Lithium iron phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

This invention discloses lithium iron phosphate cathode materials, their preparation methods, and lithium-ion batteries, relating to the field of lithium-ion battery technology. This invention utilizes first, second, and third iron phosphates of different particle sizes to form a three-level multi-scale particle size distribution system, which can improve the material's packing efficiency and compaction density. By utilizing the synergistic effect of macromolecular carbon sources and dispersants, the sand milling process is optimized, reducing slurry viscosity and improving sand milling efficiency, significantly reducing sand milling time while ensuring particle integrity and dispersibility. Simultaneously, the use of macromolecular and small-molecule carbon sources to form a double-layer carbon coating structure achieves synergistic mechanical support and conductive enhancement, simultaneously improving the material's cycle stability and rate performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to lithium iron phosphate cathode materials, their preparation methods, and lithium-ion batteries. Background Technology

[0002] Lithium iron phosphate (LFP), as one of the key cathode materials for lithium-ion batteries, has broad application prospects in power batteries and energy storage. However, its low intrinsic electronic conductivity and ion diffusion rate limit the rate performance of the material, and traditional preparation methods struggle to obtain materials that combine high compaction density with excellent electrochemical performance. Currently, the main methods for improving compaction density include particle nanosizing, spheroidization, and multi-size gradation, but these methods have significant drawbacks, specifically: nanoparticles are prone to agglomeration and have poor processing performance; micron-sized spherical particles have a loose internal structure and are easily broken during electrode compaction; and multi-size gradation lacks precise control over the particle size and strength of each particle level.

[0003] In existing technologies, the above-mentioned defects are mostly mitigated by introducing additives during the preparation process. However, the use of additives often focuses on a single function, such as dispersants improving slurry rheology and binders increasing particle strength, but lacks synergistic control over the structural evolution during particle formation. In particular, for submicron and micron-sized particles that require precise control, how to achieve precise particle size control while ensuring particle strength and improving processing efficiency is a technical challenge that urgently needs to be solved in this field.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide lithium iron phosphate cathode material, its preparation method, and lithium-ion battery, aiming to ensure particle strength while achieving precise particle size control, and to prepare a high-density lithium iron phosphate cathode material with excellent comprehensive performance.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising: Ferric phosphate (Fe1), ferric phosphate (Fe2), and ferric phosphate (Fe3) with different particle sizes were prepared using ferric phosphate raw materials. The particle size D50 of the ferric phosphate raw materials was 1.0 μm–1.5 μm, that of the first ferric phosphate was 2.9 μm–3.2 μm, that of the second ferric phosphate was 0.95 μm–1.15 μm, and that of the third ferric phosphate was 0.28 μm–0.35 μm. The mass percentages of the ferric phosphate raw materials used for preparing the first, second, and third ferric phosphates were 40%–60%, 15%–45%, and 15%–25%, respectively. Functional additives, including macromolecular carbon sources and dispersants, were introduced during the preparation of the first and second ferric phosphates. The first ferric phosphate, the second ferric phosphate and the third ferric phosphate were mixed and slurried, then mixed with a lithium source and a small molecule carbon source and granulated to obtain a composite precursor powder. The composite precursor powder is sintered.

[0007] In an optional embodiment, the process of preparing the first ferric phosphate and the second ferric phosphate both include: mixing ferric phosphate raw material, functional additives and water, grinding the resulting slurry, and then spray drying it using a two-fluid atomizer. And / or, the macromolecular carbon source is selected from at least one of phenolic resin, furfural resin, polybenzoxazine precursor and polyacrylonitrile prepolymer; the weight average molecular weight of the macromolecular carbon source is 3500~8000; more preferably, the mass ratio of the amount of macromolecular carbon source to the mass of the ferric phosphate raw material used to prepare the first ferric phosphate or the second ferric phosphate is (3.5~7.5):100. And / or, the dispersant is selected from at least one of nano-silica, nano-alumina and nano-silicon carbide; the particle size D50 of the dispersant is 10nm-30nm; more preferably, the mass ratio of the amount of dispersant to the mass of the ferric phosphate raw material used to prepare the first ferric phosphate or the second ferric phosphate is (0.5~1.0):100.

[0008] In an optional embodiment, the process for preparing the first iron phosphate includes at least one of feature A1 to feature D1: Feature A1: The solid content of the slurry used for sand milling is 45%-58%; Feature B1: During the sand milling process, the rotation speed is controlled at 1700rpm~2200rpm, the processing time is 1.5h~3.0h, and the particle size D50 of the slurry after sand milling is controlled at 0.35μm~0.45μm; Feature C1: Controlled inlet air temperature of 190℃~210℃, atomization pressure of 0.35MPa~0.50MPa, and outlet air temperature of 100℃~120℃; Feature D1: The sphericity of the first ferric phosphate obtained after spray drying is >0.9, and the loose density is 1.1 g / cm³.3 ~1.3g / cm 3 .

[0009] In an optional embodiment, the process for preparing the second iron phosphate includes at least one of feature A2 to feature D2: Feature A2: The solid content of the slurry used for sand milling is 25%~35%; Feature B2: During the sand milling process, the rotation speed is controlled at 1800rpm~2400rpm, the processing time is 2h~4h, and the particle size D50 of the slurry after sand milling is controlled at 0.3μm~0.4μm; Feature C2: Control the inlet air temperature to 170℃~190℃, the atomization pressure to 0.6MPa~0.9MPa, and the outlet air temperature to 90℃~110℃; Characteristic D2: The sphericity of the second ferric phosphate obtained after spray drying is >0.85, and the loose pack density is 0.8 g / cm³. 3 ~1.0g / cm 3 .

[0010] In an optional embodiment, the process of preparing the third ferric phosphate includes: mixing ferric phosphate raw material and water to obtain a slurry with a solid content of 40% to 55%, and then grinding it until the particle size of the particles in the slurry reaches 0.28 μm to 0.35 μm; Preferably, during the preparation of the third ferric phosphate, the milling speed is controlled at 2000 rpm to 2800 rpm, the grinding time is 6 h to 9 h, and the particle size is measured by a laser particle size analyzer every 1 h to 2 h until the particle size of the particles in the slurry meets the requirements.

[0011] In an optional embodiment, the preparation process of the composite precursor powder includes: mixing first ferric phosphate, second ferric phosphate and third ferric phosphate, adding water to adjust the solid content of the slurry to 42%~52%, and obtaining a mixed slurry; The mixed slurry is mixed and stirred with a lithium source and a small molecule carbon source to obtain a mixed coating material; The mixed coating material is spray-dried.

[0012] In an optional embodiment, the process for preparing the composite precursor powder includes at least one of feature A3 to feature F3: Feature A3: The small molecule carbon source is selected from at least one of glucose, sucrose, maltose, and citric acid; Feature B3: Based on carbon element, the amount of small molecule carbon source added accounts for 2.2wt%~4.0wt% of the total mass of iron phosphate, and the total carbon content of the final product is controlled at 1.0wt%~2.0wt%; Feature C3: When preparing the mixed slurry, control the stirring speed to be 800 rpm to 1200 rpm and the stirring time to be 30 min to 60 min; Feature D3: A lithium source is added at a lithium to iron molar ratio of (1.02~1.04):1; the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; Feature E3: After mixing with lithium source and small molecule carbon source, stir at 800 rpm to 1200 rpm for 60 min to 90 min; Feature F3: During the spray drying process of the mixed coating material, the inlet air temperature is controlled at 180℃~200℃, the outlet air temperature is controlled at 90℃~110℃, and the atomization pressure is controlled at 0.4MPa~0.7MPa.

[0013] In an optional embodiment, the process of sintering the composite precursor powder includes: performing a first sintering stage and a second sintering stage under an inert atmosphere; wherein, in the first sintering stage, the sintering temperature is controlled at 300℃~420℃ and the holding time is 4h~8h; in the second sintering stage, the sintering temperature is controlled at 650℃~710℃ and the holding time is 12h~16h. Preferably, in the first sintering stage, the heating rate is controlled to be 2℃ / min~4℃ / min; Preferably, in the second sintering stage, the heating rate is controlled to be 3℃ / min~6℃ / min; Preferably, the method further includes: crushing and sieving the sintered product.

[0014] Secondly, the present invention provides a lithium iron phosphate cathode material, which is prepared by any of the preparation methods described in the foregoing embodiments.

