Lithium iron phosphate material as well as preparation method and application thereof

Abstract

The invention provides a lithium iron phosphate material and a preparation method and application thereof, and the preparation method of the lithium iron phosphate material comprises the following steps: dispersing a lithium source, an iron source, a phosphorus source and a first carbon source in a first solvent, and carrying out grinding, first drying and first sintering to obtain primary lithium iron phosphate particles; mechanically crushing the primary lithium iron phosphate particles to obtain primary lithium iron phosphate particles with a first target particle size; dispersing the primary lithium iron phosphate particles with the first target particle size and a second carbon source in a second solvent, and performing second drying and second sintering to obtain a lithium iron phosphate material with a second target particle size; the addition mass of the second carbon source is greater than that of the first carbon source. According to the preparation method of the lithium iron phosphate material, the granularity of the lithium iron phosphate primary particles can be accurately controlled, and a carbon coating layer is uniform and complete, so that the conductivity and the electrochemical performance of the material are remarkably improved.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, and in particular to a lithium iron phosphate material, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become an important cathode material for lithium-ion batteries due to its advantages such as high safety, long cycle life, and abundant raw materials. However, its low intrinsic electronic conductivity usually needs to be improved through carbon coating.

[0003] Currently, most common carbon coating methods employ a one-step sintering process, which involves directly sintering a mixture of lithium, iron, phosphorus, and carbon sources at high temperatures. This method has significant drawbacks: First, the carbon source promotes crystal growth at high temperatures, leading to uneven particle size that is difficult to control precisely. Second, adding a large amount of carbon source at once can easily cause carbon agglomerates to form between particles, resulting in uneven coating and exposed particle surfaces that affect conductivity. Finally, the sintered particles are prone to hard agglomeration; while subsequent mechanical crushing can reduce particle size, it easily damages the already formed carbon coating layer, leading to a decrease in the material's conductivity.

[0004] To address the aforementioned issues, related technology one employs a two-stage dry carbon coating method for preparing high-compact products. While this ensures macroscopic particle gradation, it neglects the particle size uniformity of the primary particles and the quality of the carbon coating, still relying on the traditional "sintering-crushing" process to control particle size. Related technology two also uses a two-stage carbon coating method, but its core lies in the incomplete carbon coating in the first stage. The second carbon coating "modifies" the first, and the repaired carbon layer cannot achieve the completeness of a single-stage carbon layer.

[0005] Therefore, developing a preparation method that can independently control the particle size of lithium iron phosphate crystals and achieve uniform and complete carbon coating is of great significance for improving the electrochemical performance of lithium iron phosphate materials. Summary of the Invention

[0006] In view of this, one objective of this application is to provide a method for preparing lithium iron phosphate materials. This method involves adding a carbon source in two steps, with an intermediate mechanical crushing step. First, a small amount of carbon source is used to control crystal growth during the first sintering process, obtaining primary particles with controllable particle size. Then, after crushing and deagglomeration, a second carbon-coated sintering process is performed. This method effectively solves the problems of uneven particle size and incomplete carbon coating that are easily detached in the single-step sintering method. The prepared lithium iron phosphate material has the characteristics of uniform particle size, complete carbon coating, and excellent conductivity, significantly improving its electrochemical performance.

[0007] Another objective of this application is to provide a lithium iron phosphate material.

[0008] Another objective of this application is to provide a positive electrode sheet.

[0009] Another object of this application is to provide a secondary battery.

[0010] To achieve the above objectives, the first aspect of this application proposes a method for preparing lithium iron phosphate material, comprising: After dispersing lithium source, iron source, phosphorus source and first carbon source in first solvent, the mixture is ground, dried and sintered for the first time to obtain primary lithium iron phosphate particles. The primary lithium iron phosphate particles are mechanically crushed to obtain primary lithium iron phosphate particles with a first target particle size. The primary lithium iron phosphate particles with the first target particle size are dispersed with a second carbon source in a second solvent, followed by a second drying and a second sintering to obtain a lithium iron phosphate material with the second target particle size; the added mass of the second carbon source is greater than the added mass of the first carbon source.

[0011] In some embodiments, the mass of the first carbon source added is 1-3% of the mass of the iron source.

[0012] In some embodiments, the added mass of the second carbon source is 10-17% of the mass of the primary lithium iron phosphate particles.

[0013] In some embodiments, the primary lithium iron phosphate particles having a first target particle size are subjected to a second sintering with a second carbon source, including: The primary lithium iron phosphate particles with the first target particle size, the second carbon source, and lithium nitrate are dispersed in a second solvent to obtain a second slurry; The second slurry is then subjected to a second drying and a second sintering.

[0014] In some embodiments, the mass of lithium nitrate added is 2-5% of the mass of the second carbon source added.

[0015] In some embodiments, the second sintering includes: sequentially performing a first sub-sintering and a second sub-sintering on the second dried product, wherein the temperature of the first sub-sintering is lower than the temperature of the second sub-sintering.

[0016] In some embodiments, the temperature of the first sub-sintering is 300-400°C, and the sintering time of the first sub-sintering is 1-2 hours.

[0017] In some embodiments, the second sub-sintering time is 2-10 hours.

[0018] In some embodiments, when the second target particle size is DV50 and the second target particle size is 200-500 nm, the sintering temperature of the second sub-particle is 780-810 °C.

[0019] In other embodiments, when the second target particle size is a DV50 particle size and the second target particle size is 500-1500 nm, the sintering temperature of the second sub-particle is 600-750 °C.

[0020] In some embodiments, the first sintering time is 1-3 hours.

[0021] In some embodiments, when the second target particle size is a DV50 particle size and the second target particle size is 200-500 nm, the temperature of the first sintering is 300-600 °C.

[0022] In other embodiments, when the second target particle size is a DV50 particle size and the second target particle size is 500-1500 nm, the temperature of the first sintering is 650-800 °C.

[0023] In some embodiments, the mass content of carbon in the primary lithium iron phosphate particles does not exceed 0.1%.

[0024] In some embodiments, both the first target particle size and the second target particle size are DV50 particle sizes.

[0025] In some embodiments, the difference between the second target particle size and the first target particle size is less than 0.1 μm, and the particle size span is less than 2.

[0026] In some embodiments, the first carbon source includes a first component and a second component, the first component including polyethylene glycol, and the second component including at least one of starch, cellulose, polymethyl methacrylate, nylon 66, and xylitol.

[0027] In some embodiments, the mass ratio of the polyethylene glycol to the second component is (1.2-2):1.

[0028] In some embodiments, the second carbon source includes at least one of sucrose, glucose, phenolic resin, polyacrylonitrile, and asphalt.

