Nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate as well as preparation method and application thereof

Abstract

The invention discloses nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate as well as a preparation method and application thereof, and relates to the technical field of positive electrode materials of lithium ion batteries. The preparation method of the nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate comprises the following steps: taking nano iron oxide as an iron source, mixing the iron source with a lithium source, a phosphorus source and a nitrogen-containing organic matter by adding water, then adding a pH regulator to regulate the pH value of reaction slurry to 9.0-10.0, and stirring for reaction to obtain precursor slurry; mixing the precursor slurry with a cationic flocculant, and performing spray drying on the obtained mixed slurry to obtain spherical precursor powder; and performing thermal reaction on the spherical precursor powder to obtain the nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate. The nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate prepared by the method has high tap density and excellent electrochemical performance, and can be widely applied to preparation of lithium ion battery positive electrode materials.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode materials technology, and more specifically, to a nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LFP), as a cathode material for lithium-ion batteries, boasts significant advantages such as high safety, long cycle life, and low raw material cost, leading to its widespread application in power batteries and energy storage. However, existing LFP preparation processes still suffer from several shortcomings: traditional processes typically require energy-intensive sand milling to disperse the raw material into nanoscale particles, which is not only energy-intensive but also prone to particle morphology damage; during spray drying, particles agglomerate, and due to a lack of effective control measures, this agglomeration often results in loose aggregate structures with poor uniformity, affecting the sphericity and tap density of the product; furthermore, high-temperature heat treatment can easily lead to excessive grain growth, reducing the material's electrochemical performance. These problems limit further improvements in LFP material performance and reductions in production costs, necessitating the development of a new preparation method to overcome these technical bottlenecks.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to provide a nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, its preparation method, and its application.

[0005] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, comprising: Nano-iron oxide was used as the iron source, and mixed with lithium source, phosphorus source and nitrogen-containing organic matter and water. Then, a pH adjuster was added to adjust the pH value of the reaction slurry to 9.0-10.0, and the reaction was stirred to obtain the precursor slurry. The precursor slurry and cationic flocculant are mixed, and the resulting mixed slurry is spray-dried to obtain spherical precursor powder. The spherical precursor powder is thermally reacted to obtain nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate material.

[0006] In an optional embodiment, the preparation method of the nano-iron oxide includes: heat-treating ferrous oxalate dihydrate at 350-500°C for 1-3 hours under an inert atmosphere to obtain nano-iron oxide, wherein the nano-iron oxide has a particle size of 50-150 nm.

[0007] In an optional embodiment, the iron source, the lithium source, and the phosphorus source are mixed in a molar ratio of Li:Fe:P = (0.98-1.05):1:1; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium dihydrogen phosphate; And / or, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[0008] In an optional embodiment, the amount of nitrogen-containing organic matter added is 2%-10% of the total mass of the iron source, the lithium source, the phosphorus source, and the nitrogen-containing organic matter; And / or, the nitrogen-containing organic compound includes at least one of urea, melamine, histidine, polypyrrole, and sodium ethylenediaminetetramethylenephosphonate.

[0009] In an optional embodiment, the mass fraction of solid particles in the slurry is controlled at 20%-30%; And / or, the temperature of the stirring reaction is 50-70℃, the stirring speed is 300-500 r / min, and the time is 2-4 hours.

[0010] In an optional embodiment, the volume ratio of the precursor slurry to the cationic flocculant is (15-25):1; And / or, the mass percentage concentration of the solute in the cationic flocculant is 0.2%~1%; And / or, the mixing time of the precursor slurry and the cationic flocculant is 2-10 s; And / or, the solute of the cationic flocculant includes at least one of cationic polyacrylamide, polydimethyldiallylammonium chloride, polyethyleneimine, chitosan-grafted acrylamide copolymer, and starch-grafted cationic monomer copolymer.

[0011] In an optional embodiment, the inlet temperature of the spray dryer is 200-230°C, and the outlet temperature is 95-110°C.

[0012] In an optional embodiment, the thermal reaction includes holding the spherical precursor powder at 380-420°C for 3-5 hours, and then holding it at 550-620°C for 4-6 hours.

