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

Abstract

The application discloses a lithium iron phosphate positive electrode material and a preparation method and application thereof. The lithium iron phosphate positive electrode material comprises lithium iron phosphate particles, a doped element, an iron phosphide layer, a carbon layer and a coating layer. The doped element is doped in the lithium iron phosphate particles. The iron phosphide layer is coated on at least part of the surface of the lithium iron phosphate particles. The carbon layer is coated on at least part of the surface of the phosphide layer. The coating layer is coated on at least part of the surface of the carbon layer. The valence of the doped element is greater than +3. The application can promote the generation of the iron phosphide layer, effectively improve the intrinsic electron and ion conductivity of the lithium iron phosphate, and widen the lithium ion transmission channel and improve the material rate performance by doping the high-valence element. The coating layer is beneficial to inhibiting the continuous growth of the material, and the obtained material has good uniformity and dispersity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrode materials, and particularly relates to a lithium iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the increasing demand for environmental protection and renewable energy, lithium batteries, as a kind of efficient and clean energy storage method, are increasingly widely used. Among them, lithium iron phosphate is widely used as a positive electrode material of lithium batteries due to its advantages of rich raw materials, low price, good safety performance, long cycle life and the like. However, due to the low electronic conductivity and lithium ion diffusion coefficient of lithium iron phosphate, its application under high rate conditions, especially high power conditions, is seriously limited. In addition, the traditional solid-phase synthesis method still has problems of poor material consistency, uneven particle growth and performance deterioration.

[0003] In order to solve the above problems, the existing technology usually adopts methods such as carbon coating and metal element doping to improve the electronic conductivity and lithium ion diffusion coefficient of lithium iron phosphate. For example, by adding carbon black, graphene and other carbon sources, the electronic conductivity of lithium iron phosphate is improved, and the further growth of particles is hindered; some over-doped elements are added to improve the lithium ion diffusion coefficient of lithium iron phosphate. In addition, some studies use high-temperature solid-phase method, sol-gel method, hydrothermal method and the like to prepare lithium iron phosphate to improve its electrochemical performance. In the above methods, although the coating of carbon materials can improve the ability of the material to conduct electrons, there are still problems of uneven coating and local crystal overgrowth in actual industrialization. For example, in the method of coating titanium dioxide, the melting temperature of titanium dioxide is 1856.85℃, and the coating state of titanium dioxide on the surface of the material is basically point-shaped or island-shaped uneven coating. As for element doping, although the existing element doping helps to improve the lithium ion diffusion coefficient, there are still problems of low intrinsic electronic conductivity of lithium iron phosphate and poor cycle performance, for example, although Na and Mg doping can expand the lithium ion transmission channel, it is difficult to fundamentally solve the problem of low intrinsic electronic conductivity of lithium iron phosphate. In addition, the existing preparation methods are often complex in steps, high in energy consumption and high in cost, which is not conducive to large-scale production. SUMMARY

[0004] In order to overcome at least one problem existing in the prior art, one of the purposes of the present application is to provide a lithium iron phosphate positive electrode material which can improve the lithium ion diffusion coefficient while improving the intrinsic electronic conductivity of the lithium iron phosphate material, and strengthen the stability of its structure and surface interface, and the material grows uniformly and has good dispersibility.

[0005] The second object of the present application is to provide a preparation method of the lithium iron phosphate cathode material, which can obtain the lithium iron phosphate cathode material with uniform particle size, good dispersibility, good conductivity, high capacity and good stability, and the preparation method has simple steps, low energy consumption and cost, and can adapt to large-scale production.

[0006] The third object of the present application is to provide a battery comprising the lithium iron phosphate cathode material.

[0007] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0008] The first aspect of the present application provides a lithium iron phosphate cathode material, comprising: lithium iron phosphate particles, a doping element, an iron phosphide layer, a carbon layer, and a coating layer; the doping element is doped in the lithium iron phosphate particles; the iron phosphide layer is coated on at least part of the surface of the lithium iron phosphate particles; the carbon layer is coated on at least part of the surface of the iron phosphide layer; and the coating layer is coated on at least part of the surface of the carbon layer; wherein the valence of the doping element is >+3 valence.

[0009] The lithium iron phosphate cathode material according to the first aspect of the present application has at least the following beneficial effects:

[0010] By doping the high-valence doping element with a valence higher than +3 valence in the lithium iron phosphate particles, the present application causes the reduction of iron elements by using the charge compensation effect, so as to form a high-conductivity iron phosphide layer on the surface of the lithium iron phosphate particles, and the interface effect between the iron phosphide layer and the lithium iron phosphate particles also helps to improve the electrochemical performance of the material; and the doping of the doping element can also widen the lithium ion transmission channel and improve the rate performance of the material, which is conducive to improving the lithium ion diffusion coefficient of the lithium iron phosphate cathode material while ensuring the structural stability thereof. The coating of the coating layer is conducive to controlling the uneven growth of local materials in the solid-phase sintering, improving the morphology and consistency of the cathode particles, and improving the stability of the material. Through the doping and coating treatment of the present application, the obtained lithium iron phosphate cathode material has low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, good stability, and good electrochemical performance.

[0011] In some specific embodiments of the present application, the doping element comprises at least one of niobium, tantalum, vanadium or iridium.

[0012] The elements such as niobium, tantalum, vanadium and iridium are doped, which have not only high valence but also suitable atomic binding energy, atomic size and atomic activity, and have good matching with the lithium iron phosphate particles, so as to form good doping and effectively improve the capacity and performance of the material; further, the doping of niobium and tantalum in the present application has better doping effect, and can obtain the cathode material with better conductivity, higher capacity and better stability.

[0013] In some embodiments of the present application, the doping element is doped in the inner shell surface layer of the lithium iron phosphate particles.

[0014] Doping the doping element in the inner shell surface layer of the lithium iron phosphate particles without fully penetrating into the bulk structure thereof is conducive to the doping element playing a charge compensation role to realize the reduction of iron elements, thereby forming an iron phosphide layer on the surface of the lithium iron phosphate particles, and is also conducive to playing the interaction between the doping element and the iron phosphide layer, the coating layer, etc., thereby improving the material performance.

[0015] In some embodiments of the present application, the coating layer comprises at least one of a boron-containing oxide, a tungsten-containing oxide, a carbon material, a phosphate, or a lithium-containing compound.

[0016] The above coating layer can achieve good coating effect and form material particles with good consistency; in addition, the interaction between the coating layer and the doping element or the action at the interface is also conducive to improving the electrochemical performance of the material.

