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

Abstract

The invention discloses a lithium iron phosphate positive electrode material as well as a preparation method and application thereof. The lithium iron phosphate positive electrode material comprises lithium iron phosphate particles, doping elements, an iron phosphide layer, a carbon layer and a coating layer, the doping element is doped in the lithium iron phosphate particles; the phosphide layer of iron 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 coats at least part of the surface of the carbon layer; wherein the valence state of the doping element is greater than + 3. According to the invention, through doping of high-valence elements, the generation of an iron phosphide layer can be promoted while a lithium ion transmission channel is widened and the rate capability of the material is improved, and the intrinsic electronic and ionic conductivity of lithium iron phosphate is effectively improved; and the coating layer is beneficial to inhibition of continuous growth of the material, so that the material with good uniformity and dispersity is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode materials, and in particular relates to a lithium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] With the growing demand for environmental protection and renewable energy, lithium batteries are increasingly being used as an efficient and clean way of energy storage. Among them, lithium iron phosphate is widely used as the positive electrode material of lithium batteries due to its abundant raw materials, low price, good safety performance, and long cycle life. 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 severely limited. In addition, the traditional solid phase synthesis method still has problems such as poor material consistency, uneven particle growth and deteriorating performance.

[0003] In order to solve the above problems, the prior art generally adopts carbon coating, metal element doping and other methods to improve the conductivity and lithium ion diffusion coefficient of lithium iron phosphate. For example, by adding carbon sources such as carbon black and graphene, the electronic conductivity of lithium iron phosphate is improved, which hinders the further growth of particles; and by adding some over-doped elements, the lithium ion diffusion coefficient of lithium iron phosphate can be improved. In addition, there are some studies that prepare lithium iron phosphate by high-temperature solid phase method, sol-gel method, hydrothermal method and other methods to improve its electrochemical properties. In the above methods, although the coating of carbon materials can improve the ability of materials to conduct electrons, there are still problems of uneven coating and excessive growth of local crystals in actual industrialization. For example, in the method of coating with titanium dioxide, the melting temperature of titanium dioxide is 1856.85°C, and its coating state on the surface of the material is basically uneven coating in the form of dots or islands; as for element doping, although the existing element doping helps to improve the lithium ion diffusion coefficient, there are still problems such as low intrinsic electronic conductivity and poor cycle performance of lithium iron phosphate. For example, although the use of Na and Mg doping can expand the lithium ion transmission channel, it is difficult to fundamentally solve the problem of poor intrinsic electronic conductivity of lithium iron phosphate. In addition, the existing preparation methods are often complicated in steps, high in energy consumption, and high in cost, which is not conducive to large-scale production. Summary of the invention

[0004] In order to overcome at least one of the problems existing in the above-mentioned prior art, one of the objects of the present invention is to provide a lithium iron phosphate positive electrode material, which can improve the intrinsic electronic conductivity of the lithium iron phosphate material while increasing the lithium ion diffusion coefficient, and enhance the stability of its structure and surface interface, and the material grows uniformly and has good dispersion.

[0005] The second object of the present invention is to provide a method for preparing a lithium iron phosphate positive electrode material, by which a lithium iron phosphate positive electrode material with uniform particle size, good dispersibility, good conductivity, high capacity and good stability can be obtained, and the preparation method has simple steps, low energy consumption and cost, and can be adapted to large-scale production.

[0006] A third object of the present invention is to provide a battery comprising the above-mentioned lithium iron phosphate positive electrode material.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] The first aspect of the present invention 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; the coating layer is coated on at least part of the surface of the carbon layer; wherein the valence state of the doping element is greater than +3.

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

[0010] The present invention dopes high-valent doping elements with a valence state of more than +3 into lithium iron phosphate particles, utilizes the charge compensation effect to cause the reduction of iron elements, thereby forming a highly conductive 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 contributes to the improvement of the electrochemical performance of the material; and the doping of doping elements can also broaden the lithium ion transmission channel, improve the rate performance of the material, and help to improve the lithium ion diffusion coefficient of the lithium iron phosphate positive electrode material, while ensuring its structural stability. The coating of the coating layer is conducive to controlling the uneven growth of local materials in solid phase sintering, improving the morphology and consistency of the positive electrode particles, and improving the stability of the material. Through the doping and coating treatment of the present invention, the lithium iron phosphate positive electrode material obtained 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 invention, the doping element includes at least one of niobium, tantalum, vanadium or iridium.