[0013] Secondly, the present invention provides a nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, which is prepared by the method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate as described in any of the foregoing embodiments.

[0014] Thirdly, the present invention provides the application of nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate as described in the above embodiments in the preparation of cathode materials for lithium-ion batteries.

[0015] The present invention has the following beneficial effects: This invention provides a method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate. Nano-iron oxide is used as the iron source, exhibiting higher sintering reactivity and significantly improving the efficiency and effectiveness of subsequent reactions. The nano-iron oxide has a small particle size and more uniform particle distribution. It is then mixed with a lithium source, a phosphorus source, and nitrogen-containing organic matter with water to form a slurry. The nitrogen-containing organic matter acts as both a nitrogen and carbon source, enabling nitrogen to be incorporated into the carbon coating layer during subsequent heat treatment, forming a highly conductive nitrogen-doped carbon layer. This invention optimizes the surface charge of the lithium iron phosphate precursor particles by precisely controlling the pH value of the reaction slurry, laying the foundation for the subsequent electrostatic adsorption of cationic flocculants by the lithium iron phosphate precursor particles. After pH adjustment, the zeta potential on the precursor particle surface is controlled to -30 to -45 mV, ensuring that the particles carry a sufficient and stable negative charge. During subsequent mixing with the cationic flocculant, electrostatic interaction allows the negatively charged precursor particles to adsorb and bridge with the positively charged flocculant, forming secondary spherical preforms. The resulting mixed slurry is spray-dried, which instantly fixes the dense "soft agglomerate" structure formed by electrostatic adsorption, forming an ideal spherical precursor. The subsequent heat treatment stage effectively prevents excessive particle growth, facilitating the formation of high-quality nitrogen-doped carbon and simultaneously strengthening the spherical structure. The method provided by this invention is interconnected and synergistic, achieving a balance between "high tap density" and "excellent electrochemical performance." The nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate prepared by the above method exhibits high tap density and excellent electrochemical performance. This nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate can be widely used in the preparation of cathode materials for lithium-ion batteries. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a SEM image of the nano-iron oxide prepared in Example 1 of the present invention; Figure 2 This is a SEM image of the nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate obtained in Example 1 of this invention. Figure 3 This is a SEM image of the nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate obtained in Comparative Example 1 of this invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0019] This invention provides a method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, comprising: using nano-iron oxide as an iron source, mixing it with a lithium source, a phosphorus source, and nitrogen-containing organic matter, adding water, then adding a pH adjuster to adjust the pH value of the reaction slurry to 9.0-10.0, stirring the reaction to obtain a precursor slurry; mixing the precursor slurry with a cationic flocculant, spray drying the resulting mixed slurry to obtain spherical precursor powder; and subjecting the spherical precursor powder to a thermal reaction to obtain nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate material.

[0020] In this invention, nano-iron oxide is selected as the iron source due to its high activity. It is then mixed with a lithium source, a phosphorus source, and nitrogen-containing organic matter with water to form a slurry. The nitrogen-containing organic matter acts as both a nitrogen and carbon source, allowing nitrogen to be incorporated into the carbon coating layer during subsequent heat treatment, forming a highly conductive nitrogen-doped carbon layer. This invention optimizes the surface charge of the lithium iron phosphate precursor particles by precisely controlling the pH value of the reaction slurry, laying the foundation for the subsequent electrostatic adsorption of cationic flocculants by the lithium iron phosphate precursor particles. After pH adjustment, the zeta potential on the precursor particle surface is controlled to -30 to -45 mV, ensuring that the particles carry a sufficient and stable negative charge. During subsequent mixing with the cationic flocculant, electrostatic interaction allows the negatively charged precursor particles to adsorb and bridge with the positively charged flocculant, forming secondary spherical preforms. The resulting mixed slurry is then spray-dried. Spray drying instantly fixes the dense "soft agglomerate" structure formed by electrostatic adsorption, forming ideal spherical precursors. The subsequent heat treatment stage effectively prevents excessive particle growth, facilitating the formation of high-quality nitrogen-doped carbon while strengthening the spherical structure. The method provided by this invention is interconnected and works synergistically to achieve a balance between "high tap density" and "excellent electrochemical performance".

