A lithium iron phosphate cathode material with a nanoporous structure and its preparation method

Abstract

This invention provides a high-density lithium iron phosphate cathode material with a nanoporous structure and its preparation method, comprising the following steps: (1) preparing cathode material intermediate dispersion A; (2) preparing cathode material intermediate dispersion B; (3) mixing the cathode material intermediate dispersion A and the cathode material intermediate dispersion B, and spray drying to obtain lithium iron phosphate intermediate mixed particles; (4) sintering the lithium iron phosphate intermediate mixed particles in an inert gas to obtain the lithium iron phosphate cathode material. This invention can improve the electrolyte wettability of the material, shorten the lithium ion transport distance, and enable more lithium ions inside the particles to participate in insertion and extraction more quickly and effectively, thereby improving its electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of battery materials and relates to a lithium iron phosphate cathode material with a nanoporous structure and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) has become a hot material in the energy storage and passenger vehicle markets in recent years due to its excellent cycle life, safety performance, and low cost. To achieve even higher energy density, the compaction density of LFP materials must be further improved. The compaction density of LFP materials has increased from the current 2.45 g / cm³. 3 It is gradually transitioning to 2.55 g / cm³. 3 and above.

[0003] The prevailing theory in the production of high-compact-density lithium iron phosphate (LFP) posits that appropriately increasing the size of the primary particles can improve the material's compaction density. Therefore, the increasing size of the primary particle size inevitably negatively impacts the material's specific capacity and low-temperature rate performance. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a lithium iron phosphate cathode material with a nanoporous structure and its preparation method. The lithium iron phosphate cathode material with a nanoporous structure provided by this invention exhibits excellent capacity and rate performance. The lithium iron phosphate cathode material of this invention, comprising small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure, not only has a high compaction density but also a high specific capacity, thus achieving a high energy density and excellent rate performance.

[0005] Specifically, the present invention provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0006] (1) Preparation of cathode material intermediate dispersion A: Disperse phosphorus source, lithium source, iron source and pore-forming agent in water, adjust pH to 2-5, react at 100-180℃ for 6-10h, then add soluble carbon source, continue reaction for 5-24h, and obtain the cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A.

[0007] (2) Preparation of cathode material intermediate dispersion B: Phosphorus source, lithium source, iron source and soluble carbon source are dispersed in water, the pH is adjusted to 3-6, and the reaction is carried out at 120-200℃ for 5-14 hours to obtain the cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B, wherein the particle size D of the lithium iron phosphate intermediate particles B is... 50 The particle size D is smaller than that of the lithium iron phosphate intermediate particle A. 50 ;

[0008] (3) Mix the cathode material intermediate dispersion A and the cathode material intermediate dispersion B, and spray dry them to obtain lithium iron phosphate intermediate mixed particles;

[0009] (4) The lithium iron phosphate intermediate mixed particles are sintered in an inert gas to obtain the lithium iron phosphate cathode material.

[0010] This invention also provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0011] (1') Preparation of cathode material intermediate dispersion A: Disperse phosphorus source, lithium source, iron source and pore-forming agent in water, adjust pH to 2-5, react at 100-180℃ for 6-10h, then add soluble carbon source, continue reaction for 5-24h, to obtain the cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A.

[0012] (2') Spray dry the cathode material intermediate dispersion A to obtain lithium iron phosphate intermediate particles A, and sinter the lithium iron phosphate intermediate mixed particles A in an inert gas to obtain the lithium iron phosphate cathode material.

[0013] In one or more embodiments, in steps (1) and (1'), the pH is adjusted to 3-4, for example 3.1-3.5, 3.1-3.4, 3.2±0.1.

[0014] In one or more embodiments, in steps (1) and (1'), the reaction temperature is 120-140°C, for example, 120-130°C.

[0015] In one or more embodiments, in steps (1) and (1'), the reaction time before the addition of the soluble carbon source is 6-7 h.

[0016] In one or more embodiments, in steps (1) and (1'), the reaction time after the addition of the soluble carbon source is 12 ± 2 h.

[0017] In one or more embodiments, in step (2), the pH is adjusted to 3.5-4.5, for example 3.6 ± 0.1.

[0018] In one or more embodiments, the pH of the reaction system in step (2) is higher than the pH of the reaction system in step (1).

[0019] In one or more embodiments, in step (2), the reaction temperature is 120-140°C, for example 120-130°C.

[0020] In one or more embodiments, in step (2), the reaction time is 12 ± 2 h.

[0021] In one or more embodiments, the reaction time in step (2) is less than the total reaction time in step (1).

[0022] In one or more embodiments, the lithium source in step (1), the lithium source in step (2), and the lithium source in step (1') are each independently selected from one or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium dihydrogen phosphate, and lithium hydrogen phosphate, preferably lithium carbonate.

[0023] In one or more embodiments, the phosphorus source in step (1), the phosphorus source in step (2), and the phosphorus source in step (1') are each independently selected from one or more of phosphoric acid, lithium dihydrogen phosphate, and lithium hydrogen phosphate, preferably phosphoric acid.

[0024] In one or more embodiments, the iron source in step (1), the iron source in step (2), and the iron source in step (1') are each independently selected from one or more of magnetite, iron oxide, and iron powder, preferably iron powder.

[0025] In one or more embodiments, the pore-forming agent in step (1) and the pore-forming agent in step (1') are each independently selected from one or more of ammonium bicarbonate, urea, ammonium chloride, PVP, PEG and PVA, preferably selected from one or more of ammonium bicarbonate, urea and ammonium chloride.

[0026] In one or more embodiments, the soluble carbon source in step (1), the soluble carbon source in step (2), and the soluble carbon source in step (1') are each independently selected from one or more of sucrose, glucose, fructose, starch, amino acids, and citric acid, preferably selected from one or two of sucrose and glucose.

[0027] In one or more embodiments, the molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source in step (1) is (1-1.1):1:(0.9-1).

[0028] In one or more embodiments, the molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source in step (2) is (1-1.1):1:(0.9-1).

[0029] In one or more embodiments, the molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source in step (1') is (1-1.1):1:(0.9-1).

[0030] In one or more embodiments, the mass of the pore-forming agent in step (1) is 1%-10% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source.

[0031] In one or more embodiments, the mass of the pore-forming agent in step (1') is 1%-10% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source.

[0032] In one or more embodiments, the mass of the soluble carbon source in step (1) is 5%-15% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source, preferably 7%-9%.

[0033] In one or more embodiments, the mass of the soluble carbon source in step (2) is 5%-15% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source, preferably 10%-14%.

[0034] In one or more embodiments, the mass of the soluble carbon source in step (1') is 5%-15% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source, preferably 7%-9%.

[0035] In one or more embodiments, the particle size D of the lithium iron phosphate intermediate particle A is... 50 The range is 500nm-3000nm.

[0036] In one or more embodiments, the particle size D of the lithium iron phosphate intermediate particle B is... 50 The range is 100nm-800nm.

[0037] In one or more embodiments, in step (3), the cathode material intermediate dispersion A and the cathode material intermediate dispersion B are mixed according to a mass ratio of lithium iron phosphate intermediate particles A to lithium iron phosphate intermediate particles B of 5:5 to 9:1, preferably 6:4 to 8:2.