[0012] The elements such as niobium, tantalum, vanadium and iridium are used for doping. In addition to having a high valence state, they also have suitable atomic binding energy, atomic size and atomic activity, have good matching with lithium iron phosphate particles, can form good doping, and thus effectively improve the capacity and performance of the material; further, the use of niobium and tantalum for doping in the present invention has a better doping effect, and can obtain a positive electrode material with better conductivity, higher capacity and better stability.

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

[0014] Doping the doping elements in the inner shell surface layer of the lithium iron phosphate particles without completely penetrating into the bulk structure is beneficial for the doping elements to play a charge compensation role to achieve the reduction of the iron element, thereby forming an iron phosphide layer on the surface of the lithium iron phosphate particles. It is also beneficial for the doping elements to interact with the iron phosphide layer, coating layer, etc., thereby improving the material performance.

[0015] In some specific embodiments of the present invention, 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.

[0016] The use of the above-mentioned coating layer can achieve a good coating effect and form material particles with good consistency; in addition, the interaction or interface effect between the coating layer and the doping element is also beneficial to improving the electrochemical properties of the material.

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

[0018] In some specific embodiments of the present invention, the thickness of the iron phosphide layer is 1 to 20 nm.

[0019] In some specific embodiments of the present invention, the thickness of the carbon layer is 1 to 40 nm.

[0020] In some specific embodiments of the present invention, the coating layer has a thickness of 1 to 40 nm.

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

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

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

[0024] The second aspect of the present invention provides a method for preparing a lithium iron phosphate positive electrode 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 the mixture to obtain the lithium iron phosphate positive electrode material as described in the first aspect of the present invention; the additive is a compound containing the doping element; the coating agent is a compound that forms the coating layer.

[0025] The method for preparing the lithium iron phosphate positive electrode material according to the second aspect of the present invention has at least the following beneficial effects:

[0026] The iron source, phosphorus source and lithium source are used to form lithium iron phosphate; the doping elements in the additives will be doped into the formed lithium iron phosphate, and the doping elements are also beneficial to promote the reduction of iron elements on the surface or subsurface of the lithium iron phosphate, thereby generating an iron phosphide layer; the carbon source is used to form a carbon layer, which is coated on the surface of the iron phosphide; the coating agent will form a coating layer coated on the outermost surface of the material. In the preparation method of the present invention, firstly, a pre-sintering treatment is performed to make each raw material undergo a preliminary chemical reaction to form a specific composition and doping structure, and then subsequent coating and calcination treatments are performed, which is beneficial to obtain a positive electrode material with a specific layer structure, and the material has better uniformity and better electrochemical performance, and the preparation method is simple, low cost, and easy to realize industrial production.

[0027] In some specific embodiments of the present invention, 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 specific embodiments of the present invention, the mass percentage of the additive in the mixture is 1-3%.

[0029] The doping elements such as niobium, tantalum, vanadium, iridium and the like in the additives are initially doped in the pre-sintered product during the pre-sintering process to form a specific doping distribution structure, which is beneficial to the subsequent coating and calcining treatments. Furthermore, the use of niobium and tantalum for doping in the present invention has a better doping effect, and can obtain a positive electrode material with better conductivity, higher capacity and better stability. Moreover, the present invention only needs to add a small amount of additives to achieve a good doping modification effect.

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

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

[0032] During the preparation of the material, due to the high temperature reaction, the coating agent will form a coating layer on the surface of the carbon layer. For example, boric acid will generate boron-containing oxides, tungsten oxide will generate tungsten-containing oxides, polyaniline and polypyrrole will generate carbon materials, etc. Furthermore, the use of boric acid and tungsten oxide can achieve better coating effects and obtain positive electrode materials with better electrochemical properties. And only a small amount of coating agent needs to be added to achieve a good coating modification effect.

[0033] In some specific embodiments of the present invention, the temperature of the pre-sintering treatment is 400-700°C.

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

[0035] In some specific embodiments of the present invention, the calcination temperature is 700-900°C.

[0036] In some specific embodiments of the present invention, the calcination treatment time is 2 to 8 hours.

[0037] The third aspect of the present invention provides a battery, comprising the lithium iron phosphate positive electrode material described in the first aspect of the present invention, or the lithium iron phosphate positive electrode material prepared by the preparation method described in the second aspect of the present invention.

