Lithium iron phosphate, preparation method thereof and positive electrode sheet

Abstract

The application relates to the technical field of battery materials, in particular to a lithium iron phosphate, a preparation method and a positive pole piece, and the preparation method comprises the following steps: mixing an iron source, an organic phosphorus source, a dopant and an organic solvent, and performing grinding and drying treatment to obtain dry materials; performing primary calcination treatment on the dry materials to obtain carbon-coated doped iron phosphate; mixing the carbon-coated doped iron phosphate with a lithium source to obtain mixed materials, and then performing secondary calcination treatment on the mixed materials to obtain lithium iron phosphate. In the application, the iron source and the organic phosphorus source are used for primary calcination, the organic phosphorus source is carbonized and decomposed at high temperature, in-situ carbon coating and iron phosphate generation are performed, and thus the carbon-coated iron phosphate is obtained. During secondary calcination, the carbon coating layer exists, the particles cannot be fused and grown again, the graphitization degree of the carbon layer is high, the particles are more compact, the crystallinity of the product is higher, and thus the product has higher compaction density and capacity.

Description

Technical Field

[0001] This invention relates to the technical field of battery materials, specifically to a lithium iron phosphate, its preparation method, and a positive electrode sheet. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, has been widely used in power batteries and energy storage due to its advantages such as high safety, long cycle life, environmental friendliness, and abundant raw material sources. Currently, the conventional industrial process for preparing lithium iron phosphate typically involves mixing lithium, iron, phosphorus, and carbon sources, adding water or an organic solvent for stirring and dispersion to form a slurry, followed by spray drying and granulation. Finally, it undergoes high-temperature calcination under an inert atmosphere to complete the crystallization and carbon coating process.

[0003] However, this traditional method has significant technical drawbacks: during the mixing stage, the lithium source, iron source, and phosphorus source are dispersed together in the slurry and formed precursor particles after spray drying. In the subsequent high-temperature calcination process, the lithium source may melt or partially melt at relatively low temperatures, leading to easy adhesion and agglomeration between particles. Simultaneously, at high temperatures, the lithium source can penetrate into the already formed iron phosphate or other intermediate phases, further promoting particle growth and sintering, forming coarse secondary particles or hard agglomerates.

[0004] The increased particle size and inhomogeneity severely impact the electrochemical performance of lithium iron phosphate materials. On one hand, large particles prolong the diffusion path of lithium ions in the solid phase, reducing the ionic conductivity and increasing polarization. On the other hand, tight adhesion between particles reduces the effective contact area between the electrode material and the electrolyte, hindering lithium ion migration and thus leading to a decrease in the material's rate performance. Furthermore, the presence of hard agglomerates also affects the material's tap density and processing properties, ultimately limiting its application in high-end lithium-ion batteries.

[0005] Therefore, developing a method for preparing lithium iron phosphate that can effectively suppress particle adhesion, control particle size, and improve material capacity and compaction density has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] In view of the technical problems existing in the background art, this application provides a lithium iron phosphate, its preparation method and positive electrode sheet, aiming to solve the technical problem that the process of generating lithium iron phosphate produces large particles, thereby affecting the rate performance of the material.

[0007] In a first aspect, embodiments of this application provide a method for preparing lithium iron phosphate, comprising the following steps: Iron source, organophosphorus source, dopant and organic solvent are mixed and then ground and dried to obtain dried material; The dried material was subjected to a single calcination treatment to obtain carbon-coated doped iron phosphate; The carbon-coated doped iron phosphate is mixed with a lithium source to obtain a mixture, which is then subjected to a second calcination treatment to obtain lithium iron phosphate.

[0008] In the technical solution of this application embodiment, an iron source and an organophosphorus source are first reacted under high-temperature calcination. The organophosphorus source carbonizes and decomposes at high temperature, resulting in in-situ carbon coating and the formation of iron phosphate, thereby obtaining carbon-coated iron phosphate. Because the precursor has already formed a coating layer, the secondary calcination will not cause further fusion and growth between particles. This is beneficial for the uniform doping of dopants, resulting in a high degree of graphitization of the carbon layer, denser particles, and higher crystallinity of the product, thus leading to higher compaction density and volume of the product.

[0009] In some embodiments, the iron source is one or more of iron oxide red, iron oxide yellow, and iron(III) oxide; and / or The organophosphorus source is one or more of the following: trialkyl phosphate, tributyl phosphate, triethyl phosphate, trioctyl phosphate, triphenyl phosphate, xylene phosphate, and xylene diphenyl phosphate; and / or The dopant is one or more of magnesium salts, cobalt salts, nickel salts, and niobium salts.

[0010] In this embodiment, the selected iron source is mainly iron oxide, which can volatilize as water during calcination. The phosphorus source is mainly organic phosphorus, and its carbon chains partially volatilize as water vapor and carbon oxides at high temperatures, while some carbon remains, forming a carbon coating. The dopant is a metal salt, which can form metal ion doping, thereby effectively improving the electrical properties of the material.

[0011] In some embodiments, the iron-to-phosphorus ratio of the iron in the iron source to the phosphorus in the organic phosphorus source is 3:(3.05-3.10); and / or The mass of the dopant accounts for 0.5%-1.0% of the mass of the iron source.

[0012] In this embodiment, by controlling the iron-phosphorus ratio of the iron element in the iron source to the phosphorus element in the organic phosphorus source to be 3:(3.05-3.10), the slight excess of phosphorus element can effectively increase the particle size of carbon-coated iron phosphate, thereby ensuring the compaction density of the material.

[0013] By controlling the dopant content to be 0.5%-1.0% of the iron source mass, the electrical properties of the material can be effectively improved. Excessive dopant content can negatively impact the crystal structure of iron phosphate, leading to a decrease in capacity and cycle performance; conversely, insufficient dopant content will result in no doping effect.

[0014] In some embodiments, the step of mixing the iron source, organophosphorus source, dopant, and organic solvent, followed by grinding and drying to obtain a dried material, includes: The iron source, organophosphorus source, dopant and organic solvent are mixed to obtain a primary slurry; The primary slurry is then ground to obtain a secondary slurry; The secondary slurry is dried to obtain the dried material; The secondary slurry has a D50 particle size of 200-400 nm; the drying process is a spray drying process, which uses nitrogen gas at 150-350℃ as a heat source, and the dried material obtained has a particle size of 5-15 μm.