[0029] In some embodiments, the mechanical crushing method includes at least one of ball mill crushing, sand mill crushing, and air jet crushing.

[0030] In some embodiments, both the first sintering and the second sintering are carried out in a nitrogen or / and inert gas atmosphere.

[0031] In some embodiments, both the first solvent and the second solvent include at least one of water, methanol, and ethanol.

[0032] In some embodiments, the first drying method includes spray drying.

[0033] A second aspect of this application discloses a lithium iron phosphate material, prepared using the preparation method described in the first aspect of this application, wherein the lithium iron phosphate material comprises: The core includes lithium iron phosphate; The outer shell includes a carbon layer that covers at least a portion of the outer surface of the core.

[0034] In some embodiments, the thickness of the carbon layer is 2-5 nm.

[0035] In some embodiments, the resistivity of the lithium iron phosphate material powder is less than 10 Ω·cm.

[0036] In some embodiments, the DV50 particle size of the lithium iron phosphate material is 200-500 nm or 500-1500 nm.

[0037] The third aspect of this application provides a positive electrode sheet, comprising a positive electrode material; the positive electrode material comprises lithium iron phosphate material prepared by the method for preparing lithium iron phosphate material as described in the first aspect of this application, or includes lithium iron phosphate material as described in the second aspect of this application.

[0038] The fourth aspect of this application provides a secondary battery, including a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the same as the positive electrode described in the third aspect of this application.

[0039] The method for preparing lithium iron phosphate materials described in this application can bring at least the following beneficial effects: 1. Controllable particle size: By adjusting the carbon source content (1-3%) and sintering temperature during the first sintering process, the nucleation and growth process of lithium iron phosphate crystals can be effectively controlled, thereby achieving precise control of the primary particle size.

[0040] 2. Uniform and dense carbon coating: The intermediate mechanical crushing step completely deagglomerates the hard agglomerates formed after the first sintering and controls the carbon source ratio of the first sintering. The carbon source already attached to the surface of the primary lithium iron phosphate is less than 0.1%, which allows the second carbon source to be uniformly coated on the surface of each primary particle in a one-time molding process, forming a complete and dense carbon layer. This greatly improves the electronic conductivity of the material, making the powder resistivity less than 10Ω·cm.

[0041] 3. Simple process and easy to industrialize: The equipment required by this method are all conventional material preparation equipment, the process route is simple, the parameters are easy to control, and it is very suitable for large-scale industrial production.

[0042] The lithium iron phosphate material, positive electrode sheet, and secondary battery described in this application all have at least the beneficial effects of the preparation method of the lithium iron phosphate material described in this application.

[0043] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0044] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings. in: Figure 1 This is a flowchart illustrating a method for preparing lithium iron phosphate material, which is an exemplary embodiment of this application.

[0045] Figure 2 The image shows a scanning electron microscope (SEM) image of the final lithium iron phosphate product prepared in Example 1.

[0046] Figure 3 The image shows a transmission electron microscope (TEM) image of the final lithium iron phosphate product prepared in Example 1, which shows that it has only one carbon coating layer.

[0047] Figure 4 The image shows a scanning electron microscope (SEM) image of the final lithium iron phosphate product prepared in Example 2.

[0048] Figure 5 The image shows a transmission electron microscope (TEM) image of the final lithium iron phosphate product prepared in Example 2, which shows that it has only one carbon coating layer.

[0049] Figure 6 The X-ray diffraction (XRD) patterns of the final lithium iron phosphate products prepared in Examples 1 and 2 show that the peak positions are basically the same as those of lithium iron phosphate. Detailed Implementation

[0050] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0051] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] Unless otherwise specified, all raw materials and equipment involved in this application are those that can be manufactured commercially or by known methods; and all methods involved are conventional methods unless otherwise specified.

[0053] In this application, DV50 particle size refers to the particle size value corresponding to the cumulative distribution reaching 50% in the particle size distribution, also known as the median particle size. For example, if the DV50 of a sample is 10 μm, it means that among all the particles of the sample, particles with a diameter greater than 10 μm account for 50%, and particles with a diameter less than 10 μm also account for 50%.

[0054] The following describes a method for preparing lithium iron phosphate material according to an embodiment of this application, with reference to the accompanying drawings.

[0055] <Preparation Methods of Lithium Iron Phosphate Materials> Figure 1 This is a flowchart illustrating a method for preparing lithium iron phosphate material, which is an exemplary embodiment of this application.

[0056] like Figure 1 As shown, the preparation method of this lithium iron phosphate material includes the following steps: S101. After dispersing the lithium source, iron source, phosphorus source and first carbon source in the first solvent, the mixture is ground, dried and sintered for the first time to obtain primary lithium iron phosphate particles.

[0057] In the embodiments of this application, there are no restrictions on the specific selection and amount of lithium source, iron source and phosphorus source. They can be any specific selection and amount of lithium source, iron source and phosphorus source known in the art for the preparation of lithium iron phosphate.

[0058] For example, the lithium source includes, but is not limited to, at least one of lithium carbonate, lithium dihydrogen phosphate, lithium nitrate, and lithium hydroxide.

[0059] For example, the iron source includes, but is not limited to, at least one of ferrous oxalate, ferric phosphate, ferric oxide (i.e., ferric oxide), ferric sulfate, and ferric chloride.

[0060] For example, the phosphorus source includes, but is not limited to, at least one of ammonium dihydrogen phosphate, iron phosphate, lithium dihydrogen phosphate, and phosphoric acid.

[0061] It is understandable that when a substance containing both lithium and phosphorus is selected, such as lithium dihydrogen phosphate, it can serve as both a lithium source and a phosphorus source; similarly, when a substance containing both iron and phosphorus is selected, such as iron phosphate, it can serve as both an iron source and a phosphorus source.

[0062] For example, the molar ratio (Li:Fe:P) of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is (0.99-1.02):1:(0.968-1.02), including but not limited to 1.02:1:1, 0.99:1:0.968, 1.01:1:1.02, 1:1:1, 1:1:0.97, 1:1:0.98, 1:1:0.99, or 1.01:1:0.98, etc.

[0063] In some embodiments, the mass of the first carbon source added is 1-3% of the mass of the iron source, including but not limited to 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, or 2.9%.

[0064] In the embodiments of this application, controlling the mass of the first carbon source additive within the aforementioned range can control crystal growth during the first sintering process, resulting in primary lithium iron phosphate particles with controllable particle size. If the mass of the first carbon source added is too small, for example, less than 1% of the iron source mass, it may not be possible to completely reduce iron phosphate to lithium iron phosphate; while if the mass of the first carbon source added is too large, for example, greater than 3% of the iron source mass, there may be more residual carbon, reducing the particle size control capability.