[0038] The battery according to the third aspect of the present invention has at least the following beneficial effects:

[0039] The lithium iron phosphate positive electrode material provided by the present invention has good electrochemical properties such as low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, and good stability. A battery with excellent electrochemical properties can be obtained by using it as a positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a schematic diagram of the structure of the lithium iron phosphate positive electrode material of Example 1.

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

[0042] The content of the present invention is further described in detail below through specific examples. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the principles set forth in the present invention all belong to the scope of protection of the present invention. The specific process parameters and the like in the following examples are also only examples within a suitable range, that is, those skilled in the art can make a selection within a suitable range through the description herein, and are not limited to the specific data exemplified below. The raw materials, reagents or devices used in the following examples and comparative examples, unless otherwise specified, can all be obtained from conventional commercial sources, or can be obtained by existing known methods.

[0043] A first aspect of an embodiment of the present invention 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 a portion of the surface of the lithium iron phosphate particles; the carbon layer is coated on at least a portion of the surface of the iron phosphide layer; the coating layer is coated on at least a portion of the surface of the carbon layer; wherein the valence state of the doping element is greater than +3.

[0044] The present invention dopes high-valent doping elements with a valence state of more than +3 into lithium iron phosphate particles, utilizes the charge compensation effect to cause the reduction of iron elements, thereby forming a highly conductive 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 contributes to the improvement of the electrochemical performance of the material; and the doping of doping elements can also broaden the lithium ion transmission channel, improve the rate performance of the material, and help to improve the lithium ion diffusion coefficient of the lithium iron phosphate positive electrode material, while ensuring its structural stability. The coating of the coating layer is conducive to controlling the uneven growth of local materials in solid phase sintering, improving the morphology and consistency of the positive electrode particles, and improving the stability of the material. Through the doping and coating treatment of the present invention, the lithium iron phosphate positive electrode material obtained 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 invention, the carbon layer is also coated on at least a portion of the surface of the lithium iron phosphate particles.

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

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

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

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

[0050] The elements such as niobium, tantalum, vanadium and iridium are used for doping. In addition to having a high valence state, they also have suitable atomic binding energy, atomic size and atomic activity, have good matching with lithium iron phosphate particles, can form good doping, and thus effectively improve the capacity and performance of the material; further, the use of niobium and tantalum for doping in the present invention has a better doping effect, and can obtain a positive electrode material with better conductivity, higher capacity and better stability.

[0051] In some embodiments of the present invention, the doping elements are doped in the inner shell surface layer of the lithium iron phosphate particles; in some specific embodiments of the present invention, the doping depth of the doping elements in the inner shell surface layer of the lithium iron phosphate particles is 1 to 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] Doping the doping elements in the inner shell surface layer of the lithium iron phosphate particles without completely penetrating into the bulk structure is beneficial for the doping elements to play a charge compensation role to achieve the reduction of the iron element, thereby forming an iron phosphide layer on the surface of the lithium iron phosphate particles. It is also beneficial for the doping elements to interact with the iron phosphide layer, coating layer, etc., thereby improving the material performance.

[0053] In some embodiments of the present invention, the doping depth of the doping elements in the internal surface layer of the lithium iron phosphate particles is 0.001 to 1.2 μm; in some specific embodiments of the present invention, the doping depth of the doping elements in the internal surface layer of the lithium iron phosphate particles is 0.01 to 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 present invention, the iron phosphide layer is a Fe2P layer.

[0055] In some embodiments of the present invention, 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 present invention, 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 present invention, the coating layer includes a boron-containing oxide, a tungsten-containing oxide or a combination thereof.

[0056] The above coating layer can achieve a good coating effect and form material particles with good consistency; in addition, the interaction or interface between the coating layer and the doping element is also conducive to improving the electrochemical performance of the material. Furthermore, the use of boron-containing oxides and tungsten-containing oxides can achieve a better coating effect, promote the formation of positive electrode materials with uniform size and performance, and obtain positive electrode materials with better electrochemical performance.

[0057] In some embodiments of the present invention, the particle size of the lithium iron phosphate particles is 0.1-2 μm; in some specific embodiments of the present invention, the particle size of the lithium iron phosphate particles is 0.15-1.8 μm; in some examples of the present invention, 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 invention refers to the particle size of the lithium iron phosphate primary particles, that is, the particle size of a single particle.