[0015] In this embodiment, the raw materials are uniformly mixed and nano-sized through grinding, resulting in a secondary slurry with a D50 particle size of 200-400 nm. This reduces the particle size of ferric phosphate, improving rate performance. Controlling the particle size of the dried material to 5-15 μm prevents agglomeration of sintered materials and effectively shortens the high-temperature diffusion distance, which is beneficial for the formation of ferric phosphate products.

[0016] In some embodiments, the step of subjecting the dried material to a single calcination treatment to obtain carbon-coated doped iron phosphate includes: Under an inert gas atmosphere, the dried material is subjected to the first calcination treatment to obtain a first calcined material. Subsequently, the first calcined material is pulverized to obtain carbon-coated doped iron phosphate with a particle size of 0.5-1.2 μm. The temperature of the first calcination treatment is 300-450℃, and the time of the first calcination treatment is 3-6h; The spray drying temperature is 150-350℃, and the particle size of the dried material is 5-15μm.

[0017] In this embodiment, the calcination temperature is 300-450℃, which is relatively low. This allows for control of the precursor particle size, resulting in smaller particle size of the prepared carbon-coated iron phosphate, thus achieving smaller lithium iron phosphate particles and improving rate performance.

[0018] In some embodiments, the lithium source is one or more selected from lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate; and / or The lithium ratio of the lithium element in the lithium source to the iron element in the carbon-coated doped iron phosphate is 1:(1.03-1.10).

[0019] In this embodiment, by selecting the above lithium source, the elements in the anionic portion will volatilize after calcination, preventing them from being doped into the product. By controlling the iron-lithium molar ratio to 1:(1.03-1.10), a slight excess of lithium is achieved, increasing the capacity of the lithium iron phosphate product. If too much lithium is present, it becomes an impurity and cannot enter the lithium iron phosphate lattice, resulting in reduced electrical performance.

[0020] In some embodiments, the mixture further includes a supplementary dopant, wherein the supplementary dopant is at least one of a supplementary phosphorus source, a supplementary boron source, or a supplementary silicon source; and / or The supplementary phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, iron phosphate, and lithium dihydrogen phosphate. The supplementary boron source includes at least one of boric acid, boron trioxide, and methyl borate. The supplementary silicon source includes at least one of silicic acid, silane, silane, and silicon tetrafluoride.

[0021] In this embodiment, by supplementing the dopant with anions containing phosphorus, silicon, or boron, dual doping with the cations in the dopant is achieved, which can further improve the conductivity of lithium iron phosphate.

[0022] In some embodiments, the temperature of the secondary calcination treatment is 750-850°C, and the time of the secondary calcination treatment is 8-16 hours.

[0023] In this embodiment, the secondary calcination temperature is 750-850℃. The higher calcination temperature is conducive to the uniform doping of dopants, resulting in a higher degree of graphitization of the carbon layer, denser particles, and higher crystallinity of the product, thus leading to higher compaction density and capacity of the product.

[0024] Secondly, embodiments of this application provide a lithium iron phosphate battery prepared using the aforementioned lithium iron phosphate preparation method; a coin cell battery is fabricated using the lithium iron phosphate battery, wherein the coin cell battery has a maximum initial charge capacity of 162.93 mAh / g at 0.1C, a maximum initial discharge capacity of 159.53 mAh / g, a maximum initial charge capacity of 159.07 mAh / g at 0.5C, and a maximum initial discharge capacity of 144.17 mAh / g at 1C.

[0025] In this embodiment, the lithium iron phosphate material prepared in this application exhibits excellent electrochemical performance. The button battery assembled with it has an initial charge-discharge capacity of 162.93 mAh / g and 159.53 mAh / g at a 0.1C rate, respectively, which is close to the theoretical capacity, demonstrating extremely high utilization rate and reversibility of active materials. At higher rates of 0.5C and 1C, the capacity still remains at an excellent level of 159.07 mAh / g and 144.17 mAh / g, respectively, proving that the lithium iron phosphate prepared in this application has both excellent rate performance and structural stability, and can significantly improve the energy density and power output capability of the battery.

[0026] Thirdly, embodiments of this application provide a positive electrode sheet, comprising lithium iron phosphate prepared by the aforementioned preparation method.

[0027] In this embodiment, the positive electrode sheet of this application has excellent compaction density, conductivity and structural stability, which further enhances the cycle life, rate performance and safety of the battery.

[0028] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, specific embodiments of this application are given below. Attached Figure Description

[0029] To more clearly illustrate the technical solution of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0030] Figure 1 This is a SEM image of the carbon-coated doped iron phosphate prepared in Example 1 of this application; Figure 2 The XRD pattern of carbon-coated doped iron phosphate prepared in Example 1 of this application; Figure 3 Here is a SEM image of the lithium iron phosphate prepared in Example 1 of this application; Figure 4 The 0.1C and 1C charge-discharge curves of lithium iron phosphate prepared in Example 1 of this application are shown. Figure 5 This is a SEM image of the lithium iron phosphate prepared in Example 2 of this application; Figure 6 The 0.1C and 1C charge-discharge curves of lithium iron phosphate prepared in Example 1 of this application are shown. Figure 7 This is a SEM image of lithium iron phosphate prepared in Comparative Example 1 of this application. Detailed Implementation

[0031] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0033] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0034] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0035] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0036] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0037] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0038] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0039] In the existing technology, the conventional process for producing lithium iron phosphate involves mixing lithium, phosphorus, iron, and carbon sources, adding water / solvent, stirring and dispersing, then spray drying and high-temperature calcination. When lithium, iron, and phosphorus sources are mixed together and then calcined at high temperature, they tend to stick together. This causes large particles to be generated during the lithium iron phosphate production process due to the penetration and melting of the lithium source, thus affecting the rate performance of the material.

[0040] To address the technical problem of large particles being generated during the production of lithium iron phosphate in existing technologies, which affects the rate performance of the material, this invention provides lithium iron phosphate, its preparation method, and a positive electrode sheet, which can effectively suppress particle adhesion of lithium iron phosphate, control particle size, and improve material capacity and compaction density.