[0038] In one or more embodiments, the inert gas is selected from one or more of nitrogen, helium, and argon, preferably nitrogen.

[0039] In one or more embodiments, the sintering conditions in steps (4) and (2') are two-gradient plateau sintering: the first plateau is heated to 200-500℃ at a rate of 1-5℃ / min and held for 3-6h; the second plateau is heated to 600-800℃ at a rate of 5-10℃ / min and held for 8-12h.

[0040] In one or more embodiments, in step (1), the dopant is dispersed in water together with a phosphorus source, a lithium source and an iron source.

[0041] In one or more embodiments, in step (2), the dopant is dispersed in water together with a phosphorus source, a lithium source and an iron source.

[0042] In one or more embodiments, in step (1'), the dopant is dispersed in water together with a phosphorus source, a lithium source and an iron source.

[0043] In one or more embodiments, the dopant is preferably selected from one or more of titanium oxide, vanadium oxide, tungsten oxide, magnesium oxide, zirconium oxide, and aluminum oxide.

[0044] In one or more embodiments, the mass of the dopant is preferably 0.1%-1% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source.

[0045] In one or more embodiments, in steps (4) and (2'), after sintering, crushing, sieving and iron removal are performed; preferably, air jet milling is used, wherein the nozzle specification is 10±1mm, the feeding frequency is 18±5Hz, the grinding air pressure is 0.32±0.05MPa, the classifying wheel frequency is 40±5Hz, ultrasonic sieving is performed, wherein the screen mesh number is 150±10 mesh, and electromagnetic iron removal is performed, wherein the magnet strength is 20000±2000 Gauss.

[0046] The present invention also provides lithium iron phosphate cathode materials prepared by the preparation method described in any embodiment herein.

[0047] This invention also provides a lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material is a large-particle lithium iron phosphate with a nanoporous structure, the peak particle size of the large-particle lithium iron phosphate with a nanoporous structure is 1μm-100μm, and the micropore diameter D is... 50 The wavelength range is 10nm-100nm, and the porosity is 0.1%-10%.

[0048] This invention also provides a lithium iron phosphate cathode material, which comprises small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure; the peak particle size of the large-particle lithium iron phosphate with a nanoporous structure is 1μm-100μm, and the micropore diameter D is... 50 The particle size is 10nm-100nm, and the porosity is 0.1%-10%; the peak particle size of the small lithium iron phosphate particles is 200nm-2000nm.

[0049] In one or more embodiments, the peak particle size of the large lithium iron phosphate particles with a nanoporous structure is greater than the peak particle size of the small lithium iron phosphate particles.

[0050] In one or more embodiments, the lithium iron phosphate cathode material is prepared using the preparation method described in any of the embodiments herein.

[0051] In one or more embodiments, the mass of the large-particle lithium iron phosphate with a nanoporous structure accounts for 50%-100% of the total mass of the lithium iron phosphate cathode material, preferably 50%-90%, and more preferably 60%-80%.

[0052] The present invention also provides a positive electrode sheet comprising the lithium iron phosphate positive electrode material described in any embodiment herein.

[0053] The present invention also provides an electrochemical device comprising a positive electrode as described in any embodiment herein; preferably, the electrochemical device is a lithium-ion battery. Attached Figure Description

[0054] Figure 1 This is a schematic diagram showing the particle size distribution of lithium iron phosphate.

[0055] Figure 2 The image shows a scanning electron microscope (SEM) image of the lithium iron phosphate cathode material prepared in Example 1.

[0056] Figure 3 This is a SEM image of the porous, large-particle lithium iron phosphate in the lithium iron phosphate cathode material prepared in Example 1.

[0057] Figure 4 The particle size distribution diagrams are for the lithium iron phosphate cathode materials prepared in Examples 1, 5, 6, 7 and 8.

[0058] Figure 5 The particle size distribution diagrams are shown for the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 2. Detailed Implementation

[0059] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0060] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0061] In this document, the terms “contains,” “includes,” “containing,” and similar terms encompass the meanings of “basically composed of” and “composed of.” For example, when this document discloses “A contains B and C,” “A is basically composed of B and C” and “A is composed of B and C” should be considered as having been disclosed in this document.

[0062] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0063] Unless otherwise specified, percentages refer to mass percentages and proportions refer to mass ratios in this article.

[0064] In this document, when describing embodiments or examples, it should be understood that it is not intended to limit the invention to those embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein are covered within the scope defined by the claims.

[0065] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0066] This invention provides a lithium iron phosphate cathode material, which is a large-particle lithium iron phosphate with a nanoporous structure. In this invention, the large-particle lithium iron phosphate with a nanoporous structure can be obtained by spray drying and sintering of cathode material intermediate dispersion A. This invention also provides a lithium iron phosphate cathode material comprising small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure, or composed of small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure. In this invention, the lithium iron phosphate cathode material comprising small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure can be obtained by spray drying and sintering of a mixture of cathode material intermediate dispersion A and cathode material intermediate dispersion B.

[0067] In this invention, the peak particle size of the large-particle lithium iron phosphate with a nanoporous structure can be 1 μm-100 μm, for example, 2 μm, 2.5 μm, 3 μm, 4 μm, 5 μm, 10 μm, 20 μm, or 50 μm, preferably 2 μm-20 μm. In this invention, the micropore diameter D of the large-particle lithium iron phosphate with a nanoporous structure is... 50The wavelength range can be 10nm-100nm, such as 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm, 60nm, and 80nm, and the porosity can be 0.1%-10%, such as 1%, 1.5%, 2%, 2.5%, 3%, and 5%.

[0068] In this invention, the peak particle size of the small lithium iron phosphate particles can be 200nm-2000nm, such as 700nm, 800nm, 900nm, 1000nm, 1500nm, preferably 200nm-1000nm.

[0069] In this invention, the peak particle size of large lithium iron phosphate particles with a nanoporous structure is greater than that of small lithium iron phosphate particles.

[0070] In embodiments where the lithium iron phosphate cathode material comprises large-particle lithium iron phosphate and small-particle lithium iron phosphate with nanoporous structures, the large-particle lithium iron phosphate with nanoporous structures preferably accounts for ≥50% of the mass fraction of the lithium iron phosphate cathode material, for example, 50%-100%, 50%-90%, and more preferably 60%-80%, for example, 65%, 70%, or 75%.

[0071] In this invention, a phosphorus source, a lithium source, an iron source, and a pore-forming agent are dispersed in water and reacted for a period of time under certain pH and temperature conditions. Then, a soluble carbon source is added, and the reaction continues for a period of time to obtain a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. When preparing the cathode material intermediate dispersion A, the pH is controlled at 2-5, preferably 3-4, such as 3.1, 3.2, 3.3, 3.4, or 3.5; the reaction temperature is controlled at 100-180℃, preferably 120-140℃, such as 125℃, 130℃, or 135℃; before adding the soluble carbon source, the reaction time can be 6-10 hours, such as 7 hours, 8 hours, or 9 hours; after adding the soluble carbon source, the reaction time can be 5-24 hours, such as 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, or 20 hours.

[0072] In this invention, a phosphorus source, a lithium source, an iron source, and a soluble carbon source are dispersed in water and reacted for a period of time under certain pH and temperature conditions to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles A. When preparing the cathode material intermediate dispersion B, the pH is controlled at 3-6, preferably 3.5-4.5, such as 3.6, 3.7, 3.8, 3.9, or 4; the reaction temperature is controlled at 120-200℃, preferably 120-140℃, such as 125℃, 130℃, or 135℃; and the reaction time can be 5-14 hours, such as 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or 13 hours.

