Highly compacted lithium iron phosphate with doping-induced in-situ bimodal particle size distribution and preparation method thereof

Abstract

The application belongs to the technical field of lithium ion battery cathode material, and particularly relates to a high-compaction lithium iron phosphate with in-situ bimodal particle size distribution induced by doping and a preparation method thereof. The method synchronously introduces a multi-element doping system with different grain growth regulation effects in the synthesis and sintering process of lithium iron phosphate precursors, utilizes the synergistic effect of different doping ions on grain nucleation and growth to form a bimodal particle size distribution structure in which a large particle skeleton phase and a small particle filling phase coexist in a single reaction system. The application directly realizes particle grading through doping regulation without a later grading and physical mixing process, effectively improves the compaction density of lithium iron phosphate material, optimizes the ion diffusion efficiency and electronic conductivity performance, and solves the technical problems that the existing high-compaction lithium iron phosphate preparation process is complex, particle distribution is uneven, and the electrochemical performance and compaction density are difficult to balance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a high-pressure lithium iron phosphate with doping-induced in-situ bimodal particle size distribution and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as one of the core cathode materials for lithium-ion batteries, boasts advantages such as low cost, good thermal stability, and long cycle life, and is widely used in new energy vehicles, energy storage power stations, and other fields. With the increasing demands for battery energy density and volumetric energy density in new energy equipment, improving the compaction density of lithium iron phosphate materials has become a key technological breakthrough. Currently, the industry's mainstream high-compact lithium iron phosphate compaction density target has been raised to 2.55 g / cm³. 3 The above figures are for some high-end products, with some reaching 2.70 g / cm³. 3 .

[0003] Currently, the mainstream method for improving the compaction density of lithium iron phosphate is particle size distribution technology. This involves physically mixing lithium iron phosphate particles of different sizes, using large particles as a framework and small particles to fill the gaps between them to achieve close packing. However, existing processes mostly adopt a "separate preparation of large and small particles - subsequent physical mixing" approach, which suffers from problems such as cumbersome procedures, high production costs, uneven particle mixing leading to segregation, and poor batch stability. For example, the patent with publication number CN121307007A uses a compounding method of lithium iron phosphate with two different particle sizes to improve compaction density, but it is essentially still a physical mixing distribution method and fails to solve the technical pain point of uneven mixing.

[0004] Meanwhile, a single physical gradation is insufficient to simultaneously address the electrochemical performance of materials, often resulting in a contradiction between increased compaction density and decreased ion diffusion efficiency and conductivity. Furthermore, some existing technologies attempt to regulate bimodal structures through additives. For example, patent CN116924376A introduces silica aerogel to construct a bimodal structure in the iron phosphate precursor stage before preparing lithium iron phosphate. The bimodal structure is formed in the precursor stage and relies on inorganic additive regulation, which is completely different from the doping-induced regulation mechanism. Another patent, CN121565858A, uses bulk doping and carbon coating to synergistically optimize lithium iron phosphate performance, but it does not involve particle size distribution regulation and therefore cannot achieve a significant increase in compaction density.

[0005] To address these issues, in-situ gradation technology has gradually become a research hotspot. However, existing technologies struggle to precisely control the ratio and distribution of large and small particles, resulting in limited improvement in compaction density and minimal optimization of electrochemical performance.

[0006] Therefore, overcoming the uncontrollable particle size distribution in the in-situ gradation technology of lithium iron phosphate is a technical problem that urgently needs to be solved in this field.

[0007] It should be noted that the information disclosed in this background section is only for understanding the background technology of the present application concept, and therefore, the above description is not considered to constitute prior art information. Summary of the Invention

[0008] This disclosure provides at least one method for preparing high-compact lithium iron phosphate with doping-induced in-situ bimodal particle size distribution. By designing a multi-element doping system with differentiated grain growth control, a bimodal structure with large-particle framework phase and small-particle filling phase coexisting is precisely induced in a single reaction system, eliminating the need for subsequent mixing processes. A continuous carbon coating layer is then formed through dry carbon doping and low-temperature secondary sintering. This simultaneously improves compaction density and electrochemical performance without damaging the in-situ bimodal structure, solving the technical problems of cumbersome physical gradation processes, uneven mixing, and the difficulty in achieving balanced performance in single doping and additive-induced bimodal technologies.

