Preparation method of lithium iron phosphate, lithium iron phosphate and lithium ion battery

Abstract

The application provides a preparation method of lithium iron phosphate, lithium iron phosphate and a lithium ion battery, and belongs to the technical field of new energy batteries. The preparation method comprises the following steps: providing ternary composite iron phosphate; mixing the ternary composite iron phosphate, a lithium source and a ligand in a solvent to form a precursor solution, spraying a nitrogen source solution in the form of spraying into the precursor solution to obtain slurry; drying the slurry to obtain a solid precursor; and performing heat treatment on the solid precursor in an atmosphere containing red phosphorus vapor to obtain the lithium iron phosphate after cooling. By adopting the ternary composite iron phosphate with a specific proportion as a raw material, the electrochemical performance of the lithium iron phosphate is effectively improved; the preparation method is simple in process and easy to control, and can be mass-produced in an industrialized manner. The prepared lithium iron phosphate has high specific capacity, long cycle life and good rate performance, and significantly improves the overall performance of the lithium ion battery.

Description

Technical Field

[0001] This application relates to the field of new energy battery technology, and in particular to a method for preparing lithium iron phosphate, lithium iron phosphate and lithium-ion batteries. Background Technology

[0002] There are many types of lithium-ion batteries, mainly including lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, and lithium iron phosphate. Among them, lithium iron phosphate is widely used in lithium-ion batteries because of its good lattice stability, minimal impact of lithium-ion insertion and extraction on the lattice, good reversibility, and the absence of any harmful heavy metals, resulting in high safety performance. It is also more widely available and cheaper.

[0003] Currently, most lithium iron phosphate (LFP) batteries are produced by mixing and milling a ternary composite of iron phosphate, organic carbon, and other additives, followed by spray drying, sintering, and air jet milling to obtain carbon-coated LFP materials. However, due to the olivine crystal structure of LFP, its electronic conductivity is low and its ion diffusion rate is poor, thus affecting its kinetic performance. Especially during charge and discharge, LFP batteries exhibit significant polarization and poor rate performance, which in turn affects their cycle performance; the higher the charge / discharge rate, the shorter the lifespan.

[0004] In practical applications, lithium iron phosphate (LFP) batteries with larger primary particles typically have a higher compaction density when fabricated into electrodes compared to LFP batteries using smaller primary particles. However, due to the larger primary particle size, LFP batteries are prone to severe electrochemical polarization, which affects their kinetic performance. While current methods can improve the electrochemical performance of LFP batteries by reducing the primary particle size, this reduces the electrode compaction density, thus impacting the battery's energy density.

[0005] In addition, due to the inherent characteristics of lithium iron phosphate, its low-temperature performance and rate performance are relatively poor compared to others, which limits the application areas of lithium iron phosphate batteries. Summary of the Invention

[0006] This application addresses the problem that lithium iron phosphate has low conductivity due to its inherent structure, making it impossible to achieve high-rate charge and discharge. Its purpose is to provide a method for preparing lithium iron phosphate, lithium iron phosphate, and a lithium-ion battery.

[0007] Specifically, the first aspect of this application provides a method for preparing lithium iron phosphate, comprising the following steps:

[0008] (a) Provide ternary composite iron phosphate;

[0009] (b) The composite iron phosphate is mixed with a lithium source and ligand in a solvent to form a precursor solution, and a nitrogen source solution is sprayed into the precursor solution in the form of a spray to obtain a slurry;

[0010] (c) The slurry obtained in step (b) is dried to obtain a solid precursor;

[0011] (d) Vapor phase assisted multi-stage sintering: The solid precursor obtained in step (c) is subjected to a first stage heat treatment under a protective atmosphere, followed by a second stage heat treatment in an atmosphere containing red phosphorus vapor, and finally a third stage heat treatment under an inert atmosphere. After cooling, the lithium iron phosphate is obtained.

[0012] Furthermore, the ternary composite ferric phosphate comprises a first ferric phosphate with an iron-phosphorus molar ratio of 0.965-0.975:1, a second ferric phosphate with an iron-phosphorus molar ratio of 0.975-0.985:1, and a third ferric phosphate with an iron-phosphorus molar ratio of 0.985-0.995:1.