[0073] The difference in pH and temperature when preparing cathode material intermediate dispersions A and B is mainly to adjust the nucleation and growth rate of lithium iron phosphate intermediates. Lower pH and temperature are more conducive to the growth of intermediates, thus generating intermediate particles with larger particle size; higher pH and temperature are more conducive to the nucleation of intermediates, thus generating intermediate particles with smaller particle size.

[0074] In this invention, the particle size D of lithium iron phosphate intermediate particles B is... 50 The particle size D is smaller than that of lithium iron phosphate intermediate particle A. 50 In some implementations, the particle size D of the lithium iron phosphate intermediate particle A is... 50 The particle size is 500nm-3000nm, preferably 500nm-2000nm, such as 600nm, 700nm, 800nm, 900nm, and 1000nm. In some embodiments, the particle size D of the lithium iron phosphate intermediate particles B is... 50 The wavelength range is 100nm-800nm, preferably 100nm-500nm, such as 200nm, 300nm, and 400nm.

[0075] To ensure that the particle size D50 of lithium iron phosphate intermediate particles B is smaller than that of lithium iron phosphate intermediate particles A, the pH of the reaction system during the preparation of cathode material intermediate dispersion B can be controlled to be higher than the pH of the reaction system during the preparation of cathode material intermediate dispersion A, and / or the reaction time during the preparation of cathode material intermediate dispersion B can be controlled to be shorter than the total reaction time during the preparation of cathode material intermediate dispersion A. In this invention, the temperatures during the preparation of cathode material intermediate dispersions A and B can be controlled to be the same or different.

[0076] The lithium source suitable for this invention can be one or more selected from lithium carbonate, lithium hydroxide, lithium nitrate, lithium dihydrogen phosphate, and lithium hydrogen phosphate, preferably lithium carbonate.

[0077] The phosphorus source suitable for this invention can be one or more selected from phosphoric acid, lithium dihydrogen phosphate, and lithium hydrogen phosphate, preferably phosphoric acid.

[0078] The iron source suitable for this invention can be one or more selected from iron(II,III) oxide, iron oxide, and iron powder, preferably iron powder.

[0079] In this invention, when preparing the intermediate dispersions A and B of the positive electrode material, the preferred molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source is (1-1.1):1:(0.9-1).

[0080] The pore-forming agent suitable for this invention can be one or more selected from ammonium bicarbonate, urea, ammonium chloride, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and polyvinyl alcohol (PVA), preferably one or more selected from ammonium bicarbonate, urea, and ammonium chloride. When preparing the cathode material intermediate dispersion A, the mass of the pore-forming agent is preferably 1%-10% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source, for example, 2%, 3%, 4%, 5%, 6%, or 8%. The main function of the pore-forming agent is to provide active sites for crystal nucleation and growth, and simultaneously volatilizes in gaseous form during subsequent sintering, forming micropores within the bulk phase.

[0081] The soluble carbon source suitable for this invention can be one or more selected from sucrose, glucose, fructose, starch, amino acids, and citric acid, preferably one or two selected from sucrose and glucose. The mass of the soluble carbon source is preferably 5%-15% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source, for example, 6%, 8%, 10%, 12%, or 14%. When preparing cathode material intermediate dispersion A, the mass of the soluble carbon source is preferably 7%-9% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source, for example, 7.5%, 8%, or 8.5%. When preparing cathode material intermediate dispersion B, the mass of the soluble carbon source is preferably 10%-14% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source, for example, 11%, 12%, or 13%.

[0082] In this invention, when preparing the cathode material intermediate dispersion A or B, a dopant can be added to the reaction system. For example, the dopant can be dispersed together with a phosphorus source, a lithium source, and an iron source in water. Suitable dopants for this invention can be one or more selected from titanium oxide, vanadium oxide, tungsten oxide, magnesium oxide, zirconium oxide, and aluminum oxide. The mass of the dopant is preferably 0.1%-1% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source, for example, 0.2%, 0.4%, 0.5%, 0.6%, and 0.8%.

[0083] When mixing cathode material intermediate dispersion A and cathode material intermediate dispersion B, the mixing ratio can be controlled so that the mass ratio of lithium iron phosphate intermediate particles A to lithium iron phosphate intermediate particles B in the mixture is 5:5 to 9:1, for example, 5.5:4.5, 6:4, 6.5:3.5, 7:3, 7.5:2.5, 8:2, 8.5:1.5, preferably 6:4 to 8:2, for example, 6.5:3.5, 7:3, 7.5:2.5, more preferably 7:3 to 8:2, for example, 7.5:2.5. Controlling the mass ratio of lithium iron phosphate intermediate particles A to lithium iron phosphate intermediate particles B within the aforementioned range is beneficial to improving the compaction density of the cathode material and enhancing its specific capacity and rate performance. By optimizing the mixing ratio of lithium iron phosphate intermediate particles A and lithium iron phosphate intermediate particles B, the gradation ratio of large and small lithium iron phosphate particles in the lithium iron phosphate cathode material can be optimized, thereby improving energy density and rate performance. Optimizing the mixing ratio of lithium iron phosphate intermediate particles A and B to form a typical bimodal distribution is crucial for the preparation of high-pressure lithium iron phosphate.

[0084] It is understandable that when preparing lithium iron phosphate cathode materials containing small-particle lithium iron phosphate and large-particle lithium iron phosphate with nanoporous structures, the mass fraction of lithium iron phosphate intermediate particles A in lithium iron phosphate intermediate particles A and B is approximately the same as the mass fraction of large-particle lithium iron phosphate with nanoporous structures in the lithium iron phosphate cathode material.

[0085] In this invention, lithium iron phosphate intermediate particles are sintered in an inert gas. The inert gas suitable for this invention can be one or more selected from nitrogen, helium, and argon, preferably nitrogen.

[0086] In this invention, the preferred sintering conditions for the lithium iron phosphate intermediate particles are two-gradient plateau sintering: the first plateau involves heating at a rate of 1–5 °C / min (e.g., 2 °C / min, 3 °C / min) to 200–500 °C (e.g., 240 °C, 300 °C), with a holding time of 3–6 h (e.g., 4 h, 5 h); the second plateau involves heating at a rate of 5–10 °C / min (e.g., 6 °C / min, 7 °C / min) to 600–800 °C (e.g., 650 °C, 690 °C, 750 °C), with a holding time of 8–12 h (e.g., 9 h, 10 h, 11 h). The primary purpose of the first plateau is to ensure complete volatilization of the pore-forming agent, preventing adverse effects on subsequent crystal growth and carbon source carbonization; the primary purpose of the second plateau is to further promote grain growth and crystal morphology, while simultaneously forming a good carbon coating layer, thereby ensuring excellent electrochemical performance of the material.

[0087] In this invention, after sintering, crushing, sieving, and iron removal can be performed. Preferably, an air jet mill is used for crushing, wherein the nozzle specification is 10±1mm, the feeding frequency is 18±5Hz, the grinding air pressure is 0.32±0.05MPa, and the classifying wheel frequency is 40±5Hz. Ultrasonic sieving is performed, wherein the screen mesh number is 150±10 mesh, and electromagnetic iron removal is performed, wherein the magnet strength is 20000±2000 Gauss.