[0021] Specifically, it includes the following steps: S1. Using nano-iron oxide as the iron source, it is mixed with lithium source, phosphorus source and nitrogen-containing organic matter and water. Then, a pH adjuster is added to adjust the pH value of the reaction slurry to 9.0-10.0, and the reaction is stirred to obtain the precursor slurry.

[0022] Iron, lithium, and phosphorus sources are mixed in a molar ratio of Li:Fe:P = (0.98-1.05):1:1; the amount of nitrogen-containing organic matter added is 2%-10% of the total mass of iron, lithium, phosphorus, and nitrogen-containing organic matter, and the mass fraction of solid particles in the reaction slurry is controlled at 20%-30%.

[0023] The nano-iron oxide in this invention can be commercially available or prepared in-house. Preferably, the nano-iron oxide is prepared in-house, exhibiting higher sintering reactivity and significantly improving the efficiency and effectiveness of subsequent reactions. The preparation method of the nano-iron oxide includes: heat-treating ferrous oxalate dihydrate at 350-500℃ for 1-3 hours under an inert atmosphere to obtain nano-iron oxide with a particle size of 50-150 nm.

[0024] The lithium source includes, but is not limited to, at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium dihydrogen phosphate; and / or, the phosphorus source includes, but is not limited to, at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; and / or, the nitrogen-containing organic matter includes, but is not limited to, at least one of urea, melamine, histidine, polypyrrole, and sodium ethylenediaminetetramethylenephosphonate. The nitrogen-containing organic matter in this invention can simultaneously serve as both a carbon and nitrogen source, enabling nitrogen to be doped into the carbon coating layer during subsequent heat treatment of lithium iron phosphate, forming a highly conductive nitrogen-doped carbon layer. Furthermore, it should be noted that when ammonium dihydrogen phosphate or diammonium hydrogen phosphate is selected as the phosphorus source, although the phosphorus source also contains nitrogen, ammonium dihydrogen phosphate or diammonium hydrogen phosphate decomposes during high-temperature sintering, and most of the nitrogen element (N) escapes as ammonia gas, leaving very little nitrogen in the bulk phase of the material, thus contributing limited to nitrogen doping in the final material. Therefore, the main source of nitrogen doping is the additional nitrogen-containing organic matter.

[0025] In this invention, the pH of the slurry is controlled at 9.0-10.0 to ensure that the surface of the lithium iron phosphate precursor particles has a sufficient and stable negative charge (Zeta potential -30~-45 mV), which makes the subsequent electrostatic adsorption with the cationic flocculant stronger and more uniform. This is the key to forming high-density, high-sphericity secondary particles.

[0026] A stirring reaction is then carried out. Stirring promotes the formation of precursors with specific morphologies from the iron, lithium, and phosphorus sources and nitrogen-containing organic compounds. The stirring operation breaks up particle agglomeration, ensuring that the iron, lithium, and phosphorus sources achieve a uniform molecular / ionic distribution in the liquid phase. This guarantees precise control of the molar ratio of Li, Fe, and P in subsequent reactions, preventing the appearance of impurities in the final product due to localized component segregation. The stirring reaction is conducted at a temperature of 50-70℃, a stirring speed of 300-500 r / min, and a time of 2-4 hours.

[0027] S2. Mix the precursor slurry and the cationic flocculant, and spray dry the resulting mixed slurry to obtain spherical precursor powder.

[0028] In this invention, the volume ratio of the precursor slurry to the cationic flocculant is (15-25):1; the mass percentage concentration of the solute in the cationic flocculant is 0.2%~1%; the mixing time of the precursor slurry and the cationic flocculant is 2-10s; wherein, the solute of the cationic flocculant includes at least one of cationic polyacrylamide, polydimethyldiallylammonium chloride, polyethyleneimine, chitosan-grafted acrylamide copolymer, and starch-grafted cationic monomer copolymer.

