that if the carbon coating inner layer is not doped with metal phosphide, the metal phosphide doped in the present invention can improve the conductivity of lithium iron phosphate while increasing the compaction density compared with doping a simple carbon layer.

Claims

1. A composite cathode material precursor, comprising a porous iron phosphate core, a carbon-coated inner layer doped with metal phosphide coated on the surface of the core, and an iron phosphate-coated outer layer coated on the surface of the carbon-coated inner layer; At least a part of the carbon-coated inner layer is disposed in the pores of the porous iron phosphate core.

2. The composite cathode material precursor according to claim 1, wherein, The average pore diameter of the porous iron phosphate core is 10-20 nm, and the porosity is 5-20%.

3. The composite cathode material precursor according to claim 1 or 2, wherein, Based on the mass of the carbon-coated inner layer, the doping amount of the metal phosphide is 0.5-2 wt.%.

4. The composite cathode material precursor according to any one of claims 1-3, wherein, The thickness ratio of the carbon-coated inner layer to the iron phosphate-coated outer layer is 1:(20-120).

5. A method for preparing a composite cathode material precursor according to any one of claims 1-4, comprising the following steps: (1) Mix a porous iron phosphate material, a phosphorus source, a polymer monomer, an initiator, a metal salt and a solvent, and react to obtain porous iron phosphate coated with a polymer coating layer doped with phosphorus and metal; (2) Use the porous iron phosphate coated with the polymer coating layer doped with phosphorus and metal as a seed, mix it with an iron source, a phosphorus source and a catalyst, and perform a hydrothermal reaction to obtain a hydrothermal product; (3) Sinter the hydrothermal product to obtain the composite cathode material precursor.

6. The preparation method according to claim 5, wherein, The phosphorus source in step (1) includes an organic phosphorus source, and the organic phosphorus source includes hexachlorocyclotriphosphazene and / or triisopropyl phosphite; The polymer monomer in step (1) includes any one or a combination of at least two of 4,4-dihydroxybiphenyl, 4,4-dihydroxydiphenyl sulfone or 4,4-diaminobiphenyl; The initiator in step (1) includes any one or a combination of at least two of benzylamine, 4-nitro-N-methylamine or 4-dinitrosobenzeneamine; The metal salt in step (1) is a transition metal salt, and the transition metal salt includes any one or a combination of at least two of nickel nitrate, nickel sulfate, nickel chloride, cobalt nitrate, cobalt sulfate, cobalt chloride, titanium nitrate, titanium sulfate or titanium chloride.

7. The preparation method according to claim 5 or 6, wherein, The solid-liquid ratio of the porous iron phosphate material and the solvent in step (1) is (3-5) mg:1 mL.

8. The preparation method according to any one of claims 5-7, wherein, The mass ratio of the porous iron phosphate material, the phosphorus source, the polymer monomer and the metal salt in step (1) is 1:(2-3):(3-6):(8-10).

9. The preparation method according to any one of claims 5-8, wherein, The mixing method in step (1) includes: adding the porous iron phosphate material, the phosphorus source, the polymer monomer and the metal salt to the solvent for mixing to obtain a mixed solution, and then adding an initiator to the mixed solution to continue mixing.

10. The preparation method according to any one of claims 5-9, wherein, The volume ratio of the initiator to the mixed solution is (1 - 2):

40.

11. The preparation method according to any one of claims 5 - 10, wherein, the temperature of the reaction in step (1) is 150 - 250 °C and the time is 12 - 24 h.

12. The preparation method according to any one of claims 5 - 11, wherein, the catalyst in step (2) comprises any one or a combination of at least two of ethylenediamine, 2-hydroxyethylamine or malonamide.

13. The preparation method according to any one of claims 5 - 12, wherein, the molar ratio of iron atoms in the iron source, phosphorus atoms in the phosphorus source to the catalyst in step (2) is 1:(1 - 4):(15 - 20).

14. The preparation method according to any one of claims 5 - 13, wherein, the mass of the seed crystal in step (2) accounts for 10 - 20% of the mass of the composite cathode material precursor.

15. The preparation method according to any one of claims 5 - 14, wherein, the temperature of the hydrothermal reaction in step (2) is 150 - 190 °C and the time is 1 - 5 h.

16. The preparation method according to any one of claims 5 - 15, wherein, the temperature of the sintering treatment in step (3) is 550 - 750 °C and the time is 3 - 6 h.

17. The preparation method according to any one of claims 5 - 16, wherein, the preparation method comprises the following steps: (Ⅰ) Dissolve the iron source and the organic acid in water and mix evenly, then carry out a hydrothermal reaction at 120 - 150 °C for 36 - 48 h. After the reaction, wash and centrifuge the obtained precipitate, and then dry it at 130 - 140 °C for 12 - 24 h to obtain an iron-based metal-organic framework material; wherein, the solid-liquid ratio of the iron source, the organic acid and the solvent is 1 g:(3 - 6) g:(1 - 3) mL; (Ⅱ) After crushing the iron-based metal-organic framework material, soak it in a phosphate buffer solution with a concentration of 0.1 - 1 mol / L, control the pH of the solution to be 4 - 8 and stir, then carry out a synthesis reaction at 70 - 90 °C for 12 - 24 h. After the reaction, wash and dry the product to obtain a porous iron phosphate material; (Ⅲ) Add the porous iron phosphate material, the organic phosphorus source, the polymer monomer and the transition metal salt to the solvent and stir to mix evenly to obtain a mixed solution. Subsequently, add an initiator and continue to stir and mix, then react at 150 - 250 °C for 12 - 24 h. After the reaction, carry out suction filtration and washing to obtain porous iron phosphate coated with a polymer coating doped with phosphorus and metal; wherein, the solid-liquid ratio of the porous iron phosphate material to the solvent is (3 - 5) mg:1 mL, the mass ratio of the porous iron phosphate material, the phosphorus source, the polymer monomer and the metal salt is 1:(2 - 3):(3 - 6):(8 - 10), and the volume ratio of the initiator to the mixed solution is (1 - 2):40; (Ⅳ) Use the porous iron phosphate coated with a polymer coating doped with phosphorus and metal as a seed crystal, mix it with a ferric salt, phosphoric acid and a catalyst, and carry out a hydrothermal reaction at 150 - 190 °C for 1 - 5 h to obtain a hydrothermal product; Among them, the molar ratio of iron atoms in the iron source, phosphorus atoms in phosphoric acid and the catalyst is 1:(1 - 4):(15 - 20), and the mass of the seed crystal accounts for 10 - 20% of the mass of the composite cathode material precursor; (Ⅴ) In a vacuum atmosphere, the hydrothermal product is sintered to obtain the composite cathode material precursor; Among them, the temperature of the sintering treatment is 550 - 750 °C, and the time is 3 - 6 h.

18. A cathode material obtained by mixing and sintering the composite cathode material precursor according to any one of claims 1 - 4 and a lithium source.

19. A lithium-ion battery comprising the cathode material according to claim 18.

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