[0009] In a first aspect, embodiments of this disclosure provide a method for preparing high-pressure lithium iron phosphate, comprising the following steps: S1: Precursor preparation: mixing an iron source, a phosphorus source, and a lithium source in a solvent to form a uniform precursor sol or slurry; adding a main dopant and a secondary dopant to the mixture and stirring until uniformly mixed to obtain a doped precursor mixture; wherein the main dopant is at least one of TiO2, V2O5, and MgO; and the secondary dopant is at least one of Nb2O5, ZrO2, and Y2O3; S2: Drying treatment: drying the... S3: Segmented sintering: The doped precursor mixture is spray-dried to obtain doped precursor powder; S4: Segmented sintering: The doped precursor powder is placed in a sintering furnace and sintered in segments under a protective atmosphere, and low-temperature pre-sintering and high-temperature main sintering are completed in sequence. After cooling, crude lithium iron phosphate with in-situ bimodal particle size distribution is obtained; S5: Dry carbon doping and secondary sintering: Carbon source is added to the crude lithium iron phosphate and mixed dry, and then low-temperature secondary sintering is carried out under a protective atmosphere to pyrolyze the carbon source to form a carbon coating layer. After crushing and classification, high-pressure lithium iron phosphate is obtained.

[0010] In one optional embodiment, the molar ratio of the metal elements of the main dopant to the auxiliary dopant is 1:0.2 to 1:0.5; and the doping amount of the main dopant is 0.5% to 2.0% of the theoretical mass of lithium iron phosphate, and the doping amount of the auxiliary dopant is 0.1% to 0.8% of the theoretical mass of lithium iron phosphate.

[0011] In an optional embodiment, in S1, the molar ratio of the iron source, phosphorus source, and lithium source is Fe:P:Li = 1:1:1.02 to 1.08; the iron source is selected from at least one of ferrous oxalate, ferric phosphate, and iron oxide red; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium nitrate; the solvent is deionized water and / or anhydrous ethanol, and the mass ratio of the solvent to the total of the iron source, phosphorus source, and lithium source is 5:1 to 10:1.

[0012] In one optional embodiment, in S2, the spray drying conditions are: inlet air temperature 180-220°C, outlet air temperature 80-100°C, spray pressure 0.2-0.4 MPa, and feed rate 50-100 mL / min.

[0013] In an optional embodiment, in S3, the protective atmosphere is one or a mixture of nitrogen and argon; the low-temperature pre-sintering temperature is 300–400°C, and the holding time is 1–2 h, used to achieve preliminary crystallization of the precursor and volatilization of impurities; the high-temperature main sintering temperature is 650–750°C, and the holding time is 3–5 h, used to achieve synergistic regulation of grain growth promoted by the main dopant and grain growth inhibited by the auxiliary dopant; the heating rate of the segmented sintering is 2–5°C / min, used to avoid abnormal grain growth.

[0014] In an optional embodiment, in S4, the dry carbon doping uses at least one of sucrose, glucose, and carbon black as the carbon source, and the amount of carbon source added is 1% to 3% of the crude lithium iron phosphate mass; the secondary sintering temperature is 350 to 550°C, and the holding time is 1 to 3 hours; the protective atmosphere is one or a mixture of nitrogen and argon, so that the carbon source is pyrolyzed to form a continuous carbon coating layer without destroying the in-situ bimodal particle size structure.

[0015] In an optional embodiment, in S4, the particle size of the high-pressure lithium iron phosphate exhibits a bimodal distribution, with the two main peaks located at 0.4–1.0 μm and 2.0–4.0 μm, respectively, and the D50 is 1.5–2.5 μm.

[0016] Secondly, embodiments of this disclosure also provide a high-density lithium iron phosphate prepared by the method described above, wherein the high-density lithium iron phosphate has a compaction density of 2.65–2.75 g / cm³. 3 .

[0017] Thirdly, this disclosure also provides a lithium-ion battery, including the high-pressure lithium iron phosphate as described above; the lithium-ion battery has a 0.1C initial discharge specific capacity ≥160 mAh / g, a 1C rate discharge capacity retention rate ≥95%, and a 500-cycle capacity retention rate ≥90%.