[0015] Thirdly, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material of the aforementioned embodiments.

[0016] The present invention has the following beneficial effects: It utilizes first, second, and third iron phosphates of different particle sizes to form a three-level multi-scale particle gradation system, which can improve the packing efficiency and compaction density of the material; by utilizing the synergistic effect of macromolecular carbon sources and dispersants, the sand milling process is optimized, reducing slurry viscosity and improving sand milling efficiency, significantly reducing sand milling time while ensuring particle integrity and dispersibility; simultaneously, it utilizes macromolecular and small-molecule carbon sources to form a double-layer carbon coating structure, achieving synergistic mechanical support and conductive reinforcement, and simultaneously improving the material's cycle stability and rate performance. Attached Figure Description

[0017] 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.

[0018] Figure 1 SEM image of the product prepared in Example 1; Figure 2 The image shows the SEM image of the product prepared in Comparative Example 1. Detailed Implementation

[0019] 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.

[0020] To prepare high-density lithium iron phosphate cathode materials with excellent comprehensive performance, this invention optimizes the preparation method, enabling precise particle size control while ensuring particle strength.

[0021] This invention provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: S1, Controllable preparation of multi-stage precursor particles First ferric phosphate, second ferric phosphate, and third ferric phosphate with different particle sizes were prepared using ferric phosphate raw materials for later use.

[0022] Among them, iron phosphate raw material (FePO4) The particle size D50 of 2H2O is 1.0μm~1.5μm, such as 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, etc. The initial particle size D50 of the iron phosphate raw material is within the above range, and there is no obvious agglomeration phenomenon in the raw material. The first type of ferric phosphate has a particle size D50 of 2.9 μm to 3.2 μm (high-strength spherical particles), such as 2.9 μm, 3.0 μm, 3.1 μm, 3.2 μm, etc.; the second type of ferric phosphate has a particle size D50 of 0.95 μm to 1.15 μm (high-strength spherical particles), such as 0.95 μm, 0.98 μm, 1.00 μm, 1.03 μm, 1.05 μm, 1.08 μm, 1.10 μm, 1.13 μm, 1.15 μm, etc.; the third type of ferric phosphate has a particle size D50 of 0.28 μm to 0.35 μm (nanoparticles), such as 0.28 μm, 0.30 μm, 0.33 μm, 0.35 μm, etc.

[0023] In the ferric phosphate raw materials, the mass proportions of the ferric phosphate raw materials used to prepare primary ferric phosphate, secondary ferric phosphate, and tertiary ferric phosphate are 40%~60%, 15%~45%, and 15%~25%, respectively. By controlling the proportions of the three particle sizes of ferric phosphate, precise control of particle size can be achieved, constructing a three-level multi-scale particle gradation system. Scientific stacking improves the material packing efficiency, resulting in stable compaction density (≥2.70 g / cm³). 3 ).

[0024] Specifically, in the iron phosphate raw materials, the mass percentage of iron phosphate raw materials used to prepare the first iron phosphate can be 40%, 43%, 45%, 48%, 50%, 53%, 55%, 58%, 60%, etc.; the mass percentage of iron phosphate raw materials used to prepare the second iron phosphate can be 15%, 18%, 20%, 23%, 25%, 28%, 30%, 33%, 35%, 38%, 40%, 43%, 45%, etc.; and the mass percentage of iron phosphate raw materials used to prepare the third iron phosphate can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, etc.

[0025] Furthermore, functional additives are introduced during the preparation of primary and secondary ferric phosphates. These additives include macromolecular carbon sources and dispersants. The macromolecular carbon sources also act as binders, forming a three-dimensional cross-linked network after curing to firmly bond the primary ferric phosphate particles, resulting in spherical secondary particles with high mechanical strength. The dispersants effectively isolate the primary particles through steric hindrance, preventing excessive agglomeration during curing and thus precisely controlling the size of the final spherical particles.

[0026] In some embodiments, the macromolecular carbon source is selected from at least one of phenolic resin, furfural resin, polybenzoxazine precursor, and polyacrylonitrile prepolymer, and the macromolecular carbon source can be any one or more of the above. The weight-average molecular weight of the macromolecular carbon source is 3500~8000, such as 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, etc. The mass ratio of the macromolecular carbon source to the ferric phosphate raw material used to prepare the first ferric phosphate or the second ferric phosphate is (3.5~7.5):100, such as 3.5:100, 4.0:100, 4.5:100, 5.0:100, 5.5:100, 6.0:100, 6.5:100, 7.0:100, 7.5:100, etc. When preparing the first ferric phosphate, the mass ratio of the macromolecular carbon source to the ferric phosphate raw material used in this step is controlled at (3.5~7.5):100; similarly, when preparing the second ferric phosphate, the mass ratio of the macromolecular carbon source to the ferric phosphate raw material used in this step is controlled at (3.5~7.5):100. By controlling the type and amount of macromolecular carbon source, it is beneficial to further improve the mechanical strength of the secondary particles.

[0027] In some embodiments, the dispersant is selected from at least one of silica (SiO2), nano-alumina (Al2O3), and nano-silicon carbide (SiC), and the dispersant can be any one or more of the above. The particle size D50 of the dispersant is 10nm-30nm, such as 10nm, 15nm, 20nm, 25nm, 30nm, etc., and the nano-silica is preferably fumed silica with a particle size of 10-30nm. The mass ratio of the dispersant to the ferric phosphate raw material used to prepare the first ferric phosphate or the second ferric phosphate is (0.5~1.0):100, such as 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, 1.0:100, etc. When preparing the first ferric phosphate, the mass ratio of the dispersant to the ferric phosphate raw material used in this step is controlled at (0.5~1.0):100; similarly, when preparing the second ferric phosphate, the mass ratio of the dispersant to the ferric phosphate raw material used in this step is controlled at (0.5~1.0):100. By adjusting the type and amount of dispersant, particle agglomeration can be better avoided, and the final spherical particle size can be precisely controlled to be 2.9μm~3.2μm or 0.95μm~1.15μm. If the dispersant is excessive, it will affect the electrochemical performance of the product; if the dispersant is insufficient, agglomeration cannot be effectively avoided.

[0028] In some embodiments, the steps for preparing the first and second ferric phosphates are largely the same, with the main difference being parameter control. The preparation steps both include: mixing ferric phosphate raw materials, functional additives, and water; milling the resulting slurry; and then spray drying using a two-fluid atomizer. Milling controls the particle size, and subsequent spray drying using a two-fluid atomizer allows for precise control of the size and morphology of the spherical particles of the first and second ferric phosphates, improving particle mechanical strength and sphericity, thus laying the structural foundation for high compaction. During spray drying, the inlet air temperature is higher than the curing initiation temperature of the phenolic resin (approximately 150°C), causing the phenolic resin to undergo a rapid thermosetting reaction as the droplets evaporate.

[0029] It should be noted that during the sand milling process, macromolecular carbon sources such as phenolic resin have a lubricating effect, which can reduce the internal frictional resistance of the slurry. The hard grinding effect of dispersants such as nano-silica can enhance the particle refinement efficiency. The two work together to reduce the viscosity of the slurry, thereby increasing the sand milling efficiency by more than 40%, significantly shortening the sand milling time, reducing energy consumption, improving production efficiency, and ensuring particle integrity and dispersibility.

[0030] The parameters for sand milling and spray drying were controlled separately during the preparation of ferric phosphate and ferric phosphate: [Fe1 phosphate] When preparing the first ferric phosphate, the raw materials and additives are mixed and then deionized water is added to prepare a slurry with a high solid content of 45%-58% (solid content refers to the mass fraction of ferric phosphate, macromolecular carbon source and dispersant), which is then sent to a sand mill for sand milling pretreatment. The solid content of the slurry used for sand milling can be controlled to be 45%, 48%, 50%, 53%, 55%, 58%, etc.; during the sand milling process, the rotation speed is controlled to be 1700rpm~2200rpm, such as 1700rpm, 1800rpm, 1900rpm, 2000rpm, 2100rpm, 2200rpm, etc.; the sand milling time is 1.5h~3.0h, such as 1.5h, 1.8h, 2.0h, 2.3h, 2.5h, 2.8h, 3.0h, etc.; the particle size D50 of the slurry after sand milling is controlled to be 0.35μm~0.45μm, such as 0.35μm, 0.38μm, 0.40μm, 0.43μm, 0.45μm, etc.