[0029] By instantaneously mixing the precursor slurry and cationic flocculant in a very short time (only 2-10 seconds), electrostatic attraction is used to adsorb and bridge the negatively charged precursor particles with the positively charged flocculant, forming secondary pellets. This invention controls the extremely short mixing time to ensure rapid and uniform adsorption-bridging reactions, avoiding premature mixing that could lead to flocculant failure or uneven adsorption. If the mixing time is too long, the cationic flocculant molecular chains will degrade and fail due to continuous shearing, resulting in asynchronous adsorption-bridging—particles that contact earlier will form large flocs, while particles that contact later will have insufficient bridging due to localized flocculant consumption, leading to a wider size distribution and poor uniformity of the secondary pellets. Furthermore, the formed soft agglomerates will break down or compact under prolonged shearing, resulting in poor sphericity, deteriorated particle size distribution, and reduced tap density of the spray-dried powder. Excessive flocculation may also increase slurry viscosity, clogging atomizing nozzles or drying tower walls, severely impacting production continuity and final product performance.

[0030] Subsequently, the obtained mixed slurry was immediately spray-dried at an inlet temperature of 200-230℃ and an outlet temperature of 95-110℃ to obtain spherical precursor powder.

[0031] S3. Spherical precursor powder is thermally reacted to obtain nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate material.

[0032] In this invention, the thermal reaction includes first holding the spherical precursor powder at 380-420°C for 3-5 hours, and then holding it at 550-620°C for 4-6 hours.

[0033] This invention employs a segmented heat treatment process. The first segment primarily involves the crystallization reaction of LiFePO4 at a temperature of 380-420℃. This low-temperature crystallization prevents excessive particle growth and initiates the decomposition and carbonization of nitrogen-containing organic matter. The second segment involves holding the material at 550-620℃ for 4-6 hours to completely carbonize any remaining organic matter, forming a highly conductive nitrogen-doped carbon layer and strengthening the connections between primary particles, thus enhancing the spherical structure. The final product is a spherical polycrystalline lithium iron phosphate product with nitrogen-doped carbon coating. Nitrogen atoms act as electron donors, and doping them into the carbon coating layer significantly enhances the intrinsic electronic conductivity of the carbon coating layer, establishing an efficient pathway for charge transport and directly improving the material's rate performance.

[0034] The nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate prepared by the above-described method exhibits high tap density and excellent electrochemical performance. This nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate can be widely used in the preparation of cathode materials for lithium-ion batteries.

[0035] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0036] Example 1 S1. Iron source preparation: 1 kg of ferrous oxalate dihydrate was heat-treated at 400 °C for 2 hours under nitrogen protection to obtain approximately 444 g of nano-iron oxide (particle size 50-150 nm).

[0037] S2. Slurry Preparation: Mix all of the above-mentioned nano-iron oxide, 211.6g lithium carbonate, 639.5g ammonium dihydrogen phosphate, 64g histidine (containing nitrogen-containing organic matter), and 5L deionized water. Add ammonia water dropwise to adjust the pH of the slurry to 9.5, and stir at 400 r / min for 3 hours at 60℃.

[0038] S3. Spray molding: Prepare a 0.5% cationic polyacrylamide solution, mix the slurry and flocculant solution at a volume ratio of 20:1 for 5 seconds using a static mixer, and then spray dry (inlet 220℃, outlet 105℃) to collect spherical precursor powder.

[0039] S4. Segmented heat treatment: The spherical precursor powder is placed in argon gas and first heat-treated at 400℃ for 4 hours, then heat-treated at 580℃ for 5 hours to obtain the final product, denoted as S1.

[0040] Example 2 S1. Iron source preparation: 1 kg of ferrous oxalate dihydrate was heat-treated at 350 °C for 3 hours under nitrogen protection to obtain approximately 444 g of nano-iron oxide (particle size 50-150 nm).

[0041] S2. Slurry Preparation: Mix all of the above-mentioned nano-iron oxide, 137.2g lithium hydroxide, 543.9g phosphoric acid, 64g urea (containing nitrogen-containing organic matter) with 4.76L deionized water. Add ammonia dropwise to adjust the pH of the slurry to 9, and stir at 300 r / min for 4 hours at 50℃.