[0059] In some embodiments of the present invention, the thickness of the iron phosphide layer is 1-20 nm; in some specific embodiments of the present invention, the thickness of the iron phosphide layer is 3-15 nm; in some examples of the present invention, the thickness of the iron phosphide layer 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 invention, the thickness of the carbon layer is 1-40 nm; in some specific embodiments of the present invention, the thickness of the carbon layer is 1-30 nm; in some examples of the present invention, 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 invention, the coating layer has a thickness of 1 to 40 nm; in some specific embodiments of the present invention, the coating layer has a thickness of 1 to 30 nm; in some examples of the present invention, the coating layer has a thickness of 2 to 30 nm. Non-limiting specific examples include 2 nm, 3 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm or 30 nm.

[0062] In some embodiments of the present invention, 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 invention, the D10 particle size of the lithium iron phosphate positive electrode material is 0.2-0.6 μm; in some examples of the present invention, 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 invention, the D50 particle size of the lithium iron phosphate positive electrode material is 0.9-1.5 μm; in some specific embodiments of the present invention, the D50 particle size of the lithium iron phosphate positive electrode material is 0.95-1.3 μm; in some examples of the present invention, the D50 particle size of the lithium iron phosphate positive electrode material is 1-1.2 μm. Non-limiting specific examples are 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 invention, the D99 particle size of the lithium iron phosphate positive electrode material is 2-4 μm; in some specific embodiments of the present invention, the D99 particle size of the lithium iron phosphate positive electrode material is 2.5-3.5 μm; in some examples of the present invention, the D99 particle size of the lithium iron phosphate positive electrode material is 2.8-3.2 μm. Non-limiting specific examples are 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 an embodiment of the present invention provides a method for preparing a lithium iron phosphate positive electrode 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 the mixture to obtain a lithium iron phosphate positive electrode material as described in the first aspect of the embodiment of the present invention; the additive is a compound containing an doping element; and the coating agent is a compound that forms a coating layer.

[0066] The iron source, phosphorus source and lithium source are used to form lithium iron phosphate; the doping elements in the additives will be doped into the formed lithium iron phosphate, and the doping elements are also beneficial to promote the reduction of iron elements on the surface or subsurface of the lithium iron phosphate, thereby generating an iron phosphide layer; the carbon source is used to form a carbon layer, which is coated on the surface of the iron phosphide; the coating agent will form a coating layer coated on the outermost surface of the material. In the preparation method of the present invention, firstly, a pre-sintering treatment is performed to make each raw material undergo a preliminary chemical reaction to form a specific composition and doping structure, and then subsequent coating and calcination treatments are performed, which is beneficial to obtain a positive electrode material with a specific layer structure, and the material has better uniformity and better electrochemical performance, and the preparation method is simple, low cost, and easy to realize industrial production.

[0067] In some embodiments of the present invention, the additive includes 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 invention, the additive includes a niobium-containing compound, a tantalum-containing compound, or a combination thereof; in some examples of the present invention, the additive is selected from a niobium-containing compound.

[0068] In some embodiments of the present invention, 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 invention, the niobium-containing compound includes at least one of niobium pentoxide, niobium hydroxide or niobium phosphate; in some examples of the present invention, the niobium-containing compound is selected from niobium pentoxide.

[0069] In some embodiments of the present invention, 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 invention, the tantalum-containing compound includes tantalum pentoxide, tantalum hydroxide or a combination thereof; in some examples of the present invention, the tantalum-containing compound is selected from tantalum pentoxide.

[0070] In some embodiments of the present invention, 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 invention, the vanadium-containing compound includes vanadium pentoxide, vanadium trioxide or a combination thereof; in some examples of the present invention, the vanadium-containing compound is selected from vanadium pentoxide.

[0071] In some embodiments of the present invention, 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 invention, the iridium-containing compound includes iridium dioxide, iridium hydroxide or a combination thereof; in some examples of the present invention, the iridium-containing compound is selected from iridium dioxide.

[0072] In some embodiments of the present invention, the mass percentage of the additive in the mixture is 1-3%; in some specific embodiments of the present invention, the mass percentage of the additive in the mixture is 1.2-2.8%; in some examples of the present invention, the mass percentage of the additive in the mixture is 1.5-2.5%. Non-limiting specific examples are 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, iridium and the like in the additives are initially doped in the pre-sintered product during the pre-sintering process to form a specific doping distribution structure, which is beneficial to the subsequent coating and calcining treatments; further, the use of niobium and tantalum for doping in the present invention has a better doping effect, and can obtain a positive electrode material with better conductivity, higher capacity and better stability. Moreover, the present invention only needs to add a small amount of additives to achieve a good doping modification effect.

