Lithium iron phosphate positive electrode material, preparation method and application thereof

Abstract

The application discloses a lithium iron phosphate positive electrode material and a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The application utilizes the "reactivity bonding" and "in-situ pore-forming-conductive synergistic effect", and through functional pretreatment of a fine slurry containing small particles by a surfactant and a hydrophobic polymer, the fine slurry can produce a synergistic effect with a functional nitrogen source in a subsequent granulation and sintering carbonization process, a graphitized nitrogen-doped carbon coating layer with high conductivity is constructed, the internal resistance can be significantly reduced, rich mesopores are formed in the material, and the rapid transmission of lithium ions is ensured; the high-density and low-porosity spherical secondary particles are constructed through double-stage particle granulation, the tap density and processing performance can be improved, a stable hydrophobic surface is formed, iron elution is inhibited, and the service life of the material is improved. The application can solve a series of contradictory problems such as conductivity, ion migration rate, tap density and interface stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LFP) cathode materials possess advantages such as structural stability, good safety, long cycle life, and low cost, leading to their widespread application in both the power and energy storage markets. However, due to multiple factors including the material's bulk structure and manufacturing process, its rate capability and low-temperature electrochemical performance have consistently lagged behind layered oxide materials. Significantly improving the electronic conductivity and lithium-ion diffusion rate of LFP cathode materials remains a performance bottleneck. Furthermore, the cell requires high moisture content in its materials; therefore, enhancing the hydrophobicity of the material surface is also crucial.

[0003] The core challenge in analyzing lithium iron phosphate (LiFePO4) materials lies in the trade-off between electronic conductivity and ion diffusion rate, and the conflict between material nanostructuring and electrode processing performance. Existing technologies typically employ the following methods for improvement:

[0004] (1) Nano-sizing: Reducing the primary particle size to shorten the ion diffusion path. However, this leads to an increase in specific surface area, causing a series of problems such as poor slurry processability, high water absorption, and reduced tap density.

[0005] (2) Carbon coating: A carbon layer is coated on the surface of the particles to improve electronic conductivity. However, conventional carbon sources (such as glucose and sucrose) are mostly amorphous carbon after carbonization, which has limited improvement in conductivity, and the dense carbon layer may hinder lithium ion migration.

[0006] (3) Particle size distribution: The filling density is increased by mixing particles of different sizes. However, simple physical mixing is difficult to form stable and compact secondary spheres, which are easily broken in subsequent processing, resulting in a decline in performance.

[0007] Therefore, there is an urgent need to develop a comprehensive technical solution that can synergistically address a series of contradictory issues related to conductivity, ion mobility, tap density, and interface stability.

[0008] In view of this, the present invention is proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a lithium iron phosphate cathode material, its preparation method, and its application, aiming to synergistically solve problems such as conductivity, ion mobility, tap density, and interface stability, thereby improving the overall performance of the product.

[0010] This invention is implemented as follows:

[0011] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising:

[0012] Iron source, phosphorus source, lithium source, carbon source, nitrogen source and first solvent are mixed to obtain the slurry to be treated;

[0013] The slurry to be treated is first refined to reduce the particle size in the slurry. The slurry with a mass fraction of 60%-80% is taken as the matrix slurry. The remaining slurry is then further refined to obtain the refined slurry.

[0014] A functional slurry is obtained by mixing a surfactant, a second solvent, a hydrophobic polymer, and a slurry refiner.

[0015] The functional slurry and the matrix slurry are mixed and granulated to obtain precursor particles;

[0016] The precursor particles are sintered.

[0017] In an optional embodiment, the particle size in the slurry is reduced to 0.2μm-0.4μm through a single refining process;

[0018] And / or, through secondary refining treatment, the particle size in the slurry is reduced to 0.05μm-0.15μm;

[0019] And / or, a primary and secondary refining process is performed using grinding.

[0020] In an optional embodiment, the surfactant is selected from cationic surfactants;

[0021] And / or, the hydrophobic polymer is selected from at least one of polyvinylidene fluoride and fluorinated acrylic acid;

[0022] And / or, the mass ratio of surfactant to iron source is (0.008-0.015):100;

[0023] And / or, the mass ratio of hydrophobic polymer to the solid content of the refined slurry is (0.4-0.6):100;

[0024] And / or, first mix and dissolve the surfactant and the second solvent, then mix with the hydrophobic polymer, and then mix with the refined slurry;

[0025] And / or, the second solvent is water.

[0026] In an optional embodiment, the surfactant is selected from at least one of hexadecyltrimethylammonium bromide, Tween, sodium dodecyl sulfate, and dioctadecyldimethylammonium chloride.