[0031] Furthermore, during the preparation of primary ferric phosphate, the inlet air temperature is controlled at 190℃~210℃ during spray drying, such as 190℃, 195℃, 200℃, 205℃, 210℃, etc.; the atomization pressure is 0.35MPa~0.50MPa, such as 0.35MPa, 0.40MPa, 0.45MPa, 0.50MPa, etc.; and the outlet air temperature is 100℃~120℃, such as 100℃, 105℃, 110℃, 115℃, 120℃, etc. The sphericity of the primary ferric phosphate obtained after spray drying is >0.9, such as 0.91, 0.93, 0.95, 0.98, 0.99, etc.; and the loose pack density of the primary ferric phosphate obtained after spray drying is 1.1 g / cm³. 3 ~1.3g / cm 3 For example, it can be 1.1 g / cm³ 3 1.2g / cm 3 1.3g / cm 3 wait.

[0032] [Fe2 phosphate] When preparing the second ferric phosphate, the raw materials and additives are mixed and then deionized water is added to prepare a low solid content slurry with a solid content of 25% to 35%, which is then sent to a sand mill for sand milling pretreatment. The solid content of the slurry used for sand milling can be controlled at 25%, 28%, 30%, 33%, 35%, etc.; during the sand milling process, the rotation speed is controlled at 1800rpm~2400rpm, such as 1800rpm, 1900rpm, 2000rpm, 2100rpm, 2200rpm, 2300rpm, 2400rpm, etc.; the sand milling time is 2h~4h, such as 2.0h, 2.3h, 2.5h, 2.8h, 3.0h, 3.3h, 3.5h, 3.8h, 4.0h, etc.; the particle size D50 of the slurry after sand milling is controlled at 0.3μm~0.4μm, such as 0.30μm, 0.33μm, 0.35μm, 0.38μm, 0.40μm, etc.

[0033] Furthermore, during the preparation of the second ferric phosphate, the inlet air temperature is controlled at 170℃~190℃ during spray drying, such as 170℃, 175℃, 180℃, 185℃, 190℃, etc.; the atomization pressure is 0.6MPa~0.9MPa, such as 0.6MPa, 0.7MPa, 0.8MPa, 0.9MPa, etc.; and the outlet air temperature is 90℃~110℃, such as 90℃, 95℃, 100℃, 105℃, 110℃, etc. After spray drying, the sphericity of the obtained second ferric phosphate is >0.85, such as 0.86, 0.90, 0.91, 0.93, 0.95, 0.98, 0.99, etc.; and the loose packing density is 0.8 g / cm³. 3 ~1.0g / cm3 For example, it can be 0.8g / cm 3 0.9g / cm 3 1.0g / cm 3 wait.

[0034] [Third ferric phosphate] In some embodiments, the process of preparing third ferric phosphate includes: mixing ferric phosphate raw material with water to obtain a slurry with a solid content of 40% to 55% (without adding any organic additives or dispersants); feeding the slurry into a zirconia bead mill for dispersion treatment until the particle size in the slurry reaches 0.28 μm to 0.35 μm; and stopping the milling. Specifically, the solid content of the slurry can be 40%, 43%, 45%, 48%, 50%, 53%, 55%, etc. In the preparation of ferric phosphate, the milling speed is controlled at 2000 rpm to 2800 rpm (e.g., 2000 rpm, 2300 rpm, 2500 rpm, 2800 rpm, etc.), and the milling time is 6 h to 9 h (e.g., 6 h, 7 h, 8 h, 9 h, etc.). Every 1 h to 2 h, the particle size is measured by sampling with a laser particle size analyzer until the particle size D50 of the slurry reaches 0.28 μm to 0.35 μm, thus obtaining a stable dispersed nano slurry, which is then sealed and stored for later use.

[0035] S2, wet composite process and double-layer carbon coating sintering The first ferric phosphate, the second ferric phosphate, and the third ferric phosphate were mixed and slurried, then mixed with a lithium source and a small molecule carbon source and granulated to obtain a composite precursor powder; the composite precursor powder was then sintered.

[0036] It should be noted that by introducing macromolecular carbon sources such as phenolic resin and small-molecule carbon sources in a stepwise manner, a functionally differentiated bilayer carbon coating structure is constructed, achieving a synergistic effect of mechanical support and conductive enhancement, and simultaneously improving the cycling stability and rate performance of the material. Specifically, the internal framework carbon formed by the carbonization of macromolecular carbon sources such as phenolic resin provides mechanical support and buffers cyclic stress; the external dense carbon layer formed by the carbonization of small-molecule carbon sources provides efficient electron channels, and the two work synergistically to improve the electrochemical performance of the material.

[0037] [Preparation of composite precursor powder] The preparation process of the composite precursor powder includes: mixing ferric phosphate, ferric phosphate, and ferric phosphate; adding water to adjust the solid content of the slurry to 42%~52% (e.g., 42%, 45%, 48%, 50%, 52%, etc., where solid content refers to the total mass fraction of substances other than water), to obtain a mixed slurry; mixing the mixed slurry with a lithium source and a small molecule carbon source to obtain a mixed coating material; and spray-drying the mixed coating material to obtain a composite precursor powder with uniform composition and good flowability. Specifically, the ferric phosphate can be a raw material in the form of the slurry prepared above, but is not limited to this.

[0038] In some embodiments, when preparing the mixed slurry, a high-speed disperser is used for stirring, and the stirring speed is controlled at 800 rpm to 1200 rpm, such as 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, etc.; the stirring time is 30 min to 60 min, such as 30 min, 40 min, 50 min, 60 min, etc., to ensure that multi-scale particles are uniformly dispersed and there is no obvious agglomeration.

[0039] In some embodiments, the small molecule carbon source is selected from at least one of glucose, sucrose, maltose, and citric acid. The small molecule carbon source can be any one or more of the above, and all of the above small molecule carbon sources are industrial-grade low-cost raw materials with a purity ≥98.0%. The amount of small molecule carbon source added, based on carbon element, accounts for 2.2wt%~4.0wt% of the total mass of iron phosphate, such as 2.2wt%, 2.5wt%, 3.0wt%, 3.5wt%, 4.0wt%, etc.; the total carbon content of the final product is controlled at 1.0wt%~2.0wt%, such as 1.0wt%, 1.3wt%, 1.5wt%, 1.8wt%, 2.0wt%, etc.

[0040] In some embodiments, the lithium source is selected from lithium carbonate (Li2CO3) and lithium hydroxide (LiOH). The lithium source can be any one or more of H2O, lithium acetate (CH3COOLi), and lithium oxalate (Li2C2O4), with a purity ≥99.0%. The lithium source is added at a lithium to iron molar ratio of (1.02~1.04):1, such as 1.02:1, 1.03:1, 1.04:1, etc. After mixing with the lithium source and the small molecule carbon source, stir at 800rpm~1200rpm for 60min~90min to ensure thorough mixing of the lithium source, small molecule carbon source, and slurry, achieving initial coating of the particles by the small molecule carbon source. Specifically, the stirring speed can be 800rpm, 900rpm, 1000rpm, 1100rpm, 1200rpm, etc.; the stirring time can be 60min, 70min, 80min, 90min, etc.

[0041] In some embodiments, during the spray drying process of the mixed coating material, the prepared mixed slurry is conveyed to the spray drying tower, and the inlet air temperature is controlled at 180℃~200℃, such as 180℃, 185℃, 190℃, 195℃, 200℃, etc.; the outlet air temperature is controlled at 90℃~110℃, such as 90℃, 95℃, 100℃, 105℃, 110℃, etc.; and the atomization pressure is controlled at 0.4MPa~0.7MPa, such as 0.4MPa, 0.5MPa, 0.6MPa, 0.7MPa, etc. By adjusting the operating parameters of spray drying, a composite precursor powder with uniform composition and good flowability is obtained.

[0042] [sintering] The composite precursor powder is placed in an atmosphere furnace filled with inert gas and sintered according to the following procedure: first, the first sintering stage (pre-curing and pre-carbonization) is carried out, followed by the second sintering stage (crystallization and carbothermic reduction).