[0065] In some embodiments, the first carbon source includes a first component and a second component. The first component includes, but is not limited to, polyethylene glycol (PEG), and the second component includes, but is not limited to, at least one of carbon-containing organic compounds such as starch, cellulose, polymethyl methacrylate, nylon 66, and xylitol.

[0066] For example, the polyethylene glycol includes at least one of PEG-200, PEG-400, PEG-1000, PEG-2000, or PEG-10000.

[0067] In some embodiments, the mass ratio of polyethylene glycol to the second component is (1.2-2):1, including but not limited to 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1 or 1.9:1.

[0068] In the embodiments of this application, polyethylene glycol and other first components and starch and other second components are selected as the first carbon source and compounded in the above ratio, so that no additional carbon layer is generated while ensuring that iron phosphate is fully reduced to lithium iron phosphate.

[0069] In some embodiments, the first solvent includes, but is not limited to, at least one of water, methanol, ethanol, etc., and may be water.

[0070] In some embodiments, the grinding method includes, but is not limited to, at least one of ball milling, sand milling, roller milling, etc., and sand milling may be selected.

[0071] In the embodiments of this application, the slurry formed by dispersing the lithium source, iron source, phosphorus source and first carbon source in the first solvent can be depolymerized to a specified particle size range by grinding. For example, when the specified particle size is DV50, the specified particle size range can be 350-470nm.

[0072] For example, the grinding speed is 1500-2500 r / min, including but not limited to 1700 r / min, 1900 r / min, 2100 r / min or 2300 r / min; the grinding time is 20-60 min, including but not limited to 30 min or 40 min.

[0073] In some embodiments, the first drying method includes, but is not limited to, at least one of spray drying, evaporation, etc., and spray drying may be selected.

[0074] For example, when the first drying method includes spray drying, the process conditions for spray drying are: inlet air temperature 180-260℃, outlet air temperature 80-120℃. Optionally, the inlet air temperature may be, for example, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, or 250℃, and the outlet air temperature may be, for example, 90℃, 100℃, or 110℃.

[0075] In the embodiments of this application, the precursor powder of lithium iron phosphate is obtained after the first drying.

[0076] In the embodiments of this application, the first sintering is for controlling particle size formation.

[0077] In some embodiments, the first sintering is carried out in a nitrogen or / and inert gas atmosphere.

[0078] For example, the inert gas includes, but is not limited to, at least one of helium, argon, etc.

[0079] It should be noted that, in the embodiments of this application, the conditions for the first sintering are related to the target particle size (i.e., the second target particle size hereinafter referred to as the second target particle size) of the lithium iron phosphate material product ultimately required by the preparation method of the lithium iron phosphate material in this application embodiment. Generally, the larger the target particle size of the final lithium iron phosphate material product, the higher the temperature of the first sintering; conversely, the lower the temperature of the first sintering.

[0080] Optionally, the target particle size (i.e., the second target particle size below) of the final lithium iron phosphate material product is the DV50 particle size.

[0081] In some embodiments, the target particle size (i.e., the second target particle size hereinafter) DV50 of the lithium iron phosphate material product that is ultimately desired is 200-500 nm, including but not limited to 225 nm, 250 nm, 275 nm, 300 nm, 325 nm, 350 nm, 375 nm, 400 nm, 425 nm, 450 nm or 475 nm.

[0082] In other embodiments, the target particle size (i.e., the second target particle size hereinafter) DV50 of the lithium iron phosphate material product to be obtained is 500-1500nm, including but not limited to 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm or 1400nm.

[0083] As an optional example, when the target particle size (i.e., the second target particle size hereinafter) DV50 of the final lithium iron phosphate material product to be prepared is 200-500 nm, the temperature of the first sintering is 300-600℃, including but not limited to 325℃, 350℃, 375℃, 400℃, 425℃, 450℃, 475℃, 500℃, 525℃, 550℃ or 575℃, etc.; the time of the first sintering is 1-3h, including but not limited to 1.2h, 1.5h, 1.7h, 2h, 2.3h, 2.5h or 2.8h, etc.

[0084] As another optional example, when the target particle size (i.e., the second target particle size hereinafter) DV50 of the final lithium iron phosphate material product to be prepared is 500-1500 nm, the temperature of the first sintering is 650-800℃, including but not limited to 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃ or 790℃, etc.; the time of the first sintering is 1-3h, including but not limited to 1.2h, 1.5h, 1.7h, 2h, 2.3h, 2.5h or 2.8h, etc.

[0085] In some embodiments, the carbon content in the primary lithium iron phosphate particles does not exceed 0.1% by mass, including but not limited to not exceeding 0.09%, not exceeding 0.08%, not exceeding 0.07%, not exceeding 0.06%, or exceeding 0.05%.

[0086] In the embodiments of this application, the mass content of carbon element in the primary lithium iron phosphate particles is limited to no more than 0.1%, which allows for a second carbon coating without residual carbon, thereby ensuring the uniformity and integrity of the carbon layer. At the same time, it is beneficial to control the growth of primary lithium iron phosphate particles and obtain the designed particle size.

[0087] For example, the mass content of carbon in primary lithium iron phosphate particles is determined by chemical methods.

[0088] S102. The primary lithium iron phosphate particles obtained in step S101 are mechanically crushed to obtain primary lithium iron phosphate particles with a first target particle size.

[0089] In the embodiments of this application, the primary lithium iron phosphate particles are mechanically crushed in order to reduce the hard agglomerates formed during the depolymerization and sintering process to the first target particle size.

[0090] In some embodiments, the mechanical crushing method includes, but is not limited to, at least one of ball mill crushing, sand mill crushing, and air jet crushing.

[0091] In some implementations, the first target particle size is the DV50 particle size.

[0092] In some embodiments, the difference between the target particle size (i.e., the second target particle size hereinafter referred to as the second target particle size) (e.g., the DV50 particle size) of the final lithium iron phosphate material product to be prepared and the first target particle size (e.g., the DV50 particle size) is less than 0.1 μm, and the particle size span is less than 2.

[0093] It is understood that the target particle size (i.e., the second target particle size, as described below) (e.g., DV50 particle size) of the final lithium iron phosphate material product to be prepared is greater than or equal to the first target particle size (e.g., DV50 particle size), and the difference between the two is recorded as the difference between the target particle size of the final lithium iron phosphate material product to be prepared and the first target particle size.

[0094] For example, the difference between the target particle size (i.e., the second target particle size hereinafter referred to as the second target particle size) (e.g., the DV50 particle size) of the final lithium iron phosphate material product to be prepared and the first target particle size (e.g., the DV50 particle size) is including but not limited to less than 0.08 μm, less than 0.06 μm or less than 0.04 μm, and the particle size range is including but not limited to less than 1.5, less than 1 or less than 0.5.