[0041] In a first aspect, embodiments of this application provide a method for preparing lithium iron phosphate, comprising the following steps: Iron source, organophosphorus source, dopant and organic solvent are mixed and then ground and dried to obtain dried material; The dried material was subjected to a single calcination treatment to obtain carbon-coated doped iron phosphate; The carbon-coated doped iron phosphate is mixed with a lithium source to obtain a mixture, which is then subjected to a second calcination treatment to obtain lithium iron phosphate.

[0042] In the technical solution of this application embodiment, by using an organic phosphorus source and an organic solvent to achieve molecular-level uniform mixing of the iron source, phosphorus source and dopant, carbon-coated doped iron phosphate is first formed through grinding, drying and primary calcination, and then mixed with a lithium source for secondary calcination. This effectively avoids the problems of uneven mixing of lithium, iron and phosphorus and the formation of impurity phases in traditional processes. At the same time, it solves the problem of primary particle inhomogeneity caused by mutual adhesion between lithium iron phosphate particles during the calcination process in conventional processes. This significantly improves the crystal purity and primary particle size uniformity of the product. Meanwhile, the synergistic effect of in-situ carbon coating and dopant enhances electronic conductivity and ion diffusion ability. The final lithium iron phosphate has excellent rate performance and cycle stability.

[0043] Furthermore, in some embodiments, the iron source is one or more of iron oxide red, iron oxide yellow, and iron(III) oxide; and / or The organophosphorus source is one or more of the following: trialkyl phosphate, tributyl phosphate, triethyl phosphate, trioctyl phosphate, triphenyl phosphate, xylene phosphate, and xylene diphenyl phosphate; and / or The dopant is one or more of magnesium salts, cobalt salts, nickel salts, and niobium salts.

[0044] In the technical solution of this application embodiment, by using the above-mentioned iron source, organophosphorus source and dopant, a low-cost, highly dispersed and stable precursor system can be achieved. The organophosphorus source can not only generate a uniform carbon coating layer in situ to improve conductivity during calcination, but also effectively inhibit grain growth. At the same time, the dopant introduces heterogeneous ions to adjust the lattice structure, reduce antisite defects, and enhance electronic conductivity and lithium-ion diffusion ability, thereby obtaining a doped carbon-coated iron phosphate precursor, thereby synergistically improving the rate performance and cycle life of lithium iron phosphate materials.

[0045] Specifically, magnesium salts include at least one of magnesium oxide, magnesium carbonate, magnesium hydroxide, and magnesium acetate.

[0046] Cobalt salts include at least one of nickel suboxide, nickel oxide, nickel oxalate, and nickel acetate.

[0047] Nickel salts include at least one of nickel oxalate, nickel carbonate, and nickel acetate.

[0048] Niobium salts include at least one of niobium pentoxide, niobium oxalate, niobium chloride, and lithium niobate.

[0049] In the technical solution of this application embodiment, the above-mentioned magnesium salt, cobalt salt, nickel salt and niobium salt have good dispersibility in organic solvents, and during the calcination process, they can effectively release doped ions and uniformly enter the lithium iron phosphate lattice, avoiding the generation of impurity phases caused by incomplete decomposition or agglomeration of dopants, thereby precisely controlling the lattice parameters and electronic structure of the material, and further improving the rate performance and cycle stability of lithium iron phosphate.

[0050] Furthermore, in some embodiments, the iron-to-phosphorus ratio of the iron in the iron source to the phosphorus in the organophosphorus source is 3:(3.05-3.10); and / or The mass of the dopant accounts for 0.5%-1.0% of the mass of the iron source.

[0051] In the technical solution of this application embodiment, by controlling the iron-to-phosphorus ratio of the iron source and the organic phosphorus source, a slight excess of phosphorus is used to compensate for the loss of phosphorus elements during the first and second calcination processes, effectively avoiding the generation of impurity phases such as iron oxide due to phosphorus deficiency; at the same time, the mass of the dopant is controlled at 0.5%-1.0% of the iron source mass, which not only allows the dopant ions to fully enter the crystal lattice to optimize conductivity and structural stability, but also avoids crystal distortion or capacity decay caused by excessive dopant, thereby significantly improving the rate performance and cycle life of lithium iron phosphate while ensuring high specific capacity.

[0052] Specifically, the iron-to-phosphorus ratio of the iron in the iron source to the phosphorus in the organic phosphorus source is 3:3.05, 3:3.06, 3:3.07, 3:3.08, 3:3.09, 3:3.10, or any ratio within the above range.

[0053] The mass of the dopant accounts for 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0% of the mass of the iron source, or any value within the above range.

[0054] Furthermore, in some embodiments, the step of mixing the iron source, organophosphorus source, dopant, and organic solvent, and then grinding and drying them to obtain a dried material includes: The iron source, organophosphorus source, dopant and organic solvent are mixed to obtain a primary slurry; The primary slurry is then ground to obtain a secondary slurry; The secondary slurry is dried to obtain the dried material; The secondary slurry has a D50 particle size of 200-400 nm; the drying process is a spray drying process, which uses nitrogen gas at 150-350℃ as a heat source, and the dried material obtained has a particle size of 5-15 μm.

[0055] In the technical solution of this application embodiment, by controlling the D50 particle size of the secondary slurry within the above-mentioned range, the precursor particles are ensured to be small and uniform, which is beneficial to reduce lattice defects and shorten the diffusion distance during subsequent calcination. Spray drying can effectively prevent the oxidation of divalent iron during the drying process, achieve instantaneous drying, avoid segregation between different components, and obtain a dry material with a particle size of 5-15μm through spray drying. The material has a suitable size, good flowability and high bulk density, which not only facilitates subsequent process operations, but also maintains a uniform morphology and carbon coating layer during the first calcination. Ultimately, this results in lithium iron phosphate material with excellent high compaction density, good processing performance and stable electrochemical performance.

[0056] Furthermore, the organic solvent is ethanol, and the mass of ethanol added is 3-5 times the mass of the organophosphorus source.

[0057] In the technical solution of this application embodiment, the use of this organic solvent helps to achieve high dispersion and uniform mixing of iron source, phosphorus source and dopant, obtain suitable slurry viscosity and solid content, thereby ensuring that the particle size distribution and spherical morphology of the precursor after grinding and spray drying are controllable. At the same time, ethanol is easy to volatilize and recover, and has low energy consumption, effectively reducing the preparation cost while ensuring material consistency.