[0043] In the first sintering stage, the sintering temperature is controlled at 300℃~420℃, such as 300℃, 330℃, 350℃, 380℃, 400℃, 420℃, etc.; the holding time is 4h~8h, such as 4h, 5h, 6h, 7h, 8h, etc. The heating rate in this stage is 2℃ / min~4℃ / min, such as 2℃ / min, 3℃ / min, 4℃ / min, etc. In this stage, the macromolecular carbon source such as phenolic resin is completely solidified and initially carbonized, forming a three-dimensional network framework inside the first and second ferric phosphate particles, enhancing the stability of the particle structure.

[0044] The second sintering stage controls the sintering temperature to be between 650℃ and 710℃, such as 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, or 710℃; the holding time is between 12h and 16h, such as 12h, 13h, 14h, 15h, or 16h. The heating rate in this stage is 3℃ / min to 6℃ / min, such as 3℃ / min, 4℃ / min, 5℃ / min, or 6℃ / min. During this stage, the lithium iron phosphate crystalline phase is formed, and the small-molecule carbon source is fully carbonized to form a dense conductive layer on the surface, achieving a double-layer carbon coating structure.

[0045] Specifically, the type of inert atmosphere is not limited, such as high-purity nitrogen (purity >99.999%) with an oxygen content ≤50ppm, but it is not limited to these. After sintering, the cooling rate is controlled to be ≤15℃ / min.

[0046] After sintering, the sintered product is mechanically crushed and passed through a 200-mesh sieve to obtain the final lithium iron phosphate cathode material.

[0047] This invention also provides a lithium iron phosphate cathode material, which is prepared by the preparation method provided in this invention. The prepared lithium iron phosphate cathode material has excellent electrochemical performance.

[0048] It should be noted that the lithium iron phosphate cathode material provided in this embodiment of the invention has excellent electrochemical performance mainly due to the following reasons: a composite system of "structural framework + conductive network" is formed by double-layer carbon coating. The three-dimensional network formed by the carbonization of the inner phenolic resin can effectively suppress abnormal particle growth during sintering, buffer the volume stress during battery cycling, and reduce particle cracking; the outer small molecule carbon source forms a dense conductive layer, resulting in better electrochemical performance.

[0049] Tests showed that the material has an initial discharge capacity of ≥158mAh / g at 1C rate, an initial discharge capacity of ≥135mAh / g at 10C rate, and a capacity retention rate of ≥90% after 2000 cycles.

[0050] This invention also provides a positive electrode sheet, including the above-mentioned lithium iron phosphate positive electrode material, and may further include a positive electrode current collector, wherein a positive electrode active coating is formed on at least one surface of the positive electrode current collector, and the lithium iron phosphate positive electrode material exists in the positive electrode active coating as a positive electrode active material.

[0051] This invention also provides a lithium-ion battery, including the above-mentioned positive electrode, and may further include a negative electrode, electrolyte, separator, etc. to form a complete battery structure.

[0052] This invention provides a device including the aforementioned lithium-ion battery. The lithium-ion battery can serve as a power source for the device or as an energy storage unit. This device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0053] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0054] Example 1 This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: The raw materials and quantities used are as follows: Main raw material: Industrial grade iron phosphate (FePO4) 1000g of Fe:P (2H2O), purity 99.6%, Fe:P = 1.0:1.02, initial D50 = 1.2μm; Lithium source: 204.5g lithium carbonate (Li2CO3), purity 99.2%, Li / Fe = 1.03; Macromolecular carbon source: 40g of phenolic resin (Mw=6000, thermosetting) (25g used when preparing the first ferric phosphate and 15g used when preparing the second ferric phosphate); Dispersant: Fumed silica (SiO2, particle size D50 15nm, specific surface area 200m²) 2 / g) 8g (5g is used when preparing the first ferric phosphate and 3g is used when preparing the second ferric phosphate); Small molecule carbon source: 40g of glucose (accounting for 3.2% of the total mass of iron phosphate by carbon element).

[0055] Particle size distribution: The amount of ferric phosphate used in the preparation of the first, second and third ferric phosphates is different. The amount of ferric phosphate used in the preparation of the first ferric phosphate is 50% (500g) of the total amount, the amount of ferric phosphate used in the preparation of the second ferric phosphate is 30% (300g) of the total amount, and the amount of ferric phosphate used in the preparation of the third ferric phosphate is 20% (200g) of the total amount.

[0056] Preparation process: Preparation of ferric phosphate (component C): Weigh 200g of ferric phosphate, add 230g of deionized water, and adjust to a solid content of 46.5% slurry. Disperse the slurry using a zirconia bead mill for 8 hours (2600rpm). Samples were taken every 2 hours to test the particle size. The final slurry D50 = 0.32μm and viscosity 35mPa. s (25℃, shear rate 100s) -1 It is sealed and stored, and labeled as ferric phosphate slurry.

[0057] Preparation of ferric phosphate (component B): Weigh 300g of ferric phosphate, add 15g of phenolic resin, 3g of nano-silica, and 700g of deionized water to prepare a slurry with a solid content of 28.6%; pre-treat by sand milling for 3 hours (2000rpm). After sand milling, D50 = 0.35μm, viscosity 28mPa. The slurry was spray-dried using a two-fluid atomizer with an inlet air temperature of 180±2℃, an outlet air temperature of 105±3℃, and an atomization pressure of 0.75MPa. The collected iron phosphate particles had the following characteristics: D50 = 1.08μm, D10 = 0.82μm, D90 = 1.35μm, sphericity = 0.87, and loose density = 0.92g / cm³. 3 Tap density = 1.35 g / cm³ 3 The compressive strength is 12.5 MPa.

[0058] Preparation of ferric phosphate (component A): Weigh 500g of ferric phosphate, add 25g of phenolic resin, 5g of nano-silica, and 500g of deionized water to prepare a slurry with a solid content of 50.0%; pre-treat by sand milling for 2 hours (1900rpm). After sand milling, D50=0.38μm and viscosity 42mPa. The slurry was spray-dried using a two-fluid atomizer, with the inlet air temperature set at 200±2℃, the outlet air temperature at 115±3℃, and the atomization pressure at 0.4MPa. The collected iron phosphate particles had the following dimensions: D50=3.10μm, D10=2.45μm, D90=3.85μm, sphericity=0.91, and loose density=1.21g / cm³. 3 Tap density = 1.68 g / cm³ 3 The compressive strength is 14.2 MPa.

[0059] Wet gradation mixing: Mix the first ferric phosphate particles, the second ferric phosphate particles, and the third ferric phosphate slurry, add deionized water, adjust the overall solid content to 45%, and stir with a high-speed disperser at 1000 rpm for 45 minutes to ensure uniform particle dispersion.

[0060] Ingredients and carbon source addition: Add 155.2g lithium carbonate and 40g glucose, and continue stirring at 1000rpm for 90 minutes to fully mix the lithium source and the small molecule carbon source.

[0061] Composite spray drying: The mixed slurry is fed into the spray drying tower, with the inlet air temperature set at 190±2℃, the outlet air temperature at 100±3℃, and the atomization pressure at 0.5MPa. The composite precursor powder is then collected.

[0062] Program-controlled sintering: The precursor powder is placed in a high-purity nitrogen atmosphere furnace and heated to 380℃ at 3℃ / min and held for 6 hours; then heated to 690℃ at 5℃ / min and held for 14 hours; after cooling, it is crushed and passed through a 200-mesh sieve to obtain the final product, which is marked as LFP-1.

[0063] Performance test: Compacted density 2.75 g / cm³ 3 Total carbon content 1.5%, electronic conductivity 1.2×10⁻⁶ -1 S / cm; 1C discharge specific capacity 160mAh / g, 10C discharge specific capacity 138mAh / g, capacity retention rate 92% after 2000 cycles; particle breakage rate 2.5% (after rolling).

[0064] Example 2 This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: The raw materials and quantities used are as follows: Main raw material: Industrial grade iron phosphate (FePO4) 1000g of 2H2O, parameters same as in Example 1; Lithium source: Lithium hydroxide (LiOH) 234.7g of H2O, purity 99.1%, Li / Fe = 1.04; Macromolecular carbon source: furfural resin (Mw=5000) 38g (22g used when preparing the first ferric phosphate, 16g used when preparing the second ferric phosphate); Dispersant: 7g of nano-alumina (Al2O3, particle size 20nm) (4g used in the preparation of the first ferric phosphate and 3g used in the preparation of the second ferric phosphate); Small molecule carbon source: 45g sucrose (accounting for 3.6% of the total mass of iron phosphate by carbon element).