[0088] In some embodiments, the method for preparing the lithium iron phosphate cathode material of the present invention, comprising small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure, includes the following steps:

[0089] (1) Disperse phosphorus source, lithium source, iron source, pore-forming agent and optional dopant in water, react at a certain pH and temperature for 6 to 10 hours, then add soluble carbon source and continue to react for 5 to 24 hours to obtain cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A.

[0090] (2) Disperse phosphorus source, lithium source, iron source, soluble carbon source and optional dopant in water, and react at a certain pH and temperature for 5 to 12 hours to obtain cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B.

[0091] (3) Mix the cathode material intermediate dispersion A and cathode material intermediate dispersion B in a certain proportion and spray dry them to obtain lithium iron phosphate intermediate mixed particles with large and small particle size distribution.

[0092] (4) The lithium iron phosphate intermediate mixed particles are sintered in an inert gas, and then crushed by an air jet mill with a nozzle specification of 10±1mm, a feeding frequency of 18±5Hz, a grinding air pressure of 0.32±0.05MPa, a classifying wheel frequency of 40±5Hz, ultrasonic sieving with a screen mesh of 150±10 mesh, and electromagnetic iron removal with a magnet strength of 20000±2000 Gauss to obtain lithium iron phosphate cathode material.

[0093] The present invention also provides a positive electrode sheet comprising the lithium iron phosphate positive electrode material of the present invention, and an electrochemical device comprising the positive electrode sheet, such as a lithium-ion battery. A lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0094] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer formed on the surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode material, a conductive agent, and a binder. The positive electrode material layer is obtained by coating a positive electrode slurry containing a positive electrode material, a conductive agent, a binder, and a solvent onto the positive electrode current collector, followed by rolling, die-cutting, and baking. The solvent for the positive electrode slurry can be N-methylpyrrolidone (NMP). The positive electrode current collector can be copper foil, aluminum foil, titanium foil, nickel foil, iron foil, zinc foil, etc. In the positive electrode sheet of the present invention, the positive electrode material includes the lithium iron phosphate positive electrode material of the present invention, or is the lithium iron phosphate positive electrode material of the present invention. The conductive agent of the positive electrode can be one or more selected from conductive carbon black (SP), carbon fiber (CF), acetylene black, conductive graphite, graphene, carbon nanotubes, and carbon microspheres. The binder of the positive electrode can be one or more selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyvinyl alcohol, polyolefin, styrene-butadiene rubber, fluorinated rubber, polyurethane, and sodium alginate. In some embodiments, the conductive agent in the positive electrode material layer is SP, and the binder is PVDF. The content ratio of each component in the positive electrode material layer can be conventional; for example, the mass fraction of the positive electrode active material can be 90%-98%, such as 92%, 94%, 96%, 96.7%, or 97%; the mass fraction of the conductive agent can be 1%-5%, such as 1.2%, 1.5%, 2%, 3%, 3.5%, or 4%; and the mass fraction of the binder can be 1%-5%, such as 1.5%, 1.8%, 2%, 2.5%, 3%, or 4%.

[0095] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on the surface of the negative electrode current collector. The negative electrode current collector can be copper foil. The negative electrode material layer includes a negative electrode material, a conductive agent, and a binder. The negative electrode material layer is obtained by coating a negative electrode slurry containing a negative electrode material, a conductive agent, a binder, and a solvent onto the positive electrode current collector, followed by rolling, die-cutting, and baking. The solvent for the negative electrode slurry can be water. The negative electrode active material can be selected from graphite, lithium metal (including structured lithium metal), mesophase carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, and spinel-structured lithiated TiO2-Li4Ti5O. 12The negative electrode may contain one or more of the following: Li-Al alloys. The negative electrode conductive agent may be one or more selected from conductive carbon black (SP), acetylene black, carbon nanotubes, carbon nanowires, carbon microspheres, carbon fibers, and graphene. The negative electrode binder may be one or more selected from polyvinylidene fluoride, polytetrafluoroethylene, acrylonitrile copolymers, polybutylene acrylate, polyacrylonitrile, and styrene-butadiene rubber (SBR). The negative electrode material layer and the negative electrode slurry may also contain a thickener, such as sodium carboxymethyl cellulose (CMC). In some embodiments, the negative electrode material in the negative electrode material layer is graphite, the conductive agent is conductive carbon black, the binder is styrene-butadiene rubber, and the thickener is CMC. The mass ratio of each component in the negative electrode material layer can be conventional. For example, the mass fraction of the negative electrode active material can be 90%–98%, such as 93%, 95%, 96%, 96.7%, or 97%; the mass fraction of the conductive agent can be 0.5%–5%, such as 1%, 1.5%, or 2%; the mass fraction of the binder can be 0.5%–5%, such as 0.8%, 1%, 1.5%, 2%, or 3%; and the mass fraction of the thickener can be 0–5%, such as 1%, 1.5%, 1.8%, 2%, or 3%.

[0096] The positive electrode, negative electrode, and separator are stacked according to design requirements, and then assembled, injected with electrolyte, left to stand, undergo formation aging, and capacity testing to produce a lithium-ion battery. This invention has the following beneficial effects:

[0097] This invention prepares large-particle lithium iron phosphate with a nanoporous structure via a liquid-phase method. Furthermore, the particle size distribution ratio can be optimized to produce particles with a high compaction density (≥2.54 g / cm³). 3 This invention relates to lithium iron phosphate cathode materials. The liquid-phase process of this invention is simple, environmentally friendly and safe. Through simple co-precipitation, it can quickly achieve atomic-level mixing of Li, Fe and P, avoiding the introduction of complex and demanding liquid-phase processes (such as hydrothermal methods which require high temperature and high pressure, and sol-gel methods which require long reaction times).

[0098] This invention allows for the preparation of lithium iron phosphate intermediates with different particle sizes by controlling the liquid-phase co-precipitation reaction time. Generally, the longer the reaction time, the larger the intermediate particle size, resulting in larger cathode material particles prepared through sintering. Figure 1 As shown, mixing lithium iron phosphate particles with different particle sizes will form a certain particle size distribution, resulting in a material with different particle size distributions. Generally speaking, lithium iron phosphate particles with smaller particle sizes will fill the middle of lithium iron phosphate particles with larger particle sizes, forming a higher packing density, thereby forming a higher compaction density.

[0099] The large-particle lithium iron phosphate of this invention has a certain microporous structure, which can improve the electrolyte wettability of the material, shorten the lithium ion transport distance, and enable more lithium ions inside the particles to participate in insertion and extraction more quickly and effectively, thereby improving its electrochemical performance.

[0100] Compared to existing high-pile-density lithium iron phosphate cathode materials, the lithium iron phosphate cathode material of this invention, comprising both small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure, not only exhibits higher compaction density but also higher specific capacity, thus achieving higher energy density. Furthermore, the large-particle lithium iron phosphate cathode material of this invention possesses a richer pore structure, thereby also achieving better rate performance.

[0101] This invention directly introduces a carbon source during the co-precipitation reaction to achieve in-situ coating of lithium iron phosphate, thereby forming a more uniform and stable lithium iron phosphate cathode material.