[0017] In some embodiments of the present application, the particle size of the lithium iron phosphate particles is 0.1-2 μm.

[0018] In some embodiments of the present application, the thickness of the iron phosphide layer is 1-20 nm.

[0019] In some embodiments of the present application, the thickness of the carbon layer is 1-40 nm.

[0020] In some embodiments of the present application, the thickness of the coating layer is 1-40 nm.

[0021] In some embodiments of the present application, the D10 particle size of the lithium iron phosphate positive electrode material is 0.1-0.8 μm.

[0022] In some embodiments of the present application, the D50 particle size of the lithium iron phosphate positive electrode material is 0.9-1.5 μm.

[0023] In some embodiments of the present application, the D99 particle size of the lithium iron phosphate positive electrode material is 2-4 μm.

[0024] The second aspect of the present application provides a preparation method of a lithium iron phosphate positive electrode material, comprising the following steps: performing pre-sintering treatment on a mixture containing an iron source, a phosphorus source, a lithium source, a carbon source, and an additive to obtain a pre-sintered material, mixing the pre-sintered material with a coating agent, and then performing calcination treatment to obtain the lithium iron phosphate positive electrode material as described in the first aspect of the present application; the additive is a compound containing the doping element; and the coating agent is a compound forming the coating layer.

[0025] The preparation method of the lithium iron phosphate cathode material according to the second aspect of the present application has at least the following beneficial effects:

[0026] The iron source, the phosphorus source and the lithium source are used to form the lithium iron phosphate; the doping elements in the additive are doped in the formed lithium iron phosphate, and the doping elements are also beneficial to promoting the reduction of iron elements at the surface or subsurface of the lithium iron phosphate, so as to generate a layer of phosphide of iron; the carbon source is used to form a carbon layer, which is coated on the surface of the phosphide of iron; and the coating agent is used to form a coating layer, which is coated on the outermost part of the material. In the preparation method of the present application, the preliminary chemical reaction of the raw materials is performed through the pre-sintering treatment, so as to form a specific composition and doping structure, and then the coating and calcination treatment are performed, which is beneficial to obtaining the cathode material with a specific layer structure, and the material has better uniformity and electrochemical performance. Moreover, the preparation method is simple, low in cost and easy to realize industrial production.

[0027] In some embodiments of the present application, the additive includes at least one of a niobium-containing compound, a tantalum-containing compound, a vanadium-containing compound or an iridium-containing compound.

[0028] In some embodiments of the present application, the mass percentage of the additive in the mixture is 1-3%.

[0029] The doping elements such as niobium, tantalum, vanadium and iridium in the additive are preliminarily doped in the pre-sintering product in the pre-sintering process, so as to form a specific doping distribution structure, which is beneficial to the subsequent coating treatment and calcination treatment. Further, the niobium and tantalum are adopted for doping in the present application, which has a better doping effect, and the cathode material with better conductivity, higher capacity and better stability can be obtained. Moreover, the additive with a small amount is only needed to be added, and a good doping modification effect can be achieved.

[0030] In some embodiments of the present application, the coating agent includes at least one of boric acid, tungsten oxide, polyaniline, polypyrrole, phosphate or a lithium-containing compound.

[0031] In some embodiments of the present application, the mass ratio of the coating agent to the pre-sintering material is (0.1-5):100.

[0032] In the preparation process of the material, the coating agent will form a coating layer on the surface of the carbon layer due to the high-temperature reaction, for example, the boric acid will generate an oxide containing boron, the tungsten oxide will generate an oxide containing tungsten, the polyaniline and polypyrrole will generate carbon materials, etc. Further, the boric acid and tungsten oxide can be adopted to obtain a better coating effect, and the cathode material with better electrochemical performance can be obtained. Moreover, the coating agent with a small amount is only needed to be added, and a good coating modification effect can be achieved.

[0033] In some embodiments of the present application, the temperature of the pre-sintering treatment is 400-700℃.

[0034] In some embodiments of the present application, the time of the pre-sintering treatment is 1-6 hours.

[0035] In some embodiments of the present application, the temperature of the calcination treatment is 700-900℃.

[0036] In some embodiments of the present application, the time of the calcination treatment is 2-8 hours.

[0037] The third aspect of the present application provides a battery comprising the lithium iron phosphate cathode material of the first aspect of the present application, or the lithium iron phosphate cathode material prepared by the preparation method of the second aspect of the present application.

[0038] According to the battery of the third aspect of the present application, at least the following beneficial effects are achieved:

[0039] The lithium iron phosphate cathode material provided by the present application has low resistivity, good conductivity, high charge-discharge capacity, high capacity retention rate, good stability and other good electrochemical properties. Using the lithium iron phosphate cathode material as a cathode material can obtain a battery with excellent electrochemical properties. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 The structure of the lithium iron phosphate cathode material of Example 1 is shown in the figure.

[0041] Figure 2 The SEM images of the lithium iron phosphate cathode materials of Example 1 and Comparative Example 1 are shown in the figure. DETAILED DESCRIPTION

[0042] The content of the present application will be further described in detail through specific examples. It should also be understood that the following examples are only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Some non-essential improvements and adjustments made by those skilled in the art based on the principles described in the present application are within the scope of protection of the present application. The following examples of specific process parameters, etc. are only one example in the appropriate range, i.e. those skilled in the art can make appropriate choices within the scope of the present application, and are not limited to the specific data of the following examples. The raw materials, reagents or devices used in the following examples and comparative examples, if not specifically stated, can be obtained from conventional commercial channels, or can be obtained by existing known methods.

[0043] The first aspect of the embodiment of the present application provides a lithium iron phosphate positive electrode material, comprising: lithium iron phosphate particles, a doping element, an iron phosphide layer, a carbon layer, and a coating layer; the doping element is doped in the lithium iron phosphate particles; the iron phosphide layer is coated on at least part of the surface of the lithium iron phosphate particles; the carbon layer is coated on at least part of the surface of the iron phosphide layer; and the coating layer is coated on at least part of the surface of the carbon layer; wherein the valence of the doping element is > +3.