[0065] Particle size distribution: The amount of ferric phosphate used in the preparation of the first, second and third ferric phosphates is different. The amount of ferric phosphate used in the preparation of the first ferric phosphate is 45% (450g) of the total amount, the amount of ferric phosphate used in the preparation of the second ferric phosphate is 40% (400g) of the total amount, and the amount of ferric phosphate used in the preparation of the third ferric phosphate is 15% (150g) of the total amount.

[0066] Preparation process: Preparation of third ferric phosphate (component C): Weigh 150g of ferric phosphate, add 180g of deionized water, and adjust to a slurry with a solid content of 45.5%. Dispersion treatment was carried out using a zirconia bead mill for 7 hours (speed 2600rpm). The particle size was tested every 2 hours. The final slurry D50=0.30μm, and the third ferric phosphate slurry was obtained for use.

[0067] Preparation of ferric phosphate II (component B): Weigh 400g of ferric phosphate, add 16g of furfural resin, 3g of nano-alumina, and 850g of deionized water to prepare a slurry with a solid content of 32.0%; pre-treat by sand milling for 3.5 hours (2200rpm), resulting in a D50 of 0.37μm after sand milling; spray dry the slurry using a two-fluid atomizer with the following atomization parameters: inlet air temperature 185℃, outlet air temperature 100℃, and atomization pressure 0.8MPa. The collected ferric phosphate particles have the following characteristics: D50 = 1.12μm, sphericity = 0.86, and loose density = 0.88g / cm³. 3 The compressive strength is 11.8 MPa.

[0068] Preparation of ferric phosphate (component A): Weigh 450g of ferric phosphate, add 22g of furfural resin, 4g of nano-alumina, and 480g of deionized water to prepare a slurry with a solid content of 48.4%; pre-treat by sand milling for 2.5 hours (2000rpm), resulting in a D50 of 0.42μm; spray dry the slurry using a two-fluid atomizer with the following parameters: inlet air temperature 205℃, outlet air temperature 110℃, and atomization pressure 0.45MPa, yielding ferric phosphate particles with a D50 of 3.05μm, sphericity of 0.90, and a bulk density of 1.18g / cm³. 3 The compressive strength is 13.5 MPa.

[0069] Wet mixing and subsequent processes: First ferric phosphate particles, second ferric phosphate particles, and third ferric phosphate slurry are mixed, and water is added to bring the solid content to 42%. The mixture is stirred at 1000 rpm for 45 minutes using a high-speed disperser to ensure uniform particle dispersion. Lithium hydroxide and sucrose are added, and stirring continues at 1000 rpm for 90 minutes to ensure thorough mixing of the lithium source and the small-molecule carbon source. After uniform mixing, the mixture is spray-dried (inlet air 195℃, outlet air 105℃, atomization pressure 0.5MPa), and the composite precursor powder is collected. Sintering parameters: The precursor powder is placed in a high-purity nitrogen atmosphere furnace, heated to 350℃ at 3℃ / min, and held for 7 hours; then heated to 700℃ at 5℃ / min and held for 13 hours. After cooling, the powder is crushed and passed through a 200-mesh sieve to obtain the final product, labeled LFP-2.

[0070] Performance test: Compacted density 2.73 g / cm³ 3 The total carbon content is 1.7%, and the electronic conductivity is 1.1 × 10⁻⁶. -1 S / cm; 1C discharge specific capacity 159mAh / g, 10C discharge specific capacity 136mAh / g, capacity retention rate 91% after 2000 cycles; particle breakage rate 2.8%.

[0071] Example 3 This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: The raw materials and quantities used are as follows: Main raw material: Industrial grade iron phosphate (FePO4) 1000g of 2H2O, parameters same as in Example 1; Lithium source: 361.2g lithium acetate (CH3COOLi), purity 99.3%, Li / Fe=1.02; Macromolecular carbon source: Polybenzoxazine precursor (purchased from Aladdin, Mw=7000) 42g (28g used in the preparation of the first ferric phosphate and 14g used in the preparation of the second ferric phosphate); Dispersant: 9g of nano-silicon carbide (SiC, particle size 25nm) (6g used in the preparation of the first ferric phosphate and 3g used in the preparation of the second ferric phosphate); Small molecule carbon source: 38g maltose (accounting for 3.0% of the total mass of ferric phosphate by carbon element).

[0072] Particle size distribution: The amount of ferric phosphate used in the preparation of the first, second and third ferric phosphates is different. The amount of ferric phosphate used in the preparation of the first ferric phosphate is 55% (550g) of the total amount, the amount of ferric phosphate used in the preparation of the second ferric phosphate is 20% (200g) of the total amount, and the amount of ferric phosphate used in the preparation of the third ferric phosphate is 25% (250g) of the total amount.

[0073] Preparation process: Preparation of third ferric phosphate (component C): Weigh 250g of ferric phosphate, add 280g of deionized water, and adjust to a slurry with a solid content of 47.2%. Dispersion treatment was carried out using a zirconia bead mill for 9 hours (speed 2400rpm). The particle size was tested every 2 hours. The final slurry D50=0.34μm, and it was labeled as third ferric phosphate slurry.

[0074] Preparation of ferric phosphate II (component B): Weigh 200g of ferric phosphate, add 14g of polybenzoxazine precursor, 2g of nano-silicon carbide, and 450g of deionized water to prepare a slurry with a solid content of 30.8%; pre-treat by sand milling for 2.5 hours (2100rpm), resulting in a D50 of 0.33μm; spray dry the slurry using a two-fluid atomizer with the following atomization parameters: inlet air temperature 175℃, outlet air temperature 95℃, and atomization pressure 0.7MPa. The collected ferric phosphate particles have the following characteristics: D50 = 1.02μm, sphericity = 0.88, and loose density = 0.95g / cm³. 3 The compressive strength is 13.1 MPa.

[0075] Preparation of ferric phosphate (component A): Weigh 550g of ferric phosphate, add 28g of polybenzoxazine precursor, 6g of nano-silicon carbide, and 520g of deionized water to prepare a slurry with a solid content of 51.4%; pre-treat by sand milling for 1.8 hours (1800rpm), resulting in a D50 of 0.40μm; spray dry the slurry using a two-fluid atomizer with the following parameters: inlet air temperature 195℃, outlet air temperature 112℃, and atomization pressure 0.38MPa. Collect the ferric phosphate particles with the following characteristics: D50 = 3.15μm, sphericity = 0.92, and loose density = 1.25g / cm³. 3 The compressive strength is 15.3 MPa.

[0076] Wet mixing and subsequent processes: First ferric phosphate particles, second ferric phosphate particles, and third ferric phosphate slurry are mixed, and water is added to bring the solid content to 43%. The mixture is stirred at 1000 rpm for 45 minutes using a high-speed disperser to ensure uniform particle dispersion. Lithium acetate and maltose are added, and stirring continues at 1000 rpm for 90 minutes to obtain a mixed slurry, ensuring thorough mixing of the lithium source and the small-molecule carbon source. The mixed slurry is fed into a spray drying tower, with the inlet air temperature controlled at 185℃, the outlet air temperature at 98℃, and the atomization pressure at 0.5 MPa, collecting the composite precursor powder. The precursor powder is placed in a high-purity nitrogen atmosphere furnace, heated to 400℃ at 3℃ / min and held for 5 hours; then heated to 680℃ at 5℃ / min and held for 15 hours. After cooling, the powder is crushed and passed through a 200-mesh sieve to obtain the final product, labeled LFP-3.

[0077] Performance test: Compacted density 2.76 g / cm³ 3 The total carbon content is 1.4%, and the electronic conductivity is 1.3 × 10⁻⁶. -1 S / cm; 1C discharge specific capacity 158mAh / g, 10C discharge specific capacity 137mAh / g, capacity retention rate 90% after 2000 cycles; particle breakage rate 2.3%.

[0078] Comparative Example 1 (without phenolic resin and nano-silica) Raw materials and process: Except for the absence of phenolic resin and nano silica, the proportions of other raw materials, particle size distribution and process parameters are completely consistent with those in Example 1, and are marked as DLFP-1.