[0013] Furthermore, the ligand is at least one of aminotrimethylphosphonic acid, hydroxyethylidene diphosphonic acid, and diethylenetriaminepentamethylphosphonic acid.

[0014] Furthermore, the mass ratio of the ternary composite iron phosphate to the ligand is 1:(0.05-0.3).

[0015] Further, the lithium source is at least one of lithium carbonate, lithium hydroxide, or lithium acetate, and the molar ratio of lithium element in the lithium source to iron element in the ternary composite iron phosphate is (1.0-1.1):1;

[0016] The nitrogen source solution is prepared from polyvinylpyrrolidone and ammonium heptamolybdate.

[0017] Further, the mixing described in step (b) is a mechanochemical mixing performed by ball milling or high-speed shear emulsification for a mixing time of 1-6 hours.

[0018] Further, in step (d), the temperature of the first stage heat treatment is 300-500℃, the heating rate is 1-5℃ / min, and the holding time is 1-5 hours;

[0019] The second stage of heat treatment involves raising the temperature from the end temperature of the first stage heat treatment to 550-600℃ at a heating rate of 0.5-3℃ / min, and holding it at that temperature for 2-6 hours.

[0020] The third stage of heat treatment involves holding the temperature at 650-750℃ for 5-10 hours.

[0021] Further, in step (d), the red phosphorus vapor is obtained by heating solid red phosphorus to 280-320°C in a vaporization zone independent of the precursor sintering zone and then carrying it to the reaction zone by a protective gas.

[0022] The second aspect of this application provides a lithium iron phosphate, which is prepared by the method described above. The lithium iron phosphate consists of micron-sized secondary spherical particles composed of primary nanoparticles and has a nitrogen-doped carbon layer on its surface.

[0023] Furthermore, the nanoparticles have a particle size of 50-150 nm, and the nitrogen-doped carbon layer has a thickness of 2-5 nm.

[0024] A third aspect of this application provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery comprises the aforementioned lithium iron phosphate.

[0025] The present invention has the following beneficial effects:

[0026] This invention first utilizes a composite iron phosphate as a precursor, comprising three iron phosphates with different iron-phosphorus molar ratios, effectively optimizing the crystal structure of lithium iron phosphate and thereby improving its electronic conductivity and ion diffusion rate. This improvement significantly enhances the kinetic performance of lithium iron phosphate, reducing polarization during charge and discharge, improving rate performance, and ultimately extending the battery's cycle life.

[0027] During the sintering stage, this invention introduces red phosphorus vapor as a gaseous phosphorus source and reducing agent, constructing a solid-gas synergistic reaction system. This system not only allows for precise control of the stoichiometry but also achieves Fe... 3+ The gentle in-situ reduction ensures perfect synchronization between the reduction and crystallization processes. This reaction pathway effectively avoids the lag and inhomogeneity of traditional carbothermal reduction methods, significantly improving product quality. A three-stage sintering process—"low-temperature pre-carbonization - medium-temperature gas-phase reaction - high-temperature crystallization"—achieves deep synergy of process parameters. The temperature control at each stage is highly matched to the thermal decomposition characteristics of the precursor and the timing of the introduction of the gas-phase medium, thus enabling precise control of the material nucleation, growth, and carbonization processes. The final lithium iron phosphate product exhibits excellent performance characteristics: the primary particles display a uniform spherical morphology of approximately 50 nm, and the carbon coating layer possesses a high degree of graphitization and excellent conductivity. These characteristics promise significant improvements in rate performance and cycle life, providing an ideal solution for the development of high-performance lithium-ion batteries. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this drawing or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this drawing. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0029] Figure 1 Here is a SEM image of lithium iron phosphate in Example 1;

[0030] Figure 2 Here is a SEM image of the first iron phosphate in Example 1;

[0031] Figure 3 This is a SEM image of the second iron phosphate in Example 1;

[0032] Figure 4 This is a SEM image of the third ferric phosphate in Example 1.