[0027] In an optional embodiment, the carbon source is selected from at least one of cyclodextrin, glucose, and maltose;

[0028] And / or, the nitrogen source is selected from at least one of urea, polyacrylonitrile, and amino acids;

[0029] And / or, the mass ratio of carbon source to nitrogen source is 1:(0.5-1.5), and the mass ratio of the total amount of carbon source and nitrogen source to iron source is (1.0-1.5):1.

[0030] In an optional embodiment, the iron source is selected from at least one of ferric phosphate, iron oxide red, and ferrous oxalate;

[0031] And / or, the phosphorus source is selected from at least one of ferric phosphate, diammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid;

[0032] And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate;

[0033] And / or, by adjusting the amounts of iron, phosphorus, and lithium sources, the molar ratio of iron, phosphorus, and lithium is made to be 1:(0.99-1.03):(1.01-1.04).

[0034] And / or, the first solvent is water, and the amount of the first solvent is adjusted to make the solid content of the slurry to be treated 30%-40%.

[0035] In an optional implementation, granulation is carried out by spray drying, and the inlet and outlet air temperatures are controlled at 110℃-130℃.

[0036] And / or, during the sintering process, the heating rate is controlled at 2℃ / min-5℃ / min, the target holding temperature is 650℃-750℃, and the holding time is 8h-12h.

[0037] 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.

[0038] Thirdly, the present invention provides a lithium-ion battery cathode sheet, comprising the lithium iron phosphate cathode material of the aforementioned embodiments.

[0039] Fourthly, the present invention provides a lithium-ion battery, including the lithium-ion battery positive electrode sheet of the aforementioned embodiments.

[0040] This invention offers the following advantages: It utilizes a combination of reactive bonding and in-situ pore-forming-conductivity synergistic effects. By functionalizing a refined slurry containing small particles with surfactants and hydrophobic polymers, it enables the slurry to synergistically interact with a functional nitrogen source during subsequent granulation and sintering / carbonization, constructing a highly conductive graphitized nitrogen-doped carbon coating layer. This significantly reduces internal resistance, creates abundant mesopores within the material, and ensures rapid lithium-ion transport. Furthermore, the invention employs bi-level particle granulation to construct high-density, low-porosity spherical secondary particles, improving tap density and processing performance, forming a stable hydrophobic surface, inhibiting iron dissolution, and extending material lifespan. Therefore, the preparation method provided by this invention can synergistically address a series of contradictory issues related to conductivity, ion mobility, tap density, and interfacial stability, demonstrating excellent application prospects. Attached Figure Description

[0041] 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.

[0042] Figure 1 SEM image of the cathode material prepared in Example 1;

[0043] Figure 2 The image shows the XRD pattern of the cathode material prepared in Example 1. Detailed Implementation

[0044] 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.

[0045] To address the issues of conductivity, ion mobility, tap density, and interface stability in existing lithium iron phosphate cathode materials, this invention constructs a highly conductive graphitized nitrogen-doped carbon coating layer, which significantly reduces internal resistance, forms abundant mesopores within the material, and ensures rapid lithium-ion transport. Furthermore, by constructing high-density, low-porosity spherical secondary particles, it improves tap density and processing performance, forms a stable hydrophobic surface, and inhibits iron dissolution.

[0046] This invention also provides a method for preparing lithium iron phosphate cathode material, utilizing "reactive bonding" and "in-situ pore-forming-conductive synergy." A specific process is used to functionalize the small particles in the bigraded particle structure, enabling them to synergistically interact with functional nitrogen sources (such as polyacrylonitrile) during subsequent spray carbonization, thereby constructing a highly conductive graphitized nitrogen-doped carbon coating layer. The specific steps are as follows:

[0047] S1, Slurry Preparation

[0048] Iron, phosphorus, lithium, carbon, and nitrogen sources are mixed in a specific molar ratio and added to a first solvent with a certain solid content to prepare a slurry to be treated. The slurry is then subjected to a first refinement treatment to reduce the particle size. The slurry after the first refinement treatment is divided into two parts: a portion containing 60%-80% by mass is used as the matrix slurry, and the remaining slurry undergoes a second refinement treatment to further reduce the particle size, resulting in a refined slurry. This refined slurry will then proceed to step S2 for pretreatment to prepare the functional slurry.