[0095] S103. The primary lithium iron phosphate particles with the first target particle size obtained in step S102 are dispersed with the second carbon source in the second solvent, and then subjected to a second drying and a second sintering to obtain lithium iron phosphate material with the second target particle size; the added mass of the second carbon source is greater than the added mass of the first carbon source.

[0096] In some embodiments, the added mass of the second carbon source is 10-17% of the mass of the primary lithium iron phosphate particles, including but not limited to 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, or 16.5%. By controlling the amount of the second carbon source added within the above range, a uniform thickness can be obtained without excess, loosely coated carbon layers.

[0097] In the embodiments of this application, the second sintering is for the purpose of perfecting the carbon layer and the lithium iron phosphate crystals. Specifically, the function of the second sintering is to promote the formation of a stable and dense carbon layer to encapsulate the lithium iron phosphate particles and to perfect the lithium iron phosphate crystals.

[0098] As an optional example, the primary lithium iron phosphate particles having a first target particle size are subjected to a second sintering with a second carbon source, including the following steps: (1) The primary lithium iron phosphate particles with the first target particle size, the second carbon source and lithium nitrate are dispersed in a second solvent to obtain a second slurry; (2) The second slurry is subjected to a second drying and a second sintering.

[0099] In the above example, lithium nitrate can be used as a liquid-phase sintering medium to improve the uniform dispersion of the carbon source on the particle surface. Since the primary lithium iron phosphate particles with the first target particle size are obtained by mechanically crushing the primary lithium iron phosphate particles obtained in the first sintering, in the second sintering process, lithium nitrate, as a liquid-phase sintering medium, together with the lithium element in the aforementioned lithium source, serves as the source of lithium element in the final lithium iron phosphate material, which is referred to as the source of lithium element in the core lithium iron phosphate discussed later, forming core lithium iron phosphate.

[0100] For example, the added lithium nitrate is 2-5% of the added mass of the second carbon source, including but not limited to 2.2%, 2.4%, 2.6%, 2.7%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, or 4.8%. When the added amount of lithium nitrate is within the above range, it can better serve as a liquid-phase sintering medium to improve the uniform dispersion of the carbon source on the particle surface. If its added amount is too low (e.g., less than 1% of the added mass of the second carbon source), the effect of improving the uniform dispersion of the carbon source on the particle surface as a liquid-phase sintering medium is slightly reduced. If its added amount is too high (e.g., more than 5% of the added mass of the second carbon source), it may affect the compaction density of the final product.

[0101] For example, when the raw material for the second sintering contains lithium nitrate, the second sintering includes: sequentially subjecting the second dried product to a first sub-sintering and a second sub-sintering, wherein the temperature of the first sub-sintering is lower than the temperature of the second sub-sintering. The function of the first sub-sintering is to melt the lithium nitrate and promote uniform dispersion of the carbon source, while the function of the second sub-sintering is to promote the formation of a stable and dense carbon layer to encapsulate the lithium iron phosphate particles and to improve the lithium iron phosphate crystal structure.

[0102] Optionally, the sintering temperature of the first sub-sintering element is 300-400℃, including but not limited to 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, or 390℃. It should be emphasized that in the embodiments of this application, lithium nitrate (LiNO3) is only used as an additive and is relatively stable within this temperature range under an inert gas atmosphere.

[0103] Optionally, the sintering time of the first sub-sub ...

[0104] Optionally, the sintering time of the second sub-sub ...

[0105] Optionally, when the second target particle size (i.e. the target particle size of the final lithium iron phosphate material product to be prepared as described above) is a DV50 particle size, and the second target particle size is 200-500 nm, the second sub-sintering temperature is 780-810℃, including but not limited to 785℃, 790℃, 795℃, 800℃ or 805℃.

[0106] Optionally, when the second target particle size (i.e., the target particle size of the final lithium iron phosphate material product to be prepared as described above) is a DV50 particle size, and the second target particle size is 500-1500 nm, the sintering temperature of the second sub-sub ...

[0107] As another alternative example, when the raw materials for the second sintering do not contain lithium nitrate, the second sintering only requires the aforementioned second sub-sintering.

[0108] In some embodiments, the second carbon source includes, but is not limited to, at least one of sucrose, glucose, phenolic resin, polyacrylonitrile, asphalt, etc.

[0109] In some embodiments, the second sintering is carried out in nitrogen and / or an inert gas.

[0110] For example, the inert gas includes, but is not limited to, at least one of helium, argon, etc.

[0111] In some embodiments, the second solvent includes, but is not limited to, at least one of water, methanol, ethanol, etc., and may be water.

[0112] In some embodiments, the lithium iron phosphate material having the second target particle size has a core-shell structure, wherein the core comprises lithium iron phosphate and the shell comprises a carbon layer covering at least a portion of the outer surface of the core. When the core is lithium iron phosphate, the carbon layer covers at least a portion of the outer surface of the lithium iron phosphate.

[0113] It should be noted that the lithium iron phosphate material prepared by the method described in this application has a second target particle size, and the core surface is coated with only one layer of carbon. The preparation mechanism of this method is as follows: When the first carbon source was added for the first time, the amount added was low enough that it only served a reducing function, so no excess carbon layer remained after the first sintering. However, after the second carbon source was added and a second sintering was performed, the amount added was higher, ensuring that the carbon source could directly coat the surface of the lithium iron phosphate, thus forming only one carbon coating layer.

[0114] The method for preparing lithium iron phosphate material according to the embodiments of this application can bring at least the following beneficial effects: 1. Controllable particle size: By adjusting the carbon source content (1-3%) and sintering temperature during the first sintering process, the nucleation and growth process of lithium iron phosphate crystals can be effectively controlled, thereby achieving precise control of the primary particle size.

[0115] 2. Uniform and dense carbon coating: The intermediate mechanical crushing step completely deagglomerates the hard agglomerates formed after the first sintering and controls the carbon source ratio of the first sintering. The carbon source already attached to the surface of the primary lithium iron phosphate is less than 0.1%, which allows the second carbon source to be uniformly coated on the surface of each primary particle in a one-time molding process, forming a complete and dense carbon layer. This greatly improves the electronic conductivity of the material, making the powder resistivity less than 10Ω·cm.

[0116] 3. Simple process and easy to industrialize: The equipment required by this method are all conventional material preparation equipment, the process route is simple, the parameters are easy to control, and it is very suitable for large-scale industrial production.

[0117] Lithium iron phosphate materials The second aspect of this application discloses a lithium iron phosphate material prepared using the preparation method described in the first aspect of this application. The lithium iron phosphate material includes a core and a shell; the core includes lithium iron phosphate; the shell includes a carbon layer that covers at least a portion of the outer surface of the core.