[0033] The purpose, features, and advantages of this accompanying drawing will be further explained in conjunction with the embodiments and with reference to the accompanying drawing. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0035] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0036] The first aspect of this application provides a method for preparing lithium iron phosphate, comprising the following steps:

[0037] (a) Provide ternary composite iron phosphate;

[0038] (b) The composite iron phosphate is mixed with a lithium source and ligand in a solvent to form a precursor solution, and a nitrogen source solution is sprayed into the precursor solution in the form of a spray to obtain a slurry;

[0039] (c) The slurry obtained in step (b) is dried to obtain a solid precursor;

[0040] (d) Vapor phase assisted multi-stage sintering: The solid precursor obtained in step (c) is subjected to a first stage heat treatment under a protective atmosphere, followed by a second stage heat treatment in an atmosphere containing red phosphorus vapor, and finally a third stage heat treatment under an inert atmosphere. After cooling, the lithium iron phosphate is obtained.

[0041] In this embodiment, the ternary composite ferric phosphate comprises a first ferric phosphate with an iron-phosphorus molar ratio of 0.965-0.975:1, a second ferric phosphate with an iron-phosphorus molar ratio of 0.975-0.985:1, and a third ferric phosphate with an iron-phosphorus molar ratio of 0.985-0.995:1.

[0042] The composite ferric phosphate has a first ferric phosphate mass ratio of 5%-50%, a second ferric phosphate mass ratio of 10%-50%, and a third ferric phosphate mass ratio of 0%-30%.

[0043] This invention achieves precise optimization of the final lithium iron phosphate material crystal structure by controlling the ratio of three different iron-phosphorus molar ratios of iron phosphate. This composite structure effectively enhances the diffusion channels of lithium ions in the material, strengthens electronic conductivity, and thus significantly improves the material's kinetic performance.

[0044] The preparation method of ternary composite iron phosphate in step (a) includes:

[0045] Synthesis of ferric phosphate:

[0046] Solution preparation: Dissolve 0.965 mol of FeSO4·7H2O in 500 mL of deionized water to form solution A1; dissolve 1.0 mol of H3PO4 in 500 mL of deionized water to form solution B1.

[0047] Coprecipitation reaction: In a reaction vessel, solutions A and B are simultaneously added dropwise to 1 L of deionized water (preheated to 50°C) and mixed under mechanical stirring (500 rpm). During the addition, H₂O₂ (1.1 times the molar amount of FeSO₄) is slowly added to oxidize Fe. 2+ to Fe 3+ Adjust the pH of the reaction system to 2.5 with ammonia, maintain the reaction temperature at 50°C, and stir continuously for 2 hours.

[0048] Post-processing: The precipitated slurry was aged for 1 hour, then centrifuged, washed with deionized water until neutral, and the precipitate was vacuum dried at 80°C for 12 hours to obtain first ferric phosphate powder.

[0049] Synthesis of iron phosphate:

[0050] Solution preparation: Dissolve 0.980 mol of FeSO4·7H2O in 500 mL of deionized water to form solution A2; dissolve 1.0 mol of H3PO4 in 500 mL of deionized water to form solution B2.

[0051] Coprecipitation reaction: The procedure is the same as for ferric phosphate, but the pH is adjusted to 2.5, the reaction temperature is 55℃, and other conditions remain unchanged.

[0052] Post-processing: Same as the first ferric phosphate, dried to obtain the second ferric phosphate powder.

[0053] Synthesis of ferric phosphate:

[0054] Solution preparation: Dissolve 0.990 mol of FeSO4·7H2O in 500 mL of deionized water to form solution A3; dissolve 1.0 mol of H3PO4 in 500 mL of deionized water to form solution B3.

[0055] Coprecipitation reaction: The procedure is the same as for ferric phosphate, but the pH is adjusted to 2.5, the reaction temperature is 60℃, and other conditions remain unchanged.

[0056] Post-processing: Same as for the first ferric phosphate, dried to obtain the third ferric phosphate powder.