[0079] Key process data: Pre-treatment by sand milling in the preparation of ferric phosphate: Sand milling for 6 hours is required to achieve D50=0.36μm (100% longer than in Example 1), and the viscosity of the slurry after sand milling is 65mPa. s (132% higher than in Example 1); The second ferric phosphate particles obtained after spray drying have the following characteristics: D50 = 1.42 μm, sphericity = 0.65, and bulk density = 0.62 g / cm³. 3 Compressive strength 3.8MPa (without solidified network support, extremely poor strength); The first step in the preparation of ferric phosphate involves sand milling pretreatment: sand milling for 4.5 hours is required to achieve D50 = 0.40 μm (125% longer than in Example 1), and the viscosity of the slurry after sand milling is 82 mPa. s (95% higher than in Example 1); The first ferric phosphate obtained after spray drying had the following characteristics: D50 = 3.85 μm, sphericity = 0.72, and bulk density = 0.85 g / cm³. 3 The compressive strength is 4.5 MPa.

[0080] Performance test: Compacted density 2.52 g / cm³ 3 The total carbon content is 1.4%, and the electronic conductivity is 0.8 × 10⁻⁶. -2 S / cm; 1C discharge specific capacity 152mAh / g, 10C discharge specific capacity 118mAh / g, capacity retention rate 81% after 2000 cycles; particle breakage rate 8.7%.

[0081] Comparative Example 2 (without nano-silica) Raw materials and process: except for the absence of nano silica, the other raw material ratios, particle size distribution and process parameters are completely consistent with those of Example 1, and are labeled as DLFP-2.

[0082] Key process data: The sand milling pretreatment in the preparation of ferric phosphate requires 4.5 hours of sand milling to achieve D50=0.37μm (50% longer than in Example 1). The viscosity of the slurry after sand milling is 49mPa. s (75% higher than in Example 1); The second ferric phosphate particles obtained after spray drying have the following characteristics: D50 = 1.25 μm, sphericity = 0.76, and loose density = 0.78 g / cm³. 3 The compressive strength is 7.2 MPa (without steric hindrance, significant agglomeration, and decreased strength). The sand milling pretreatment in the preparation of the first ferric phosphate requires 3 hours of sand milling to achieve D50=0.41μm (50% longer than in Example 1), and the viscosity of the slurry after sand milling is 63mPa. s (50% higher than in Example 1); The first batch of ferric phosphate particles obtained after spray drying had the following characteristics: D50 = 3.48 μm, sphericity = 0.82, and bulk density = 1.02 g / cm³. 3 The compressive strength is 8.5 MPa.

[0083] Performance test: Compacted density 2.61 g / cm³ 3 Total carbon content 1.5%, electronic conductivity 1.5×10 -2 S / cm; 1C discharge specific capacity 155mAh / g, 10C discharge specific capacity 126mAh / g, capacity retention rate 85% after 2000 cycles; particle breakage rate 5.3%.

[0084] Comparative Example 3 (using a single-fluid centrifugal atomizer) Raw materials and process: The first and second ferric phosphates were prepared using a single fluid centrifugal atomizer (atomization pressure 1.2 MPa). The proportions of other raw materials, particle size distribution and process parameters were completely consistent with those in Example 1, and it was labeled as DLFP-3.

[0085] Key process data: The second ferric phosphate particles obtained after spray drying have the following characteristics: D50 = 1.58 μm, sphericity = 0.70, wide particle size distribution (D90 / D10 = 2.85), and loose packing density = 0.75 g / cm³. 3 Compressive strength 6.8MPa (uneven atomization, irregular cured network); The first iron phosphate particles obtained after spray drying have the following characteristics: D50 = 4.12 μm, sphericity = 0.75, wide particle size distribution (D90 / D10 = 3.02), loose density = 0.98 g / cm³, and compressive strength = 7.9 MPa.

[0086] Performance test: Compacted density 2.63 g / cm³ 3 Total carbon content 1.5%, electronic conductivity 2.2×10⁻⁶ -2 S / cm; 1C discharge specific capacity 156mAh / g, 10C discharge specific capacity 129mAh / g, capacity retention rate 86% after 2000 cycles; particle breakage rate 4.8%.

[0087] Comparative Example 4 (single carbon source coating, no phenolic resin) Raw materials and process: No phenolic resin was added, and the amount of glucose, a small molecule carbon source, was adjusted to 55g (total carbon content was maintained at 1.5%). The proportions of other raw materials, particle size distribution and process parameters were completely consistent with those in Example 1, and it was labeled as DLFP-4.

[0088] Key process data: During the preparation of primary and secondary ferric phosphates, after spray drying: the particles lack a solidified network support, have low strength (compressive strengths of 4.2 MPa and 3.5 MPa, respectively), are brittle, and have a loose bulk density of 0.95 g / cm³. 3 (Fe1 phosphate), 0.72 g / cm³ 3 (Fe2 phosphate); Sintered particles: Some showed abnormal growth, D50=3.52μm, no inner framework carbon, and loose structure.

[0089] Performance test: Compacted density 2.65 g / cm³ 3 Total carbon content 1.5%, electronic conductivity 3.5×10 -2 S / cm; 1C discharge specific capacity 157mAh / g, 10C discharge specific capacity 130mAh / g, capacity retention rate after 2000 cycles 83%; particle breakage rate 6.2%.

[0090] Comparative Example 5 (single carbon source coating, no small molecule carbon source) Raw materials and process: No small molecule carbon source was added, and the amount of phenolic resin added was adjusted to 55g (total carbon content was kept at 1.5%). The other raw material ratios, particle size distribution and process parameters were completely consistent with those in Example 1, and it was labeled as DLFP-5.

[0091] Key process data: The carbon layer after sintering: only a large molecular carbon skeleton exists, without a dense outer carbon layer; the carbon layer is uneven, porous, and has low electronic conductivity (0.5×10). -2 S / cm); Particle surface: high roughness, discontinuous conductive channels.

[0092] Performance test: Compacted density 2.71 g / cm³ 3 Total carbon content 1.5%, electronic conductivity 0.5×10 -2 S / cm; 1C discharge specific capacity 154mAh / g, 10C discharge specific capacity 115mAh / g, capacity retention rate after 2000 cycles 88%; particle breakage rate 3.1%.

[0093] Comparative Example 6 (Particle Size Distribution Control) Only iron phosphate raw material with a single particle size (D50=1.2μm, i.e. the particle size of the raw material itself, without graded preparation) was used, without A / B / C particle size gradation, and the rest of the process was the same as in Example 1.

[0094] Performance test: Compacted density 2.50 g / cm³ 3 Total carbon content 1.5%, electronic conductivity 0.6×10 -2 S / cm; 1C discharge specific capacity 150mAh / g, 10C discharge specific capacity 110mAh / g, capacity retention rate after 2000 cycles 78%; particle breakage rate 9.0%.

[0095] The sphericity was determined using image analysis, with the following steps: A suitable amount of the sample powder was taken and evenly dispersed on conductive tape. At least five photographs with different fields of view were taken using a scanning electron microscope at 1000-5000x magnification. The sphericity of at least 200 particles was statistically analyzed using image analysis software (such as Image-Pro Plus or Nano Measurer). The formula for calculating the sphericity of a single particle is: Sphericity = ; Where A is the area of ​​the particle in the two-dimensional projection, and P is the perimeter of the particle. The final sphericity value is the arithmetic mean of the sphericities of all statistically analyzed particles.

[0096] The compressive strength was determined using the single-particle mechanical compression method. The specific steps are as follows: The sample powder to be tested was dispersed on a rigid substrate. Using a nanoindenter or micromechanical testing instrument (such as a Fischerscope H100 or Agilent Nano Indenter G200) equipped with a flat-head indenter (5-10 μm in diameter), isolated particles within the target size range (2.9-3.2 μm for first ferric phosphate and 0.95-1.15 μm for second ferric phosphate) were selected under an optical microscope and subjected to a compression test at a constant loading rate (e.g., 0.5 mN / s) until the particles fractured. The critical load at fracture was recorded. At least 30 particles were randomly selected from each sample for testing, and the average value of the critical load at fracture was taken as the compressive strength (unit: MPa) of the sample.