[0049] In some embodiments, the iron source is selected from at least one of ferric phosphate, iron oxide red, and ferrous oxalate, and the iron source can be any one or more of the above. The phosphorus source is selected from at least one of ferric phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid, and the phosphorus source can be any one or more of the above. Both the iron source and the phosphorus source can be ferric phosphate, but are not limited to this. The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate, and the lithium source can be any one or more of the above. By adjusting the amounts of the iron source, phosphorus source, and lithium source, the molar ratio of iron, phosphorus, and lithium in each raw material is made to be 1:(0.99-1.03):(1.01-1.04), such as 1:0.99:1.01, 1:1.00:1.02, 1:1.01:1.02, 1:1.02:1.03, 1:1.03:1.04, etc.

[0050] In some embodiments, the carbon source is selected from at least one of cyclodextrin, glucose, and maltose, and the carbon source can be any one or more of the above. Cyclodextrin is used as the main carbon source because it is a molecule with unique properties of being hydrophobic inside and hydrophilic outside. Its inner cavity has high molecular recognition and enrichment capabilities, while its outer cavity has good solubility. Cyclodextrin has a long carbon chain, resulting in a high degree of graphitization of the carbon layer after carbonization. This is beneficial for improving the rate performance of the material.

[0051] Furthermore, the nitrogen source is selected from at least one of urea, polyacrylonitrile, and amino acids, and can be any one or more of these. The mass ratio of carbon source to nitrogen source is 1:(0.5-1.5), such as 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, etc. The mass ratio of the total amount of carbon source and nitrogen source to iron source is adjusted to (1.0-1.5):1, such as 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc. By adjusting the amount of carbon source and nitrogen source, the content of carbon and nitrogen in the product is made more suitable, which is beneficial to further improve conductivity and enhance the electrochemical performance of the cathode material.

[0052] In some embodiments, the first solvent can be water, specifically distilled water, but not limited thereto. The amount of the first solvent is adjusted so that the solid content of the prepared slurry is 30%-40%, such as 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, etc.

[0053] Furthermore, a primary refining process reduces the particle size of the slurry to 0.2 μm-0.4 μm, such as 0.20 μm, 0.23 μm, 0.25 μm, 0.28 μm, 0.30 μm, 0.33 μm, 0.35 μm, 0.38 μm, and 0.40 μm. A secondary refining process reduces the particle size of the slurry to 0.05 μm-0.15 μm, resulting in particle sizes of 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, 0.10 μm, 0.11 μm, 0.12 μm, 0.13 μm, 0.14 μm, and 0.15 μm. Adjusting the particle size of the slurry improves the pretreatment effect of step S2 and enhances the bonding function of small particles in the functional slurry prepared in step S2. There are no restrictions on the operation methods for primary and secondary refinement processes; for example, grinding can be used to reduce particle size.

[0054] S2, Pretreatment of small particle slurry

[0055] The surfactant, second solvent, hydrophobic polymer, and slurry refiner are mixed evenly to obtain a functional slurry for later use.

[0056] The surfactant is selected from cationic surfactants, and the specific type is not limited. In some embodiments, the surfactant is selected from at least one of hexadecyltrimethylammonium bromide (CTAB), Tween, sodium dodecyl sulfate, and dioctadecyldimethylammonium chloride. The surfactant can be any one or more of the above, and all of the above hydrophilic surfactants are suitable for the pretreatment process of the embodiments of the present invention.

[0057] In some embodiments, the hydrophobic polymer is selected from at least one of polyvinylidene fluoride (PVDF) and fluorinated acrylic acid, and the hydrophobic polymer can be any one or more of the above. In a preferred embodiment, the surfactant is hexadecyltrimethylammonium bromide (CTAB), and the hydrophobic polymer is polyvinylidene fluoride (PVDF). The inventors have found that this combination can form a more stable "oil-in-water" structure and has a better effect on improving the hydrophobicity and conductivity of the final coating layer.

[0058] In practice, the surfactant and a second solvent (such as water) can be mixed in a specific tank first, and then a hydrophobic polymer can be added to it. The mixture is then added to a sand mill containing the refined slurry to create a functional slurry containing small particles. The second solvent can be water, but is not limited to this.

[0059] It should be noted that the hydrophilic surfactant is first dissolved in the second solvent, and then the hydrophobic polymer (such as PVDF) powder is added. Mechanical stirring is then used to form a stable "oil-in-water" micromicelle structure. When this pretreated hydrophobic polymer (such as PVDF) emulsion is added to small particle slurry, the slurry viscosity increases significantly. The "oil-in-water" structure of PVDF quickly anchors to the surface of the small particles, giving them extremely strong adhesive properties.