[0102] The present invention will be described below by way of specific embodiments. It should be understood that these embodiments are merely illustrative and are not intended to limit the scope of the invention. The methods, reagents, and materials used in the embodiments are conventional methods, reagents, and materials in the art, unless otherwise stated. The raw material compounds in the embodiments are all commercially available.

[0103] In this invention, scanning electron microscopy (SEM) and ImageJ were used to statistically analyze the particle size distribution and median particle size D of lithium iron phosphate intermediate particles. 50 .

[0104] In this invention, the peak particle size of the lithium iron phosphate cathode material was measured using a Mastersizer 3000 laser particle size analyzer, and the median pore size D of the micropores in the lithium iron phosphate cathode material was statistically analyzed using SEM and ImageJ. 50 and porosity.

[0105] Example 1

[0106] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium bicarbonate (3% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose (8% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0107] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0108] The two dispersions, A and B, were mixed at a mass ratio of 7.5:2.5 and spray-dried to obtain mixed lithium iron phosphate intermediate particles. These particles were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 240°C at a rate of 2°C / min and held for 4 hours; the second plateau was heated to 690°C at a rate of 5°C / min and held for 10 hours. The particles were then crushed using an air jet mill, with the nozzle specification being [specified missing]. The feed rate was 10mm, the grinding air pressure was 18Hz, the classifying wheel frequency was 40Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibited a bimodal distribution, with peak particle sizes of 2.58μm and 0.82μm, respectively. The micropore diameter D of the porous large-particle lithium iron phosphate was... 50 It has a wavelength of 40 nm and a porosity of 2%.

[0109] The SEM images of the lithium iron phosphate cathode material prepared in Example 1 and the SEM images of the porous large-particle lithium iron phosphate therein are shown below. Figure 2 and Figure 3 As shown.

[0110] Example 2

[0111] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Urea, at 4% of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.3, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Glucose, at 8% of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder, was then added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0112] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0113] The two dispersions, A and B, were mixed at a mass ratio of 7.5:2.5 and spray-dried to obtain mixed lithium iron phosphate intermediate particles. These particles were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 300℃ at a rate of 2℃ / min and held for 4 hours; the second plateau was heated to 690℃ at a rate of 5℃ / min and held for 10 hours. The particles were then crushed using an air jet mill, with the nozzle specification being [specified missing]. The feed rate was 10mm, the grinding air pressure was 18Hz, the classifying wheel frequency was 40Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibited a bimodal distribution, with peak particle sizes of 2.58μm and 0.82μm, respectively. The micropore diameter D of the porous large-particle lithium iron phosphate was... 50 It has a wavelength of 38 nm and a porosity of 1.9%.

[0114] Example 3

[0115] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium chloride, at 5% of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Glucose, at 8% of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder, was then added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0116] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of the lithium iron phosphate intermediate particles B is... 50 300nm;

[0117] The two dispersions, A and B, were mixed at a mass ratio of 7.5:2.5 and spray-dried to obtain mixed lithium iron phosphate intermediate particles. These particles were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 320°C at a rate of 2°C / min and held for 4 hours; the second plateau was heated to 690°C at a rate of 5°C / min and held for 10 hours. The particles were then crushed using an air jet mill, with the nozzle specification being [specified missing]. The feed rate was 10mm, the grinding air pressure was 18Hz, the classifying wheel frequency was 40Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibited a bimodal distribution, with peak particle sizes of 2.58μm and 0.82μm, respectively. The micropore diameter D of the porous large-particle lithium iron phosphate was... 50 It has a wavelength of 36 nm and a porosity of 1.8%.

[0118] Example 4

[0119] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. PVP, at 5% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.3, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Glucose, at 8% of the total mass of phosphorus in the phosphorus, lithium in the lithium carbonate, and iron in the iron powder, was then added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0120] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0121] The two dispersions, A and B, were mixed at a mass ratio of 7.5:2.5 and spray-dried to obtain mixed lithium iron phosphate intermediate particles. These particles were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 360°C at a rate of 2°C / min and held for 4 hours; the second plateau was heated to 690°C at a rate of 5°C / min and held for 10 hours. The particles were then crushed using an air jet mill, with the nozzle specification being [specified missing]. The feed rate was 10mm, the grinding air pressure was 18Hz, the classifying wheel frequency was 40Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibited a bimodal distribution, with peak particle sizes of 2.58μm and 0.82μm, respectively. The micropore diameter D of the porous large-particle lithium iron phosphate was... 50 It has a wavelength of 39 nm and a porosity of 1.9%.

[0122] Example 5

[0123] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium bicarbonate (3% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose (8% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0124] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0125] The two dispersions, A and B, were mixed at a mass ratio of 8:2 to obtain mixed lithium iron phosphate intermediate particles. The mixture was then spray-dried to obtain mixed lithium iron phosphate intermediate particles, which were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 240℃ at a rate of 2℃ / min and held for 4 hours; the second plateau was heated to 700℃ at a rate of 5℃ / min and held for 10 hours. The particles were then crushed using an air jet mill with a nozzle specification of 10. The feed rate was 18 Hz, the grinding air pressure was 0.32 MPa, the classifying wheel frequency was 40 Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle lithium iron phosphate and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibited a bimodal distribution, with peak particle sizes of 11.98 μm and 1.98 μm, respectively. The micropore diameter D of the porous large-particle lithium iron phosphate was... 50 The wavelength is 30 nm, and the porosity is 1.6%.

[0126] Example 6

[0127] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium bicarbonate, dissolved in water at 3% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose, at 8% of the total mass of phosphorus in the phosphorus phosphate, lithium in the lithium carbonate, and iron in the iron powder, was added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0128] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0129] The two dispersions, A and B, were mixed at a mass ratio of 7:3 to obtain mixed lithium iron phosphate intermediate particles. The mixture was then spray-dried to obtain mixed lithium iron phosphate intermediate particles, which were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first platform was heated to 240℃ at a rate of 2℃ / min and held for 4 hours; the second platform was heated to 685℃ at a rate of 5℃ / min and held for 10 hours. The particles were then crushed using an air jet mill with a nozzle specification of 1. The feed rate was 0 mm, the grinding air pressure was 18 Hz, the classifying wheel frequency was 40 Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle lithium iron phosphate and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibits a bimodal distribution, with peak particle sizes of 2.93 μm and 0.72 μm, respectively. The pore size D of the porous large-particle lithium iron phosphate is... 50 It has a wavelength of 40 nm and a porosity of 2%.

[0130] Example 7

[0131] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium bicarbonate (3% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose (8% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0132] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0133] The two dispersions A and B were mixed at a mass ratio of 6.5:3.5, and spray-dried to obtain mixed lithium iron phosphate intermediate particles. These particles were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 240℃ at a rate of 2℃ / min and held for 4 hours; the second plateau was heated to 680℃ at a rate of 5℃ / min and held for 10 hours. The particles were then crushed using an air jet mill. The feed rate is 18Hz, the grinding air pressure is 0.32MPa, the classifying wheel frequency is 40Hz, and then ultrasonic sieving is performed with a 150-mesh sieve. Electromagnetic iron removal is then performed with a magnet strength of 20000 Gauss. Finally, the product is packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibits a bimodal distribution, with peak particle sizes of 2.58μm and 0.72μm, respectively. The pore size D of the porous large-particle lithium iron phosphate is... 50 It has a wavelength of 45 nm and a porosity of 2%.