[0058] Furthermore, in some embodiments, the step of subjecting the dried material to a single calcination treatment to obtain carbon-coated doped iron phosphate includes: Under an inert gas atmosphere, the dried material is subjected to the first calcination treatment to obtain a first calcined material. Subsequently, the first calcined material is pulverized to obtain carbon-coated doped iron phosphate with a particle size of 0.5-1.2 μm. The temperature of the first calcination treatment is 300-450℃, and the time of the first calcination treatment is 3-6h; The spray drying temperature is 150-350℃, and the particle size of the dried material is 5-15μm.

[0059] In the technical solution of this application embodiment, the organic phosphorus source can be fully decomposed at a lower temperature and a uniform carbon coating layer can be generated in situ, effectively suppressing excessive grain growth. Subsequently, it is pulverized to 0.5-1.2μm to obtain fine carbon-coated doped iron phosphate. These submicron particles have high specific surface area and short ion diffusion paths. After being mixed with a lithium source and calcined twice, lithium iron phosphate with uniform particle size and good crystallinity can be obtained, which significantly improves the rate performance and cycle stability of the material.

[0060] In some specific embodiments, during the first calcination, nitrogen gas is introduced for protection to maintain the oxygen content in the sintering furnace below 10 ppm, the heating rate is 50-100℃ / h, and then calcination is carried out at a temperature of 300-450℃ for 3-6 hours, the furnace pressure is 10-30 Pa, the humidity in the holding section of the furnace is ≤0.5%, and then the material is discharged after cooling. The discharged material is then crushed to a particle size of 0.5-1.2μm.

[0061] Furthermore, in some embodiments, the lithium source is one or more selected from lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate; and / or The lithium ratio of the lithium element in the lithium source to the iron element in the carbon-coated doped iron phosphate is 1:(1.03-1.10).

[0062] In the technical solution of this application embodiment, by selecting the above-mentioned lithium source, the elements in the anionic portion will volatilize after calcination, avoiding doping into the product. By controlling the iron-lithium molar ratio to 1:(1.03-1.10), a slight excess of lithium can be achieved, which can increase the capacity of the lithium iron phosphate product. At the same time, the excess lithium can also promote uniform grain growth and suppress iron-lithium antisite defects, thereby effectively improving the specific capacity, initial coulombic efficiency, and cycle stability of the material. If too much lithium is present, it becomes an impurity and cannot enter the lithium iron phosphate lattice, resulting in a decrease in electrical performance.

[0063] Specifically, the lithium ratio of the lithium element in the lithium source to the iron element in the carbon-coated doped iron phosphate is 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.10, or any ratio within the above range.

[0064] Furthermore, in some embodiments, the mixture further includes a supplementary dopant, which is at least one of a supplementary phosphorus source, a supplementary boron source, or a supplementary silicon source; and / or The supplementary phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, iron phosphate, and lithium dihydrogen phosphate. The supplementary boron source includes at least one of boric acid, boron trioxide, and methyl borate. The supplementary silicon source includes at least one of silicic acid, silane, silane, and silicon tetrafluoride.

[0065] In the technical solution of this application embodiment, by adding at least one of the above-mentioned supplementary dopants to the mixture, the rate performance, cycle life and high temperature stability of the material can be significantly improved while maintaining a high specific capacity.

[0066] Furthermore, the lithium source and supplementary phosphorus / boron / silicon source need to be pulverized to a particle size of 0.5-1.0 μm before being added.

[0067] In the technical solution of this application embodiment, by pretreating the lithium source, its specific surface area and reactivity can be significantly increased, and a more uniform solid phase distribution can be achieved when mixed with carbon-coated doped iron phosphate. This shortens the diffusion path of lithium ions and doped ions during the secondary calcination process, promotes full lattice reaction and reduces local component segregation, thereby effectively suppressing the generation of impurity phases. Finally, lithium iron phosphate materials with uniform particle size and electrochemical performance are obtained, which greatly improves its rate performance and cycle stability.

[0068] Furthermore, the number of moles of the added supplementary phosphorus source / supplementary boron source / supplementary silicon source is 0.01-0.05 times the number of moles of lithium in the added lithium source.

[0069] In the technical solution of this application embodiment, the amount of added supplementary dopant is controlled within the above range, which can improve the rate performance and cycle life of lithium iron phosphate material while avoiding impurities and capacity decay.

[0070] Furthermore, in some embodiments, the temperature of the secondary calcination treatment is 750-850°C, and the time of the secondary calcination treatment is 8-16 hours.

[0071] In the technical solution of this application embodiment, the secondary calcination is controlled at a high temperature and maintained for an appropriate time. This ensures that lithium elements diffuse into the iron phosphate material to generate high-purity and well-crystallized lithium iron phosphate, while avoiding excessive grain growth or damage to the carbon coating layer due to excessively high temperature or long time. This results in a lithium iron phosphate cathode material with uniform particle size, stable structure, and excellent electrochemical performance.

[0072] In some specific embodiments, during the secondary calcination, nitrogen gas is introduced for protection to maintain the oxygen content in the sintering furnace below 5 ppm. The heating rate is 100-200℃ / h, and then calcination is carried out at 750-850℃ for 8-16 hours. Simultaneously, the nitrogen introduction rate and exhaust rate are controlled to maintain the furnace pressure at 30-90 Pa, and the humidity in the holding section is ≤0.3%. The material is then cooled to a temperature ≤120℃ before being discharged. After discharge, the material is pulverized by airflow to a particle size of 0.5-1.5 μm. The gas source is nitrogen gas at 80-150℃ and 0.5-1.0 MPa. Sieving is performed using a 100-200 mesh screen. Iron removal and vacuum packaging are both carried out in a constant temperature and humidity room (temperature 25±1℃, humidity ≤10%) to control the moisture content of the lithium iron phosphate product to below 800 ppm.

[0073] Secondly, embodiments of this application provide a lithium iron phosphate battery prepared using the aforementioned lithium iron phosphate preparation method; a coin cell battery is fabricated using the lithium iron phosphate battery, wherein the coin cell battery has a maximum initial charge capacity of 162.93 mAh / g at 0.1C, a maximum initial discharge capacity of 159.53 mAh / g, a maximum initial charge capacity of 159.07 mAh / g at 0.5C, and a maximum initial discharge capacity of 144.17 mAh / g at 1C.

[0074] Thirdly, embodiments of this application provide a positive electrode sheet, comprising lithium iron phosphate prepared by the aforementioned preparation method.