[0044] The present application forms a high-conductivity iron phosphide layer on the surface of the lithium iron phosphate particles by doping a high-valence doping element with a valence > +3 in the lithium iron phosphate particles and causing the reduction of iron elements by charge compensation, and the interface effect between the iron phosphide layer and the lithium iron phosphate particles also helps to improve the electrochemical performance of the material; and the doping of the doping element can also widen the lithium ion transmission channel and improve the rate performance of the material, which is conducive to improving the lithium ion diffusion coefficient of the lithium iron phosphate positive electrode material while ensuring the structural stability thereof. The coating of the coating layer is conducive to controlling the uneven growth of local materials in the solid-phase sintering, improving the morphology and consistency of the positive electrode particles, and improving the stability of the material. The lithium iron phosphate positive electrode material obtained through the doping and coating treatment of the present application has low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, good stability, and good electrochemical performance.

[0045] In some embodiments of the present application, the carbon layer is also coated on at least part of the surface of the lithium iron phosphate particles.

[0046] In some embodiments of the present application, the coating layer is also coated on at least part of the surface of the lithium iron phosphate particles, and / or the coating layer is also coated on at least part of the surface of the iron phosphide layer.

[0047] In some embodiments of the present application, the valence of the doping element is ≥ +4; in some specific embodiments of the present application, the valence of the doping element is +4 to +8; and in some examples of the present application, the valence of the doping element is +4, +5, +6, +7 or +8.

[0048] Based on the charge compensation effect, the doping of the high-valence element is conducive to promoting the reduction of iron elements, thereby forming an iron phosphide layer with good conductivity.

[0049] In some embodiments of the present application, the doping element comprises at least one of niobium (Nb), tantalum (Ta), vanadium (V) or iridium (Ir); in some specific embodiments of the present application, the doping element comprises niobium, tantalum or a combination thereof; and in some examples of the present application, the doping element is selected from niobium.

[0050] The elements such as niobium, tantalum, vanadium and iridium are doped, and in addition to high valence, the elements have suitable atomic binding energy, atomic size and atomic activity, and have good matching with the lithium iron phosphate particles, so that good doping is formed, and the capacity and performance of the material are effectively improved; further, the niobium and tantalum are doped in the application, and better doping effect can be obtained, and the positive electrode material with better conductivity, higher capacity and better stability can be obtained.

[0051] In some embodiments of the application, the doping elements are doped in the inner shell surface layer of the lithium iron phosphate particles; in some specific embodiments of the application, the doping depth of the doping elements in the inner shell surface layer of the lithium iron phosphate particles is 1-30% of the particle size of the lithium iron phosphate particles; non-limiting specific examples are 1%, 5%, 10%, 15%, 20%, 25% or 30%.

[0052] The doping elements are doped in the inner shell surface layer of the lithium iron phosphate particles, and do not completely penetrate into the bulk structure, which is beneficial to the doping elements to play the charge compensation role to realize the reduction of iron elements, so as to form an iron phosphide layer on the surface of the lithium iron phosphate particles, and is also beneficial to the interaction between the doping elements and the iron phosphide layer, the coating layer and the like, so as to improve the material performance.

[0053] In some embodiments of the application, the doping depth of the doping elements in the inner shell surface layer of the lithium iron phosphate particles is 0.001-1.2 μm; in some specific embodiments of the application, the doping depth of the doping elements in the inner shell surface layer of the lithium iron phosphate particles is 0.01-1 μm; non-limiting specific examples are 0.01 μm, 0.05 μm, 0.1 μm, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm or 1 μm.

[0054] In some embodiments of the application, the iron phosphide layer is a Fe2P layer.

[0055] In some embodiments of the application, the coating layer includes at least one of a boron-containing oxide, a tungsten-containing oxide, a carbon material, a phosphate or a lithium-containing compound; in some specific embodiments of the application, the coating layer includes at least one of a boron-containing oxide, a tungsten-containing oxide, a carbon material or a phosphate; in some examples of the application, the coating layer includes a boron-containing oxide, a tungsten-containing oxide or a combination thereof.

[0056] The above coating layer can achieve good coating effect and form material particles with good consistency; in addition, the interaction between the coating layer and the doping elements or the effect at the interface is also beneficial to improve the electrochemical performance of the material. Further, the use of boron-containing oxide and tungsten-containing oxide can obtain better coating effect, promote the formation of positive electrode material with uniform size and performance, and obtain positive electrode material with better electrochemical performance.

[0057] In some embodiments of the present application, the particle size of the lithium iron phosphate particles is 0.1-2 μm; in some specific embodiments of the present application, the particle size of the lithium iron phosphate particles is 0.15-1.8 μm; in some examples of the present application, the particle size of the lithium iron phosphate particles is 0.2-1.5 μm. Non-limiting specific examples are 0.2 μm, 0.3 μm, 0.5 μm, 0.8 μm, 1 μm, 1.2 μm or 1.5 μm.

[0058] The particle size of the lithium iron phosphate particles in the present application refers to the particle size of the primary particles of lithium iron phosphate, i.e. the particle size of the individual particles.

[0059] In some embodiments of the present application, the thickness of the phosphide layer of iron is 1-20 nm; in some specific embodiments of the present application, the thickness of the phosphide layer of iron is 3-15 nm; in some examples of the present application, the thickness of the phosphide layer of iron is 5-10 nm. Non-limiting specific examples are 5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm.

[0060] In some embodiments of the present application, the thickness of the carbon layer is 1-40 nm; in some specific embodiments of the present application, the thickness of the carbon layer is 1-30 nm; in some examples of the present application, the thickness of the carbon layer is 2-30 nm. Non-limiting specific examples are 3 nm, 5 nm, 10 nm, 15 nm, 20 nm or 25 nm.

[0061] In some embodiments of the present application, the thickness of the coating layer is 1-40 nm; in some specific embodiments of the present application, the thickness of the coating layer is 1-30 nm; in some examples of the present application, the thickness of the coating layer is 2-30 nm. Non-limiting specific examples are 2 nm, 3 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm or 30 nm.

[0062] In some embodiments of the present application, the D10 particle size of the lithium iron phosphate positive electrode material is 0.1-0.8 μm; in some specific embodiments of the present application, the D10 particle size of the lithium iron phosphate positive electrode material is 0.2-0.6 μm; in some examples of the present application, the D10 particle size of the lithium iron phosphate positive electrode material is 0.3-0.5 μm. Non-limiting specific examples are 0.3 μm, 0.32 μm, 0.35 μm, 0.4 μm, 0.42 μm, 0.45 μm or 0.5 μm.

[0063] In some embodiments of the present application, the D50 particle size of the lithium iron phosphate cathode material is 0.9-1.5 μm; in some specific embodiments of the present application, the D50 particle size of the lithium iron phosphate cathode material is 0.95-1.3 μm; in some examples of the present application, the D50 particle size of the lithium iron phosphate cathode material is 1-1.2 μm. Non-limiting specific examples include 1 μm, 1.02 μm, 1.05 μm, 1.1 μm, 1.12 μm, 1.15 μm or 1.2 μm.