[0057] After the reaction is complete, the first, second and third ferric phosphate powders are mixed in the following mass ratios (5%-50% for the first ferric phosphate, 10%-50% for the second ferric phosphate, and 0%-30% for the third ferric phosphate). The mixed powders are then placed in a ball mill and ball-milled for 2 hours at a ball-to-powder ratio of 10:1 and a speed of 200 rpm to ensure uniform mixing without over-grinding (to maintain submicron-sized particles). Ultrasonic dispersion (300W power, 30 minutes) is then used to further mix the powders to prevent particle agglomeration and form a ternary composite ferric phosphate.

[0058] Step (b) involves mixing the ternary composite iron phosphate with a lithium source and a ligand in a solvent to form a precursor solution. Under vigorous stirring (600 rpm), a nitrogen source solution is sprayed into the precursor solution in a spray form. After spraying, stirring continues for 1 hour to ensure sufficient interaction between the PVP and the precursor solution, resulting in a slurry. The ligand is at least one of aminotrimethylphosphonic acid, hydroxyethylidene diphosphonic acid, and diethylenetriaminepentamethylphosphonic acid. Preferably, it is aminotrimethylphosphonic acid. The mass ratio of the ternary composite iron phosphate to the ligand is 1:(0.05-0.3). The lithium source is at least one of lithium carbonate, lithium hydroxide, or lithium acetate.

[0059] The nitrogen source solution is prepared from polyvinylpyrrolidone (PVP) and ammonium heptamolybdate. The preparation method is as follows: weigh PVP and ammonium heptamolybdate, dissolve them in warm water at 60°C, and stir magnetically until completely clear to obtain a composite nitrogen source solution for later use. The mass percentage of PVP is 3.0 wt%, and the mass percentage of ammonium heptamolybdate is 1.5 wt%.

[0060] In the nitrogen source, the long-chain molecules of polyvinylpyrrolidone can entangle around the precursor solution network, forming a physical coating. Upon pyrolysis, the pyrrolidone provides highly reactive nitrogen atoms, which are particularly effective in improving the electronic conductivity of carbon materials. Ammonium heptamolybdate decomposes to provide NH3, which serves as a gaseous nitrogen source for bulk doping. Molybdenum (Mo) is a known metal that catalyzes the transformation of amorphous carbon to graphitized carbon; its introduction can significantly improve the graphitization degree and conductivity of the carbon coating layer.

[0061] The mixing is a mechanochemical mixing performed by ball milling or high-speed shear emulsification, with a mixing time of 1-6 hours.

[0062] The slurry obtained in step (c) is dried at a temperature of 80-120℃ for 4-8 hours to ensure complete evaporation of the solvent and obtain a homogeneous solid mixture.

[0063] In step (d), the first stage of heat treatment involves heating the dried precursor to 300-500℃ at a rate of 1-5℃ / min under a flowing argon atmosphere and holding it at that temperature for 1-5 hours. This forms a porous carbon framework rich in nitrogen and oxygen heteroatoms, fixing Fe, Li, and P elements within it. Simultaneously, polyvinylpyrrolidone (PVP) and ammonium heptamolybdate begin to decompose. The pyrrolidone structure of PVP is initially fixed into the carbon framework, while ammonium heptamolybdate decomposes to generate MoO3 and NH3. NH3 then performs initial nitrogen doping on the material.

[0064] The second stage of heat treatment involves switching the atmosphere to a mixture of argon and red phosphorus vapor (this can be achieved by placing the red phosphorus in the low-temperature zone upstream of the tube furnace, such as 300°C, and using a carrier gas to carry the vapor into the reaction zone). The temperature is then slowly increased from 450°C to 550-600°C at a rate of 0.5-3°C / min and held for 2-6 hours.

[0065] The red phosphorus vapor is obtained by heating solid red phosphorus to 280-320°C in a vaporization zone independent of the precursor sintering zone, and then carrying it to the reaction zone with a protective gas. Red phosphorus vapor (P4) has a high vapor pressure below 580°C, making it suitable as a dynamic phosphorus source and reducing agent. As a dynamic phosphorus source, it precisely compensates for phosphorus loss from the solid precursor at moderate temperatures, ensuring a strict 1:1 iron-to-phosphorus ratio in the final product and avoiding the formation of impurities such as Fe2P. As a mild reducing agent, its reducing properties are gentler than hydrogen and more controllable than solid carbon. It can reduce Fe... 3+In-situ, gradual reduction to Fe 2+ This reduction process matches the formation kinetics of LiFePO4 crystal nuclei, promoting the formation of a large number of fine, uniform nuclei. Furthermore, red phosphorus vapor not only reduces Fe... 3+ Furthermore, its strong reducing atmosphere can partially reduce the MoO3 generated in the first stage to metallic molybdenum (Mo) or molybdenum carbide (Mo2C) nanoparticles. These nanoparticles, acting as highly efficient catalysts, greatly promote the transformation of surrounding amorphous carbon into graphene-like ordered structures.

