Lithium manganese iron phosphate material and preparation method thereof, positive pole piece and secondary battery

Abstract

The invention provides a lithium manganese iron phosphate material and a preparation method thereof, a positive pole piece and a secondary battery, and belongs to the technical field of lithium ion batteries, the lithium manganese iron phosphate material comprises an inner core and a carbon layer coated outside the inner core, the material of the inner core comprises LiMnxFe1-xPO4, a nanocrystal domain is formed on the surface of the inner core, and the nanocrystal domain is made of LiMnxFe1-xPO4. The nanocrystal domain contains an iron element and a phosphorus element, where 0 lt; xlt; 1. The invention aims to solve the technical problem of low interface electron transmission capability of the existing carbon-coated lithium manganese iron phosphate material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a lithium manganese iron phosphate material and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] Lithium manganese iron phosphate (LMP) has gradually attracted attention due to its higher redox platform compared to lithium iron phosphate (LFP). To further improve the conductivity of LMP materials, a carbon layer is typically coated onto the surface. However, the carbon coating process is prone to problems such as uneven carbon layer or excessively thick carbon layer. These issues further affect the interfacial electron transport capability of the material, limiting the improvement of its conductivity. Summary of the Invention

[0003] In view of the technical problems existing in the background art, this application provides a lithium manganese iron phosphate material and its preparation method, a positive electrode sheet and a secondary battery, aiming to solve the technical problem of low interfacial electron transport capability of existing carbon-coated lithium manganese iron phosphate materials.

[0004] In a first aspect, embodiments of this application provide a lithium manganese iron phosphate material, comprising a core and a carbon layer covering the core, wherein the core material comprises LiMn. x Fe 1-x PO4, wherein the surface of the core is formed with nanocrystalline domains, and the nanocrystalline domains contain iron and phosphorus elements, wherein 0 <x<1。

[0005] In the technical solution of this application embodiment, nanocrystalline domains are formed on the surface of the material. These nanocrystalline domains are Fe-P systems with high conductivity, which helps to enhance the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of lithium manganese iron phosphate material. When using this lithium manganese iron phosphate material to prepare secondary batteries, it helps to improve the capacity and rate performance of the secondary batteries.

[0006] In some embodiments, the material of the nanocrystal domains includes one or more of FeP, Fe2P, Fe3P, and FeP2.

[0007] In this embodiment, the above-mentioned compound has high conductivity, and the components of the nanocrystal domains are selected from the above-mentioned compound, which is more conducive to enhancing the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of lithium manganese iron phosphate material.

[0008] In some embodiments, the average size of the nanocrystal domains is 5 nm to 30 nm, and the maximum geometric size is less than or equal to 30 nm.

[0009] In this embodiment, the size of the nanocrystal domains is controlled within the above-mentioned range, which can give full play to the high conductivity of the nanocrystal domains while avoiding the obstruction of lithium ion diffusion due to excessively large nanocrystal domains.

[0010] In some embodiments, the D50 particle size of the lithium manganese iron phosphate material is 0.5 μm to 1 μm.

[0011] In this embodiment, the lithium manganese iron phosphate material within the above-mentioned range has a suitable particle size, which can comprehensively improve the electrochemical performance and dispersion performance of the lithium manganese iron phosphate material, and is beneficial to improving the charge and discharge efficiency and rate performance of the secondary battery.

[0012] In some embodiments, the specific surface area of ​​the lithium manganese iron phosphate material is 12 m². 2 / g~32m 2 / g.

[0013] In this embodiment, the lithium manganese iron phosphate material within the above-mentioned range has a suitable specific surface area, which can comprehensively improve the electrochemical performance and dispersion performance of the lithium manganese iron phosphate material, and is beneficial to improving the charge and discharge efficiency and rate performance of the secondary battery.

[0014] In some embodiments, the compaction density of the lithium manganese iron phosphate material under 3T pressure is 1.98 g / cm³. 3 ~2.35g / cm 3 .

[0015] In this embodiment, the high compaction density of the lithium manganese iron phosphate material is beneficial for improving the energy density of the secondary battery.

[0016] In some embodiments, the carbon layer in the lithium manganese iron phosphate material has a mass percentage content of 1.1% to 2.5%.

[0017] In this embodiment, the mass percentage of the carbon layer is controlled within the above-mentioned range, which can ensure that the core is fully coated to fully improve the conductivity and electrochemical stability of the lithium manganese iron phosphate material, while avoiding the diffusion of lithium ions during the insertion and extraction process due to excessive carbon layer thickness.

[0018] Secondly, embodiments of this application provide a method for preparing lithium manganese iron phosphate material, comprising the following steps: A carbon-coated component and lithium manganese iron phosphate particles are provided. The lithium manganese iron phosphate particles include a core and a carbon layer covering the core. The core material includes LiMn. x Fe 1-x PO4, 0 <x<1; The carbon-coated component comes into contact with the lithium manganese iron phosphate particles to form an assembly; In an environment with an oxygen content of less than 15 ppm, an electric field is applied to the composite material for pulse heating to obtain lithium manganese iron phosphate material.

[0019] In the technical solution of this application embodiment, lithium manganese iron phosphate particles are encapsulated with a carbon-based coating. Then, an electric current is applied to the assembly to induce Joule heating, thereby creating a high-temperature environment for heating the lithium manganese iron phosphate particles. Based on this, by controlling the current, the lithium manganese iron phosphate particles can be rapidly heated and cooled, promoting the in-situ synthesis of highly conductive Fe-P nanocrystalline domains on their surface. This helps enhance the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of the lithium manganese iron phosphate material. When using this lithium manganese iron phosphate material to prepare secondary batteries, it helps to improve the capacity and rate performance of the secondary batteries. The preparation method used in this application is simple, stable, controllable, and easy to mass-produce.