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

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

[0076] In some embodiments of the present invention, the mass ratio of the coating agent to the pre-sintered material is (0.1-5):100; in some specific embodiments of the present invention, the mass ratio of the coating agent to the pre-sintered material is (0.3-3):100; in some examples of the present invention, the mass ratio of the coating agent to the pre-sintered material is (0.5-2):100. Non-limiting specific examples are 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 will form a coating layer on the surface of the carbon layer. For example, boric acid will generate boron-containing oxides, tungsten oxide will generate tungsten-containing oxides, polyaniline and polypyrrole will generate carbon materials, etc. Furthermore, the use of boric acid and tungsten oxide can achieve better coating effects and obtain positive electrode materials with better electrochemical properties. And only a small amount of coating agent needs to be added to achieve a good coating modification effect.

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

[0079] In some embodiments of the present invention, the phosphorus source includes at least one of monoammonium hydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium phosphate or iron phosphate; in some specific embodiments of the present invention, the phosphorus source includes at least one of monoammonium hydrogen phosphate, diammonium hydrogen phosphate or iron phosphate; in some examples of the present invention, the phosphorus source is selected from iron phosphate.

[0080] In some embodiments of the present invention, the mass ratio of the iron source to the phosphorus source is 1:(0.5-2); in some embodiments of the present invention, the mass ratio of the iron source to the phosphorus source is 1:(0.7-1.5); in some embodiments of the present invention, the mass ratio of the iron source to the phosphorus source is 1:(0.8-1.2); non-limiting specific examples are 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 invention, 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 invention, the lithium source includes at least one of lithium carbonate, lithium hydroxide or lithium phosphate; in some examples of the present invention, the lithium source is selected from lithium carbonate.

[0082] In some embodiments of the present invention, the mass ratio of the iron source to the lithium source is 1:(0.5-2); in some embodiments of the present invention, the mass ratio of the iron source to the lithium source is 1:(0.8-1.5); in some embodiments of the present invention, the mass ratio of the iron source to the lithium source is 1:(1-1.2); non-limiting specific examples are 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 invention, the carbon source includes at least one of sugars, alcohols, esters, acids or resins; in some specific embodiments of the present invention, the carbon source includes sugars, alcohols or a combination thereof; in some examples of the present invention, the carbon source is selected from sugars.

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

[0085] The carbon source of the present invention is rich in sources and can be substances such as sugars or resins. Among them, sugars, especially glucose, are easily soluble, have a small molecular weight, and are easier to achieve a uniform coating effect at low temperatures; resin carbon sources can also achieve a good coating effect, but compared with resin carbon sources, sugars, especially glucose, have better dispersion performance and uniform coating performance.

[0086] In some embodiments of the present invention, the mass ratio of the iron source to the carbon source is 1:(0.1-1.5); in some embodiments of the present invention, the mass ratio of the iron source to the carbon source is 1:(0.2-1.2); in some embodiments of the present invention, the mass ratio of the iron source to the carbon source is 1:(0.3-1); non-limiting specific examples are 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 invention, the mass ratio of the iron source to the additive is 1:(0.01-0.2); in some embodiments of the present invention, the mass ratio of the iron source to the additive is 1:(0.02-0.15); in some embodiments of the present invention, the mass ratio of the iron source to the additive is 1:(0.03-0.1); non-limiting specific examples are 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 invention, before the pre-sintering process, the mixture is further subjected to a ball milling process; in some specific embodiments of the present invention, the method of ball milling the mixture is selected from wet ball milling, and the liquid medium is selected from water.

[0089] In some embodiments of the present invention, the mixture is ball-milled for 12 to 36 hours, with non-limiting examples being 12 hours, 16 hours, 20 hours, 24 hours, 30 hours, 34 hours or 36 hours.

[0090] In some embodiments of the present invention, the mixture is ball-milled at a ball-milling speed of 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 invention, after the ball milling process and before the pre-sintering process, a drying step is further included; in some specific embodiments of the present invention, the drying method is selected from spray drying. Granulation by spray drying can obtain a granular mixture, which is conducive to more complete mixing of the raw materials, thereby facilitating the pre-sintering process.

[0092] In some embodiments of the present invention, the temperature of the pre-sintering process is 400-700°C; in some specific embodiments of the present invention, the temperature of the pre-sintering process is 500-680°C; in some examples of the present invention, the temperature of the pre-sintering process is 550-650°C. Non-limiting specific examples are 550°C, 560°C, 580°C, 600°C, 620°C, 640°C or 650°C.