[0134] Example 8

[0135] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium bicarbonate (3% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose (8% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added to the reaction solution, and the reaction was carried out for 12 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 800nm;

[0136] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0137] The two dispersions, A and B, were mixed at a mass ratio of 6:4 to obtain mixed lithium iron phosphate intermediate particles. The mixture was then spray-dried to obtain mixed lithium iron phosphate intermediate particles, which were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 240℃ at a rate of 2℃ / min and held for 4 hours; the second plateau was heated to 675℃ at a rate of 5℃ / min and held for 10 hours. The particles were then crushed using an air jet mill with a nozzle specification of 1. The feed rate was 0 mm, the grinding air pressure was 18 Hz, the classifying wheel frequency was 40 Hz, and then ultrasonic sieving was performed with a 150-mesh sieve. Electromagnetic iron removal was then performed with a magnet strength of 20,000 Gauss. Finally, the product was packaged to obtain a lithium iron phosphate cathode material containing both porous large-particle lithium iron phosphate and small-particle lithium iron phosphate. The particle size of this lithium iron phosphate cathode material exhibits a bimodal distribution, with peak particle sizes of 8.14 μm and 0.72 μm, respectively. The pore size D of the porous large-particle lithium iron phosphate is... 50 It has a wavelength of 50 nm and a porosity of 2%.

[0138] Example 9

[0139] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a phosphorus, lithium, and iron molar ratio of 1:1:0.96. Then, ammonium bicarbonate (3% by mass of the total phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) and TiO2 (0.5% by mass) were added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose (8% by mass of the total phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added to the reaction solution, and the reaction was carried out for another 8 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 600nm;

[0140] The above dispersion was spray-dried to obtain lithium iron phosphate intermediate particles A, which were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first platform was heated to 240℃ at a rate of 2℃ / min and held for 4 hours; the second platform was heated to 680℃ at a rate of 5℃ / min and held for 12 hours. The particles were then crushed using an air jet mill with a nozzle size of 10mm, a feeding frequency of 18Hz, a grinding air pressure of 0.32MPa, and a classifying wheel frequency of 40Hz. Ultrasonic sieving with a 150-mesh sieve followed by electromagnetic iron removal with a magnet strength of 20,000 Gauss was then performed. Finally, the particles were packaged to obtain porous, large-particle lithium iron phosphate cathode material. The particle size of this lithium iron phosphate cathode material exhibits a single-peak distribution with a peak particle size of 2μm and a micropore size D. 50 It has a wavelength of 40 nm and a porosity of 2%.

[0141] Example 10

[0142] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Ammonium bicarbonate (3% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃. The reaction was carried out for 6 hours. Then, glucose (8% by mass of the total mass of phosphorus in phosphoric acid, lithium in lithium carbonate, and iron in iron powder) was added to the reaction solution, and the reaction was carried out for another 8 hours. This yielded a cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. The particle size D of lithium iron phosphate intermediate particles A was... 50 It is 600nm;

[0143] The above dispersion was spray-dried to obtain lithium iron phosphate intermediate particles A, which were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first platform was heated to 240℃ at a rate of 2℃ / min and held for 4 hours; the second platform was heated to 680℃ at a rate of 5℃ / min and held for 12 hours. The particles were then crushed using an air jet mill with a nozzle size of 10mm, a feeding frequency of 18Hz, a grinding air pressure of 0.32MPa, and a classifying wheel frequency of 40Hz. Ultrasonic sieving with a 150-mesh sieve followed by electromagnetic iron removal with a magnet strength of 20,000 Gauss was then performed. Finally, the particles were packaged to obtain porous, large-particle lithium iron phosphate cathode material. The particle size of this lithium iron phosphate cathode material exhibits a single-peak distribution with a peak particle size of 2μm and a micropore size D. 50 It has a wavelength of 40 nm and a porosity of 2%.

[0144] Comparative Example 1

[0145] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 8% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃ for 18 hours to obtain a cathode material intermediate dispersion A' containing lithium iron phosphate intermediate particles A'. The particle size D of the lithium iron phosphate intermediate particles A' was... 50 It is 800nm;

[0146] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 12% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.6, and the temperature was controlled at 120℃ for 12 hours to obtain a cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B. The particle size D of lithium iron phosphate intermediate particles B is... 50 300nm;

[0147] The two dispersions, A' and B, were mixed at a mass ratio of 7.5:2.5 and spray-dried to obtain mixed lithium iron phosphate intermediate particles. These particles were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first plateau was heated to 240°C at a rate of 2°C / min and held for 4 hours; the second plateau was heated to 690°C at a rate of 5°C / min and held for 10 hours; then, an airflow was used... The material is ground and crushed using a 10mm nozzle, a feeding frequency of 18Hz, a grinding air pressure of 0.32MPa, and a classifying wheel frequency of 40Hz. It is then ultrasonically sieved using a 150-mesh screen, followed by electromagnetic iron removal using a magnet with a strength of 20,000 Gauss. Finally, it is packaged to obtain a lithium iron phosphate cathode material containing both large and small lithium iron phosphate particles. The particle size of this lithium iron phosphate cathode material exhibits a bimodal distribution, with peak particle sizes of 2.58μm and 0.82μm, respectively.

[0148] Comparative Example 2

[0149] A phosphoric acid aqueous solution with a concentration of 8.9 wt%, lithium carbonate, and iron powder were mixed evenly according to a molar ratio of phosphorus, lithium, and iron of 1:1:0.96. Glucose, at a mass of 8% of the total mass of phosphorus in the phosphoric acid, lithium in the lithium carbonate, and iron in the iron powder, was added and mixed evenly. Nitric acid was added to adjust the pH to 3.2, and the temperature was controlled at 120℃ for 14 hours to obtain a cathode material intermediate dispersion A' containing lithium iron phosphate intermediate particles A'. The particle size D of the lithium iron phosphate intermediate particles A' was... 50 It is 600nm;

[0150] The above dispersion was spray-dried to obtain lithium iron phosphate intermediate particles A', which were then packed into a roller kiln and sintered under a nitrogen atmosphere. The gradient sintering conditions were as follows: the first platform was heated to 240°C at a rate of 2°C / min and held for 4 hours; the second platform was heated to 680°C at a rate of 5°C / min and held for 12 hours. The particles were then crushed using an air jet mill with a nozzle size of 10 mm, a feeding frequency of 18 Hz, a grinding air pressure of 0.32 MPa, and a classifying wheel frequency of 40 Hz. The particles were then ultrasonically sieved with a 150-mesh screen and subjected to electromagnetic iron removal with a magnet strength of 20,000 Gauss. Finally, the particles were packaged to obtain large-particle lithium iron phosphate cathode material. The particle size of this lithium iron phosphate cathode material exhibited a single-peak distribution with a peak particle size of 2 μm.