[0018] The beneficial effects of this invention are as follows: the high-density lithium iron phosphate with doping-induced in-situ bimodal particle size distribution and its preparation method, by setting a main dopant that promotes grain growth and an auxiliary dopant that inhibits grain growth, directly induces the formation of an in-situ bimodal particle size distribution during the synthesis process. The doping elements not only regulate the particle size distribution but also enter the interstitial space to stabilize the crystal structure. Combined with dry carbon doping and secondary sintering, a continuous and robust carbon coating layer can be formed, while improving ion diffusion efficiency and electronic conductivity, thus resolving the technical contradiction of "high density - low performance". In addition, the large-particle framework phase and the small-particle filling phase formed in-situ by doping induce synergistically achieve close packing, effectively reducing the porosity of the material. The compaction density is increased by 15% to 20% compared with traditional single-particle-size lithium iron phosphate and by 2.7% to 6.6% compared with existing physically mixed graded lithium iron phosphate, significantly increasing the battery compaction density to over 2.65 g / cm³, which is higher than the performance indicators of most existing high-density lithium iron phosphate.

[0019] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained through the structures particularly pointed out in the description and the drawings.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a SEM image of a high-pressure LiFePO4 material provided in Example 1; Figure 2 This is a particle size distribution curve of a high-pressure compacted LiFePO4 material provided in Example 1. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] As used herein, the phrases “in one embodiment,” “according to one embodiment,” “in some embodiments,” etc., generally refer to the fact that a particular feature, structure, or characteristic following the phrase can be included in at least one embodiment of this disclosure. Therefore, a particular feature, structure, or characteristic can be included in more than one embodiment of this disclosure, such that these phrases do not necessarily refer to the same embodiment. As used herein, the terms “example,” “exemplary,” etc., are used to “serve as an example, instance, or illustration.” Any implementation, aspect, or design described herein as “example” or “exemplary” is not necessarily to be construed as preferred or superior to other implementations, aspects, or designs. Rather, the use of the terms “example,” “exemplary,” etc., is intended to present concepts in a specific manner.

[0025] In this document, exemplary embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. As used herein, expressions such as “at least one of…” modify the entire list of elements when following a list of elements, rather than individual elements in the list. For example, the expression “at least one of a, b, and c” should be understood to include only a, only b, only c, both a and b, both a and c, both b and c, or all of a, b, and c.

[0026] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0027] The main dopant and auxiliary dopant described in this application are both fed in the form of metal oxides. By utilizing the strong chemical stability and resistance to moisture absorption and decomposition of metal oxides, it is possible to achieve precise metering of dopant components, avoid the rapid dissolution of soluble salts that causes local ion segregation, and match the segmented sintering temperature rise regime, so that the atoms slowly diffuse and dissolve into the lithium iron phosphate lattice during high-temperature sintering, and stably achieve differentiated grain control. At the same time, it avoids the large-scale decomposition of dopant salts at low temperatures that would generate gas and increase the porosity of the powder, thus ensuring the high-compaction characteristics of the final material.

[0028] This disclosure provides a method for preparing high-pressure lithium iron phosphate, comprising the following steps: S1: Precursor preparation: Iron source, phosphorus source, and lithium source are mixed in a solvent to form a uniform precursor sol or slurry; a main dopant and a secondary dopant are added to the mixture, and the mixture is stirred until homogeneous to obtain a doped precursor mixture; the main dopant is at least one of TiO2, V2O5, and MgO; the secondary dopant is at least one of Nb2O5, ZrO2, and Y2O3; S2: Drying treatment: The doped precursor... S3: Segmented sintering: The doped precursor powder is placed in a sintering furnace and sintered in segments under a protective atmosphere, successively completing low-temperature pre-sintering and high-temperature main sintering. After cooling, crude lithium iron phosphate with in-situ bimodal particle size distribution is obtained; S4: Dry carbon doping and secondary sintering: Carbon source is added to the crude lithium iron phosphate and mixed dry, and then low-temperature secondary sintering is carried out under a protective atmosphere to pyrolyze the carbon source to form a carbon coating layer. After crushing and classification, high-pressure lithium iron phosphate is obtained.

[0029] In some embodiments, specifically, the molar ratio of the metal elements of the main dopant to the auxiliary dopant is 1:0.2 to 1:0.5; and the doping amount of the main dopant is 0.5% to 2.0% of the theoretical mass of lithium iron phosphate, and the doping amount of the auxiliary dopant is 0.1% to 0.8% of the theoretical mass of lithium iron phosphate.