[0060] Furthermore, the mass ratio of surfactant to iron source is (0.008-0.015):100, such as 0.008:100, 0.009:100, 0.010:100, 0.011:100, 0.012:100, 0.013:100, 0.014:100, 0.015:100, etc. The mass ratio of hydrophobic polymer to the solid content of refined slurry is (0.4-0.6):100, such as 0.40:100, 0.43:100, 0.45:100, 0.48:100, 0.50:100, 0.53:100, 0.55:100, 0.58:100, 0.60:100, etc. By adjusting the amount of surfactant and hydrophobic polymer, a stable "water-in-oil" structure can be formed and small particles can be given better adhesion; this facilitates the achievement of the optimal window for effective filling, pore formation and the formation of a complete hydrophobic coating.

[0061] S3. Preparation of precursor particles

[0062] The functional slurry obtained in step S2 and the matrix slurry obtained in step S1 are mixed and granulated to obtain high-density precursor particles. The granulation method is not limited; spray drying is an option. During spray drying, the inlet and outlet air temperatures are controlled between 110℃ and 130℃, such as 110℃, 113℃, 115℃, 118℃, 120℃, 123℃, 125℃, 128℃, or 130℃.

[0063] It should be noted that the pretreated small-particle functional slurry is remixed evenly with the ordinary large-particle matrix slurry and then spray-dried. During this process, the particles in the small-particle functional slurry act as "functionalized bonding points," actively filling and firmly bonding to the voids formed by the particle accumulation in the matrix slurry through their enhanced surface adhesion, forming dense and robust spherical secondary particles (polycrystalline spheres).

[0064] S4, sintering

[0065] The precursor particles obtained in step S3 are sintered to form a highly conductive graphitized nitrogen-doped carbon coating layer. During sintering, the heating rate is controlled at 2℃ / min-5℃ / min, the target holding temperature is 650℃-750℃, and the holding time is 8h-12h.

[0066] Specifically, the temperature is increased from room temperature (e.g., 25℃) at rates of 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, etc., with target holding temperatures of 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, etc., and holding times of 8h, 9h, 10h, 11h, 12h, etc. The sintering process can be carried out under a protective atmosphere, such as nitrogen or argon.

[0067] It should be noted that, under a protective atmosphere, the secondary spherical precursor obtained in step S3 is subjected to programmed temperature sintering: in the range of 300-500℃, the nitrogen source (such as polyacrylonitrile) undergoes cyclization and preliminary carbonization, forming a stable nitrogen-containing cross-linked structure. In the range of 450-650℃, the pretreated hydrophobic polymer undergoes staged decomposition and carbonization: the surfactant decomposes, and the hydrophobic polymer it encapsulates partially vaporizes to create pores, while partially carbonizes to form a carbon layer. This process is intertwined with the carbonization process of polyacrylonitrile. Finally, at 700-750℃, the final lithium iron phosphate crystal is formed, whose surface is coated with a composite coating layer composed of polyacrylonitrile-derived nitrogen-doped carbon, main carbon source-derived carbon, and hydrophobic polymer-derived carbon. This composite coating layer contains abundant mesopores and has a high degree of graphitization. In addition, the presence of the surfactant-coated hydrophobic polymer regulates its decomposition behavior: part of it decomposes to produce gas, creating mesopores in situ in the carbon layer, which is conducive to ion transport; the other part carbonizes to form a hydrophobic carbon layer. Cyclodextrin and other materials serve as the main carbon source, and after carbonization, they form a carbon skeleton with a high degree of graphitization, which together with the CN coating layer constitutes a three-dimensional conductive network.

[0068] In summary, the method for preparing lithium iron phosphate cathode material provided in this embodiment of the invention, through steps S1-S4, forms a highly conductive graphitized nitrogen-doped carbon coating layer. The working principle of the preparation method is as follows:

[0069] (1) In the embodiment of the present invention, stable and dense polycrystalline secondary spheres are formed in the spray granulation stage. The single crystal small particles with high power performance are first prepared into polycrystalline materials similar to ternary materials. The small particles are in close contact with each other, which can greatly solve the BET processing problem.