[0042] S3. Spray molding: Prepare a 0.2% polydimethyldiallylammonium chloride solution. Mix the slurry and flocculant solution at a volume ratio of 15:1 for 2 seconds using a static mixer. Then spray dry (inlet 200℃, outlet 95℃) and collect spherical precursor powder.

[0043] S4. Segmented heat treatment: The spherical precursor powder is placed in argon gas and first heat-treated at 380℃ for 5 hours, then heat-treated at 550℃ for 6 hours to obtain the final product, denoted as S2.

[0044] Example 3 S1. Iron source preparation: 1 kg of ferrous oxalate dihydrate was heat-treated at 500 °C for 1 hour under nitrogen protection to obtain approximately 444 g of nano-iron oxide (particle size 50-150 nm).

[0045] S2. Slurry Preparation: Mix all of the above-mentioned nano-iron oxide, 394.44g lithium nitrate, 732.6g diammonium hydrogen phosphate, 45.12g polypyrrole (containing nitrogen-containing organic matter) with 6.64L deionized water. Add ammonia dropwise to adjust the pH of the slurry to 10, and stir at 500 r / min for 2 hours at 70℃.

[0046] S3. Spray molding: Prepare a 1% chitosan-grafted acrylamide copolymer solution. Mix the slurry and flocculant solution at a volume ratio of 25:1 for 10 seconds using a static mixer. Then spray dry (inlet 230℃, outlet 110℃) and collect spherical precursor powder.

[0047] S4. Segmented heat treatment: The spherical precursor powder is placed in argon gas and first heat-treated at 420℃ for 3 hours, then heat-treated at 620℃ for 4 hours to obtain the final product, denoted as S3.

[0048] Comparative Example 1 This comparative example is basically the same as Example 1, except that the decomposition of ferrous oxalate dihydrate in step S1 is omitted, and micron-sized Fe2O3 is used directly. The specific operation is as follows: S1. Weigh 444g of micron-sized Fe2O3.

[0049] S2. Slurry Preparation: Mix 444g of micron-sized Fe2O3, 211.6g of lithium carbonate, 639.5g of ammonium dihydrogen phosphate, and 64g of histidine (containing nitrogen-containing organic matter) with 5L of deionized water. Add ammonia dropwise to adjust the pH of the slurry to 9.5, and stir at 60℃ for 3 hours.

[0050] S3-S4 are the same as in Example 1.

[0051] The resulting product is denoted as D1.

[0052] This comparative example does not use nano-iron oxide derived from the decomposition of ferrous oxalate dihydrate, but instead uses an equimolar amount of ordinary micron-sized ferric oxide as the iron source. Due to the low activity and large particle size of the iron source, the slurry cannot form a stable colloid, resulting in poor electrostatic adsorption.

[0053] Comparative Example 2 This comparative example is basically the same as Example 1, except that the decomposition treatment of ferrous oxalate dihydrate in step S1 is omitted in this comparative example, and FeC2O4 is used directly. Using 2H2O as the iron source, the specific operation is as follows: S1. Weigh 1 kg of FeC2O4 2H2O.

[0054] S2. Slurry preparation: Take the above 1 kg FeC2O4... Mix 2H₂O, 211.6g lithium carbonate, 639.5g ammonium dihydrogen phosphate, 64g histidine (containing nitrogen-containing organic matter), and 7.66L deionized water. Add ammonia dropwise to adjust the pH of the slurry to 9.5, and stir at 60℃ for 3 hours.

[0055] S3-S4 are the same as in Example 1.

[0056] The resulting product is denoted as D2.

[0057] This comparative example does not involve the low-temperature decomposition step of ferrous oxalate dihydrate; instead, it directly uses equimolar amounts of ferrous oxalate dihydrate (FeC2O4). When ferrous oxalate dihydrate (2H2O) is added to the slurry as an iron source, the undecomposed ferrous oxalate dihydrate has low activity and poor particle dispersibility, and cannot provide effective nanoscale reaction sites.