[0064] In some embodiments of the present application, the D99 particle size of the lithium iron phosphate cathode material is 2-4 μm; in some specific embodiments of the present application, the D99 particle size of the lithium iron phosphate cathode material is 2.5-3.5 μm; in some examples of the present application, the D99 particle size of the lithium iron phosphate cathode material is 2.8-3.2 μm. Non-limiting specific examples include 2.8 μm, 2.85 μm, 2.9 μm, 2.95 μm, 3 μm, 3.05 μm, 3.1 μm, 3.15 μm or 3.2 μm.

[0065] The second aspect of the embodiments of the present application provides a preparation method of a lithium iron phosphate cathode material, comprising the following steps: pre-sintering a mixture containing an iron source, a phosphorus source, a lithium source, a carbon source and an additive to obtain a pre-sintered material, mixing the pre-sintered material with a coating agent and then calcining to obtain the lithium iron phosphate cathode material according to the first aspect of the embodiments of the present application; the additive is a compound containing a doping element; and the coating agent is a compound forming a coating layer.

[0066] The iron source, the phosphorus source and the lithium source are used to form lithium iron phosphate; the doping element in the additive is doped in the formed lithium iron phosphate, and the doping element is also conducive to promoting the reduction of iron elements at the surface or subsurface of the lithium iron phosphate, thereby generating a layer of iron phosphide; the carbon source is used to form a carbon layer coated on the surface of the iron phosphide; and the coating agent forms a coating layer coated on the outermost part of the material. In the preparation method of the present application, the raw materials are first subjected to preliminary chemical reactions through pre-sintering to form a specific composition and doping structure, and then subjected to subsequent coating and calcining, which is conducive to obtaining a cathode material with a specific layer structure, and the material has better uniformity and better electrochemical performance. Moreover, the preparation method is simple, low in cost and easy to realize industrial production.

[0067] In some embodiments of the present application, the additive comprises at least one of a niobium-containing compound, a tantalum-containing compound, a vanadium-containing compound or an iridium-containing compound; in some specific embodiments of the present application, the additive comprises a niobium-containing compound and a tantalum-containing compound; and in some examples of the present application, the additive is selected from a niobium-containing compound.

[0068] In some embodiments of the present application, the niobium-containing compound includes at least one of niobium pentoxide (Nb2O5), niobium hydroxide, niobium acetate, niobium carbonate or niobium phosphate; in some specific embodiments of the present application, the niobium-containing compound includes at least one of niobium pentoxide, niobium hydroxide or niobium phosphate; in some examples of the present application, the niobium-containing compound is selected from niobium pentoxide.

[0069] In some embodiments of the present application, the tantalum-containing compound includes at least one of tantalum pentoxide (Ta2O5), tantalum hydroxide, tantalum pentafluoride, tantalum pentachloride, tantalum pentabromide or tantalum pentaiodide; in some specific embodiments of the present application, the tantalum-containing compound includes tantalum pentoxide, tantalum hydroxide or a combination thereof; in some examples of the present application, the tantalum-containing compound is selected from tantalum pentoxide.

[0070] In some embodiments of the present application, the vanadium-containing compound includes at least one of vanadium pentoxide (V2O5), vanadium trioxide (V2O3), vanadium dioxide (VO2), ammonium metavanadate (NH4VO3) or vanadium pentachloride; in some specific embodiments of the present application, the vanadium-containing compound includes vanadium pentoxide, vanadium trioxide or a combination thereof; in some examples of the present application, the vanadium-containing compound is selected from vanadium pentoxide.

[0071] In some embodiments of the present application, the iridium-containing compound includes at least one of iridium dioxide (IrO2), iridium hydroxide (Ir(OH)4) or iridium tetrachloride; in some specific embodiments of the present application, the iridium-containing compound includes iridium dioxide, iridium hydroxide or a combination thereof; in some examples of the present application, the iridium-containing compound is selected from iridium dioxide.

[0072] In some embodiments of the present application, the mass percentage of the additive in the mixture is 1-3%; in some specific embodiments of the present application, the mass percentage of the additive in the mixture is 1.2-2.8%; in some examples of the present application, the mass percentage of the additive in the mixture is 1.5-2.5%. Non-limiting specific examples include 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4% or 2.5%.

[0073] The doping elements such as niobium, tantalum, vanadium and iridium in the additive are preliminarily doped in the pre-sintering product in the pre-sintering process, forming a specific doping distribution structure, which is conducive to the subsequent coating treatment and calcination treatment; further, the present application has better doping effect by using niobium and tantalum for doping, and can obtain a positive electrode material with better conductivity, higher capacity and better stability. Moreover, the present application only needs to add a small amount of additive to achieve good doping modification effect.

[0074] In some embodiments of the present application, the coating agent comprises at least one of boric acid, tungsten oxide, polyaniline (PANI), polypyrrole (PPy), phosphate or lithium-containing compound; in some specific embodiments of the present application, the coating agent comprises at least one of boric acid, tungsten oxide, polyaniline (PANI) or phosphate; in some examples of the present application, the coating agent is selected from boric acid, tungsten oxide or a combination thereof.

[0075] In some embodiments of the present application, non-limiting examples of the phosphate include aluminum phosphate, calcium phosphate and the like; in some specific embodiments of the present application, the phosphate is selected from aluminum phosphate.

[0076] In some embodiments of the present application, the mass ratio of the coating agent to the pre-sintered material is (0.1-5):100; in some specific embodiments of the present application, the mass ratio of the coating agent to the pre-sintered material is (0.3-3):100; in some examples of the present application, the mass ratio of the coating agent to the pre-sintered material is (0.5-2):100. Non-limiting specific examples include 0.5:100, 0.8:100, 1:100, 1.2:100, 1.5:100, 1.8:100 or 2:100.

[0077] During the preparation of the material, due to the high-temperature reaction, the coating agent forms a coating layer on the surface of the carbon layer, for example, boric acid generates boron-containing oxides, tungsten oxide generates tungsten-containing oxides, polyaniline and polypyrrole generate carbon materials, etc. Further, using boric acid and tungsten oxide can achieve better coating effect and obtain positive electrode material with better electrochemical performance. Moreover, only a small amount of coating agent needs to be added to achieve good coating modification effect.