[0151] The specific surface area (BET), compaction density, and particle size distribution of the lithium iron phosphate cathode materials in Examples 1-8 and Comparative Examples 1-2 were tested using the following methods. Batteries were then fabricated, and their specific capacity and discharge capacity were tested. The results are shown in Table 2. Figure 4 and Figure 5 As shown:

[0152] 1. BET testing method:

[0153] 1) Take a clean quartz tube, cool it, weigh the empty tube mass m0, weigh about 0.4-0.6g of sample into the quartz tube, and seal it with a rubber stopper;

[0154] 2) Start the degassing station, open the main nitrogen valve and pressure reducing valve of the degassing station, maintain the pressure at 1.0-1.2 MPa, and set the temperature of the degassing station to 200 degrees Celsius;

[0155] 3) Insert the vent tube into the quartz tube containing the sample, being careful not to touch the sample. Open the vent valve and plug it with a rubber stopper.

[0156] 4) Place the quartz tube in the drying oven, cover it with the heat insulation jacket, keep it at a constant temperature for 2 hours, then take it out and place it on the cooling rack to cool for 20 minutes to room temperature. Weigh the sample mass and tube mass m1 again, and calculate the actual sample mass m2 = m1 - m0.

[0157] 5) Turn on the vacuum pump and instruments, and preheat for 30 minutes;

[0158] 6) Install the pre-treated quartz tube containing the sample, tighten the screws (to ensure no air leakage), and enter the sample mass m2 in the software window;

[0159] 7) Fill the Dewar flask with liquid nitrogen (handle with care to prevent frostbite). The liquid nitrogen level should be below the small hole of the liquid nitrogen level measuring device. After the liquid boils slowly, place it on the lifting platform of the instrument's Dewar flask, click to run the program to start the test, and calculate the test results.

[0160] 2. Compacted density test method: Weigh approximately 1g of sample and place it into a pre-cleaned mold. Then, gradually apply pressure to the powder up to 200MPa according to a specific procedure. The density is calculated using the formula D = m 粉末 / S 模具 / H 粉末 Calculate the compacted density of the sample, where m 粉末 For sample quality, S 模具 H is the cross-sectional area of ​​the mold. 粉末 The height of the sample inside the mold after pressure is applied.

[0161] 3. Particle size distribution test method: Using a laser particle size analyzer, weigh about 0.2g of positive electrode material powder sample and add it to 100mL of water. At the same time, sonicate it for 10min in an external ultrasonic device at a power of 120W. Then, inject the sample and sonicate it internally (about 20W) continuously, keeping the light-blocking degree at 5-15% for testing. The dispersant is water with a refractive index of 1.333.

[0162] 4. Battery manufacturing method:

[0163] 1) Lithium iron phosphate cathode material, conductive carbon black, and PVDF were dry-mixed at a mass ratio of 96:2:2, and then N-methylpyrrolidone (NMP) was added for kneading, high-speed dispersion, and viscosity adjustment to prepare cathode slurry;

[0164] 2) Graphite, conductive carbon black, and sodium carboxymethyl cellulose (CMC) dry powder were mixed in a mass ratio of 96.7:1:1.5. Then H2O was added for kneading, high-speed dispersion, and viscosity adjustment. Finally, styrene-butadiene rubber (SBR) accounting for 0.8% of the total mass of the solid components of the negative electrode slurry was added for dispersion to prepare the negative electrode slurry.

[0165] 3) Aluminum foil is used as the positive current collector and copper foil is used as the negative current collector. The positive and negative electrode slurries are coated onto the positive and negative current collectors respectively. After rolling, die cutting and baking, positive and negative electrode sheets are obtained. The positive and negative electrode sheets are stacked, injected with electrolyte, left to stand, formed and aged and tested for capacity to make a 5Ah small soft pack. The electrolyte formula is as follows: the solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a mass ratio of 2:5:3. The additives are vinylene carbonate (VC) at 2.5% of the total mass of electrolyte and vinyl sulfate (DTD) at 1%. The lithium salt is LiPF6 and the lithium salt concentration in the electrolyte is 1.1 mol / L.

[0166] 5. 0.33C capacity test method: Place the battery in a constant temperature room at 25℃±2℃ and test according to the procedure and parameters shown in Table 1.

[0167] Table 1: Capacity Testing Procedure and Parameters

[0168]

[0169]

[0170] 6. 3C Discharge Capacity / 0.33C Discharge Capacity Test Method: Test the discharge capacity of step 6 and step 10 according to the process and parameters described in Table 1. 3C discharge capacity / 0.33C discharge capacity = discharge capacity of step 10 / discharge capacity of step 6. A higher 3C discharge capacity / 0.33C discharge capacity indicates better rate capability.

[0171] Table 2: BET, compaction density, specific capacity, and discharge capacity of lithium iron phosphate cathode materials in Examples 1-10 and Comparative Examples 1-2

[0172]

[0173]

[0174] As can be seen from Examples 1, 2, 3, and 4, different pore-forming agents can affect the micropore size and porosity of large-particle lithium iron phosphate, and thus affect the improvement of the specific capacity and rate performance of the cathode material.

[0175] As can be seen from Examples 1, 5, 6, 7, and 8, the ratio of lithium iron phosphate intermediate particles of different sizes will affect the improvement of the compaction density of the cathode material, and thus affect the improvement of the specific capacity and rate performance of the cathode material.

[0176] As can be seen from Example 1 (large lithium iron phosphate particles have a nanoporous structure) and Comparative Example 1 (large lithium iron phosphate particles do not have a nanoporous structure), the nanoporous structure plays an important role in improving the capacity and rate performance of the cathode material.

[0177] As can be seen from Comparative Example 1 (the particle size of the lithium iron phosphate cathode material is bimodal) and Comparative Example 2 (the particle size of the lithium iron phosphate cathode material is bimodal), compared with the single-peaked particle size distribution, the bimodal particle size distribution plays an important role in improving the compaction density of the cathode material.

[0178] As can be seen from Example 9 (large-particle lithium iron phosphate containing doped elements) and Example 10 (large-particle lithium iron phosphate without doped elements), appropriate element doping has an improving effect on rate performance.

[0179] As can be seen from Example 10 (large lithium iron phosphate particles with nanoporous structure) and Comparative Example 2 (large lithium iron phosphate particles without nanoporous structure), large lithium iron phosphate particles with nanoporous structure have better capacity and rate performance.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The preparation method includes the following steps: (1) Preparation of cathode material intermediate dispersion A: Disperse phosphorus source, lithium source, iron source and pore-forming agent in water, adjust pH to 2-5, react at 100-180℃ for 6-10h, add soluble carbon source, and continue to react for 5-24h to obtain the cathode material intermediate dispersion A containing lithium iron phosphate intermediate particles A. (2) Preparation of cathode material intermediate dispersion B: Phosphorus source, lithium source, iron source and soluble carbon source are dispersed in water, pH is adjusted to 3-6, and reacted at 120-200℃ for 5-14h to obtain the cathode material intermediate dispersion B containing lithium iron phosphate intermediate particles B, wherein the particle size D of the lithium iron phosphate intermediate particles B is... 50 The particle size D is smaller than that of the lithium iron phosphate intermediate particle A. 50 ; (3) Mix the cathode material intermediate dispersion A and the cathode material intermediate dispersion B, and spray dry them to obtain lithium iron phosphate intermediate mixed particles; (4) The lithium iron phosphate intermediate mixed particles are sintered in an inert gas to obtain the lithium iron phosphate cathode material; The sintering conditions in step (4) are two-gradient plateau sintering: the first plateau is heated to 200-500℃ at a rate of 1-5℃ / min; the second plateau is heated to 600-800℃ at a rate of 5-10℃ / min. The pH of the reaction system in step (2) is higher than the pH of the reaction system in step (1); The reaction time in step (2) is less than the total reaction time in step (1).