[0030] Specifically, both the primary and secondary dopants are added in the form of oxides, which are slowly dispersed in the precursor sol or slurry to avoid excessively high local concentrations that could lead to abnormal grain growth. During the high-temperature main sintering stage at 650-750℃, the primary dopants Ti, V, and Mg dissolve into the LiFePO4 lattice, accelerating the grain boundary migration rate and promoting grain growth. The secondary dopants Nb, Zr, and Y are enriched at the grain boundaries, generating steric hindrance and inhibiting grain growth. This invention achieves differentiated grain growth by leveraging the synergistic effect of dual doping during the high-temperature sintering stage, which differs from existing technologies that rely on single doping and physical mixing.

[0031] In some embodiments, specifically in S1, the molar ratio of the iron source, phosphorus source, and lithium source is Fe:P:Li = 1:1:1.02 to 1.08; the iron source is selected from at least one of ferrous oxalate, ferric phosphate, and iron oxide red; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium nitrate; the solvent is deionized water and / or anhydrous ethanol, and the mass ratio of the solvent to the total of the iron source, phosphorus source, and lithium source is 5:1 to 10:1.

[0032] In some embodiments, specifically in S2, the spray drying conditions are: inlet air temperature 180-220°C, outlet air temperature 80-100°C, spray pressure 0.2-0.4 MPa, and feed rate 50-100 mL / min.

[0033] Specifically, the drying process parameters have been optimized to ensure the drying effect of the precursor powder while avoiding the agglomeration of dopant ions caused by high temperature, which is different from the problem of poor particle morphology caused by excessively high or low parameters in existing spray drying.

[0034] In some embodiments, specifically in S3, the protective atmosphere is one or a mixture of nitrogen and argon; the low-temperature pre-sintering temperature is 300–400°C, and the holding time is 1–2 h, used to achieve preliminary crystallization of the precursor and volatilization of impurities; the high-temperature main sintering temperature is 650–750°C, and the holding time is 3–5 h, used to achieve synergistic regulation of the main dopant promoting grain growth and the auxiliary dopant inhibiting grain growth; the heating rate of the segmented sintering is 2–5°C / min, used to avoid abnormal grain growth.

[0035] Specifically, in the segmented sintering stage, the main dopant promotes the rapid growth of some grains to form a large-particle framework phase, while the auxiliary dopant inhibits the growth of some grains, retaining a small-particle filling phase. The synergistic effect of these two dopants achieves an in-situ bimodal particle size distribution. After sintering, the material is cooled to room temperature in the furnace. This segmented sintering process is precisely matched to the doping system. Through the synergy of low-temperature pre-sintering and high-temperature main sintering, it ensures normal grain crystallization while achieving in-situ differentiation of large and small particles, unlike existing single-temperature sintering methods that cannot precisely control particle size distribution. In some embodiments, specifically in S4, the dry carbon doping uses at least one of sucrose, glucose, and carbon black as the carbon source, and the amount of carbon source added is 1% to 3% of the crude lithium iron phosphate mass; the secondary sintering temperature is 350 to 550°C, and the holding time is 1 to 3 hours; the protective atmosphere is one or a mixture of nitrogen and argon, so that the carbon source is pyrolyzed to form a continuous carbon coating layer without destroying the in-situ bimodal particle size structure.

[0036] Specifically, the dry carbon doping process simplifies procedures and reduces costs. At the same time, in synergy with the in-situ bimodal structure, it not only improves the conductivity of the material but also avoids particle agglomeration caused by wet carbon doping, further optimizing the balance between compaction density and electrochemical performance.

[0037] In some embodiments, specifically in S4, the particle size of the high-pressure lithium iron phosphate exhibits a bimodal distribution, with the two main peaks located at 0.4–1.0 μm and 2.0–4.0 μm, respectively, and the D50 is 1.5–2.5 μm.

[0038] This disclosure also provides a high-density lithium iron phosphate prepared by the method described above, wherein the high-density lithium iron phosphate has a compaction density of 2.65–2.75 g / cm³. 3 .