[0097] Discharge specific capacity (1C, 10C) and cycle retention rate testing methods: Electrochemical performance testing was conducted using coin cells (CR2032). The positive electrode composition was as follows: lithium iron phosphate positive electrode material: conductive carbon black: polyvinylidene fluoride (PVDF) = 8:1:1 by mass. A slurry was prepared using N-methylpyrrolidone (NMP) as the solvent, coated onto aluminum foil, and vacuum dried (120℃, 12h) before being stamped into a 12mm diameter positive electrode sheet. The negative electrode used a lithium metal sheet. The electrolyte was a mixture of 1 mol / L LiPF6 dissolved in a 1:1:1 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The separator was Celgard 2400. The battery was assembled in an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm).

[0098] Charge and discharge tests were conducted on LAND or NEWARE battery testing systems, with a voltage range of 2.0V to 4.2V (vs. Li). + / Li). The test procedure is as follows: first, activate at 0.1C for 2 weeks; then perform constant current charge and discharge tests at 1C and 10C respectively, and record the first discharge specific capacity (unit: mAh / g).

[0099] 2000-cycle capacity retention test conditions: 2000 constant current charge-discharge cycles at 1C rate, and the capacity retention rate is calculated using the following formula: .

[0100] Electronic conductivity testing method: The electronic conductivity of lithium iron phosphate cathode material powder was determined using the four-probe method. The testing instrument was an ST-2258C multifunctional digital four-probe tester or a similar device. Approximately 2g of the powder sample was placed in a special mold and pressed into a circular sheet with a diameter of 13mm under a pressure of 20 MPa. The sheet was placed on the four-probe test stage, and the voltage drop was measured under a constant current (e.g., 10 µA). The resistivity was automatically calculated according to the instrument's preset formula, and its reciprocal was taken as the electronic conductivity (unit: S / cm). Each sample was measured three times at different locations, and the average value was taken as the final result.

[0101] Test method for particle breakage rate: The particle breakage rate is determined by simulating the electrode rolling process. The specific steps are as follows: Lithium iron phosphate cathode material, conductive agent, and binder are mixed according to the actual battery formulation to form a slurry, which is then coated onto aluminum foil to form a cathode sheet. The cathode sheet is placed in a roller mill (roller gap set to 80%~85% of the original electrode sheet thickness) for a single cold pressing. The rolled cathode sheet is collected, and the binder is dissolved by soaking in N-methylpyrrolidone (NMP) solvent. The particles are then recovered by centrifugation. Appropriate amounts of the original powder and the recovered powder are taken, and their particle size distribution is determined using a laser particle size analyzer. The particle breakage rate is calculated using the following formula:

[0102] Among them, D 50 The median volume average particle size is used. Each sample was tested at least three times, and the average value was taken as the final particle breakage rate (in %).

[0103] Table 1. Comparison of A / B components in each embodiment and comparative example.

[0104] Table 2 Comparison of product parameters for each embodiment and comparative example

[0105] According to Tables 1 and 2: (1) Synergistic effect of macromolecular carbon source and dispersant on sand milling efficiency: In Example 1, when the two components are used together, the grinding time for component A is only 2.0 hours and for component B is only 3.0 hours. The viscosity of the slurry after grinding is low (42 mPa for component A). Components S and B, 28 mPa s); In Comparative Example 1 (without macromolecular carbon source and dispersant), the sand milling time of components A and B was extended to 4.5 h and 6.0 h, respectively, resulting in a decrease in efficiency of more than 50% and a significant increase in slurry viscosity (95%~132% higher than in Example 1). In Comparative Example 2 (without dispersant), the milling time of components A and B was extended to 3.0 h and 4.5 h, respectively, resulting in a 50% decrease in efficiency and a significant increase in slurry viscosity (50% to 75% higher than in Example 1). The results demonstrate that the lubricating effect of macromolecular carbon sources (such as phenolic resins) and the hard abrasive effect of dispersants (such as nano-silica) work synergistically to significantly reduce particle agglomeration and slurry viscosity, greatly improve grinding efficiency, and achieve rapid particle refinement.

[0106] (2) The controlling effect of macromolecular carbon sources and dispersants on particle size and morphology: In Example 1, component A has a D50 of 3.10 μm, a sphericity of 0.91, and a compressive strength of 14.2 MPa, while component B has a D50 of 1.08 μm, a sphericity of 0.87, and a compressive strength of 12.5 MPa. The components are dimensionally accurate, have regular morphology, and high strength. In Comparative Example 1 (without macromolecular carbon source and dispersant), the particle size of components A / B is relatively large (D50 is 3.85μm and 1.42μm, respectively), the sphericity is significantly reduced (0.72 and 0.65), the compressive strength is only 30%~35% of that of Example 1, and the loose density is greatly reduced; This indicates that the three-dimensional network formed by the curing of macromolecular carbon sources (such as phenolic resin) can fix the particle structure, and the steric hindrance effect of dispersants (such as nano-silica) can precisely control the particle size. The two work together to ensure the dimensional accuracy, high sphericity and mechanical strength of the particles, laying the structural foundation for high compaction density.

[0107] (3) The key role of two-fluid atomization in particle size control: Example 1 uses two-fluid atomization, with narrow particle size distribution of components A / B (D90 / D10 are 1.57 and 1.65 respectively), high sphericity, and excellent compressive strength; Comparative Example 3 used single-fluid atomization, resulting in a wide particle size distribution for components A and B (D90 / D10 were 3.02 and 2.85, respectively), decreased sphericity (0.75 and 0.70), and a reduction in compressive strength of over 45%, leading to a decrease in compacted density of 0.12 g / cm³. 3 ; This confirms that two-fluid atomization can precisely control droplet size and drying rate. Combined with a curing control mechanism, it can achieve precise control of particle size and morphology, which is superior to single-fluid atomization technology.

[0108] (4) The effect of double-layer carbon coating on improving electrochemical performance: Example 1 (bilayer carbon) has an electronic conductivity of 1.2 × 10⁻⁶. -1 S / cm, 10C capacity 138mAh / g, 2000 cycles retention rate 92%; Comparative Example 4 (single small molecule carbon source) has slightly higher conductivity (3.5 × 10⁻⁶).-2 However, lacking the structural support of a large molecular carbon source, the particles have low compressive strength (only 4.2 MPa), a cycle retention rate of only 83%, and a particle breakage rate of 6.2%. Comparative Example 5 (single macromolecular carbon source) exhibits extremely poor conductivity (0.5 × 10⁻⁶). -2 (S / cm), with a 10C capacity of only 115mAh / g, which cannot meet the requirements of high-rate applications; This study demonstrates that bilayer carbon coating can both form a framework structure through macromolecular carbon sources, enhancing mechanical strength and cycling stability, and form a dense conductive layer through small molecule carbon sources, strengthening electron transport, thus achieving synergistic optimization of electrochemical performance and structural stability.

[0109] (5) The controlling effect of particle size distribution on compaction and electrochemical performance The compaction density dropped sharply: the compaction density of Example 1 (three particle size distribution) reached 2.75 g / cm³, while that of Comparative Example 6 (single particle size) was only 2.50 g / cm³, a decrease of about 9%. The reason is that a single particle size cannot form a dense packing of "large particle skeleton + medium particle filling + small particle gap filling", and the porosity between particles is significantly increased.

[0110] Rate performance degradation: 10C discharge specific capacity decreased from 138 mAh / g to 110 mAh / g, a drop of approximately 20%. The single particle size was either too large (Li...). + The diffusion path is either too long or too small (loose structure), making it impossible to balance ion transport and structural stability, resulting in increased polarization at high magnification.

[0111] Cycle life and mechanical strength declined: the retention rate after 2000 cycles decreased from 92% to 78%, and the particle breakage rate increased from 2.5% to 9.0%. The lack of large particle skeleton support and the easy breakage of small particles led to the collapse of the electrode structure and a significant decrease in cycle stability.

[0112] Electronic conductivity decreased: electronic conductivity decreased from 1.2 × 10⁻⁶. - ¹S / cm decreased to 0.6×10 - The carbon density decreased by approximately 95% to 2S / cm. The uniform particle size resulted in uneven carbon coating, discontinuous conductive network, and impeded electron transport.

[0113] Compared with other comparative examples: The performance degradation of Comparative Example 6 is significantly greater than that of other comparative examples (such as Comparative Examples 1 / 2 / 4 / 5), indicating that particle size distribution is a more fundamental core design than carbon source and dispersant. Once the distribution fails, other optimization measures are difficult to make up for the performance loss.