[0075] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0076] I. Preparation Method Example 1 A method for preparing lithium iron phosphate includes the following steps: (1) Iron oxide red, magnesium oxide, tributyl phosphate and ethanol were mixed and slurried. The slurry particle size was controlled to be 300 nm by grinding. Then, it was spray dried at 230 °C to obtain a spray-dried material with a particle size of 10.2 μm. The iron-phosphorus molar ratio of iron oxide red and tributyl phosphate was 3:3.08, the mass of magnesium oxide was 0.75% of the mass of iron oxide red, and the mass of ethanol added was 4 times the mass of tributyl phosphate. (2) The dried material was placed in a nitrogen atmosphere, the oxygen content in the system was controlled to be less than 10 ppm and the pressure was 20 Pa. The temperature was raised to 380 ℃ at a rate of 75 ℃ / h for one calcination. The humidity content of the system was controlled to be ≤0.5%. The material was calcined at a constant temperature for 5 h. After calcination, the material was cooled and discharged. The product was pulverized to obtain carbon-coated doped iron phosphate with a particle size of 0.85 μm. (3) Carbon-coated doped iron phosphate and lithium carbonate and phosphoric acid with a particle size of 0.8 μm are mixed. The number of moles of lithium in lithium carbonate is 1.06 times the number of moles of iron in carbon-coated doped iron phosphate, and the number of moles of phosphoric acid is 0.03 times the number of moles of lithium in lithium carbonate. The mixture is placed in a nitrogen atmosphere, and the oxygen content in the system is controlled to be less than 5 ppm and the pressure is 60 Pa. The temperature is raised to 800 ℃ at a rate of 150 ℃ / h for secondary calcination. The humidity content of the system is controlled to be ≤0.3%. The mixture is calcined at a constant temperature for 12 h. Then it is cooled to the material temperature ≤120 ℃ and discharged. After discharge, the material is crushed to a particle size of 1.0 μm by a nitrogen gas flow of 120 ℃ and 0.8 MPa. The material is then sieved through a 120 mesh screen. Iron removal and vacuum packaging are carried out in a constant temperature and humidity room (temperature is 25±1 ℃, humidity ≤10%). The moisture content of the lithium iron phosphate product is controlled to be less than 800 ppm.

[0077] Example 2 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (1), niobium pentoxide is used instead of magnesium oxide. The rest is basically the same as in Example 1.

[0078] Example 3 A method for preparing lithium iron phosphate includes the following steps: (1) Iron oxide yellow, nickel oxide, triethyl phosphate and ethanol were mixed and slurried. The slurry particle size was controlled to be 200 nm by grinding. Then, it was spray dried at 150 °C to obtain a spray-dried material with a particle size of 5 μm. The iron-phosphorus molar ratio of iron oxide yellow and triethyl phosphate was 3:3.05, the mass of nickel oxide was 0.5% of the mass of iron oxide yellow, and the mass of ethanol added was 3 times the mass of triethyl phosphate. (2) The dried material was placed in a nitrogen atmosphere, the oxygen content in the system was controlled to be less than 10 ppm and the pressure was 30 Pa. The temperature was raised to 300 ℃ at a rate of 50 ℃ / h for one calcination. The humidity content of the system was controlled to be ≤0.5%. The material was calcined at a constant temperature for 6 h. After calcination, the material was cooled and discharged. The product was pulverized to obtain carbon-coated doped iron phosphate with a particle size of 0.5 μm. (3) Carbon-coated doped iron phosphate and lithium acetate and silica with a particle size of 0.5 μm are mixed, wherein the molar number of lithium in lithium acetate is 1.03 times the molar number of iron in carbon-coated doped iron phosphate, and the molar number of silicon in silica is 0.01 times the molar number of lithium in lithium acetate; the mixture is placed in a nitrogen atmosphere, the oxygen content in the system is controlled to be less than 5 ppm and the pressure is 90 Pa, and the temperature is increased at a rate of 100 °C / h to The material is calcined twice at 750℃, with the humidity content of the system controlled at ≤0.3%, and calcined at a constant temperature for 16 hours. Then, it is cooled to the material temperature ≤120℃ before being discharged. After discharge, the material is pulverized to a particle size of 0.5μm by a nitrogen gas flow at 80℃ and 1.0MPa, and then sieved through a 150-mesh screen. Iron removal and vacuum packaging are carried out in a constant temperature and humidity room (temperature 25±1℃, humidity ≤10%), and the moisture content of the lithium iron phosphate product is controlled to be less than 800ppm.

[0079] Example 4 A method for preparing lithium iron phosphate includes the following steps: (1) Iron oxide, cobalt oxalate, xylene phosphate and ethanol were mixed and slurried. The slurry particle size was controlled to be 400 nm by grinding. Then, the slurry was spray-dried at 350 °C to obtain a spray-dried material with a particle size of 15 μm. The iron-phosphorus molar ratio of iron oxide and xylene phosphate was 3:3.10, the mass of cobalt oxalate was 1.0% of the mass of iron oxide, and the mass of ethanol added was 5 times the mass of xylene phosphate. (2) The dried material was placed in a nitrogen atmosphere, the oxygen content in the system was controlled to be less than 10 ppm and the pressure was 10 Pa. The temperature was raised to 450 ℃ at a rate of 100 ℃ / h for one calcination. The humidity content of the system was controlled to be ≤0.5%. The material was calcined at a constant temperature for 3 h. After calcination, the material was cooled and discharged. The product was pulverized to obtain carbon-coated doped iron phosphate with a particle size of 1.2 μm. (3) Carbon-coated doped iron phosphate and lithium hydroxide with a particle size of 1.0 μm and boric acid are mixed, wherein the number of moles of lithium in the lithium hydroxide is 1.10 times the number of moles of iron in the carbon-coated doped iron phosphate, and the number of moles of phosphorus in the boric acid is 0.05 times the number of moles of lithium in the added lithium hydroxide; the mixture is placed in a nitrogen atmosphere, the oxygen content in the system is controlled to be less than 5 ppm and the pressure is 30 Pa, and the temperature is increased at a rate of 200 °C / h. The material is calcined at 850℃ for a second time, with the humidity content of the system controlled to be ≤0.3%. The calcination is carried out at a constant temperature for 8 hours, and then cooled to a material temperature of ≤120℃ before being discharged. After discharge, the material is pulverized with nitrogen gas at 150℃ and 0.5MPa until the particle size is 1.0μm. Then, it is sieved through a 100-mesh sieve. Iron removal and vacuum packaging are carried out in a constant temperature and humidity room (temperature 25±1℃, humidity ≤10%), and the moisture content of the lithium iron phosphate product is controlled to be below 800ppm.