[0070] While ensuring material performance, this invention avoids the problem of large BET (Break-Earth Scale) by forming polycrystalline spheres. Specifically, a dual-particle gradation treatment method is adopted, with the particle size of the matrix slurry being 0.2μm-0.4μm and the particle size of the refined slurry being 0.05μm-0.15μm. The two slurries are treated differently. In order to form small particles filling the large particles during spraying, the refined slurry is specially treated. At the end of sand milling, hydrophobic polymers such as PVDF are added. Because PVDF is hydrophobic and not easily dissolved, a surfactant is selected to pre-treat it. The hydrophilic surfactant encapsulates it to form an oil-in-water structure. After addition, the viscosity gradually increases, and the surface of the small particles is quickly anchored by the oil-in-water structure of PVDF, resulting in strong adhesion on the surface of the small particles. After treatment, it is compounded with the matrix slurry containing large particles. After the large and small particles are mixed evenly, it is sprayed. During spraying, the adhesion of the small particles fills the gaps in the accumulation of large particles, forming dense polycrystalline spheres. During the sintering process, a carbon source with a low carbonization temperature is first reduced and carbonized. The nitrogen source (such as polyacrylonitrile) is gradually carbonized to form a graphitized N and C layer, which together form a CN coating layer, improving the conductivity of the material and benefiting its rate performance. The PVDF layer, coated with a surfactant, carbonizes and becomes a gas that escapes, playing a role in mesoporous structure creation and releasing PVDF. The carbonized PVDF forms a carbon layer that coats the particle surface, creating a hydrophobic layer.

[0071] (2) Constructing a multifunctional synergistic coating layer: To improve the power performance of the material, nitrogen sources such as polyacrylonitrile are introduced because polyacrylonitrile forms pyridine N and graphite N after carbonization, which can improve the electronic conductivity of the material, reduce internal resistance, and reduce the voltage difference polarization of the charge and discharge plateau. The lone pair electrons of pyridine N can form coordinate bonds with iron, thereby inhibiting iron dissolution and reducing the formation of iron phosphide. The hydrophobicity of LFP can be achieved by using some silicon-based carbon sources (such as polydimethylsiloxane, polyphenylmethylsiloxane) and PVDF to form a coating layer, forming a hydrophobic carbon layer.

[0072] The steps in the embodiments of this invention are not simply superimposed. The synergistic effect of the special pretreatment process for refining the slurry and the introduction of nitrogen sources such as polyacrylonitrile allows the components to interact under a specific spatiotemporal sequence, producing a synergistic effect of "1+1>2". Specifically, this is manifested in:

[0073] (1) Structural synergy: The "functionalized small particles" not only solve the physical filling problem, but the pretreated polymer they carry also becomes a "bridge" connecting particles of different sizes and a "template" for subsequent pore formation, realizing structural optimization from the micro to the meso scale.

[0074] (2) Functional synergy: Polyacrylonitrile and pretreated hydrophobic polymers (such as PVDF) interact during carbonization. The introduction of polyacrylonitrile increases the graphitization degree of the overall carbon layer, while the controlled decomposition of PVDF creates mesopores, and its decomposition products (such as HF) may activate the carbonization process of polyacrylonitrile, optimize the formation efficiency of pyridine N, and thus more effectively inhibit iron dissolution.

[0075] (3) Process synergy: The entire process can be completed on conventional equipment. Through ingenious material handling and program design, multiple functional steps such as granulation, coating, doping, and pore formation are integrated into a single sintering process, which simplifies the process, reduces costs, and is suitable for industrial production.

[0076] This invention also provides a lithium iron phosphate cathode material, prepared by the method provided in this invention, which has a highly conductive graphitized nitrogen-doped carbon coating layer, and can synergistically solve a series of problems such as conductivity, ion mobility, tap density and interface stability.

[0077] This invention also provides a lithium-ion battery positive electrode sheet, comprising the aforementioned lithium iron phosphate positive electrode material, using the aforementioned lithium iron phosphate positive electrode material as the positive electrode active material. It may further include a positive electrode current collector, on at least one surface of which a positive electrode active coating is formed, wherein the lithium iron phosphate positive electrode material exists as the positive electrode active material in the positive electrode active coating.

[0078] This invention also provides a lithium-ion battery, including the above-mentioned lithium-ion battery positive electrode, and may also include a negative electrode, electrolyte, separator, etc., to form a complete battery structure with good electrochemical performance.

[0079] Specifically, the types of negative electrode, electrolyte, and separator are not limited and can be common materials used in lithium iron phosphate batteries.

[0080] This invention also 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.

[0081] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0082] Example 1

[0083] This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows:

[0084] Ferric phosphate was used as both the phosphorus and iron source (iron-phosphorus molar ratio approximately 1:1), lithium carbonate as the lithium source, cyclodextrin as the carbon source, and polyacrylonitrile as the nitrogen source. The lithium source was weighed according to an iron-to-lithium molar ratio of 1:1.03, and the total mass ratio of carbon and nitrogen sources to the iron source was controlled at 1.2:1, and the mass ratio of carbon and nitrogen sources was 1:1. The lithium, carbon, and nitrogen sources were added to the ferric phosphate raw material, and distilled water was added at a solid content of 40%. After milling until the average particle size in the slurry reached 0.3 μm, 70% of the slurry was taken as the matrix slurry. The remaining 30% of the slurry was ground until the average particle size reached 0.1 μm, resulting in a refined slurry.