[0114] The particle breakage rate of Comparative Example 6 (9.0%) was the highest among all groups, directly confirming the design logic of "three-size gradation provides mechanical support".

[0115] Comprehensive comparison Figure 1 (Example 1) and Figure 2 The SEM image (50,000×) of (Comparative Example 1) shows that the lithium iron phosphate cathode material prepared in Example 1 of this invention ( Figure 1 The particles exhibit a regular, dense spherical morphology with smooth surfaces and tight packing. This is attributed to the synergistic curing and steric hindrance effect of the macromolecular carbon source (phenolic resin) and dispersant (nano silica) during the spray drying process, effectively constructing a high-strength three-dimensional network structure; while Comparative Example 1 ( Figure 2 Because the aforementioned functional additives were not added, the particles had irregular morphology, rough and loose surfaces, poor sphericity, and obvious microcracks. The significant difference in morphology between the two directly confirms the key role of the technical solution of the present invention in improving particle sphericity, structural density, and mechanical strength. This is also the structural reason why Example 1 is superior to Comparative Example 1 in terms of compaction density (2.75 vs. 2.52 g / cm³), compressive strength (14.2 vs. 4.5 MPa), and cyclic stability (92% retention rate after 2000 cycles vs. 81%).

[0116] In summary, this invention successfully overcomes the technical bottleneck of simultaneously improving the compaction density and electrochemical performance of traditional lithium iron phosphate materials through a combination of "synergistic solidification and dispersion mechanism for precise multi-scale particle control + two-fluid atomization molding + double-layer carbon coating sintering". The synergistic effect of macromolecular carbon sources (such as phenolic resins) and dispersants (such as nano-silica) is the core to achieving improved milling efficiency, precise particle size control, and enhanced mechanical strength. Two-fluid atomization helps ensure particle morphology, and double-layer carbon coating has a significant impact on electrochemical performance.

[0117] 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 a lithium iron phosphate cathode material, characterized in that, include: First ferric phosphate, second ferric phosphate, and third ferric phosphate with different particle sizes were prepared using ferric phosphate raw materials. The particle size D50 of the ferric phosphate raw materials was 1.0 μm to 1.5 μm, the first ferric phosphate had a particle size D50 of 2.9 μm to 3.2 μm, the second ferric phosphate had a particle size D50 of 0.95 μm to 1.15 μm, and the third ferric phosphate had a particle size of 0.28 μm to 0.35 μm. The mass percentages of the ferric phosphate raw materials used to prepare the first ferric phosphate, second ferric phosphate, and third ferric phosphate were 40% to 60%, 15% to 45%, and 15% to 25%, respectively. Functional additives, including macromolecular carbon sources and dispersants, were introduced during the preparation of the first ferric phosphate and second ferric phosphate. The first ferric phosphate, the second ferric phosphate and the third ferric phosphate are mixed and slurried, then mixed with a lithium source and a small molecule carbon source and granulated to obtain a composite precursor powder. The composite precursor powder is sintered.

2. The preparation method according to claim 1, characterized in that, The process of preparing the first ferric phosphate and the second ferric phosphate both include: mixing the ferric phosphate raw material, functional additives and water, grinding the resulting slurry, and then spray drying it using a two-fluid atomizer; And / or, the macromolecular carbon source is selected from at least one of phenolic resin, furfural resin, polybenzoxazine precursor and polyacrylonitrile prepolymer; the weight average molecular weight of the macromolecular carbon source is 3500~8000; more preferably, the mass ratio of the amount of the macromolecular carbon source to the mass of the ferric phosphate raw material used to prepare the first ferric phosphate or the second ferric phosphate is (3.5~7.5):100; And / or, the dispersant is selected from at least one of nano-silica, nano-alumina and nano-silicon carbide; the particle size D50 of the dispersant is 10nm-30nm; more preferably, the mass ratio of the amount of the dispersant to the mass of the ferric phosphate raw material used to prepare the first ferric phosphate or the second ferric phosphate is (0.5~1.0):

100.

3. The preparation method according to claim 2, characterized in that, The process for preparing the first iron phosphate has at least one of the following characteristics: A1-D1 Feature A1: The solid content of the slurry used for sand milling is 45%-58%; Feature B1: During the sand milling process, the rotation speed is controlled at 1700rpm~2200rpm, the processing time is 1.5h~3.0h, and the particle size D50 of the slurry after sand milling is controlled at 0.35μm~0.45μm; Feature C1: Controlled inlet air temperature of 190℃~210℃, atomization pressure of 0.35MPa~0.50MPa, and outlet air temperature of 100℃~120℃; Feature D1: The sphericity of the first ferric phosphate obtained after spray drying is >0.9, and the loose pack density is 1.1 g / cm³. 3 ~1.3g / cm 3 .

4. The preparation method according to claim 1, characterized in that, The process for preparing the second iron phosphate has at least one of the following characteristics: A2-D2 Feature A2: The solid content of the slurry used for sand milling is 25%~35%; Feature B2: During the sand milling process, the rotation speed is controlled at 1800rpm~2400rpm, the processing time is 2h~4h, and the particle size D50 of the slurry after sand milling is controlled at 0.3μm~0.4μm; feature C2: Control the inlet air temperature to 170℃~190℃, the atomization pressure to 0.6MPa~0.9MPa, and the outlet air temperature to 90℃~110℃; Feature D2: The sphericity of the second ferric phosphate obtained after spray drying is >0.85, and the loose pack density is 0.8 g / cm³. 3 ~1.0g / cm 3 .

5. The preparation method according to claim 1, characterized in that, The process of preparing the third ferric phosphate includes: mixing the ferric phosphate raw material with water to obtain a slurry with a solid content of 40% to 55%, and then grinding it until the particle size of the particles in the slurry reaches 0.28 μm to 0.35 μm; Preferably, during the preparation of the third ferric phosphate, the milling speed is controlled at 2000 rpm to 2800 rpm, the milling time is 6 h to 9 h, and the particle size is measured by a laser particle size analyzer every 1 h to 2 h until the particle size of the particles in the slurry meets the requirements.

6. The preparation method according to claim 1 or 5, characterized in that, The preparation process of the composite precursor powder includes: mixing the first ferric phosphate, the second ferric phosphate and the third ferric phosphate, adding water to adjust the solid content of the slurry to 42%~52% to obtain a mixed slurry; The mixed slurry is mixed and stirred with the lithium source and the small molecule carbon source to obtain a mixed coating material; The mixed coating material is spray-dried.

7. The preparation method according to claim 6, characterized in that, The process for preparing the composite precursor powder comprises at least one of feature A3 to feature F3: Feature A3: The small molecule carbon source is selected from at least one of glucose, sucrose, maltose, and citric acid; Feature B3: The amount of the small molecule carbon source added accounts for 2.2wt%~4.0wt% of the total mass of iron phosphate, and the total carbon content of the final product is controlled at 1.0wt%~2.0wt%; Feature C3: When preparing the mixed slurry, the stirring speed is controlled at 800 rpm to 1200 rpm and the stirring time is 30 min to 60 min; Feature D3: The lithium source is added at a lithium to iron molar ratio of (1.02~1.04):1; the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; Feature E3: After mixing with the lithium source and the small molecule carbon source, stir at a speed of 800 rpm to 1200 rpm for 60 min to 90 min; Feature F3: During the spray drying process of the mixed coating material, the inlet air temperature is controlled at 180℃~200℃, the outlet air temperature is controlled at 90℃~110℃, and the atomization pressure is controlled at 0.4MPa~0.7MPa.

8. The preparation method according to claim 1, characterized in that, The process of sintering the composite precursor powder includes: performing a first sintering stage and a second sintering stage under an inert atmosphere; wherein, in the first sintering stage, the sintering temperature is controlled at 300℃~420℃ and the holding time is 4h~8h; in the second sintering stage, the sintering temperature is controlled at 650℃~710℃ and the holding time is 12h~16h. Preferably, in the first sintering stage, the heating rate is controlled to be 2℃ / min~4℃ / min; Preferably, in the second sintering stage, the heating rate is controlled to be 3℃ / min~6℃ / min; Preferably, the method further includes: crushing and sieving the sintered product.

9. A lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. A lithium-ion battery, characterized in that, Including the lithium iron phosphate cathode material as described in claim 9.

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