[0080] Example 5 A method for preparing lithium iron phosphate includes the following steps: (1) Iron oxide red, nickel oxalate, triphenyl phosphate and ethanol were mixed to form a slurry. The slurry particle size was controlled to be 300 nm by grinding. Then, it was spray-dried at 200 °C to obtain a spray-dried material with a particle size of 12 μm. The iron-phosphorus molar ratio of iron oxide red and triphenyl phosphate was 3:3.06, the mass of nickel oxalate was 0.85% of the mass of iron oxide red, and the mass of ethanol added was 4 times the mass of triphenyl phosphate. (2) The dried material was placed in a nitrogen atmosphere, the oxygen content in the system was controlled to be less than 10 ppm and the pressure was 20 Pa. The temperature was raised to 400 ℃ at a rate of 80 ℃ / h for one calcination. The humidity content of the system was controlled to be ≤0.5%. The material was calcined at a constant temperature for 5 h. After calcination, the material was cooled and discharged. The product was pulverized to obtain carbon-coated doped iron phosphate with a particle size of 0.85 μm. (3) Carbon-coated doped iron phosphate and lithium carbonate and phosphoric acid with a particle size of 0.8 μm are mixed. The number of moles of lithium in lithium carbonate is 1.06 times the number of moles of iron in carbon-coated doped iron phosphate, and the number of moles of phosphoric acid is 0.03 times the number of moles of lithium in lithium carbonate. The mixture is placed in a nitrogen atmosphere, and the oxygen content in the system is controlled to be less than 5 ppm and the pressure is 60 Pa. The temperature is raised to 820 °C at a rate of 150 °C / h for secondary calcination. The humidity content of the system is controlled to be ≤0.3%. The mixture is calcined at a constant temperature for 12 h. Then it is cooled to the material temperature ≤120 °C and discharged. After discharge, the material is pulverized by a nitrogen gas flow of 120 °C and 0.8 MPa to a particle size of 1.0 μm. The material is then sieved through a 120 mesh screen. Iron removal and vacuum packaging are carried out in a constant temperature and humidity room (temperature is 25±1 °C, humidity ≤10%). The moisture content of the lithium iron phosphate product is controlled to be less than 800 ppm.

[0081] Example 6 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (1), the mass of magnesium oxide is 0.5% of the mass of iron oxide red, and the rest is basically the same as in Example 1.

[0082] Example 7 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (1), the mass of magnesium oxide is 1.0% of the mass of iron oxide red, and the rest is basically the same as in Example 1.

[0083] Example 8 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (2), the calcination temperature is 300°C. The rest is basically the same as in Example 1.

[0084] Example 9 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (2), the calcination temperature is 450°C. The rest is basically the same as in Example 1.

[0085] Example 10 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the lithium element contained in lithium carbonate is 1.03 times the molar amount of iron element contained in carbon-coated doped iron phosphate. The rest is basically the same as in Example 1.

[0086] Example 11 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the lithium element contained in lithium carbonate is 1.10 times the molar amount of iron element contained in carbon-coated doped iron phosphate. The rest is basically the same as in Example 1.

[0087] Example 12 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the secondary calcination temperature is 750°C, and the rest is basically the same as Example 1.

[0088] Example 13 This embodiment provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the secondary calcination temperature is 850°C, and the rest is basically the same as Example 1.

[0089] Comparative Example 1 After mixing iron phosphate, lithium carbonate, phosphoric acid, magnesium oxide and glucose, water was added to slurry. The amount of iron phosphate, lithium carbonate, phosphoric acid and magnesium oxide added, based on their iron, lithium, phosphorus and magnesium content, was the same as in Example 1. The amount of glucose added was 11.3% of the mass of iron phosphate. The solid content of the final slurry was 25%. Then it was ground to 300 nm and spray-dried to a particle size of 10.2 μm. Calcination, crushing, iron removal and vacuum packaging were carried out in accordance with the calcination process of step (3) in Example 1.

[0090] Comparative Example 2 Comparative Example 2 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (1), the mass of magnesium oxide is 0.4% of the mass of iron oxide red, and the rest is basically the same as in Example 1.

[0091] Comparative Example 3 Comparative Example 3 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (1), the mass of magnesium oxide is 1.1% of the mass of iron oxide red, and the rest is basically the same as Example 1.

[0092] Comparative Example 4 Comparative Example 4 provides a method for preparing lithium iron phosphate. The only difference from Example 1 is that in step (2), the calcination temperature is 280°C. The rest is basically the same as Example 1.

[0093] Comparative Example 5 Comparative Example 5 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (2), the calcination temperature is 480°C. The rest is basically the same as Example 1.

[0094] Comparative Example 6 Comparative Example 6 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the lithium element contained in lithium carbonate is 1.02 times the molar amount of iron element contained in carbon-coated doped iron phosphate. The rest is basically the same as Example 1.

[0095] Comparative Example 7 Comparative Example 7 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the lithium element contained in lithium carbonate is 1.12 times the molar amount of iron element contained in carbon-coated doped iron phosphate. The rest is basically the same as Example 1.

[0096] Comparative Example 8 Comparative Example 8 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the secondary calcination temperature is 700°C, and the rest is basically the same as Example 1.

[0097] Comparative Example 9 Comparative Example 9 provides a method for preparing lithium iron phosphate. Compared with Example 1, the only difference is that in step (3), the secondary calcination temperature is 900°C, and the rest is basically the same as Example 1.

[0098] Comparative Example 10 Comparative Example 10 provides a method for preparing lithium iron phosphate. The only difference from Example 1 is that phosphoric acid is not added in step (3), and the rest is basically the same as Example 1.

[0099] II. Testing Methods (a) The following properties were tested on the carbon-coated doped iron phosphate materials and lithium iron phosphate prepared in the examples and comparative examples.

[0100] 1. SEM testing: The morphology of carbon-coated doped iron phosphate materials and lithium iron phosphate was characterized using a Zeiss MERLIN Compact Quanta 200FEG field emission and scanning electron microscope (SEM).