[0085] Hexadecyltrimethylammonium bromide (CTAB) at 0.01% of the iron source mass (i.e., a mass ratio of hexadecyltrimethylammonium bromide to iron source of 0.01:100) is first dissolved in water, and then PVDF at 0.5% of the slurry solid content (i.e., a mass ratio of PVDF to slurry solid content of 0.5:100) is added and stirred evenly to obtain a PVDF pretreatment solution. The PVDF pretreatment solution is added to a tank for storing the slurry refinement. The resulting slurry solution is then added to a sand mill tank for storing the matrix slurry. The mixture with different particle sizes is spray-dried, with the inlet and outlet air temperatures controlled at 120℃ to obtain spherical sprayed material. This material is then fed into a roller kiln, where the sintering temperature is set to 700℃, the heating rate is 2℃ / min, and the sintering time is 10 hours.

[0086] SEM image of the lithium iron phosphate cathode material prepared in Example 1 is shown below. Figure 1 As shown, lithium iron phosphate is composed of a mixture of large and small particles, with the small particles clearly embedded in the gaps between the large particles.

[0087] The XRD pattern of the lithium iron phosphate cathode material prepared in Example 1 is shown below. Figure 2 As shown, its diffraction peaks correspond one-to-one with the standard card, indicating good crystallization properties.

[0088] Example 2

[0089] This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows:

[0090] Ammonium dihydrogen phosphate was used as the phosphorus source, ferrous oxalate as the iron source, lithium hydroxide as the lithium source, cyclodextrin as the carbon source, and urea as the nitrogen source. The raw materials were weighed according to a molar ratio of iron, phosphorus, and lithium of 1:0.99:1.01, a mass ratio of the total amount of carbon and nitrogen sources to the iron source of 1.0:1, and a mass ratio of carbon to nitrogen sources of 1:0.5. The phosphorus and iron sources were mixed, and the lithium, carbon, and nitrogen sources were added. Distilled water was added at a solid content of 30%. After milling until the average particle size in the slurry reached 0.2 μm, 60% of the slurry was taken out as the matrix slurry. The remaining 40% of the slurry was ground until the average particle size reached 0.05 μm, resulting in a refined slurry.

[0091] Tween, comprising 0.008% of the iron source mass (i.e., a Tween-to-iron source mass ratio of 0.008:100), is first dissolved in water. Then, PVDF, comprising 0.4% of the slurry solids content (i.e., a PVDF-to-slurry solids content mass ratio of 0.4:100), is added and stirred until homogeneous to obtain a PVDF pretreatment solution. This PVDF pretreatment solution is added to a tank storing the slurry. The resulting slurry solution is then added to a sand mill tank storing the matrix slurry. The mixture with varying particle sizes is spray-dried, with the inlet and outlet air temperatures controlled at 110℃, to obtain spherical spray-dried material. This material is then fed into a roller kiln, where the sintering temperature is set to 650℃, the heating rate is 2℃ / min, and the sintering time is 12 hours.

[0092] Example 3

[0093] This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows:

[0094] Phosphoric acid was used as the phosphorus source, iron oxide red as the iron source, lithium dihydrogen phosphate as the lithium source, cyclodextrin as the carbon source, and amino acids as the nitrogen source. The raw materials were weighed according to a molar ratio of iron, phosphorus, and lithium of 1:1.03:1.04, a total carbon and nitrogen source to iron source mass ratio of 1.5:1, and a carbon to nitrogen source mass ratio of 1:1.5. The phosphorus and iron sources were mixed, and the lithium, carbon, and nitrogen sources were added. Distilled water was added at a solid content of 40%. After milling until the average particle size in the slurry reached 0.4 μm, 80% of the slurry was taken as the matrix slurry. The remaining 20% ​​of the slurry was ground until the average particle size reached 0.15 μm, resulting in a refined slurry.

[0095] Sodium dodecyl sulfate (0.015% by mass of iron source, i.e., a mass ratio of sodium dodecyl sulfate to iron source of 0.015:100) is first dissolved in water, and then PVDF (0.6% by mass of the slurry solid content, i.e., a mass ratio of PVDF to slurry solid content of 0.6:100) is added and stirred evenly to obtain a PVDF pretreatment solution. The PVDF pretreatment solution is added to a tank for storing the slurry refinement. The resulting slurry solution is then added to a sand mill tank for storing the matrix slurry. The mixture with different particle sizes is spray-dried, with the inlet and outlet air temperatures controlled at 130℃ to obtain spherical sprayed material. This material is then fed into a roller kiln, where the sintering temperature is set to 750℃, the heating rate is 5℃ / min, and the sintering time is 8 hours.