[0118] In some embodiments, the thickness of the carbon layer is 2-5 nm, including but not limited to 2.5 nm, 3 nm, 3.5 nm, 4 nm, or 4.5 nm. A carbon layer thickness within this range can significantly improve the conductivity of lithium iron phosphate and reduce powder resistance.

[0119] In some embodiments, the resistivity of the lithium iron phosphate material powder is less than 10 Ω·cm, including but not limited to less than 9 Ω·cm, less than 8 Ω·cm, less than 7 Ω·cm, less than 6 Ω·cm, or less than 5 Ω·cm.

[0120] In some embodiments, the DV50 particle size of the lithium iron phosphate material is 200-500 nm, including but not limited to 225 nm, 250 nm, 275 nm, 300 nm, 325 nm, 350 nm, 375 nm, 400 nm, 425 nm, 450 nm or 475 nm.

[0121] In some embodiments, the DV50 particle size of the lithium iron phosphate material is 500-1500nm, including but not limited to 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm or 1400nm.

[0122] The lithium iron phosphate material described in this application has at least the beneficial effects of the preparation method of the lithium iron phosphate material described in this application.

[0123] <Positive Electrode> The positive electrode sheet of this application embodiment includes a positive electrode material; the positive electrode material includes lithium iron phosphate material prepared by the preparation method of lithium iron phosphate material of this application embodiment, or includes lithium iron phosphate material of this application embodiment.

[0124] It is understood that the lithium iron phosphate material prepared by the method of preparing lithium iron phosphate material in the embodiments of this application or the lithium iron phosphate material in the embodiments of this application plays the role of positive electrode active material in positive electrode material.

[0125] In some embodiments, the positive electrode sheet further includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer comprising the positive electrode material.

[0126] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0127] In addition to the lithium iron phosphate material described in this application, cathode materials also include other cathode active materials. These other cathode active materials can be selected from materials capable of absorbing and releasing lithium.

[0128] Other positive electrode active materials are not specifically limited and can be selected according to requirements. For example, positive electrode active materials may include, but are not limited to, lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), spinel-type lithium manganese oxide (LiMn2O4), and spinel-type lithium nickel manganese oxide (LiNi). 0.5 Mn 1.5O4), layered lithium manganate (LiMnO2), lithium nickelate (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium manganate (LiMgO2), lithium calcium oxide (LiCaO2), lithium copper oxide (LiCuO2), lithium zinc oxide (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al<统一格式,保留标签 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 0.8 Co 0.15 Al 0.05 O2), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi[[ID=!]] 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, etc.), lithium-rich materials (such as lithium-rich nickel cobalt manganese oxide), manganese dioxide (MnO2), vanadium oxide, sulfur oxide, silicate oxide, and at least one of their respective modified compounds. These materials can be used alone or in combination of two or more.

[0129] The modified compounds of the above other positive electrode active materials can be doping modification, surface coating modification, or simultaneous doping and coating modification of the above other positive electrode active materials.

[0130] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0131] 说明:原文中“ 1-x-y ”等标签格式不规范,统一按照保留标签的要求进行了保留。对于“<统一格式,保留标签 1-x-y ”部分,不太明确具体意图,按照要求保留了原标签格式。对于“ 1 / 3 ”疑似重复标签中的一个,保留了原格式。你可以根据实际情况进行调整。In some embodiments, the cathode material may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0132] In some embodiments, the cathode material may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0133] In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as all positive active materials, conductive agents, binders and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto a positive current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0134] Secondary batteries The secondary battery of this application embodiment includes a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the positive electrode of this application embodiment.

[0135] In some embodiments, the secondary battery includes lithium-ion batteries, etc.

[0136] In some embodiments, the secondary battery also includes an electrolyte.

[0137] [Negative electrode plate] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0138] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0139] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0140] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0141] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0142] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0143] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0144] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0145] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0146] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0147] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0148] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0149] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0150] [Isolation membrane] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0151] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0152] The positive electrode and the secondary battery of the present application embodiments both have at least the beneficial effects of the preparation method of lithium iron phosphate material of the present application embodiments.

[0153] The following non-limiting embodiments further illustrate certain features of the present technology.

[0154] I. Examples and Comparative Examples Example 1 The preparation method of lithium iron phosphate material in this embodiment includes the following steps: (1) First slurry and first sintering: Lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate and the first carbon source were mixed in deionized water to prepare a first slurry with a solid content of 40%. Then, the first slurry was milled to DV50=300nm using a sand mill, followed by spray drying to obtain precursor powder. Finally, the precursor powder was sintered in a nitrogen atmosphere at a temperature of 5℃ / min to 700℃ for 2 hours to obtain primary lithium iron phosphate particles.

[0155] in: The first carbon source consists of PEG-1000 and glucose, with a mass ratio of PEG-1000 to glucose of 1.36:1; the added mass of the first carbon source is 3% of the mass of ferrous oxalate.

[0156] The molar ratio (Li:Fe:P) of lithium in lithium carbonate, iron in ferrous oxalate, and phosphorus in ammonium dihydrogen phosphate is 1.02:1:1.

[0157] The conditions for spray drying are: inlet air temperature 230℃ and outlet air temperature 110℃.

[0158] (2) Intermediate crushing: The primary lithium iron phosphate particles obtained in step (1) are crushed by an air jet mill and classified to obtain powder with DV50=1.39μm.

[0159] (3) Second slurry and second sintering: The crushed powder obtained in step (2) is mixed evenly with the second carbon source and lithium nitrate in deionized water to prepare a second slurry with a solid content of 35%. Then, the second slurry is spray-dried and sintered at 350°C for 1.5 hours in a nitrogen atmosphere, and then sintered at 650°C for 5 hours. After natural cooling, the final product is obtained.

[0160] The second carbon source is glucose, and the added mass of the second carbon source is 12% of the mass of the primary lithium iron phosphate particles obtained in step (1); the added mass of lithium nitrate is 4% of the added mass of the second carbon source; the spray drying conditions are: inlet air temperature 250℃ and outlet air temperature 100℃.

[0161] The X-ray diffraction pattern of the product was determined, and the test pattern is shown below. Figure 6 As shown, the peak position matches well with that of lithium iron phosphate, indicating that the product is indeed lithium iron phosphate. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed, and the test images are shown below. Figure 2 and Figure 3 As shown in the figure. Among them: the SEM image shows that the final product lithium iron phosphate material particles prepared in this embodiment are spherical, with good dispersibility and no obvious agglomeration; the TEM test image shows that the final product lithium iron phosphate material prepared in this embodiment has a core-shell structure, with the core being lithium iron phosphate and the outer shell being a carbon layer. The carbon layer uniformly and completely covers the outer surface of the core, and the thickness of the carbon layer is 2nm.