[0058] Comparative Example 3 This comparative example is basically the same as Example 1, except that histidine is not used in this comparative example. Instead, sucrose of equal carbon content is physically mixed into the spray-dried spherical precursor powder. The specific operation is as follows: S1. Iron source preparation: 1 kg of ferrous oxalate dihydrate was heat-treated at 400°C for 2 hours under nitrogen protection to obtain approximately 444 g of nano-iron oxide.

[0059] S2. Slurry Preparation: Mix all the above-mentioned nano-iron oxide, 211.6g lithium carbonate, 639.5g ammonium dihydrogen phosphate, and 5L deionized water. Add ammonia water dropwise to adjust the pH of the slurry to 9.5, and stir at 60℃ for 3 hours.

[0060] S3. Spray molding: Prepare a 0.5% cationic polyacrylamide solution, mix the slurry and flocculant solution at a volume ratio of 20:1 for 5 seconds using a static mixer, and then spray dry (inlet 220℃, outlet 105℃) to collect spherical precursor powder.

[0061] S4. Segmented heat treatment: The spherical precursor powder is mixed with sucrose (70.6g) and then placed in argon gas. It is first heat-treated at 400℃ for 4 hours and then heat-treated at 580℃ for 5 hours to obtain the final product.

[0062] The resulting product is designated as D3.

[0063] Comparative Example 4 This comparative example is essentially the same as Example 1, except that the pH value is not adjusted in this comparative example, and the pH adjustment step in step S1 is omitted. The natural pH of the slurry is approximately 6.0. The remaining steps are exactly the same as in Example 1. The resulting product is denoted as D4.

[0064] Comparative Example 5 This comparative example is basically the same as Example 1, except that the two heat treatment stages in step S4 are combined into one: direct heat treatment at 580°C for 9 hours. The resulting product is denoted as D5.

[0065] Comparative Example 6 This comparative example is essentially the same as Example 1, except that step S4 differs from that in Example 1. In this comparative example, the spherical precursor powder is placed in argon gas and kept at 350°C for 4 hours, followed by a heating at 650°C for 5 hours. The remaining steps are exactly the same as in Example 1. The resulting product is designated D6.

[0066] Comparative Example 7 This comparative example is basically the same as Example 1, except that in this comparative example, the slurry and flocculant are not instantaneously mixed using a static mixer. Instead, the flocculant solution is added directly during the preparation of the precursor slurry in step S1. That is, the flocculant is mixed synchronously with the raw materials such as the lithium source, phosphorus source, and iron source. The remaining steps are exactly the same as in Example 1. The resulting product is denoted as D7.

[0067] Comparative Example 8 This comparative example is basically the same as Example 1, except that in this comparative example, the slurry and flocculant solution are mixed at a volume ratio of 20:1 for 1 minute using a static mixer. The resulting product is denoted as D8.

[0068] Experimental Example (1) The tap density of the spherical lithium iron phosphate cathode materials obtained in Examples 1-3 and Comparative Examples 1-8 was tested, and the test results are shown in Table 1; (2) The spherical lithium iron phosphate cathode materials obtained in Examples 1-3 and Comparative Examples 1-8 were used to prepare coin cells, and the electrochemical performance was tested. The test results are shown in Table 1. Specifically, the positive electrode preparation involves mixing the prepared lithium iron phosphate positive electrode material, carbon black, polyvinylidene fluoride, and N-methylpyrrolidone in a mass ratio of 90:5:5:40. After high-speed mixing for 30 minutes, a positive electrode slurry is obtained. This slurry is then evenly coated onto aluminum foil using a scraper and placed in a forced-air drying oven at 120°C for 20 minutes. The dried electrode sheet is then rolled and cut to obtain the positive electrode. The coin cell test uses equal volumes of LiPF6 and diethyl carbonate (DEC) as electrolytes, with a LiPF6 concentration of 1 mol / L. A lithium metal sheet is used as the negative electrode to prepare a coin cell half-cell. The test is conducted using a LAND battery programmable controller (LAND). The CT2001A is used to test battery performance. The coin cell rate test conditions are as follows: at room temperature of 25℃, the 0.1C three-cycle, 0.5C three-cycle, and 10C three-cycle tests are performed, with a voltage range of 2-4.3V. The 0.1C three-cycle test is used to activate the coin cell, followed by the testing of the 0.5C and 10C capacities.