[0039] This disclosure also provides a lithium-ion battery, including the high-pressure lithium iron phosphate as described above; the lithium-ion battery has a 0.1C initial discharge specific capacity ≥160 mAh / g, a 1C rate discharge capacity retention rate ≥95%, and a 500-cycle capacity retention rate ≥90%.

[0040] Example 1: S1, Precursor preparation: Ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate were dissolved in deionized water at a molar ratio of Fe:P:Li = 1:1:1.05, with a solvent-to-solute mass ratio of 8:1, to form a homogeneous slurry; TiO2 (doping amount of 1.0% of the theoretical mass of lithium iron phosphate) and Nb2O5 (doping amount of 0.4% of the theoretical mass of lithium iron phosphate) were added to the slurry, and the mixture was stirred for 3 hours until homogeneous to obtain the precursor slurry.

[0041] S2, Drying treatment: The precursor slurry is spray-dried, with the inlet air temperature controlled at 200℃, the outlet air temperature at 90℃, the spray pressure at 0.3 MPa, and the feed rate at 80 mL / min, to obtain the doped precursor powder.

[0042] S3, Segmented sintering: The precursor powder is placed under a nitrogen protective atmosphere and heated to 350℃ for low-temperature pre-sintering at a heating rate of 3℃ / min, and held for 1.5 h. Then, it is heated to 700℃ for high-temperature main sintering and held for 4 h. After the holding period, it is cooled to room temperature with the furnace. After cooling, crude lithium iron phosphate is obtained.

[0043] S4, Dry carbon doping and secondary sintering: 2wt% carbon black is added to crude lithium iron phosphate and mixed evenly by dry method. Secondary low-temperature sintering is carried out at 450℃ under nitrogen atmosphere and held for 2 hours. After cooling, it is crushed and graded to obtain the finished product.

[0044] Example 2: S1, Precursor preparation: Iron phosphate, diammonium hydrogen phosphate, and lithium hydroxide were dissolved in anhydrous ethanol at a molar ratio of Fe:P:Li = 1:1:1.02, with a solvent-to-solute mass ratio of 5:1, to form a homogeneous sol; the main dopant V2O5 (doping amount of 0.5% of the theoretical mass of lithium iron phosphate) and the auxiliary dopant ZrO2 (doping amount of 0.3% of the theoretical mass of lithium iron phosphate) were added to the sol, and the mixture was stirred for 2 hours until homogeneous to obtain a precursor slurry.

[0045] S2, Drying treatment: The precursor slurry is spray-dried, with the inlet air temperature controlled at 180℃, the outlet air temperature at 80℃, the spray pressure at 0.2 MPa, and the feed rate at 50 mL / min, to obtain the doped precursor powder.

[0046] S3, Segmented sintering: The precursor powder is placed under an argon protective atmosphere and heated to 300℃ for low-temperature pre-sintering at a heating rate of 2℃ / min, and held for 2h. Then, it is heated to 650℃ for high-temperature main sintering and held for 5h. After the holding period, it is cooled to room temperature with the furnace. After cooling, crude lithium iron phosphate is obtained.

[0047] S4, Dry carbon doping and secondary sintering: 1 wt% sucrose is added to crude lithium iron phosphate and mixed. Secondary low-temperature sintering is carried out at 400℃ under argon atmosphere and held for 2 hours. After cooling, it is crushed and graded to obtain the finished product. Example

[0048] S1, Precursor Preparation: Iron oxide red, phosphoric acid, and lithium carbonate were dissolved in a deionized water-anhydrous ethanol mixed solvent at a molar ratio of Fe:P:Li = 1:1:1.08, with a solvent-solute mass ratio of 10:1, to form a homogeneous slurry. The main dopant MgO (doping amount of 1.0% of the theoretical mass of lithium iron phosphate) and the auxiliary dopant Y2O3 (doping amount of 0.8% of the theoretical mass of lithium iron phosphate) were added to the slurry, and the mixture was stirred for 4 hours until homogeneous to obtain the precursor slurry.

[0049] S2, Drying treatment: The precursor slurry is spray-dried, with the inlet air temperature controlled at 220℃, the outlet air temperature at 100℃, the spray pressure at 0.4 MPa, and the feed rate at 100 mL / min, to obtain the doped precursor powder.