[0162] Example 2 The preparation method of lithium iron phosphate material in this embodiment includes the following steps: (1) First slurry and first sintering: Lithium carbonate, iron phosphate and the first carbon source were mixed in deionized water to prepare a first slurry with a solid content of 38%. Then, the first slurry was milled to DV50=400nm using a sand mill, followed by spray drying to obtain precursor powder. Finally, the precursor powder was sintered in a nitrogen atmosphere at a temperature of 5℃ / min to 600℃ for 2 hours to obtain primary lithium iron phosphate particles.

[0163] in: The first carbon source consists of PEG-500 and starch, with a mass ratio of PEG-500 to starch of 1.42:1; the added mass of the first carbon source is 1.6% of the mass of iron phosphate.

[0164] The molar ratio (Li:Fe:P) of lithium in lithium carbonate, iron in iron phosphate, and phosphorus in iron phosphate is 0.99:1:0.968.

[0165] The conditions for spray drying are: inlet air temperature 240℃ and outlet air temperature 100℃.

[0166] (2) Intermediate crushing: The primary lithium iron phosphate particles obtained in step (1) are crushed by an air jet mill and classified to obtain powder with DV50=0.50μm.

[0167] (3) Second slurry and second sintering: The crushed powder obtained in step (2) is mixed evenly with the second carbon source and lithium nitrate in deionized water to prepare a second slurry with a solid content of 43%. After spray drying, the second slurry is sintered at 350°C for 1.5 hours in a nitrogen atmosphere, and then sintered at 780°C for 2 hours. After natural cooling, the final product is obtained.

[0168] The second carbon source is a mixture of sucrose and polyacrylonitrile in a mass ratio of 1:1.25. The added mass of the second carbon source is 11% of the mass of the primary lithium iron phosphate particles obtained in step (1). The added mass of lithium nitrate is 3.5% of the added mass of the second carbon source. The spray drying conditions are: inlet air temperature 250℃ and outlet air temperature 100℃.

[0169] The X-ray diffraction pattern of the product was determined, and the test pattern is shown below. Figure 6 As shown, the peak position matches well with that of lithium iron phosphate, indicating that the product is indeed lithium iron phosphate. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed, and the test images are shown below. Figure 4 and Figure 5As shown in the figure. Among them: the SEM image shows that the final product lithium iron phosphate material particles prepared in this embodiment are spherical, with good dispersibility and no obvious agglomeration; the TEM test image shows that the final product lithium iron phosphate material prepared in this embodiment has a core-shell structure, with the core being lithium iron phosphate and the outer shell being a carbon layer. The carbon layer uniformly and completely covers the outer surface of the core, and the thickness of the carbon layer is 2nm.

[0170] Example 3 The preparation method of lithium iron phosphate material in this embodiment includes the following steps: (1) First slurry and first sintering: Iron oxide, lithium dihydrogen phosphate and the first carbon source were mixed in deionized water to prepare a first slurry with a solid content of 20%. Then, the first slurry was milled to DV50=350nm using a sand mill, followed by spray drying to obtain precursor powder. Finally, the precursor powder was sintered in a nitrogen atmosphere at a temperature of 10℃ / min to 500℃ for 2 hours to obtain primary lithium iron phosphate particles.

[0171] in: The first carbon source consists of PEG-1500 and nylon 66, with a mass ratio of PEG-1500 to nylon 66 of 1.51:1; the added mass of the first carbon source is 2.4% of the mass of iron oxide.

[0172] The molar ratio (Li:Fe:P) of lithium in lithium dihydrogen phosphate, iron in iron oxide, and phosphorus in lithium dihydrogen phosphate is 1.01:1:1.02.

[0173] The conditions for spray drying are: inlet air temperature 200℃ and outlet air temperature 100℃.

[0174] (2) Intermediate crushing: The primary lithium iron phosphate particles obtained in step (1) are crushed by an air jet mill and classified to obtain powder with DV50=0.45μm.

[0175] (3) Second slurry and second sintering: The crushed powder obtained in step (2) is mixed evenly with the second carbon source and lithium nitrate in deionized water to prepare a second slurry with a solid content of 41%. Then, the second slurry is spray-dried and sintered in a nitrogen atmosphere at 350°C for 1.5 hours, and then sintered at 780°C for 4 hours. After natural cooling, the final product is obtained.

[0176] The second carbon source is a mixture of phenolic resin and sucrose in a mass ratio of 1:1.3. The added mass of the second carbon source is 11% of the mass of the primary lithium iron phosphate particles obtained in step (1). The added mass of lithium nitrate is 4.6% of the added mass of the second carbon source. The spray drying conditions are: inlet air temperature 240℃ and outlet air temperature 100℃.

[0177] Example 4 This embodiment is basically the same as Embodiment 1, except that: In step (1), the mass of the first carbon source added is 1% of the mass of ferrous oxalate.

[0178] Example 5 This embodiment is basically the same as Embodiment 1, except that: In step (1), the mass of the first carbon source added is 1.5% of the mass of ferrous oxalate.

[0179] Example 6 This embodiment is basically the same as Embodiment 1, except that: In step (3), the mass of lithium nitrate added is 2% of the mass of the second carbon source added.

[0180] Example 7 This embodiment is basically the same as Embodiment 1, except that: In step (3), the mass of lithium nitrate added is 5% of the mass of the second carbon source added.

[0181] Example 8 This embodiment is basically the same as Embodiment 1, except that: In step (3), the sample is first sintered at 300°C for 2 hours, and then sintered at 750°C for 7 hours.

[0182] Example 9 This embodiment is basically the same as Embodiment 1, except that: In step (3), the material is first sintered at 400°C for 1 hour, and then sintered at 600°C for 10 hours.

[0183] Example 10 This embodiment is basically the same as Embodiment 1, except that: In step (3), the mass of the second carbon source added is 10% of the mass of the primary lithium iron phosphate particles obtained in step (1).

[0184] Example 11 This embodiment is basically the same as Embodiment 1, except that: In step (3), the mass of the second carbon source added is 17% of the mass of the primary lithium iron phosphate particles obtained in step (1).

[0185] Example 12 This embodiment is basically the same as Embodiment 1, except that: In step (3), the mass of the second carbon source added is 13.5% of the mass of the primary lithium iron phosphate particles obtained in step (1).

[0186] Example 13 This embodiment is basically the same as Embodiment 1, except that: In step (1), the mass ratio of PEG-1000 to glucose is 1.8:1; the precursor powder is sintered in a nitrogen atmosphere at a temperature of 5℃ / min to 800℃ for 1 hour to obtain primary lithium iron phosphate particles.