[0069] Table 1. Statistical table of test results for different samples

[0070] The samples prepared in Examples 1-3 of this application have higher tap density, and their 0.5C capacity and 10C capacity are significantly better than those prepared in Comparative Examples 1-8.

[0071] A comparison of S1 with D1 and D2 illustrates the crucial importance of nano-iron oxide: D1, due to its low iron source activity and large particle size, suffers from incomplete reaction, poor morphology, and a sharp deterioration in various properties. Figure 1 The SEM images of the nano-iron oxide obtained in Example 1 show that the derived nano-iron oxide particles have high dispersibility, small particle size (50-100 nm), larger specific surface area, and high sintering reactivity. Furthermore, the SEM images of the finished products from Example 1 and Comparative Example 1 (…) Figure 2 and Figure 3 The results also showed that the particle distribution of Example 1 was more uniform and the spherical shape was more complete, while Comparative Example 1 was more dispersed and contained more abnormally large particles. D2, due to the direct use of undecomposed ferrous oxalate dihydrate, had insufficient activity and poor dispersibility, and could not form a stable electrostatic adsorption structure. The tap density and electrochemical performance of both were significantly inferior to those of S1, proving that only nano-Fe2O3 obtained by low-temperature decomposition of ferrous oxalate dihydrate can meet the high-performance requirements.

[0072] The comparison between S1 and D3 demonstrates the superiority of in-situ addition of nitrogen-containing organic matter (histidine). D3, which uses sucrose for physical mixing, has uneven carbon layer coating and no nitrogen doping, resulting in poor conductivity and a significantly lower high-rate (10C) capacity than S1. This indicates that in-situ addition of nitrogen-containing organic matter can form a superior carbon-nitrogen network.

[0073] The comparison between S1 and D4 demonstrates the crucial role of precise pH control. At natural pH (6.0), D4 has insufficient surface charge (Zeta potential -10 to -15 mV), resulting in poor electrostatic adsorption, poor sphericity, loose structure, and a significant decrease in tap density and capacity, especially rate performance.

[0074] The comparison between S1 and D5 and D6 demonstrates the special nature and necessity of the low-temperature segmented heat treatment in this application: D5 adopts a one-stage heat treatment, resulting in uneven graphitization of the carbon layer and weak interparticle connections; although D6 is a two-stage heat treatment, the temperature exceeds the range defined in this application, the low-temperature stage (350℃) has incomplete crystallization, and the high-temperature stage (650℃) leads to excessive particle growth. The overall performance of both is inferior to that of S1, indicating that the temperature range defined in this application is the key to achieving optimal performance.

[0075] The comparison between S1 and D7 proves the rationality of the flocculant addition position: D7 adds the flocculant simultaneously during slurry preparation, causing the flocculant to react with various ions in advance and become ineffective, failing to achieve effective electrostatic adsorption bridging, resulting in a loose spherical structure, a significant decrease in tap density and electrochemical performance, proving that instantaneous mixing in a static mixer is the key process to ensure flocculation effect.

[0076] The comparison between S1 and D8 demonstrates the importance of flocculant mixing time. Excessive mixing time leads to flocculant shear degradation and asynchronous bridging, resulting in uneven secondary pellet size distribution, and the breakage or densification of soft agglomerates. This will cause poor powder sphericity, deteriorated particle size distribution, reduced tap density, and worsened electrical properties after spray drying. This proves the importance of controlling flocculant mixing time.