[0066] Third stage heat treatment: Stop the flow of red phosphorus vapor and switch back to pure argon gas. Rapidly raise the temperature to 650-700℃ at 5℃ / min and hold for 5-10 hours.

[0067] This stage aims to further grow the LiFePO4 nuclei formed in the previous step into well-crystallized nanocrystals. The catalyst Mo / Mo2C nanoparticles are completely encapsulated in the graphitized carbon layer, avoiding their negative impact on electrochemical performance. Simultaneously, the higher temperature causes partial graphitization of the amorphous carbon coating layer, forming a highly conductive nitrogen-doped graphitized carbon layer.

[0068] After sintering, the product is cooled to room temperature in the furnace. The product is then lightly ground and passed through a 400-mesh sieve to obtain the final lithium iron phosphate product.

[0069] An embodiment of the second aspect of this application provides a lithium iron phosphate, prepared by the aforementioned method for preparing lithium iron phosphate. The lithium iron phosphate consists of micron-sized secondary spherical particles composed of primary nanoparticles, and the surface is coated with a nitrogen-doped carbon layer. The particle size of the nanoparticles is 50-150 nm, and the thickness of the nitrogen-doped carbon layer is 2-5 nm.

[0070] A third aspect of this application provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery comprises the aforementioned lithium iron phosphate.

[0071] Example

[0072] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight. Unless otherwise stated, all reagents used in the examples are available commercially or synthesized using conventional methods and are ready for use without further processing. Unless otherwise stated, all instruments used in the examples are available commercially.

[0073] Example 1

[0074] A method for preparing lithium iron phosphate includes the following steps:

[0075] (a) Providing a ternary composite ferric phosphate; wherein the first ferric phosphate has a mass ratio of 40%, the second ferric phosphate has a mass ratio of 40%, and the third ferric phosphate has a mass ratio of 20%;

[0076] Synthesis of ferric phosphate:

[0077] Solution preparation: Dissolve 0.965 mol of FeSO4·7H2O in 500 mL of deionized water to form solution A1; dissolve 1.0 mol of H3PO4 in 500 mL of deionized water to form solution B1;

[0078] Coprecipitation reaction: In a reaction vessel, solutions A and B are simultaneously added dropwise to 1 L of deionized water (preheated to 50°C) and mixed under mechanical stirring (500 rpm). During the addition, H₂O₂ (1.1 times the molar amount of FeSO₄) is slowly added to oxidize Fe. 2+ to Fe 3+ Adjust the pH of the reaction system to 2.5 with ammonia, maintain the reaction temperature at 50°C, and stir continuously for 2 hours;

[0079] Post-processing: The precipitated slurry was aged for 1 hour, then centrifuged, washed with deionized water until neutral, and the precipitate was vacuum dried at 80°C for 12 hours to obtain first ferric phosphate powder; the iron-phosphorus molar ratio of the first ferric phosphate powder was 0.97.

[0080] Synthesis of iron phosphate:

[0081] Solution preparation: Dissolve 0.980 mol of FeSO4·7H2O in 500 mL of deionized water to form solution A2; dissolve 1.0 mol of H3PO4 in 500 mL of deionized water to form solution B2;

[0082] Coprecipitation reaction: The procedure is the same as for ferric phosphate, but the pH is adjusted to 2.5, the reaction temperature is 55℃, and other conditions remain unchanged;

[0083] Post-processing: Same as the first ferric phosphate, dried to obtain the second ferric phosphate powder; the iron-phosphorus molar ratio of the second ferric phosphate powder is 0.985;

[0084] Synthesis of ferric phosphate:

[0085] Solution preparation: Dissolve 0.990 mol of FeSO4·7H2O in 500 mL of deionized water to form solution A3; dissolve 1.0 mol of H3PO4 in 500 mL of deionized water to form solution B3.