[0187] Example 14 This embodiment is basically the same as Embodiment 1, except that: In step (1), the first slurry is milled to DV50=350nm using a sand mill; the precursor powder is sintered in a nitrogen atmosphere at a temperature of 5℃ / min to 650℃ for 3 hours to obtain primary lithium iron phosphate particles.

[0188] In step (2), after classification, powder with DV50=0.64μm is obtained.

[0189] Example 15 This embodiment is basically the same as Embodiment 1, except that: In step (3), no lithium nitrate is added; a second slurry with a solid content of 35% is prepared; and it is sintered at 650°C for 5 hours in a nitrogen atmosphere.

[0190] Example 16 This embodiment is basically the same as embodiment 2, except that: In step (1), the mass of the first carbon source added is 1% of the mass of iron phosphate.

[0191] Example 17 This embodiment is basically the same as embodiment 2, except that: In step (1), the mass of the first carbon source added is 3% of the mass of iron phosphate.

[0192] Example 18 This embodiment is basically the same as embodiment 2, except that: In step (3), the mass of lithium nitrate added is 2% of the mass of the second carbon source added.

[0193] Example 19 This embodiment is basically the same as embodiment 2, except that: In step (3), the material is first sintered at 300°C for 2 hours, and then sintered at 795°C for 10 hours.

[0194] Example 20 This embodiment is basically the same as embodiment 2, except that: In step (3), the sample is first sintered at 400°C for 1 hour, and then sintered at 810°C for 5 hours.

[0195] Example 21 This embodiment is basically the same as embodiment 2, except that: In step (3), lithium nitrate is not added; a second slurry with a solid content of 43% is prepared; and it is sintered at 780°C for 7.5 h in a nitrogen atmosphere.

[0196] Comparative Example 1 All raw materials of the first slurry in Example 1 (the amount of glucose added as the first carbon source was increased to 15% of the mass of the iron source) were mixed at one time, and after being sand-milled and dried using the same process, they were directly sintered at 780°C for 10 hours to obtain the lithium iron phosphate material of this comparative example.

[0197] Comparative Example 2 This comparative example is basically the same as Example 1, except that: In step (1), the mass of the first carbon source added is 12% of the mass of the primary lithium iron phosphate particles.

[0198] Comparative Example 3 This comparative example is basically the same as Example 1, except that: Step (2) is excluded; In step (3), the primary lithium iron phosphate particles obtained in step (1) are mixed evenly with the second carbon source and lithium nitrate in deionized water.

[0199] II. Material Characterization and Performance Testing 1. Material Characterization (1) Morphology and structure The morphology of the lithium iron phosphate materials prepared in each example and comparative example was tested using a scanning electron microscope (SEM) of Hitachi Quanta FEG 250 and a transmission electron microscope (TEM) of NEC JEM-2100F.

[0200] The SEM testing method is as follows: (1) Sample preparation: Ensure the sample is suitable for SEM observation, and perform necessary processing and fixation. (2) Degassing and sample placement: Place the sample under the electron gun and perform vacuum degassing. (3) Turn on the electron gun: Start the electron gun and set the spot size. (4) Instrument debugging: Adjust the contrast and brightness, and select an appropriate magnification. (5) Observe and save the image: Observe the image and take a picture, and save the required image.

[0201] The TEM testing method is as follows: (1) Check the equipment: Ensure that the indicator lights on the sample stage are normal, and check the working status of the air conditioner, cooling water machine, air compressor and other equipment. (2) Sample preparation: Select the appropriate preparation method according to the sample properties to ensure high sample quality, so as to ensure the accuracy and precision of observation. (3) Instrument settings: Select the appropriate acceleration voltage, adjust the focus and defect alignment parameters to ensure that the observation area is clearly visible. (4) Observation and analysis: Select the required TEM operation mode, set the image capture parameters, and ensure image quality and signal-to-noise ratio.

[0202] Some test results are shown in the corresponding examples and comparative examples.

[0203] (2)Phase of matter The phase composition of lithium iron phosphate materials prepared in each example and comparative example was tested using a Bruker D8 Advance X-ray diffractometer (XRD). The test method was as follows: first, the powder sample was crushed to below 10 μm to ensure the uniformity of the diffraction peaks. A Cu target was used to scan at a speed of 0.02° / s in the range of 5-80°. The diffraction intensity as a function of angle was collected and plotted.

[0204] Some test results are shown after the corresponding implementation examples.

[0205] (3) Powder resistivity The resistivity of the lithium iron phosphate powder prepared in each embodiment and comparative example was tested using a four-probe tester (model ST2722) manufactured by Suzhou Jingge Electronics Co., Ltd. The test method was as follows: Prepare the powder sample: Take a certain mass of powder (usually about 2.5 g), ensuring it is dry and free of impurities, and record the mass. Filling and pre-compressing: Load the powder into a special mold, and apply pressure (e.g., 10 kN) using a manual or automatic loading device to form a dense, flat disc, typically 2 cm in diameter. Place the probes: Press four equidistant probes (e.g., tungsten rods, 1 mm apart) vertically onto the powder disc surface, ensuring good contact. Connect the circuit: Pass a constant current (e.g., 1 μA–100 mA) through the two outer probes, and use the two inner probes to measure the voltage. Read the data: Record the voltage value, and calculate the volume resistivity using the formula, combining the current, probe spacing, and sample thickness. Multi-point measurement: To reduce error, repeat measurements at different locations and take the average value.

[0206] The test results are shown in Table 1.

[0207] (4) DV50 particle size The particle size of lithium iron phosphate materials DV10, DV50, and DV90 prepared in each example or comparative example was determined using a Malvern Mastersizer 3000 laser particle size analyzer manufactured by Malvern Panaco.

[0208] Test steps: 1) Select "Manual Measurement" and set sample information. 2) Initialize the instrument and automatically adjust the light. 3) After collecting the background, manually add the dispersed sample until the shading level reaches the required range, then start the test. 4) After the test, clean the system and calculate the particle size distribution based on DV10, DV50, and DV90.

[0209] The formula for calculating particle size span is: Particle size span = (DV90 - DV10) / DV50.

[0210] The test results are shown in the examples and comparative examples and Table 1.

[0211] (5) Carbon content Carbon content was determined using a Shanghai Dekai carbon-sulfur analyzer (HCS-140) and the test method was combustion method.

[0212] Test Procedure: 1) Weigh the dried sample and add flux. 2) Place the sample in the container and begin the test. The sample burns completely in an oxygen stream, producing carbon dioxide. 3) The detector determines the carbon dioxide concentration by measuring the intensity of infrared light absorption. 4) The instrument automatically calculates the percentage of carbon content in the sample.