[0096] Comparative Example 1

[0097] The only difference from Example 1 is that polyacrylonitrile is not added, and it is replaced with an equal amount of cyclodextrin.

[0098] Comparative Example 2

[0099] The only difference from Example 1 is that PVDF, which has hydrophobic and binding properties, is not added to the refined slurry (i.e., hexadecyltrimethylammonium bromide (CTAB) is added, but PVDF is not added).

[0100] Comparative Example 3

[0101] The only difference from Example 1 is that no gradation is performed, and the particle size is 0.3 μm. That is, after grinding until the average particle size in the slurry reaches 0.3 μm, grinding is stopped, and the whole slurry enters the modification stage of hexadecyltrimethylammonium bromide and PVDF.

[0102] Comparative Example 4

[0103] The only difference from Example 1 is the amount of PVDF added to the slurry. The amount of PVDF added is 0.3% of the solid content of the slurry.

[0104] Comparative Example 5

[0105] The only difference from Example 1 is the amount of PVDF added to the refined slurry. The amount of PVDF added accounts for 0.7% of the solid content of the refined slurry.

[0106] Comparative Example 6

[0107] The only difference from Example 1 is that the amount of CTAB added to the refined slurry is 0.005%.

[0108] Comparative Example 7

[0109] The only difference from Example 1 is that the amount of CTAB added to the refined slurry is 0.02%.

[0110] Comparative Example 8

[0111] The only difference from Example 1 is that the PVDF pretreatment solution (as in Example 1) is added before the remaining 30% slurry is ground, instead of being added after grinding.

[0112] Comparative Example 9

[0113] The only difference from Example 1 is that the sintering temperature is 800°C.

[0114] The performance parameters of the lithium iron phosphate cathode materials prepared in the test examples and comparative examples are shown in Table 1.

[0115] Test method:

[0116] 1. Compacted density test, specifically, is conducted using a Sansi compaction density meter, based on the principle that compacted density = mass / volume. Vibratory density is also based on this principle; a corresponding mass is weighed into a graduated cylinder, vibrated 1000 times by a vibratory density meter, and the volume is recorded. The ratio of mass to volume is the vibratory density.

[0117] 2. Moisture increase after 24 hours: The gravimetric method was used to test the increase in moisture content after placing the product in an environment with 70% humidity and 25°C for 24 hours.

[0118] 3. (1) Battery assembly: negative electrode shell - lithium sheet - LiPF6 electrolyte - Celgard 2500 separator - LiPF6 electrolyte - positive electrode sheet - gasket - spring sheet - positive electrode shell; (2) Test conditions: The assembled coin cell was placed in a constant temperature chamber at 25℃ for 10 h. After the standing period, the electrochemical performance was tested using the LAND battery test system. The voltage range was 2.0~3.75V. The results are recorded in Table 1.

[0119] Table 1 Performance test results of the examples and comparative examples

[0120]

[0121] The above data illustrates that:

[0122] Comparison of Comparative Example 1 and Example 1: Comparative Example 1 (without polyacrylonitrile) lagged behind in both high-rate capacity and compaction. This demonstrates that the N-doped carbon layer derived from polyacrylonitrile is crucial for constructing a highly conductive network and stabilizing the interface through Fe-N coordination bonds, and its effect cannot be replaced by conventional carbon sources.

[0123] Comparison of Comparative Example 2 and Example 1: The tap density and compacted density of Comparative Example 2 (small particles without pretreatment) were significantly reduced, and the moisture content surged. This proves that without PVDF, the "reactive bonding" function of small particles cannot be achieved, resulting in poor dense filling effect.

[0124] Comparison of Comparative Example 3 and Example 1: The performance of Comparative Example 3 (without gradation) is worse in all aspects, which confirms that the combination of dual particle gradation and special pretreatment of the present invention is necessary to obtain a high-density, high-performance polycrystalline sphere structure, and a single particle size cannot achieve the same effect.

[0125] Comparison of Comparative Example 4 and Example 1: Insufficient PVDF results in insufficient bonding points on the surface of small particles, failing to effectively fill the gaps between large particles, thus limiting the improvement in tap density. Simultaneously, the hydrophobic carbon layer formed after carbonization is discontinuous, with insufficient mesopores, leading to excessive moisture gain and decreased rate performance.