[0101] 2. XRD test: The phase structure of carbon-coated doped iron phosphate material was characterized using a Smartlab X-ray diffractometer from Rigaku, Japan.

[0102] 3. D50 particle size: Tested using a laser particle size analyzer.

[0103] 4. BET specific surface area: determined by gas adsorption BET method.

[0104] 5. Compacted density: Tested using a compaction density meter with a test pressure of 3T and a compaction time of 30s.

[0105] 6. Powder internal resistance: Tested using the four-probe method at a pressure of 10 MPa.

[0106] (ii) Properties of secondary batteries The lithium iron phosphate materials prepared in each embodiment and comparative example were mixed with ultrafine carbon powder (Super P, SP) and polyvinylidene fluoride (PVDF) at a mass ratio of 90:5:5, respectively. N-methylpyrrolidone (NMP) was added to form a slurry, yielding a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil, dried, and then pressed into a sheet, with the compaction density controlled at 2.25 g / cm³. 3 A positive electrode was obtained; a lithium sheet was used as the negative electrode, lithium hexafluorophosphate (LiPF6) was used as the lithium salt, and ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 1:1:1 were used as organic solvents to dissolve the lithium salt in the organic solvents to prepare an electrolyte with a concentration of 1M (mol / L). A button cell was then prepared. Its charge specific capacity and discharge specific capacity were measured under 0.1C charge-discharge, 0.5C charge and 1C discharge conditions. The test temperature was 25±0.1℃ and the voltage range was 2.00V~3.75V.

[0107] III. Analysis of Test Results for Each Embodiment and Comparative Example The results obtained using the above testing methods are shown in Tables 1, 2, and 3, respectively.

[0108] Table 1. Detection data of carbon-coated doped iron phosphate materials prepared in the examples. Table 2. Detection data of lithium iron phosphate prepared in the examples and comparative examples. Table 3. Electrical performance test data of coin cells assembled from lithium iron phosphate prepared in the examples and comparative examples. Figure 1 The image shows a SEM image of the carbon-coated doped iron phosphate prepared in Example 1 of this application. It can be seen from the image that the carbon-coated doped iron phosphate has a uniform particle size distribution and a particle size of about 220 nm.

[0109] Figure 2 The image shows the XRD pattern of carbon-coated doped iron phosphate prepared in Example 1 of this application. As can be seen from the image, iron phosphate was successfully prepared. Since the carbon coating layer is amorphous carbon, it is not shown in the XRD pattern.

[0110] Figure 3 This is a SEM image of lithium iron phosphate prepared in Example 1 of this application; it can be seen from the image that the carbon-coated doped iron phosphate particles have a uniform particle size distribution and a particle size of about 235 nm.

[0111] Figure 4 The 0.1C and 1C charge-discharge curves of lithium iron phosphate prepared in Example 1 of this application are shown.

[0112] Figure 5 This is a SEM image of lithium iron phosphate prepared in Example 2 of this application; it can be seen from the image that the carbon-coated doped iron phosphate particles have a uniform particle size distribution and a particle size of about 240 nm.

[0113] Figure 6 The 0.1C and 1C charge-discharge curves of lithium iron phosphate prepared in Example 1 of this application are shown.

[0114] Figure 7 The image shows a SEM image of lithium iron phosphate prepared in Comparative Example 1 of this application. As can be seen from the image, the lithium iron phosphate material prepared in Comparative Example 1 exhibits a certain degree of agglomeration, and the primary particle size distribution is extremely uneven, showing a clear coexistence of large and small particles.

[0115] As can be seen from the data in Table 1, the mass percentage of Fe in the doped coated iron phosphate prepared in Examples 1-5 is between 35.89% and 36.11%, the mass percentage of P is between 20.38% and 20.52%, and the Fe:P ratio is between 0.971 and 0.984, which is close to the theoretical value. Furthermore, the mass percentage of C is between 2.53% and 2.99%, and the BET specific surface area is 16.49 m². 2 / g -25.43m 2 The tap density is between 0.72 g / mL and 0.86 g / cm³.3 This range is conducive to the subsequent addition of lithium sources, while the high-temperature moisture content is between 0.53% and 1.23%, Fe 2+ The mass percentage is between 0.43% and 1.12%, indicating that the carbon-coated doped iron phosphate material prepared in this application has high purity, low oxidation degree, stable overall performance, and uniform and relatively small primary particles, providing a high-quality precursor for the preparation of high-rate, long-cycle lithium iron phosphate cathode materials.

[0116] As can be seen from the data in Table 2, the lithium iron phosphate material prepared by the method of this application has a much lower internal resistance than Comparative Example 1, and a higher compaction density. This indicates that the preparation method of lithium iron phosphate in this application can form a more uniform carbon coating and doping distribution, thereby significantly improving the electronic conductivity and compactness of lithium iron phosphate material, and thus obtaining a cathode material with better electrochemical performance.

[0117] In conjunction with Examples 1-5 and Comparative Example 1, Figure 3 , 5 As shown in Tables 7 and 3, the 0.1C and 1C capacities of Example 1 are significantly better than those of Comparative Example 1, indicating that the lithium iron phosphate material prepared by mixing carbon-coated doped iron phosphate with a lithium source has excellent specific capacity and rate performance. The reason for this may be that Comparative Example 1 uses a traditional one-time mixing process, directly calcining after nano-grinding. Because the lithium salt melts at high temperatures, it easily agglomerates the iron phosphate particles, forming some large agglomerates, resulting in uneven particle size distribution and the coexistence of large and small particles. This uneven morphology deteriorates the electrochemical performance of the material, especially the 1C discharge capacity. Figure 7 As shown, the particle size difference in Comparative Example 1 is obvious. The lithium ion diffusion path inside the large particles is too long, which leads to increased electrode polarization and reduced capacity, especially at high rates where the capacity decay is more severe. In contrast, Example 1 first prepares carbon-coated doped iron phosphate with uniform particle size, and then mixes it with a lithium source and calcines it. The resulting material has uniform particle size, short lithium ion diffusion path, and small polarization, thus exhibiting higher capacity and better rate performance.