[0050] S3, Segmented sintering: The precursor powder is placed under a nitrogen-argon mixed gas and heated to 400℃ for low-temperature pre-sintering at a heating rate of 5℃ / min and held for 1 hour. Then, it is heated to 750℃ for high-temperature main sintering and held for 3 hours. After the holding period, it is cooled to room temperature with the furnace. After cooling, crude lithium iron phosphate is obtained.

[0051] S4, Dry carbon doping and secondary sintering: 3wt% glucose / carbon black mixed carbon source is added to crude lithium iron phosphate, and secondary low-temperature sintering is carried out at 500℃ under nitrogen-argon mixed gas and held for 1.5h. After cooling, it is crushed and graded to obtain the finished product.

[0052] Comparative Example 1, traditional single-particle-size preparation, specifically including: S1, Precursor preparation: Ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate were dissolved in deionized water at a molar ratio of Fe:P:Li = 1:1:1.05 without adding any dopants. The mixture was stirred for 3 h until homogeneous to obtain the precursor slurry.

[0053] S2, Drying treatment: The precursor slurry is spray-dried, with the inlet air temperature controlled at 200℃, the outlet air temperature at 90℃, the spray pressure at 0.3 MPa, and the feed rate at 80 mL / min, to obtain precursor powder.

[0054] S3, Segmented sintering: The precursor powder is placed under a nitrogen protective atmosphere and heated to 350℃ for low-temperature pre-sintering at a heating rate of 3℃ / min, and held for 1.5 h. Then, it is heated to 700℃ for high-temperature main sintering and held for 4 h. After the holding period, it is cooled to room temperature with the furnace to obtain crude lithium iron phosphate with single particle size.

[0055] S4, Dry carbon doping and secondary sintering: After crushing and classifying the crude lithium iron phosphate with single particle size, 2% carbon black is added for dry carbon doping. The product is then subjected to secondary low-temperature sintering at 450℃ under a nitrogen atmosphere and held at that temperature for 3 hours. After crushing and classifying, the finished lithium iron phosphate with single particle size is obtained.

[0056] Comparative Example 2, traditional physical mixing gradation, specifically includes: S1, prepare large-particle lithium iron phosphate products and small-particle lithium iron phosphate products respectively, the specific process is as follows: Precursor preparation: Ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate were dissolved in deionized water at a molar ratio of Fe:P:Li = 1:1:1.05, with a solvent-to-solute mass ratio of 8:1, to obtain a precursor slurry.

[0057] Drying process: The precursor slurry is spray-dried with the inlet air temperature controlled at 200℃, the outlet air temperature at 90℃, the spray pressure at 0.3 MPa, and the feed rate at 80 mL / min to obtain precursor powder.

[0058] Segmented sintering: 30% by weight of the precursor powder was placed in a nitrogen protective atmosphere and heated to 350℃ for low-temperature pre-sintering at a heating rate of 3℃ / min and held for 1.5 h. Then, the temperature was raised to 650℃ for high-temperature main sintering and held for 4 h. After the holding period, the powder was cooled to room temperature with the furnace to obtain crude lithium iron phosphate with small particles and a particle size of 0.45 μm. 70% by weight of the precursor powder was placed under a nitrogen protective atmosphere and heated to 350℃ for low-temperature pre-sintering at a heating rate of 3℃ / min and held for 1.5 h. Then it was heated to 750℃ for high-temperature main sintering and held for 4 h. After the holding period, it was cooled to room temperature with the furnace to obtain crude lithium iron phosphate with large particles and a particle size of 2.39 μm. Dry carbon doping and secondary sintering: 2wt% carbon black was added to both small-particle crude lithium iron phosphate and large-particle crude lithium iron phosphate and mixed evenly by dry method. The mixture was then subjected to secondary low-temperature sintering at 450℃ under a nitrogen atmosphere and held at that temperature for 2 hours. After cooling, the mixture was crushed and graded to obtain small-particle and large-particle finished lithium iron phosphate products.

[0059] S2, large-particle finished products and small-particle finished products are physically mixed at a mass ratio of 7:3 to obtain physically mixed graded lithium iron phosphate.

[0060] Comparative Example 3, preparation by single doping, specifically including: S1, Precursor preparation: Ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate were dissolved in deionized water at a molar ratio of Fe:P:Li = 1:1:1.05, with a solvent-to-solute mass ratio of 8:1, to form a homogeneous slurry; only the main dopant TiO2 (doping amount 1.0%) was added, without adding any auxiliary dopant, and the mixture was stirred for 3 h until homogeneous to obtain the precursor slurry.