[0078] In some embodiments of the present application, the iron source comprises at least one of iron phosphate, iron hydroxide, iron oxide or iron oxyhydroxide; in some specific embodiments of the present application, the iron source comprises at least one of iron phosphate, iron hydroxide or iron oxide; in some examples of the present application, the iron source is selected from iron phosphate.

[0079] In some embodiments of the present application, the phosphorus source comprises at least one of monobasic ammonium phosphate, dibasic ammonium phosphate, ammonium phosphate, lithium phosphate or iron phosphate; in some specific embodiments of the present application, the phosphorus source comprises at least one of monobasic ammonium phosphate, dibasic ammonium phosphate or iron phosphate; in some examples of the present application, the phosphorus source is selected from iron phosphate.

[0080] In some embodiments of the present application, the mass ratio of the iron source to the phosphorus source is 1:(0.5-2); in some embodiments of the present application, the mass ratio of the iron source to the phosphorus source is 1:(0.7-1.5); in some embodiments of the present application, the mass ratio of the iron source to the phosphorus source is 1:(0.8-1.2); non-limiting specific examples include 1:0.8, 1:0.85, 1:0.9, 1:0.95, 1:1, 1:1.05, 1:1.1, 1:1.15 or 1:1.2.

[0081] In some embodiments of the present application, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate or lithium acetate; in some specific embodiments of the present application, the lithium source includes at least one of lithium carbonate, lithium hydroxide or lithium phosphate; in some examples of the present application, the lithium source is selected from lithium carbonate.

[0082] In some embodiments of the present application, the mass ratio of the iron source to the lithium source is 1:(0.5-2); in some embodiments of the present application, the mass ratio of the iron source to the lithium source is 1:(0.8-1.5); in some embodiments of the present application, the mass ratio of the iron source to the lithium source is 1:(1-1.2); non-limiting specific examples include 1:1, 1:1.02, 1:1.05, 1:1.08, 1:1.1, 1:1.12, 1:1.15 or 1:1.2.

[0083] In some embodiments of the present application, the carbon source includes at least one of a sugar, an alcohol, an ester, an acid or a resin; in some specific embodiments of the present application, the carbon source includes a sugar, an alcohol or a combination thereof; in some examples of the present application, the carbon source is selected from a sugar.

[0084] In some embodiments of the present application, the carbon source includes at least one of glucose, sucrose, starch, polyvinyl alcohol, xylitol, ethyl acetate, an amino acid, malic acid, citric acid or a phenolic resin; in some specific embodiments of the present application, the carbon source includes at least one of glucose, sucrose, starch, polyvinyl alcohol or xylitol; in some examples of the present application, the carbon source is selected from glucose.

[0085] The carbon source of the present application is abundant in sources and can be a sugar or a resin and the like, wherein the sugar, especially glucose, is easy to dissolve and has a small molecular weight and is more likely to achieve uniform coating at low temperatures; the resin carbon source can also achieve good coating effect, but compared with the resin carbon source, the dispersion performance and uniform coating performance of the sugar, especially glucose, are better.

[0086] In some embodiments of the present application, the mass ratio of the iron source to the carbon source is 1:(0.1-1.5); in some embodiments of the present application, the mass ratio of the iron source to the carbon source is 1:(0.2-1.2); in some embodiments of the present application, the mass ratio of the iron source to the carbon source is 1:(0.3-1); non-limiting specific examples include 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, or 1:1.

[0087] In some embodiments of the present application, the mass ratio of the iron source to the additive is 1:(0.01-0.2); in some embodiments of the present application, the mass ratio of the iron source to the additive is 1:(0.02-0.15); in some embodiments of the present application, the mass ratio of the iron source to the additive is 1:(0.03-0.1); non-limiting specific examples include 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, or 1:0.1.

[0088] In some embodiments of the present application, before the pre-sintering treatment, the mixture is further subjected to a ball milling treatment; in some specific embodiments of the present application, the ball milling treatment of the mixture is selected from wet ball milling, and the liquid medium is selected from water.

[0089] In some embodiments of the present application, the ball milling time of the ball milling treatment of the mixture is 12-36 hours; non-limiting specific examples include 12 hours, 16 hours, 20 hours, 24 hours, 30 hours, 34 hours, or 36 hours.

[0090] In some embodiments of the present application, the ball milling speed of the ball milling treatment of the mixture is 400-600 rpm; non-limiting specific examples include 400 rpm, 420 rpm, 450 rpm, 500 rpm, 520 rpm, 550 rpm, or 600 rpm.

[0091] In some embodiments of the present application, after the ball milling treatment and before the pre-sintering treatment, a drying step is further included; in some specific embodiments of the present application, the drying method is selected from spray drying. By spray drying, granulated mixture can be obtained, which is conducive to the more sufficient mixing of raw materials, thereby facilitating the pre-sintering treatment.

[0092] In some embodiments of the present application, the temperature of the pre-sintering treatment is 400-700℃; in some specific embodiments of the present application, the temperature of the pre-sintering treatment is 500-680℃; in some embodiments of the present application, the temperature of the pre-sintering treatment is 550-650℃. Non-limiting specific examples include 550℃, 560℃, 580℃, 600℃, 620℃, 640℃, or 650℃.

[0093] The high-temperature treatment is performed in the range of 400-700℃, and the large-radius transition metal cannot be completely incorporated into the bulk phase of the material, so that the doping element can be doped in the inner shell surface layer of the lithium iron phosphate particle without completely penetrating into the bulk phase structure, which is beneficial to promoting the reduction of iron element at the surface or subsurface of the lithium iron phosphate, so as to generate a layer of iron phosphide of iron; in addition, the doping in the inner shell surface layer of the lithium iron phosphate particle is also beneficial to the interaction between the doping element and the layer of iron phosphide of iron, the coating layer and the like, so as to improve the performance of the material.

[0094] In some embodiments of the present application, the pre-sintering treatment time is 1-6 hours; in some specific embodiments of the present application, the pre-sintering treatment time is 2-6 hours; in some examples of the present application, the pre-sintering treatment time is 3-5 hours. Non-limiting specific examples are 3 hours, 3.2 hours, 3.5 hours, 4 hours, 4.2 hours, 4.5 hours or 5 hours.

[0095] In some embodiments of the present application, the mixing time of the pre-sintered material and the coating agent is 10-60 minutes; non-limiting specific examples are 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes or 60 minutes.