[0213] The test results are shown in Table 1.

[0214] 2. Electrochemical performance The lithium iron phosphate material prepared using the above embodiments or comparative examples was mixed with conductive carbon black (Super P) and polyvinylidene fluoride binder in a mass ratio of 75:15:10 to form a slurry. This slurry was then uniformly coated onto the opposing surfaces of an aluminum foil with a thickness of 16 micrometers. After drying in an 80°C vacuum oven for 12 hours, a working electrode with a diameter of 15.8 mm and a positive active material loading of 6 mg / cm³ was obtained. 2 The negative electrode uses a 0.5mm thick lithium metal sheet, which is directly punched into a 15.8mm diameter sheet. Then, a 20-micron thick polyethylene (PE) separator and a 1.0 mol / L LiPF6 electrolyte are selected (in which the non-aqueous organic solvent is a mixture of ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio). Finally, it is assembled into a button cell (referred to as a coin cell), and the button cell model is LIR2025.

[0215] The 1C first-cycle discharge capacity was evaluated using the Blue Electric Test System (CT2001A).

[0216] Button battery test procedure: Install the test battery on the test instrument and place it in a test environment of (25±1)℃. Set the following program: let stand for 10 min; charge at constant voltage to 3.75 V, let stand for 10 min, then discharge at constant current of 0.1C to 2.0V, let stand for 10 min, then charge at constant voltage again to 3.75V, and stop charging; let stand for 10 min; then discharge at constant current of 1.0C to 3.0V; repeat the above charge and discharge steps 5 times. Record the first 1C discharge capacity.

[0217] The electrochemical performance test results are shown in Table 1.

[0218] Table 1. Partial characterization and electrochemical performance test results of lithium iron phosphate materials.

[0219]

[0220] Note: In Table 1, "-" indicates that there is none.

[0221] As can be seen from Table 1, compared with the comparative example, the preparation method of lithium iron phosphate material in this application can effectively control the particle size of the finished lithium iron phosphate product, and the product also maintains very good electrical performance.

[0222] In summary, the method for preparing lithium iron phosphate material provided in this application can precisely control the particle size of primary lithium iron phosphate particles and ensure that the carbon coating layer is uniform and intact, thereby significantly improving the conductivity and electrochemical performance of the material.

[0223] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion. In the description of the embodiments of this application, technical terms such as "first", "second", "I", "II" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0224] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0225] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0226] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces), unless otherwise explicitly specified.

Claims

1. A method for preparing lithium iron phosphate material, characterized in that, include: After dispersing lithium source, iron source, phosphorus source and first carbon source in first solvent, the mixture is ground, dried and sintered for the first time to obtain primary lithium iron phosphate particles. The primary lithium iron phosphate particles are mechanically crushed to obtain primary lithium iron phosphate particles with a first target particle size. The primary lithium iron phosphate particles with the first target particle size are dispersed with a second carbon source in a second solvent, followed by a second drying and a second sintering to obtain a lithium iron phosphate material with the second target particle size; the added mass of the second carbon source is greater than the added mass of the first carbon source.

2. The preparation method according to claim 1, characterized in that, The mass of the first carbon source added is 1-3% of the mass of the iron source; And / or, the added mass of the second carbon source is 10-17% of the mass of the primary lithium iron phosphate particles; And / or, subjecting the primary lithium iron phosphate particles having the first target particle size to a second sintering with a second carbon source, comprising: The primary lithium iron phosphate particles with the first target particle size, the second carbon source, and lithium nitrate are dispersed in a second solvent to obtain a second slurry; The second slurry is then subjected to a second drying and a second sintering.

3. The preparation method according to claim 2, characterized in that, The mass of lithium nitrate added is 2-5% of the mass of the second carbon source added; And / or, the second sintering includes: sequentially performing a first sub-sintering and a second sub-sintering on the second dried product, wherein the temperature of the first sub-sintering is lower than the temperature of the second sub-sintering.

4. The preparation method according to claim 3, characterized in that, The sintering temperature of the first component is 300-400℃, and the sintering time of the first component is 1-2 hours; And / or, the sintering time of the second sub-sub is 2-10 hours; And / or, when the second target particle size is DV50 and the second target particle size is 200-500 nm, the sintering temperature of the second sub-particle is 780-810 °C; or, When the second target particle size is DV50 and the second target particle size is 500-1500nm, the sintering temperature of the second sub-particle is 600-750℃.

5. The preparation method according to claim 1, characterized in that, The first sintering time is 1-3 hours; And / or, when the second target particle size is a DV50 particle size and the second target particle size is 200-500 nm, the temperature of the first sintering is 300-600 °C; or, When the second target particle size is DV50 and the second target particle size is 500-1500nm, the temperature of the first sintering is 650-800℃. And / or, the mass content of carbon in the primary lithium iron phosphate particles does not exceed 0.1%; And / or, both the first target particle size and the second target particle size are DV50 particle size; And / or, the difference between the second target particle size and the first target particle size is less than 0.1 μm, and the particle size span is less than 2.

6. The preparation method according to any one of claims 1 to 5, characterized in that, The first carbon source includes a first component and a second component. The first component includes polyethylene glycol, and the second component includes at least one of starch, cellulose, polymethyl methacrylate, nylon 66, and xylitol. Optionally, the mass ratio of polyethylene glycol to the second component is (1.2-2):

1. And / or, the second carbon source includes at least one of sucrose, glucose, phenolic resin, polyacrylonitrile, and pitch; And / or, the mechanical crushing method includes at least one of ball mill crushing, sand mill crushing, and air jet crushing; And / or, both the first sintering and the second sintering are carried out in a nitrogen and / or inert gas atmosphere; And / or, both the first solvent and the second solvent include at least one of water, methanol, and ethanol; And / or, the first drying method includes spray drying.

7. A lithium iron phosphate material, characterized in that, The lithium iron phosphate material is prepared by the preparation method according to any one of claims 1 to 6, and comprises: The core includes lithium iron phosphate; The outer shell includes a carbon layer that covers at least a portion of the outer surface of the core.

8. The lithium iron phosphate material according to claim 7, characterized in that, The thickness of the carbon layer is 2-5 nm; And / or, the resistivity of the lithium iron phosphate material powder is less than 10 Ω·cm; And / or, the DV50 particle size of the lithium iron phosphate material is 200-500nm or 500-1500nm.

9. A positive electrode sheet, characterized in that, Includes a cathode material, wherein the cathode material comprises lithium iron phosphate material prepared by the preparation method according to any one of claims 1 to 8 or lithium iron phosphate material according to claim 7 or 8.

10. A secondary battery, comprising a positive electrode, a separator, and a negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 9.

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