[0118] Based on the data from Examples 1, 6-7, Comparative Examples 2-3, and Table 3, it can be seen that: If the doping amount is too low, the number of dopant ions is insufficient, making it difficult to effectively improve lattice conductivity and suppress grain growth. The material still has many defects, and the rate performance improvement is limited. If the doping amount is too high, excessive dopant ions can cause lattice distortion and even generate impurity phases, damaging the integrity of the carbon coating layer. It may also hinder the normal insertion and extraction of lithium ions, leading to a decrease in specific capacity and cycle performance. Therefore, when the dopant addition is controlled at 0.5%-1.0% of the iron source, the battery performance is better; both too low and too high amounts result in a decrease in capacity.

[0119] Based on the data from Examples 1, 8-9, Comparative Examples 4-5, and Table 3, it can be seen that setting the primary calcination temperature range to 300-450℃ effectively decomposes the organophosphorus source and forms a uniform carbon coating. Temperature deviations reduce the conductivity and capacity of the material. However, when the primary calcination temperature is below 300℃, the organophosphorus source is not completely decomposed, and the residual organic matter continues to decompose and escape during subsequent secondary calcination, resulting in a loose, porous, and unevenly distributed carbon coating. At the same time, phosphorus is not completely released, easily forming phosphorus-deficient impurities, reducing the electronic conductivity and specific capacity of the material. Furthermore, when the temperature is above 450℃, iron phosphate grains grow excessively, the particles coarsen, the specific surface area decreases, and the in-situ generated carbon coating may become over-graphitized or even oxidized and ineffective. Dopant ions are also prone to premature diffusion and aggregation, making it impossible to effectively control the crystal structure, ultimately leading to a decrease in capacity and rate performance.

[0120] Based on the data from Examples 1, 10-11, Comparative Examples 6-7, and Table 3, it can be seen that during the secondary calcination process, lithium elements are lost due to high-temperature volatilization. If the lithium source is insufficient, it can easily lead to the formation of a lithium-deficient phase, reducing specific capacity and initial coulombic efficiency. However, when the lithium source is controlled within an appropriate excess range of 1:1.03-1.10 (LiFe:L), lithium loss can be effectively compensated, ensuring sufficient lithium site filling in the lithium iron phosphate lattice. Simultaneously, excess lithium can suppress LiFe inversion defects, promote uniform grain growth, and improve structural stability and electronic conductivity. If the lithium source is excessive, impurity phases may be generated, hindering lithium-ion migration and leading to a decrease in battery rate performance and cycle stability.

[0121] Based on the relevant data from Examples 1, 12-13, Comparative Examples 8-9 and Table 3, it can be seen that controlling the secondary calcination temperature at 750-850℃ can provide sufficient heat energy for lithium to fully embed into the crystal lattice, while avoiding incomplete reaction and poor crystallinity due to excessively low temperature, or excessive grain growth, carbon coating layer damage and impurity phase formation due to excessively high temperature, thereby ensuring that the material has high capacity, good rate performance and structural stability.

[0122] Based on the relevant data in Example 1, Comparative Example 10 and Table 3, it can be seen that adding supplementary dopants to the system can further compensate for element volatilization and optimize the lattice structure, thereby improving capacity and rate performance.

[0123] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, Includes the following steps: Iron source, organophosphorus source, dopant and organic solvent are mixed and then ground and dried to obtain dried material; The dried material was subjected to a single calcination treatment to obtain carbon-coated doped iron phosphate; The carbon-coated doped iron phosphate is mixed with a lithium source to obtain a mixture, which is then subjected to a second calcination treatment to obtain lithium iron phosphate.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The iron source is one or more of iron oxide red, iron oxide yellow, and iron(III) oxide; and / or The organophosphorus source is one or more of the following: trialkyl phosphate, tributyl phosphate, triethyl phosphate, trioctyl phosphate, triphenyl phosphate, xylene phosphate, and xylene diphenyl phosphate; and / or The dopant is one or more of magnesium salts, cobalt salts, nickel salts, and niobium salts.

3. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The iron-to-phosphorus ratio of the iron in the iron source to the phosphorus in the organic phosphorus source is 3:(3.05-3.10); and / or The mass of the dopant accounts for 0.5%-1.0% of the mass of the iron source.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The step of mixing an iron source, an organophosphorus source, a dopant, and an organic solvent, followed by grinding and drying to obtain a dried material, includes: The iron source, organophosphorus source, dopant and organic solvent are mixed to obtain a primary slurry; The primary slurry is then ground to obtain a secondary slurry; The secondary slurry is dried to obtain the dried material; The secondary slurry has a D50 particle size of 200-400 nm; the drying process is a spray drying process, which uses nitrogen gas at 150-350℃ as a heat source, and the dried material obtained has a particle size of 5-15 μm.

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The step of calcining the dried material once to obtain carbon-coated doped iron phosphate includes: Under an inert gas atmosphere, the dried material is subjected to the first calcination treatment to obtain a first calcined material. Subsequently, the first calcined material is pulverized to obtain carbon-coated doped iron phosphate with a particle size of 0.5-1.2 μm. The temperature of the first calcination treatment is 300-450℃, and the time of the first calcination treatment is 3-6h; The spray drying temperature is 150-350℃, and the particle size of the dried material is 5-15μm.

6. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate; and / or The lithium molar ratio of the lithium element in the lithium source to the iron element in the carbon-coated doped iron phosphate is 1:(1.03-1.10).

7. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The mixture also includes a supplementary dopant, which is at least one of a supplementary phosphorus source, a supplementary boron source, or a supplementary silicon source; and / or The supplementary phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, iron phosphate, and lithium dihydrogen phosphate. The supplementary boron source includes at least one of boric acid, boron trioxide, and methyl borate. The supplementary silicon source includes at least one of silicic acid, silane, silane, and silicon tetrafluoride.

8. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The temperature of the secondary calcination treatment is 750-850℃, and the time of the secondary calcination treatment is 8-16h.

9. A lithium iron phosphate, characterized in that, The lithium iron phosphate is prepared by any one of claims 1-8; a coin cell battery is made using the lithium iron phosphate, wherein the coin cell battery has a maximum initial charge capacity of 162.93 mAh / g at 0.1C and a maximum initial discharge capacity of 159.53 mAh / g. The maximum initial charge capacity at 0.5C is 159.07mAh / g, and the maximum initial discharge capacity at 1C is 144.17mAh / g.

10. A positive electrode plate, characterized in that, Lithium iron phosphate prepared by the preparation method according to any one of claims 1-8.

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