[0086] Coprecipitation reaction: The procedure is the same as for ferric phosphate, but the pH is adjusted to 2.5, the reaction temperature is 60℃, and other conditions remain unchanged;

[0087] Post-processing: Same as the first ferric phosphate, after drying, the third ferric phosphate powder is obtained, and the iron-phosphorus molar ratio of the third ferric phosphate powder is 0.99.

[0088] After the reaction was completed, the first, second and third ferric phosphate powders were mixed in proportion, and the mixed powder was put into a ball mill and ball milled for 2 hours at a ball-to-powder ratio of 10:1 and a speed of 200 rpm to ensure uniform mixing but not over-grinding (to maintain submicron size); ultrasonic dispersion (300W power, 30 minutes) was used to mix to avoid particle agglomeration and form ternary composite ferric phosphate;

[0089] (b) The ternary composite ferric phosphate, lithium carbonate, and aminotrimethylphosphonic acid are mechanically and chemically mixed in a solvent by ball milling or high-speed shear emulsification for 3 hours to form a precursor solution. A nitrogen source solution is sprayed into the precursor solution in the form of a spray to obtain a slurry. The mass ratio of the ternary composite ferric phosphate to aminotrimethylphosphonic acid is 1:0.15; the molar ratio of the ternary composite ferric phosphate to lithium carbonate is 1:1.05; and the nitrogen source solution is prepared from polyvinylpyrrolidone and ammonium heptamolybdate.

[0090] (c) The slurry obtained in step (b) is dried at 90°C for 6 hours to obtain a solid precursor;

[0091] (d) Vapor-phase assisted multi-stage sintering:

[0092] Sintering equipment: Dual-zone tube furnace system;

[0093] Main reaction zone: used to place precursors;

[0094] Gas phase source area: Located upstream of the main reaction area, used to place red phosphorus (P4, purity ≥99.5%).

[0095] Protective gas: High-purity argon (Ar, 99.999%).

[0096] The precursor powder was evenly spread in an alumina crucible and placed in the main reaction zone of a tube furnace; argon gas was introduced at a flow rate of 300 mL / min to purge air; the temperature was increased from room temperature to 400℃ at a rate of 3℃ / min; and the temperature was held at 400℃ for 2 hours to obtain a black, porous intermediate in which the precursor had been initially carbonized to form a nitrogen-doped carbon framework, and Fe, Li, and P were highly dispersed in it in an amorphous form.

[0097] Maintain the main reaction zone temperature at 400℃; heat the gaseous source zone (containing 1.0 g red phosphorus) to 320℃ at a rate of 10℃ / min, causing the red phosphorus to sublimate and generate P4 vapor, which is then carried into the main reaction zone by argon gas (flow rate adjusted to 100 mL / min); slowly raise the main reaction zone temperature from 400℃ to 580℃ at a rate of 2℃ / min; after reaching 580℃, hold at this temperature for 3 hours. During this stage, turn off the heating of the gaseous source zone and allow it to cool naturally. The red phosphorus vapor acts as a dynamic phosphorus source to compensate for possible phosphorus loss and as a mild reducing agent to reduce Fe... 3+ In-situ reduction to Fe 2+ Simultaneously, a large number of uniform LiFePO4 crystal nuclei are generated.

[0098] The supply of red phosphorus vapor was stopped, and the gas flow was switched to pure argon, with the flow rate restored to 300 mL / min. The temperature was increased from 580℃ to 680℃ at a rate of 5℃ / min. The temperature was held at 680℃ for 8 hours to complete the perfect growth of LiFePO4 crystals and the graphitization of the carbon coating. After sintering, the furnace was allowed to cool naturally to room temperature. The sintered product was then lightly ground in a mortar and passed through a 400-mesh (38μm) sieve to obtain the final nitrogen-doped carbon-coated nanocrystalline lithium iron phosphate with a tap density of 1.25 g / cm³. 3 .