[0126] Comparison of Comparative Example 5 and Example 1: Excessive PVDF leads to excessively high viscosity of the small-particle slurry, making it difficult to mix evenly with the large-particle slurry, resulting in poor atomization during spraying and easy nozzle clogging. After carbonization, an excessively thick carbon layer hinders lithium-ion diffusion, leading to a significant reduction in specific capacity, especially at high rates. Simultaneously, excessive carbon may also coat the surface of the active material, affecting lithium-ion insertion / extraction and resulting in increased polarization.

[0127] Comparison of Comparative Example 6 and Example 1: The surfactant was insufficient to completely encapsulate the PVDF, resulting in uneven dispersion of PVDF in the slurry and a tendency for agglomeration. This not only leads to uneven distribution of adhesive force, affecting the filling effect, but also results in uneven carbon coating after carbonization, with some areas being too thick or too thin, thus affecting the overall electrochemical performance.

[0128] Comparison of Comparative Example 7 and Example 1: Excessive surfactant introduces too many impurities, leaving a large amount of ash after carbonization, increasing the internal resistance of the electrode. At the same time, excessive surfactant may excessively reduce the surface tension of the slurry, which in turn affects the formation of spherical particles during spray granulation, resulting in poor sphericity and also affecting the tap density.

[0129] Comparison of Comparative Example 8 and Example 1: The PVDF pretreatment liquid was added before sand milling the small particles. During the sand milling process, the particle viscosity would surge, resulting in a poor filling effect and easy agglomeration.

[0130] Comparison of Example 9 and Example 1: Higher sintering temperature will deteriorate its power performance.

[0131] 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: An iron source, phosphorus source, lithium source, carbon source, nitrogen source, and a first solvent are mixed to obtain a slurry to be treated. The carbon source is selected from at least one of cyclodextrin, glucose, and maltose. The nitrogen source is selected from at least one of urea, polyacrylonitrile, and amino acids. The mass ratio of the carbon source to the nitrogen source is 1:(0.5-1.5), and the mass ratio of the total amount of the carbon source and the nitrogen source to the iron source is (1.0-1.5):

1. The first solvent is water, and the solid content of the slurry to be treated is adjusted to 30%-40% by controlling the amount of the first solvent. The slurry to be treated undergoes a first refining treatment to reduce the particle size. A portion of the slurry with a mass fraction of 60%-80% is used as the matrix slurry. The remaining slurry undergoes a second refining treatment to obtain a refined slurry. Both the first and second refining treatments are performed by grinding. The first refining treatment reduces the particle size in the slurry to 0.2μm-0.4μm; the second refining treatment reduces the particle size in the slurry to 0.05μm-0.15μm. First, the surfactant and the second solvent are mixed and dissolved, then mixed with a hydrophobic polymer, and finally mixed with the refined slurry to obtain a functional slurry; the surfactant is selected from cationic surfactants; the hydrophobic polymer is selected from at least one of polyvinylidene fluoride and fluorinated acrylic acid; the mass ratio of the surfactant to the iron source is (0.008-0.015):100; the mass ratio of the hydrophobic polymer to the solid content of the refined slurry is (0.4-0.6):100; the second solvent is water; The functional slurry and the matrix slurry are mixed and granulated to obtain precursor particles; The precursor particles are sintered.

2. The preparation method according to claim 1, characterized in that, The surfactant is selected from at least one of hexadecyltrimethylammonium bromide and dioctadecyldimethylammonium chloride.

3. The preparation method according to claim 1, characterized in that, The iron source is selected from at least one of ferric phosphate, iron oxide red, and ferrous oxalate; And / or, the phosphorus source is selected from at least one of ferric phosphate, diammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate; And / or, by adjusting the amounts of the iron source, the phosphorus source and the lithium source, the molar ratio of iron, phosphorus and lithium is made to be 1:(0.99-1.03):(1.01-1.04).

4. The preparation method according to claim 1, characterized in that, Granulation is carried out by spray drying, and the inlet and outlet air temperatures are controlled at 110℃-130℃. And / or, during the sintering process, the heating rate is controlled at 2℃ / min-5℃ / min, the target holding temperature is 650℃-750℃, and the holding time is 8h-12h.

5. A lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method according to any one of claims 1-4.

6. A positive electrode sheet for a lithium-ion battery, characterized in that, Including the lithium iron phosphate cathode material as described in claim 5.

7. A lithium-ion battery, characterized in that, Including the lithium-ion battery positive electrode sheet as described in claim 6.

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