[0093] By carrying out high-temperature treatment in the range of 400-700°C, large-radius transition metals will not be completely incorporated into the bulk phase of the material, so that the doping elements can be doped in the inner shell surface layer of the lithium iron phosphate particles without completely penetrating into its bulk structure, which is beneficial to promote the reduction of iron elements on the surface or subsurface of the lithium iron phosphate, thereby generating an iron phosphide layer; in addition, doping in the inner shell surface layer of the lithium iron phosphate particles is also beneficial to the interaction between the doping elements and the iron phosphide layer, coating layer, etc., thereby improving the material performance.

[0094] In some embodiments of the present invention, the pre-sintering treatment time is 1 to 6 hours; in some specific embodiments of the present invention, the pre-sintering treatment time is 2 to 6 hours; in some examples of the present invention, the pre-sintering treatment time is 3 to 5 hours. Non-limiting specific examples include 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 invention, the time for mixing the pre-sintered material and the coating agent is 10 to 60 minutes; non-limiting specific examples include 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes or 60 minutes.

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

[0097] In some embodiments of the present invention, the calcination time is 2 to 8 hours; in some specific embodiments of the present invention, the calcination time is 4 to 8 hours; in some examples of the present invention, the calcination time is 5 to 7 hours. Non-limiting specific examples include 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 lithium iron phosphate positive electrode material with a specific surface doping and coating structure of the present invention can be well formed. If the calcination temperature is too high or the time is too long, it may cause the doping and coating process of the material to be difficult to control, affecting the uniformity of material growth and the specific distribution of doping elements.

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

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

[0101] The present invention is further described below in conjunction with specific embodiments and comparative examples.

[0102] Example 1

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

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

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

[0106] Step 2: Mix the 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 500rpm. After the ball milling is completed, the mixture is spray-dried and granulated to obtain a granular mixture.

[0107] Step 3: Pre-sintering. The granulated mixture is placed in a high temperature furnace for pre-sintering. The pre-sintering temperature is 600°C and the pre-sintering time is 4 hours. After the pre-sintering is completed, the pre-sintered material is crushed to obtain a powdered material.

[0108] Step 4: Coating treatment. Add boric acid as a coating agent to the crushed material, and the amount of coating agent added is 1% of the mass of the pre-sintered material. Mix the materials with the coating agent for 30 minutes.

[0109] Step 5: Calcination treatment. The mixed materials are placed in a high-temperature furnace for calcination at a temperature of 800°C for 6 hours. After calcination, a powder product, namely, 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 by the coating agent polyaniline (PANI) during the calcination process;

[0113] In the preparation method, polyaniline (PANI) is used as a coating agent in step 4 of this example.

[0114] The other structures, raw materials and preparation steps are the same as those 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 a tungsten-containing oxide, which is formed by the coating agent tungsten oxide during the calcination process;

[0118] In the preparation method, tungsten oxide is used as a coating agent in step 4 of this example.

[0119] The other structures, raw materials and preparation steps are the same as those 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 by the coating agent aluminum phosphate during the calcination process;

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

[0124] The other structures, raw materials and preparation steps are the same as those 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 example is a titanium-containing oxide, which is formed by the coating agent titanium dioxide during the calcination process;

[0128] In the preparation method, titanium dioxide is used as a coating agent in step 4 of this example.

[0129] The other structures, raw materials and preparation steps are the same as those 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, and the specific steps are as follows:

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

[0133] Step 2: 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 pre-sintered materials.

[0134] Step 3: Calcination treatment. The pre-sintered material is placed in a high-temperature furnace for calcination at a temperature of 800°C for 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, which differs from Example 1 in that:

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

[0138] In the preparation method, in this example, magnesium oxide is used as an additive in step one, 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 those in Example 1.

[0140] Comparative Example 3

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

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

[0143] In the preparation method, in this example, no additive is added in step 1, and titanium dioxide is used as a coating agent in step 4.

[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 differs from Example 1 in that:

[0147] In the structural composition, in this example, no elements are doped inside the lithium iron phosphate particles; no coating layer is provided;

[0148] In the preparation method, in this example, no additive is added in step 1, step 4 (coating treatment) is omitted, and step 5 is to calcine 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 differs from Example 1 in that:

[0152] In the structural composition, the doping elements in this example are completely doped into the bulk phase of the lithium iron phosphate particles, and the specific doping and coating structure of Example 1 cannot be formed.