[0077] In summary, the method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate provided by this invention uses nano-iron oxide as the iron source, which has higher sintering reactivity and can significantly improve the efficiency and effect of subsequent reactions. The nano-iron oxide has a small particle size and more uniform particle distribution. Subsequently, it is mixed with a lithium source, a phosphorus source, and nitrogen-containing organic matter with water to form a slurry. The nitrogen-containing organic matter acts as both a nitrogen source and a carbon source, enabling nitrogen to be incorporated into the carbon coating layer during subsequent heat treatment, forming a highly conductive nitrogen-doped carbon layer. This invention optimizes the surface charge of the lithium iron phosphate precursor particles by precisely controlling the pH value of the reaction slurry, laying the foundation for the subsequent electrostatic adsorption of cationic flocculants by the lithium iron phosphate precursor particles. After pH adjustment, the zeta potential on the surface of the precursor particles is controlled to -30 to -45 mV, ensuring that the particles carry a sufficient and stable negative charge. During subsequent mixing with the cationic flocculant, electrostatic interaction allows the negatively charged precursor particles to adsorb and bridge with the positively charged flocculant, forming secondary spherical preforms. The resulting mixed slurry is spray-dried, which instantly fixes the dense "soft agglomerate" structure formed by electrostatic adsorption, forming an ideal spherical precursor. The subsequent heat treatment stage effectively prevents excessive particle growth, facilitating the formation of high-quality nitrogen-doped carbon and simultaneously strengthening the spherical structure. The method provided by this invention is interconnected and synergistic, achieving a balance between "high tap density" and "excellent electrochemical performance." The nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate prepared by the above method exhibits high tap density and excellent electrochemical performance. This nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate can be widely used in the preparation of cathode materials for lithium-ion batteries.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, characterized in that, It includes: Nano-iron oxide was used as the iron source, and mixed with lithium source, phosphorus source and nitrogen-containing organic matter and water. Then, a pH adjuster was added to adjust the pH value of the reaction slurry to 9.0-10.0, and the reaction was stirred to obtain the precursor slurry. The precursor slurry and cationic flocculant are mixed, and the resulting mixed slurry is spray-dried to obtain spherical precursor powder. The spherical precursor powder is thermally reacted to obtain nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate.

2. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The preparation method of the nano-iron oxide includes: heat-treating ferrous oxalate dihydrate at 350-500℃ for 1-3 hours under an inert atmosphere to obtain nano-iron oxide, wherein the particle size of the nano-iron oxide is 50-150 nm.

3. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The iron source, the lithium source, and the phosphorus source are mixed in a molar ratio of Li:Fe:P = (0.98-1.05):1:1; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium dihydrogen phosphate; And / or, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

4. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The amount of nitrogen-containing organic matter added is 2%-10% of the total mass of the iron source, the lithium source, the phosphorus source, and the nitrogen-containing organic matter; And / or, the nitrogen-containing organic compound includes at least one of urea, melamine, histidine, polypyrrole, and sodium ethylenediaminetetramethylenephosphonate.

5. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The mass fraction of solid particles in the reaction slurry is controlled at 20%-30%; And / or, the temperature of the stirring reaction is 50-70℃, the stirring speed is 300-500 r / min, and the time is 2-4 hours.

6. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The volume ratio of the precursor slurry to the cationic flocculant is (15-25):1; And / or, the mass percentage concentration of the solute in the cationic flocculant is 0.2%~1%; And / or, the mixing time of the precursor slurry and the cationic flocculant is 2-10 s; And / or, the solute of the cationic flocculant includes at least one of cationic polyacrylamide, polydimethyldiallylammonium chloride, polyethyleneimine, chitosan-grafted acrylamide copolymer, and starch-grafted cationic monomer copolymer.

7. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The spray dryer has an inlet temperature of 200-230℃ and an outlet temperature of 95-110℃.

8. The method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate according to claim 1, characterized in that, The thermal reaction involves first holding the spherical precursor powder at 380-420°C for 3-5 hours, and then holding it at 550-620°C for 4-6 hours.

9. A nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate, characterized in that, It is prepared by the method for preparing nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate as described in any one of claims 1-8.

10. The application of nitrogen-doped carbon-coated spherical polycrystalline lithium iron phosphate as described in claim 9 in the preparation of cathode materials for lithium-ion batteries.

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