[0061] S2, Drying treatment: The precursor slurry is spray-dried, with the inlet air temperature controlled at 200℃, the outlet air temperature at 90℃, the spray pressure at 0.3 MPa, and the feed rate at 80 mL / min, to obtain the doped precursor powder.

[0062] S3, Segmented sintering: The precursor powder is placed under a nitrogen protective atmosphere and heated to 350℃ for low-temperature pre-sintering at a heating rate of 3℃ / min, and held for 1.5 h. Then, it is heated to 700℃ for high-temperature main sintering and held for 4 h. After the holding period, it is cooled to room temperature with the furnace to obtain crude lithium iron phosphate.

[0063] S4, Dry carbon doping and secondary sintering: 2wt% carbon black is added to crude lithium iron phosphate and mixed evenly by dry method. Secondary low-temperature sintering is carried out at 450℃ under nitrogen atmosphere and held for 2 hours. After cooling, it is crushed and graded to obtain single-doped lithium iron phosphate finished product.

[0064] Performance testing The performance of lithium iron phosphate prepared in each embodiment and comparative example was tested using the following methods: 1. Compacted density test: The material, binder, and conductive agent are mixed at a mass ratio of 90:5:5 to prepare the electrode sheet, which is then compacted under a pressure of 10t, and the electrode sheet density is tested. 2. Electrochemical performance test: Assemble CR2032 coin cells with lithium metal as the negative electrode, 1 mol / L LiPF6 / EC+DEC as the electrolyte (volume ratio 1:1), and Celgard2400 microporous polypropylene membrane as the separator. Assemble the coin cells in an argon-filled glove box and perform electrochemical performance tests. See Table 1 for details. Table 1. Electrical performance test results of the coin cells prepared in the examples and comparative examples.

[0065] Please see Figure 1 , Figure 1This is a SEM image of the high-pressure compacted LiFePO4 material prepared in Example 1 of this invention. The image clearly distinguishes between large dark gray particles and small light-colored particles, representing the in-situ bimodal particle size distribution prepared in this invention. Further, from... Figure 2 As can be seen from the particle size distribution curve, the sample forms a distinct bimodal particle size distribution, with large particles ranging from 2.0 to 4.0 μm and small particles ranging from 0.4 to 1.0 μm. The large particles form the packing framework, while the small particles fill the gaps in the framework. The particles have regular morphology and good dispersibility, and the particle surface is coated with a uniform carbon layer. This demonstrates that the bimodal structure can be obtained by in-situ control of grain growth through the Ti-Nb dual-doping system. Low-temperature secondary sintering does not destroy the particle size distribution, which is beneficial to improving the compaction density and electrochemical performance of lithium iron phosphate materials.

[0066] The test results show that the lithium iron phosphate prepared in this embodiment of the invention has a significantly higher compaction density than Comparative Example 1 (single particle size), Comparative Example 2 (traditional physical mixing), and Comparative Example 3 (single doping). Furthermore, its initial discharge specific capacity, rate performance, and cycle stability are also superior. This demonstrates that the present invention effectively achieves a synergistic improvement in compaction density and electrochemical performance by synergistically inducing in-situ bimodal gradation through the synergistic action of primary and secondary dopants. This is distinct from existing single-doping and physical mixing gradation techniques, exhibiting significant technical advantages.

[0067] In summary, this high-density lithium iron phosphate (LFP) with doping-induced in-situ bimodal particle size distribution and its preparation method, by setting a main dopant to promote grain growth and an auxiliary dopant to inhibit grain growth, induces an in-situ bimodal particle size distribution directly during the synthesis process. The doping elements not only regulate the particle size distribution but also enter the interstitial space to stabilize the crystal structure. Combined with dry carbon doping and secondary sintering, a continuous and robust carbon coating layer can be formed, while improving ion diffusion efficiency and electronic conductivity, thus resolving the technical contradiction of "high density - low performance". In addition, the large-particle framework phase and small-particle filling phase formed in-situ by doping induce a close packing, effectively reducing the porosity of the material. The compaction density is 15% to 20% higher than that of traditional single-particle LFP and 2.7% to 6.6% higher than that of existing physically mixed graded LFP, significantly increasing the battery compaction density to over 2.65 g / cm³, which is higher than the performance indicators of most existing high-density LFPs.