2. The preparation method according to claim 1, characterized in that, The preparation method has one or more of the following characteristics: In step (1), adjust the pH to 3-4; In step (1), the reaction temperature is 120-140℃; In step (1), the reaction time before adding the soluble carbon source is 6-7 hours; In step (1), the reaction time after adding the soluble carbon source is 12±2h.

3. The preparation method according to claim 2, characterized in that, In step (1), adjust the pH to 3.1-3.

5.

4. The preparation method according to claim 2, characterized in that, In step (1), the reaction temperature is 120-130℃.

5. The preparation method according to claim 1, characterized in that, The preparation method has one or more of the following characteristics: In step (2), adjust the pH to 3.5-4.5; In step (2), the reaction temperature is 120-140℃; In step (2), the reaction time is 12±2h.

6. The preparation method according to claim 5, characterized in that, In step (2), adjust the pH to 3.6 ± 0.

1.

7. The preparation method according to claim 5, characterized in that, In step (2), the reaction temperature is 120-130℃.

8. The preparation method according to claim 1, characterized in that, The preparation method has one or more of the following characteristics: The lithium source in step (1) and the lithium source in step (2) are each independently selected from one or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium dihydrogen phosphate and lithium hydrogen phosphate; The phosphorus source in step (1) and the phosphorus source in step (2) are each independently selected from one or more of phosphoric acid, lithium dihydrogen phosphate and lithium hydrogen phosphate; The iron source in step (1) and the iron source in step (2) are each independently selected from one or more of magnetite, iron oxide and iron powder; The pore-forming agent in step (1) is selected from one or more of ammonium bicarbonate, urea, ammonium chloride, PVP, PEG and PVA; The soluble carbon source in step (1) and the soluble carbon source in step (2) are each independently selected from one or more of sucrose, glucose, fructose, starch, amino acids, and citric acid; The molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source in step (1) is (1-1.1):1:(0.9-1); The molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source in step (2) is (1-1.1):1:(0.9-1); In step (1), the mass of the pore-forming agent is 1%-10% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source; In step (1), the mass of the soluble carbon source is 5%-15% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source; In step (2), the mass of the soluble carbon source is 5%-15% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source; The particle size D of the lithium iron phosphate intermediate particle A 50 The range is 500nm-3000nm; The particle size D of the lithium iron phosphate intermediate particles B 50 The range is 100nm-800nm; In step (3), the cathode material intermediate dispersion A and the cathode material intermediate dispersion B are mixed according to a mass ratio of lithium iron phosphate intermediate particles A to lithium iron phosphate intermediate particles B of 5:5 to 9:

1. The inert gas is selected from one or more of nitrogen, helium, and argon; The sintering conditions in step (4) are two-gradient platform sintering: the first platform is heated to 200-500℃ at a rate of 1-5℃ / min and held for 3-6h; the second platform is heated to 600-800℃ at a rate of 5-10℃ / min and held for 8-12h.

9. The preparation method according to claim 8, characterized in that, The lithium source in step (1) and the lithium source in step (2) are lithium carbonate.

10. The preparation method according to claim 8, characterized in that, The phosphorus source in step (1) and the phosphorus source in step (2) are phosphoric acid.

11. The preparation method according to claim 8, characterized in that, The iron source in step (1) and the iron source in step (2) are iron powder.

12. The preparation method according to claim 8, characterized in that, The pore-forming agent in step (1) is selected from one or more of ammonium bicarbonate, urea and ammonium chloride.

13. The preparation method according to claim 8, characterized in that, The soluble carbon source in step (1) and the soluble carbon source in step (2) are each independently selected from one or two of sucrose and glucose.

14. The preparation method according to claim 8, characterized in that, In step (1), the mass of the soluble carbon source is 7%-9% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source.

15. The preparation method according to claim 8, characterized in that, In step (2), the mass of the soluble carbon source is 10%-14% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source.

16. The preparation method according to claim 8, characterized in that, In step (3), the cathode material intermediate dispersion A and the cathode material intermediate dispersion B are mixed according to a mass ratio of lithium iron phosphate intermediate particles A to lithium iron phosphate intermediate particles B of 6:4 to 8:

2.

17. The preparation method according to claim 8, characterized in that, The inert gas is nitrogen.

18. The preparation method according to claim 1, characterized in that, In step (1) and / or step (2), the dopant is dispersed in water together with a phosphorus source, a lithium source and an iron source.

19. The preparation method according to claim 18, characterized in that, The dopant is selected from one or more of titanium oxide, vanadium oxide, tungsten oxide, magnesium oxide, zirconium oxide, and aluminum oxide.

20. The preparation method according to claim 18, characterized in that, The mass of the dopant is 0.1%-1% of the total mass of lithium in the lithium source, phosphorus in the phosphorus source, and iron in the iron source.

21. The preparation method according to claim 1, characterized in that, In step (4), after sintering, the metal is crushed, sieved and iron is removed.

22. The preparation method according to claim 21, characterized in that, In step (4), an air jet mill is used for crushing, with a nozzle specification of 10±1mm, a feeding frequency of 18±5Hz, a grinding air pressure of 0.32±0.05MPa, a grading wheel frequency of 40±5Hz, ultrasonic sieving with a screen mesh of 150±10 mesh, and electromagnetic iron removal with a magnet strength of 20000±2000 Gauss.

23. The lithium iron phosphate cathode material prepared by any one of claims 1-22.

24. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material comprises small-particle lithium iron phosphate and large-particle lithium iron phosphate with a nanoporous structure; the peak particle size of the large-particle lithium iron phosphate with a nanoporous structure is 1μm-100μm, and the micropore diameter D is... 50 The particle size is 10nm-100nm, and the porosity is 0.1%-10%; the peak particle size of the small lithium iron phosphate particles is 200nm-2000nm; the lithium iron phosphate cathode material is prepared by the preparation method according to any one of claims 1-22.

25. The lithium iron phosphate cathode material as described in claim 24, characterized in that, The peak particle size of the large lithium iron phosphate particles with nanoporous structure is greater than that of the small lithium iron phosphate particles.

26. The lithium iron phosphate cathode material as described in claim 24, characterized in that, In the lithium iron phosphate cathode material, the mass of the large-particle lithium iron phosphate with a nanoporous structure accounts for 50%-100% of the total mass of the lithium iron phosphate cathode material.

27. The lithium iron phosphate cathode material as described in claim 26, characterized in that, In the lithium iron phosphate cathode material, the mass of the large-particle lithium iron phosphate with a nanoporous structure accounts for 50%-90% of the total mass of the lithium iron phosphate cathode material.

28. The lithium iron phosphate cathode material as described in claim 26, characterized in that, In the lithium iron phosphate cathode material, the mass of the large-particle lithium iron phosphate with a nanoporous structure accounts for 60%-80% of the total mass of the lithium iron phosphate cathode material.

29. A positive electrode plate, characterized in that, The positive electrode comprises the lithium iron phosphate positive electrode material according to any one of claims 23-28.

30. An electrochemical device, characterized in that, The electrochemical device comprises the positive electrode as described in claim 29.

31. The electrochemical device as described in claim 30, characterized in that, The electrochemical device is a lithium-ion battery.

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