[0099] Example 2

[0100] This embodiment is basically the same as Embodiment 1, except that the mass ratio of the ternary composite iron phosphate to aminotrimethylphosphonic acid is 1:0.2, and the tap density of lithium iron phosphate is 1.22 g / cm³. 3 .

[0101] Example 3

[0102] This embodiment is basically the same as Embodiment 1, except that the mass ratio of the first iron phosphate in the composite iron phosphate is 35%, the mass ratio of the second iron phosphate is 42%, and the mass ratio of the third iron phosphate is 23%.

[0103] Example 4

[0104] This embodiment is basically the same as embodiment 1, except that the drying temperature in step (c) is 100°C and the drying time is controlled at 5 hours.

[0105] Example 5

[0106] This embodiment is basically the same as Embodiment 1, except that step (d) is finally held at 700°C for 8 hours; the tap density of lithium iron phosphate is 1.28 g / cm³. 3 .

[0107] Comparative Example 1

[0108] This comparative example is basically the same as Example 1, except that step (a) uses a ternary composite iron phosphate instead of a conventional single iron phosphate.

[0109] Comparative Example 2

[0110] This comparative example is basically the same as Example 1, except that red phosphorus vapor is not used in the sintering process, and three-step sintering is carried out only in a pure Ar atmosphere.

[0111] Comparative Example 3

[0112] This comparative example is basically the same as Example 1, except that a nitrogen source solution is not sprayed into the precursor solution in step (b).

[0113] Experimental Case

[0114] The lithium iron phosphate prepared in Examples 1-5 and Comparative Examples 1-3 was used to prepare electrode sheets, wherein the active material was lithium iron phosphate prepared in Examples 1-5 and Comparative Examples 1-3; the conductive agent was acetylene black; the binder was polyvinylidene fluoride; and the mass ratio was: active material: conductive agent: binder = 90:5:5.

[0115] Assemble CR2032 button cells.

[0116] Electrochemical performance testing: All tests were conducted on the LAND CT2001A battery testing system in a 25°C constant temperature chamber.

[0117] Rate performance testing: The battery was first charged to 4.2 V at a constant current of 0.2C (1C = 170 mA / g), and then charged at a constant voltage of 4.2 V until the current dropped to 0.05C, completing the charging process. Subsequently, it was discharged to 2.5 V at a constant current of 0.2C, and the initial discharge specific capacity was recorded. Charge-discharge tests were then performed sequentially at rates of 0.5C, 1C, and 5C, with 5 cycles at each rate. Finally, the battery was cycled 5 times at the 0.2C rate to examine its capacity recovery capability.

[0118] Cycle life testing: The battery was activated by three charge-discharge cycles at 0.2C. Subsequently, a long-term cycle test was conducted at 1C. The charging regime was constant current (1C) constant voltage (4.2 V, cutoff current 0.05C), and the discharging regime was constant current (1C) to 2.5 V. The battery underwent 500 consecutive cycles.

[0119] The capacity retention rate (%) is calculated using the following formula:

[0120] Capacity retention rate = (Discharge capacity in the Nth cycle / Discharge capacity in the 3rd cycle) × 100%.

[0121] The test results are shown in Table 1.

[0122] Table 1 Test results of Examples 1-5 and Comparative Examples 1-3

[0123]

[0124] As shown in Table 1, the lithium iron phosphate prepared in Examples 1 to 5 exhibited significantly higher discharge specific capacities at all rates than those in Comparative Examples 1 to 3, especially at high rates (5C), where the performance advantage was even more pronounced. Furthermore, the capacity retention rate of the Example group after 500 cycles at 1C exceeded 97%, while the capacity retention rates of Comparative Examples 1 and 2 decreased to 93.5% and 89.3%, respectively. This result fully demonstrates that lithium iron phosphate prepared using a ternary composite ternary composite lithium iron phosphate and gas-phase assisted multi-stage sintering process can effectively improve the structural stability and electrochemical performance of the material. Through the synergistic effect of dynamic phosphorus source compensation and a mild reducing agent, not only was the iron-phosphorus ratio optimized, but the formation of a nitrogen-doped carbon coating layer was also promoted, significantly improving interfacial electronic conductivity, thereby achieving a dual improvement in high specific capacity and long cycle life.