[0096] In some embodiments of the present application, the calcination treatment temperature is 700-900℃; in some specific embodiments of the present application, the calcination treatment temperature is 720-880℃; in some examples of the present application, the calcination treatment temperature is 750-850℃. Non-limiting specific examples are 750℃, 760℃, 780℃, 800℃, 820℃, 840℃ or 850℃.

[0097] In some embodiments of the present application, the calcination treatment time is 2-8 hours; in some specific embodiments of the present application, the calcination treatment time is 4-8 hours; in some examples of the present application, the calcination treatment time is 5-7 hours. Non-limiting specific examples are 5 hours, 5.2 hours, 5.5 hours, 6 hours, 6.2 hours, 6.5 hours or 7 hours.

[0098] After pre-sintering, combined with high-temperature short-time calcination treatment, the specific surface doping and coating structure of the lithium iron phosphate positive electrode material of the present application can be well formed, and if the calcination treatment temperature is too high or the time is too long, the doping and coating process of the material can be difficult to control, which affects the uniformity of the material growth and the specific distribution of the doping element.

[0099] The third aspect of the embodiments of the present application provides a battery comprising the lithium iron phosphate positive electrode material of the first aspect of the embodiments of the present application or the lithium iron phosphate positive electrode material prepared by the preparation method of the second aspect of the embodiments of the present application.

[0100] The lithium iron phosphate positive electrode material has low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, good stability and other good electrochemical properties, and a battery with excellent electrochemical properties can be obtained by using the lithium iron phosphate positive electrode material as a positive electrode material.

[0101] The application is further described below in combination with specific examples and comparative examples.

[0102] Example 1

[0103] A lithium iron phosphate positive electrode material has a structure as shown in the schematic diagram Figure 1 , which comprises lithium iron phosphate particles 1, doped niobium element 2, Fe2P layer 3, carbon layer 4, and coating layer 5. The doped element 2 is doped in the lithium iron phosphate particles 1. The Fe2P layer 3 is coated on at least part of the surface of the lithium iron phosphate particles 1. The carbon layer 4 is coated on at least part of the surface of the Fe2P layer 3. The coating layer 5 is coated on at least part of the surface of the carbon layer 4. The lithium iron phosphate particles are obtained by high-temperature solid-phase sintering of a phosphorus source, an iron source and a lithium source, and the particle size is in the range of 0.2-3 μm. The niobium element is introduced by adding an additive niobium oxide. The Fe2P layer is mainly formed due to excessive reduction of iron elements, and the thickness of the Fe2P layer is in the range of 5-10 nm. The carbon layer is introduced by a carbon source, and the thickness of the carbon layer is in the range of 2-30 nm. The main component of the coating layer is an oxide containing boron, and the coating layer is formed by a coating agent boric acid during calcination. The thickness of the coating layer is about 10 nm.

[0104] The above-mentioned lithium iron phosphate positive electrode material is prepared by the following steps:

[0105] Step one: preparing raw materials. First, accurately weigh a certain proportion of the weight of the phosphorus source, iron source, carbon source, lithium source and additive. Among them, the phosphorus source and iron source choose iron phosphate, the carbon source chooses glucose, the lithium source chooses lithium carbonate, and the additive chooses niobium oxide. The mass ratio of iron phosphate, glucose, lithium carbonate and niobium oxide is 1:0.5:1.05:0.05.

[0106] Step two: mixing raw materials. Put the weighed raw materials into a ball mill and add an appropriate amount of water for wet ball milling. The ball milling time is 24 hours and the ball milling speed is 500 rpm. After ball milling, the mixture is spray dried and granulated to obtain a granular mixture.

[0107] Step three: pre-sintering treatment. Put the granulated mixture into a high-temperature furnace for pre-sintering. The pre-sintering temperature is 600℃, and the pre-sintering time is 4 hours. After pre-sintering, the pre-sintered material is crushed to obtain a powder material.

[0108] Step four: coating treatment. Add coating agent boric acid to the crushed material, and the amount of coating agent added is 1% of the mass of the pre-sintered material. Mix the material with the added coating agent for 30 minutes.

[0109] Step five: calcination treatment. Place the mixed material in a high-temperature furnace for calcination, with a calcination temperature of 800°C and a calcination time of 6 hours. After calcination, the powder product, i.e., the lithium iron phosphate positive electrode material, is obtained.

[0110] Example 2

[0111] A lithium iron phosphate positive electrode material, which differs from Example 1 in that:

[0112] In the structural composition, the main component of the coating layer in this example is carbon material, which is formed from the coating agent polyaniline (PANI) during calcination;

[0113] In the preparation method, polyaniline (PANI) is used as the coating agent in step four in this example.

[0114] The other structures, raw materials, and preparation steps are the same as in Example 1.

[0115] Example 3

[0116] A lithium iron phosphate positive electrode material, which differs from Example 1 in that:

[0117] In the structural composition, the main component of the coating layer in this example is tungsten-containing oxide, which is formed from the coating agent tungsten oxide during calcination;

[0118] In the preparation method, tungsten oxide is used as the coating agent in step four in this example.

[0119] The other structures, raw materials, and preparation steps are the same as in Example 1.

[0120] Example 4

[0121] A lithium iron phosphate positive electrode material, which differs from Example 1 in that:

[0122] In the structural composition, the main component of the coating layer in this example is aluminum phosphate, which is formed from the coating agent aluminum phosphate during calcination;

[0123] In the preparation method, aluminum phosphate is used as the coating agent in step four in this example.

[0124] The other structures, raw materials, and preparation steps are the same as in Example 1.

[0125] Example 5

[0126] A lithium iron phosphate positive electrode material, which differs from Example 1 in that:

[0127] In the structural composition, the main component of the coating layer in this case is titanium-containing oxide, which is formed by titanium dioxide as the coating agent during calcination.

[0128] In the preparation method, titanium dioxide is used as the coating agent in step four in this case.

[0129] The other structures, raw materials, and preparation steps are the same as in Example 1.

[0130] Comparative Example 1

[0131] A lithium iron phosphate positive electrode material is prepared by a traditional sintering process without doping and coating treatment. The specific steps are as follows:

[0132] Step one: Prepare the raw materials. First, accurately weigh a certain proportion of the weight of the phosphorus source, iron source, carbon source, and lithium source. Among them, the phosphorus source and iron source choose iron phosphate, the carbon source chooses glucose, and the lithium source chooses lithium carbonate. The mass ratio of iron phosphate, glucose, and lithium carbonate is 1:0.5:1.05.