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

[0155] Performance Testing

[0156] 1) Morphology characterization: The morphology of the lithium iron phosphate positive electrode materials obtained in each embodiment and comparative example was characterized by scanning electron microscopy (SEM).

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

[0158] 3) Electrochemical performance test: A two-probe four-wire pressure test method (test pressure of 10000N, holding time of 30s) was used to test the powder resistivity of the lithium iron phosphate positive electrode material obtained in each embodiment and comparative example; the lithium iron phosphate positive electrode material obtained in each embodiment and comparative example was mixed with a conductive agent Super-P, a binder PVDF and a solvent NMP to form a slurry, and the slurry was coated on a foil, and each positive electrode sheet was obtained after drying. The obtained positive electrode sheets were used as the positive electrode and metallic lithium was used as the negative electrode to assemble a simulated battery or button battery, and the first cycle charge gram capacity and discharge gram capacity, as well as the 100-cycle capacity retention rate were tested under the conditions of 23°C±2°C and 0.1C. The 100-cycle capacity retention rate is calculated by dividing the 100th cycle discharge gram capacity by the first cycle discharge gram capacity.

[0159] The SEM images of the lithium iron phosphate positive electrode materials of Example 1 and Comparative Example 1 are as follows: Figure 2 As shown, wherein a is Example 1 and b is Comparative Example 1. Figure 2 As can be seen, Example 1 adopts a unique doping and coating process. First, the lithium insertion reaction degree of the material and the doping degree of the element are adjusted by pre-sintering at a specific temperature and time. Generally, within the range of 400-700°C, the large-radius transition metal will not be completely doped into the material phase, so that the niobium element is doped in the surface layer of the lithium iron phosphate particles; then, combined with high-temperature short-time calcination, it can be well formed as shown in FIG. Figure 1 The material with the surface doping and coating structure shown. In Example 1, the material is coated uniformly, the particle growth is isotropic, the material is more uniform, the consistency is higher, and the performance is better; while Comparative Example 1 adopts a conventional sintering synthesis process, without doping and coating treatment, the sintering time is longer, resulting in poor material growth consistency, particle agglomeration, large particles and other problems are more serious.

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

[0161] Table 1 Particle size data of lithium iron phosphate positive electrode materials of Example 1 and Comparative Examples 2 to 4

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

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

[0164] Table 2 Electrochemical performance data of lithium iron phosphate positive electrode materials of Examples 1 to 5 and Comparative Examples 2 to 5

[0165]

[0166] As can be seen from Table 2, Examples 1 to 5 are coated with a coating layer and doped with specific elements, and pre-sintered and calcined under specific conditions to obtain a lithium iron phosphate positive electrode material with a specific doping and coating structure, wherein the formation of the Fe2P layer is mainly due to the reduction of the iron element, which is related to the synthesis temperature, doping elements, synthesis atmosphere and other conditions. The niobium element in the additive niobium pentoxide of Examples 1 to 4 is +5 valence, and its doping is conducive to the formation of the Fe2P layer. The main reason is that in the lithium iron phosphate particles, high-valent metal elements are doped, especially large atoms, high-valent metal Nb elements, and the charge compensation effect will lead to the reduction of the iron element, and then form a highly conductive Fe2P layer on the micro-surface of the particles; Fe2P itself has good electronic conductivity and can form a high conductive network. In addition, the interface effect between the Fe2P layer and the lithium iron phosphate particles also contributes to the improvement of electrochemical performance. In addition, the doping of niobium metal elements broadens the lithium ion transmission channel, improves the rate performance of the material, and is conducive to improving the lithium ion diffusion coefficient of the lithium iron phosphate positive electrode material, while ensuring its structural stability. The coating of the coating layer is beneficial to controlling the uneven growth of local materials during solid-phase sintering, improving the morphology and consistency of the positive electrode particles, and enhancing the stability of the material. In addition, there is an interaction or interface effect between the coating layer and the doped niobium element, which is also beneficial to improving the electrochemical properties of the material. The materials obtained in Examples 1 to 5 have low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, good stability, and good electrochemical properties. Different coating agents were used for coating in Examples 1 to 5, and good coating effects could be achieved. Among the coating agents, using boric acid and oxides as coating agents has better effects, and the obtained materials have lower resistivity, higher charge and discharge capacity, and higher capacity retention rate.