[0125] Comparative Example 1, using conventional single iron phosphate as raw material, suffers from inferior elemental distribution uniformity compared to the ternary composite iron phosphate system. This results in uneven LiFePO4 nucleus growth rates during sintering, leading to localized particle agglomeration. This microstructural defect exacerbates the polarization effect during high-rate charge-discharge, significantly reducing the utilization rate of the active material. The cause of Comparative Example 2 is attributed to the lack of dynamic phosphorus replenishment via red phosphorus vapor and in-situ reduction, leading to three consequences: firstly, Fe... 3+ Not fully reduced to Fe 2+ The reasons for this are threefold: first, the presence of defect sites in the crystal structure; second, the loss of phosphorus source due to volatilization leading to a shift in stoichiometry; and third, insufficient graphitization of the carbon coating layer. In Comparative Example 3, the lack of nitrogen doping resulted in poor interfacial electronic conductivity of the carbon coating layer, hindering rapid electron transport. During charge and discharge, increased electron migration resistance within the electrode impeded the kinetics of the electrode reaction. This not only reduced the utilization rate of the active material, significantly decreasing the battery's discharge specific capacity at various rates, but also, during long-term cycling, intensified polarization within the electrode due to poor electron conduction, accelerating structural damage and performance degradation of the electrode material, thus leading to a substantial decrease in capacity retention.

[0126] 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 a lithium iron phosphate cathode material, characterized in that, Includes the following steps: (a) Provide ternary composite iron phosphate; The ternary composite ferric phosphate comprises a first ferric phosphate with an iron-phosphorus molar ratio of 0.965-0.975:1, a second ferric phosphate with an iron-phosphorus molar ratio of 0.975-0.985:1, and a third ferric phosphate with an iron-phosphorus molar ratio of 0.985-0.995:

1. (b) The ternary composite iron phosphate is mixed with a lithium source and ligand in a solvent to form a precursor solution. A nitrogen source solution is sprayed into the precursor solution in the form of a spray to obtain a slurry. The nitrogen source solution is prepared from polyvinylpyrrolidone and ammonium heptamolybdate; (c) The slurry obtained in step (b) is dried to obtain a solid precursor; (d) The solid precursor obtained in step (c) is subjected to a first-stage heat treatment under a protective atmosphere for low-temperature pre-carbonization, followed by a second-stage heat treatment in an atmosphere containing red phosphorus vapor for a medium-temperature gas phase reaction, and finally a third-stage heat treatment under an inert atmosphere to achieve high-temperature crystallization. After cooling, the lithium iron phosphate cathode material is obtained.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The ligand is at least one of aminotrimethylphosphonic acid, hydroxyethylidene diphosphonic acid, and diethylenetriaminepentamethylphosphonic acid.

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The mass ratio of the ternary composite iron phosphate to the ligand is 1:(0.05-0.3).

4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium source is at least one of lithium carbonate, lithium hydroxide, or lithium acetate.

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step (d), the temperature of the first stage of heat treatment is 300-500℃, the heating rate is 1-5℃ / min, and the holding time is 1-5 hours. The second stage of heat treatment involves raising the temperature from the end temperature of the first stage heat treatment to 550-600℃ at a heating rate of 0.5-3℃ / min, and holding it at that temperature for 2-6 hours. The third stage of heat treatment involves holding the temperature at 650-750℃ for 5-10 hours.

6. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In step (d), the red phosphorus vapor is obtained by heating solid red phosphorus to 280-320°C in a vaporization zone independent of the precursor sintering zone and then carrying it to the reaction zone by a protective gas.

7. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by the method described in any one of claims 1-6, wherein the lithium iron phosphate cathode material is a micron-sized secondary spherical particle composed of primary nanoparticles and has a nitrogen-doped carbon layer coated on its surface.

8. The lithium iron phosphate cathode material according to claim 7, characterized in that, The nanoparticles have a particle size of 50-150 nm, and the nitrogen-doped carbon layer has a thickness of 2-5 nm.

9. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery comprises the lithium iron phosphate positive electrode material as described in claim 8.

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