[0133] Step two: Pre-sintering treatment. Put the weighed raw materials into a high-temperature furnace for pre-sintering. The pre-sintering temperature is 600°C, and the pre-sintering time is 4 hours to obtain a pre-sintered material.

[0134] Step three: Calcination treatment. Put the pre-sintered material into a high-temperature furnace for calcination. The calcination temperature is 800°C, and the calcination time is 12 hours. After calcination, a lithium iron phosphate positive electrode material is obtained.

[0135] Comparative Example 2

[0136] A lithium iron phosphate positive electrode material differs from Example 1 in that:

[0137] In the structural composition, the magnesium element is doped inside the lithium iron phosphate particles in this case, and the main component of the coating layer is titanium-containing oxide, which is formed by titanium dioxide as the coating agent during calcination.

[0138] In the preparation method, magnesium oxide is used as an additive in step one in this case, and titanium dioxide is used as a coating agent in step four.

[0139] The other structures, raw materials, and preparation steps are the same as in Example 1.

[0140] Comparative Example 3

[0141] A lithium iron phosphate positive electrode material differs from Example 1 in that:

[0142] In the structural composition, no elements are doped inside the lithium iron phosphate particles in this case, and the main component of the coating layer is titanium-containing oxide, which is formed by titanium dioxide as the coating agent during calcination.

[0143] In the preparation method, no additive is added in step one, and titanium dioxide is used as the coating agent in step four.

[0144] The other structures, raw materials and preparation steps are the same as those in Example 1.

[0145] Comparative Example 4

[0146] A lithium iron phosphate positive electrode material, which is different from Example 1 in that:

[0147] In the structural composition, no element is doped in the lithium iron phosphate particle in this example; and no coating layer is provided.

[0148] In the preparation method, no additive is added in step one, and step four (coating treatment) is omitted, and step five is calcination of the pre-sintered material.

[0149] The other structures, raw materials and preparation steps are the same as those in Example 1.

[0150] Comparative Example 5

[0151] A lithium iron phosphate positive electrode material, which is different from Example 1 in that:

[0152] In the structural composition, the doped element in this example is completely doped into the lithium iron phosphate particle phase, and cannot form the specific doping and coating structure of Example 1.

[0153] In the preparation method, step three (pre-sintering treatment) is omitted in this example, and step four directly uses the mixture obtained in step two for coating treatment.

[0154] The other raw materials and preparation steps are the same as those in Example 1.

[0155] Performance test

[0156] 1) Morphology characterization: Scanning electron microscopy (SEM) is used to characterize the morphology of the lithium iron phosphate positive electrode material obtained in each example and comparative example.

[0157] 2) Particle size test: The particle size of the lithium iron phosphate positive electrode material obtained in each example and comparative example is tested, including D10, D50 and D99 particle sizes.

[0158] 3) Electrochemical performance testing: The powder resistivity of the lithium iron phosphate cathode materials obtained in each example and comparative example was tested using a two-probe four-wire pressure test method (test pressure of 10000N, holding time of 30s). The lithium iron phosphate cathode materials obtained in each example and comparative example were mixed with conductive agent Super-P, binder PVDF and solvent NMP to form a slurry. The slurry was coated on foil and dried to obtain each cathode sheet. Using each cathode sheet as the cathode and lithium metal as the anode, a simulated battery or coin cell was assembled. The charge capacity and discharge capacity of the first cycle and the capacity retention rate after 100 cycles were tested under the conditions of 23℃±2℃ and 0.1C. The capacity retention rate after 100 cycles was calculated by dividing the discharge capacity of the 100th cycle by the discharge capacity of the first cycle.

[0159] SEM images of the lithium iron phosphate cathode materials of Example 1 and Comparative Example 1 are shown below. Figure 2 As shown, a is Example 1, and b is Comparative Example 1. From Figure 2 As can be seen, Example 1 employs a unique doping and coating process. First, through pre-sintering at specific temperatures and times, the degree of lithium intercalation and elemental doping in the material is adjusted. Generally, within the range of 400–700°C, large-radius transition metals are not completely incorporated into the bulk phase of the material, thus allowing niobium to be doped onto the surface of the lithium iron phosphate particles. Then, combined with high-temperature, short-time calcination, a good coating is formed... Figure 1 The materials shown have surface doping and coating structures. In Example 1, the material coating is uniform, the particle growth is isotropic, the material is more uniform, the consistency is higher, and the performance is better; while in Comparative Example 1, a conventional sintering process was used without doping and coating treatment, resulting in a longer sintering time, which led to poor material growth consistency, particle agglomeration, and more serious problems such as large particles.

[0160] The particle size data of the lithium iron phosphate cathode materials of Examples 1 and Comparative Examples 2-4 are shown in Table 1. The electrochemical performance data of the lithium iron phosphate cathode materials of Examples 1-5 and Comparative Examples 2-5 are shown in Table 2.

[0161] Table 1. Particle size data of lithium iron phosphate cathode materials in Example 1 and Comparative Examples 2-4

[0162] Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 D10 / pm 0.33 0.3 0.3 0.4 D50 / pm 1.06 1.01 1.01 1.23 D99 / pm 3.0 4.3 4.7 7.5

[0163] As can be seen from Table 1, the material obtained in Example 1 has a small difference in particle size between D10, D50 and D99, small particle size difference in the particle system, and good dispersibility and uniformity of the material; while the material obtained in Comparative Examples 2 to 4 has a large difference in particle size between D10, D50 and D99, large particle size difference in the particle system, and poor dispersibility and uniformity of the material.

[0164] Table 2 Electrochemical performance data of lithium iron phosphate cathode materials in Examples 1-5 and Comparative Examples 2-5

[0165]

[0166] As can be seen from Table 2, the lithium iron phosphate positive electrode materials with specific doping and coating structure are obtained by coating with a coating layer and specific element doping and pre-sintering and calcining under specific conditions in Examples 1-5, wherein the formation of the Fe2P layer is mainly due to the reduction of iron element, which is related to the conditions such as synthesis temperature, doping element and synthesis atmosphere. The niobium element in the additive niobium pentoxide in Examples 1-4 is in +5 valence. The main reason why the doping of the high-valence metal element, especially the metal Nb element like large atom and high valence, is conducive to the formation of the Fe2P layer is that the charge compensation effect will cause the reduction of iron element, thereby forming a high-conductivity Fe2P layer on the micro-surface of the lithium iron phosphate particle. The Fe2P itself has good electronic conductivity and can form a high-conductivity network. In addition, the interface effect between the Fe2P layer and the lithium iron phosphate particle is also conducive to the improvement of the electrochemical performance. In addition, the doping of the niobium metal element widens the lithium ion transmission channel and improves the rate performance of the material, which is conducive to improving the lithium ion diffusion coefficient of the lithium iron phosphate positive electrode material while ensuring the structural stability. The coating of the coating layer is conducive to controlling the uneven growth of the local material in the solid-phase sintering, improving the morphology and consistency of the positive electrode particles and improving the stability of the material. In addition, the interaction or interface effect between the coating layer and the doped niobium element is also conducive to improving the electrochemical performance of the material. The materials obtained in Examples 1-5 have low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate and good stability, and have good electrochemical performance. Different coating agents are used for coating in Examples 1-5, and good coating effects can be achieved. Among the coating agents, boric acid and oxide as the coating agent have better effects, and the obtained material has lower resistivity, higher charge and discharge capacity and higher capacity retention rate.