[0068] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing high-pressure lithium iron phosphate, characterized in that, Includes the following steps: S1: Precursor preparation: Iron source, phosphorus source and lithium source are mixed in a solvent to form a uniform precursor sol or slurry. The main dopant and auxiliary dopant are added to it and stirred to mix evenly to obtain a doped precursor mixture. The main dopant is at least one of TiO2, V2O5, and MgO; The auxiliary dopant is at least one of Nb2O5, ZrO2, and Y2O3; S2: Drying treatment: Spray drying the doped precursor mixture to obtain doped precursor powder; S3: Segmented sintering: The doped precursor powder is placed in a sintering furnace and sintered in segments under a protective atmosphere. The low-temperature pre-sintering and high-temperature main sintering are completed in sequence. After cooling, crude lithium iron phosphate with in-situ bimodal particle size distribution is obtained. S4: Dry carbon doping and secondary sintering: A carbon source is added to the crude lithium iron phosphate and mixed dry. Then, a low-temperature secondary sintering is carried out under a protective atmosphere to pyrolyze the carbon source to form a carbon coating layer. After crushing and classification, high-pressure lithium iron phosphate is obtained.

2. The method for preparing high-pressure lithium iron phosphate as described in claim 1, characterized in that, The molar ratio of the metal elements in the primary dopant to the secondary dopant is 1:0.2 to 1:0.

5. Furthermore, the doping amount of the main dopant is 0.5% to 2.0% of the theoretical mass of lithium iron phosphate, and the doping amount of the auxiliary dopant is 0.1% to 0.8% of the theoretical mass of lithium iron phosphate.

3. The method for preparing high-pressure lithium iron phosphate as described in claim 1, characterized in that, In S1, The molar ratio of the iron source, phosphorus source, and lithium source is Fe:P:Li = 1:1:1.02 to 1.08; The iron source is selected from at least one of ferrous oxalate, ferric phosphate, and iron oxide red. The phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid. The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium nitrate. The solvent is deionized water and / or anhydrous ethanol, and the mass ratio of the solvent to the total of the iron source, phosphorus source and lithium source is 5:1 to 10:

1.

4. The method for preparing high-pressure lithium iron phosphate as described in claim 1, characterized in that, In S2, The spray drying conditions are as follows: inlet air temperature 180-220℃, outlet air temperature 80-100℃, spray pressure 0.2-0.4 MPa, and feed rate 50-100 mL / min.

5. The method for preparing high-pressure lithium iron phosphate as described in claim 1, characterized in that, In S3, The protective atmosphere is one or a mixture of nitrogen and argon; The low-temperature pre-sintering temperature is 300–400℃, and the holding time is 1–2 h; The high-temperature main sintering temperature is 650-750℃, and the holding time is 3-5 h; The heating rate for the segmented sintering is 2–5 °C / min.

6. The method for preparing high-pressure lithium iron phosphate as described in claim 1, characterized in that, In S4, The dry carbon doping method uses at least one carbon source selected from sucrose, glucose, and carbon black, and the amount of carbon source added is 1% to 3% of the crude lithium iron phosphate mass. The secondary sintering temperature is 350–550℃, and the holding time is 1–3 hours; The protective atmosphere is one or a mixture of nitrogen and argon.

7. The method for preparing high-pressure lithium iron phosphate as described in claim 1, characterized in that, In S4, The high-pressure lithium iron phosphate has a bimodal particle size distribution, with the two main peaks located at 0.4–1.0 μm and 2.0–4.0 μm, respectively, and a D50 of 1.5–2.5 μm.

8. A high-pressure lithium iron phosphate prepared by the method according to any one of claims 1-7, characterized in that, The compacted density of the high-pressure lithium iron phosphate is 2.65–2.75 g / cm³. 3 .

9. A lithium-ion battery, characterized in that, Including the high-density lithium iron phosphate as described in claim 8; The lithium-ion battery has a first discharge specific capacity of ≥160 mAh / g at 0.1C, a capacity retention rate of ≥95% at 1C rate discharge, and a capacity retention rate of ≥90% after 500 cycles.

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