[0167] In Comparative Example 2, magnesium doping combined with titanium dioxide coating was used, and the obtained material had a higher resistivity, lower discharge capacity, and lower capacity retention rate; in Comparative Example 3, no doping treatment was performed, and only titanium dioxide was used for coating, and the obtained material had a higher resistivity, lower charge and discharge capacity, and lower capacity retention rate; in Comparative Example 4, no doping and coating were performed, and the difference between the D10, D50, and D99 particle sizes of the material was large, the particle size uniformity was poor, and the resistivity was high, the charge and discharge capacity was low, and the capacity retention rate was low; in Comparative Example 5, no pre-sintering treatment was performed, and it was difficult to dope the niobium element at a lower temperature to form the specific doping structure in Example 1. It was difficult to form the specific doping and coating structure in Example 1 by directly calcining the raw materials, and the obtained material had a higher resistivity, lower charge and discharge capacity, and lower capacity retention rate.

[0168] The lithium iron phosphate positive electrode material provided by the embodiment of the present invention can improve the intrinsic electronic conductivity of the lithium iron phosphate material while improving the lithium ion diffusion coefficient, and strengthen the stability of its structure and surface interface, and the material grows evenly and has good dispersibility. The preparation method provided by the embodiment of the present invention is based on the traditional solid phase synthesis method, through special doping element treatment, while broadening the lithium ion transmission channel and improving the material rate performance, the doping material used can construct a stable iron phosphide (Fe2P) high conductive protective layer on the surface or micro surface of the positive electrode material, thereby effectively improving the intrinsic electronic and ionic conductivity of lithium iron phosphate; then, through the 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 particles under high temperature sintering, and at the same time, it can also play a good physical isolation and protection role, 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 output of lithium iron phosphate, and reduce energy consumption and cost; and can simplify the preparation process, improve production efficiency, reduce energy consumption and cost, and adapt to large-scale production.

[0169] In summary, the present invention can promote the formation of iron phosphide layer while broadening the lithium ion transmission channel and improving the material rate performance through the doping of high-priced elements, effectively improving the intrinsic electronic and ionic conductivity of lithium iron phosphate; and the coating layer is conducive to inhibiting the continuous growth of the material, obtaining a material with good uniformity and dispersibility, and at the same time it can also play a good physical isolation and protection role, significantly improving the electrochemical properties of the material. The lithium iron phosphate positive electrode material with a specific doping and coating structure of the present invention has good electrochemical properties such as low resistivity, good conductivity, high charge and discharge capacity, high capacity retention rate, and good stability, and has good application prospects in the preparation of batteries, especially lithium batteries.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: include: Lithium iron phosphate particles, doping elements, an iron phosphide layer, a carbon layer, and a coating layer; the doping elements are doped in the lithium iron phosphate particles; The iron phosphide layer is coated on at least a portion of the surface of the lithium iron phosphate particles; the carbon layer is coated on 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 doping element is greater than +3.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The doping element includes at least one of niobium, tantalum, vanadium or iridium; And / or, the doping element is doped in the inner shell surface layer of the lithium iron phosphate particles.

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: 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.

4. The lithium iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The particle size of the lithium iron phosphate particles is 0.1 to 2 μm; and / or, the thickness of the iron phosphide layer is 1 to 20 nm; And / or, the thickness of the carbon layer is 1 to 40 nm; And / or, the coating layer has a thickness of 1 to 40 nm.

5. The lithium iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The D10 particle size of the lithium iron phosphate positive electrode material is 0.1 to 0.8 μm; And / or, the D50 particle size of the lithium iron phosphate positive electrode material is 0.9 to 1.5 μm; And / or, the D99 particle size of the lithium iron phosphate positive electrode material is 2 to 4 μm.

6. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: A mixture containing an iron source, a phosphorus source, a lithium source, a carbon source and an additive is pre-sintered to obtain a pre-sintered material, and the pre-sintered material is mixed with a coating agent and then calcined to obtain the lithium iron phosphate positive electrode material as described in any one of claims 1 to 5; the additive is a compound containing the doping element; the coating agent is a compound that forms the coating layer.

7. The preparation method according to claim 6, characterized in that: The additive includes at least one of a niobium-containing compound, a tantalum-containing compound, a vanadium-containing compound, or an iridium-containing compound; And / or, the mass percentage of the additive in the mixture is 1-3%.

8. The preparation method according to claim 6, 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.

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

10. A battery, characterized in that: It comprises the lithium iron phosphate positive electrode material according to any one of claims 1 to 5, or the lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 6 to 9.

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