[0167] The material obtained in Comparative Example 2 has high resistivity, low discharge capacity and low capacity retention rate by using magnesium doping combined with titanium dioxide coating. The material obtained in Comparative Example 3 has high resistivity, low charge and discharge capacity and low capacity retention rate by not using doping treatment and only using titanium dioxide coating. The material obtained in Comparative Example 4 has large difference between the D10, D50 and D99 particle sizes, poor particle size uniformity, high resistivity, low charge and discharge capacity and low capacity retention rate by not using doping and coating. The material obtained in Comparative Example 5 has high resistivity, low charge and discharge capacity and low capacity retention rate by not using pre-sintering treatment, which makes it difficult for the niobium element to be doped at a low temperature to form the specific doping structure in Example 1. It is difficult to form the specific doping and coating structure as in Example 1 by directly calcining the raw materials, and the obtained material has high resistivity, low charge and discharge capacity and low capacity retention rate.

[0168] The lithium iron phosphate positive electrode material provided by the embodiment of the present application can improve the lithium ion diffusion coefficient, improve the intrinsic electronic conductivity of the lithium iron phosphate material, strengthen the stability of the structure and surface interface of the lithium iron phosphate material, and has uniform material growth and good dispersibility. The preparation method provided by the embodiment of the present application is based on the traditional solid-phase synthesis method, and through special doping element treatment, the lithium ion transmission channel is widened, the material rate performance is improved, and the doping material used can construct a stable iron phosphide (Fe2P) high-conductivity protective layer on the surface or micro-surface of the positive electrode material, thereby effectively improving the intrinsic electronic and ionic conductivity of the lithium iron phosphate. Then, through optimization of the coating process, a relatively uniform coating layer is constructed on the surface of the positive electrode material, which is conducive to inhibiting the continuous growth of the particles under high-temperature sintering, and also has a good physical isolation and protection effect, thereby significantly improving the electrochemical performance of the material. In addition, the preparation method can effectively control the proportion of raw materials, improve the purity and yield of the lithium iron phosphate, reduce energy consumption and cost, simplify the preparation process, improve production efficiency, reduce energy consumption and cost, and is suitable for large-scale production.

[0169] In summary, through doping of high-valence elements, the lithium ion transmission channel is widened, the material rate performance is improved, the generation of the iron phosphide layer is promoted, the intrinsic electronic and ionic conductivity of the lithium iron phosphate is effectively improved, the coating layer is conducive to inhibiting the continuous growth of the material, and a material with good uniformity and dispersibility is obtained, and the coating layer also has a good physical isolation and protection effect, thereby significantly improving the electrochemical performance of the material. The lithium iron phosphate positive electrode material with the specific doping and coating structure has good electrochemical performance such as low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, and good stability, and has a good application prospect in the preparation of batteries, especially lithium batteries.

Claims

1. A lithium iron phosphate cathode material, characterized in that, include: Lithium iron phosphate particles, doping elements, iron phosphide layer, carbon layer, and coating layer; the doping elements are doped into the lithium iron phosphate particles; The iron phosphide layer coats at least a portion of the surface of the lithium iron phosphate particles; the carbon layer coats at least a portion of the surface of the iron phosphide layer; the coating layer coats at least a portion of the surface of the carbon layer; wherein the valence state of the dopant element is greater than +3; the coating layer includes at least one of boron-containing oxides, tungsten-containing oxides, carbon materials, phosphates, or lithium-containing compounds; the dopant element is doped into the lithium iron phosphate particles; the doping depth of the dopant element in the lithium iron phosphate particles is 1-30% of the particle size of the lithium iron phosphate particles; the dopant element includes at least one of niobium, tantalum, vanadium, or iridium; the dopant element plays a charge compensation role to reduce the iron element, thereby forming the iron phosphide layer on the surface of the lithium iron phosphate particles.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate particles have a particle size of 0.1~2μm; And / or, the thickness of the iron phosphide layer is 1~20 nm; And / or, the thickness of the carbon layer is 1~40 nm; And / or, the thickness of the coating layer is 1~40nm.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The D10 particle size of the lithium iron phosphate cathode material is 0.1~0.8μm; And / or, the D50 particle size of the lithium iron phosphate cathode material is 0.9~1.5μm; And / or, the D99 particle size of the lithium iron phosphate cathode material is 2~4μm.

4. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: A mixture containing an iron source, a phosphorus source, a lithium source, a carbon source, and additives is pre-sintered to obtain a pre-sintered material. The pre-sintered material is then mixed with a coating agent and calcined to obtain a lithium iron phosphate cathode material as described in any one of claims 1 to 3. The additive is a compound containing the doping element. The coating agent is a compound that forms the coating layer. The additive includes at least one of a niobium-containing compound, a tantalum-containing compound, a vanadium-containing compound, or an iridium-containing compound.

5. The preparation method according to claim 4, characterized in that, The additive is present in the mixture at a mass percentage of 1-3%.

6. The preparation method according to claim 4, characterized in that, The coating agent includes at least one of boric acid, tungsten oxide, polyaniline, polypyrrole, phosphate, or lithium-containing compound; And / or, the mass ratio of the coating agent to the pre-sintered material is (0.1~5):

100.

7. The preparation method according to claim 4, characterized in that, The temperature of the pre-sintering treatment is 400~700℃; And / or, the pre-sintering treatment time is 1 to 6 hours; And / or, the calcination treatment temperature is 700~900℃; And / or, the calcination treatment time is 2 to 8 hours.

8. A battery, characterized in that, It includes the lithium iron phosphate cathode material according to any one of claims 1 to 3, or the lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 4 to 7.

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