[0020] In some embodiments, in the step of applying an electric field to the assembly and performing pulse heating in an environment with an oxygen content of less than 15 ppm to obtain lithium manganese iron phosphate material, the pulse current of the electric field varies in the range of 0A to 250A.

[0021] In this embodiment, by controlling the pulse current of the electric field to vary within the above-mentioned range, the Joule heat generated by the material during the energization process can be precisely adjusted, thereby achieving heat treatment of the material at different temperatures.

[0022] In some embodiments, in the step of applying an electric field to the assembly and pulse heating it in an environment with an oxygen content of less than 15 ppm to obtain lithium manganese iron phosphate material, the pulse heating step includes a cyclic heating program, which includes first heating to T1 at a heating rate, holding at that temperature for a first time, and then cooling to T2 at a cooling rate, holding at that temperature for a second time. The heating rate is 500℃~1000℃ / s, the cooling rate is 200℃ / s~500℃ / s, T1 is 1000℃~2000℃, the first time is 1s~3s, T2 is 500℃~800℃, the second time is 1s~5s, and the number of cycles is 3~20.

[0023] In this embodiment, lithium manganese iron phosphate particles are subjected to multiple pulse heating cycles. Within each pulse cycle, the temperature is rapidly increased and then rapidly decreased. The rapid heating improves processing efficiency, while the rapid cooling allows for multiple pulses without pulse accumulation. Multiple pulses facilitate the rapid reaction and formation of Fe-P nanocrystalline domains. Optimizing T1 and controlling it within the range of 1000~2000℃ accelerates the reaction between the material and the carbon layer. Optimizing T2 and controlling it within the range of 500~800℃ shortens the time to reach the high-temperature pulse. Controlling the number of cycles to 3~20 promotes the dispersion and distribution of nanocrystalline domains on the material surface and prevents the material from overheating.

[0024] In some embodiments, in the step of applying an electric field to the assembly and pulse heating it in an environment with an oxygen content of less than 15 ppm to obtain lithium manganese iron phosphate material, the environment with an oxygen content of less than 15 ppm is a vacuum environment or a protective gas atmosphere environment, wherein the gas of the protective gas atmosphere environment includes at least one of nitrogen and argon, or a mixture of at least one of nitrogen and argon and hydrogen.

[0025] In this embodiment, high-temperature pulse heating in an oxygen-free or extremely low-oxygen environment can prevent material oxidation and the introduction of impurity phases, thereby reducing the electrochemical performance of lithium manganese iron phosphate material.

[0026] In some embodiments, the carbon coating includes one of carbon sheet, carbon paper, carbon felt, and carbon crucible.

[0027] In this embodiment, the carbon coating is mainly used to conduct electricity and form a high-temperature environment through Joule heating. Accordingly, the carbon coating can be a material that can directly hold particles, such as carbon sheet, carbon paper, carbon felt, or carbon crucible, or a material that is easy to process into a wrapping shape, such as carbon sheet, carbon paper, or carbon felt.

[0028] In some embodiments, the material of the carbonaceous cladding includes graphite.

[0029] In this embodiment, graphite has high conductivity. Using a carbon-based coating made of graphite to coat lithium manganese iron phosphate particles can improve Joule thermal processing efficiency while preventing the lithium manganese iron phosphate particles from reacting with the carbon-based coating.

[0030] In some embodiments, prior to the step of providing the carbon-coated component and lithium manganese iron phosphate particles, the method further includes: A first precursor solution is obtained by mixing lithium source, phosphorus source, manganese source, iron source, first carbon source and water and grinding them. The first precursor liquid is subjected to a first spray drying process to obtain a first powder; The first powder is subjected to a first sintering to obtain a first precursor, wherein the mass percentage of carbon in the first precursor is less than or equal to 0.2%; The first precursor, the second carbon source, and the solvent are mixed and ground to obtain the second precursor liquid. The second precursor liquid is subjected to a second spray drying to obtain a second powder; The second powder is subjected to a second sintering process, followed by pulverization to obtain lithium manganese iron phosphate particles.

[0031] In this embodiment, the preparation process of lithium manganese iron phosphate particles is optimized to synthesize lithium manganese iron phosphate particles with good density, suitable particle size, good dispersibility, uniform particle distribution and uniform carbon layer coating. Combined with the high-temperature pulse heating process, it helps to comprehensively improve the compaction density, capacity and rate performance of lithium manganese iron phosphate materials.

[0032] In some embodiments, the D50 particle size of the solid particles in the first precursor solution is less than or equal to 200 nm.

[0033] In this embodiment, when the first precursor liquid is mixed and ground, the particle size after grinding is controlled within the above-mentioned range. In this way, the raw material particle size is small enough and the mixing is sufficient, which helps to ensure that the raw material is mixed evenly, increases the mutual contact and reaction of multiple raw material components, and is beneficial to obtaining lithium manganese iron phosphate with better phase structure and reducing the particle size of lithium manganese iron phosphate material.

[0034] In some embodiments, the mass percentage of the first carbon source to the total mass of the first carbon source, the lithium source, the phosphorus source, the manganese source, and the iron source is greater than 0% and less than or equal to 6%.

[0035] In this embodiment, the first carbon source mainly acts as a reducing agent to inhibit the oxidation of divalent manganese and divalent iron and to prevent primary particle agglomeration. Therefore, the amount of the first carbon source added should not be too much or too little. If it is too little, it will not be enough to effectively inhibit oxidation and agglomeration. If it is too much, it will easily lead to residual carbon on the surface of the material, causing uneven secondary coating.

[0036] In some embodiments, the water content in the first powder is less than or equal to 1% by mass.

[0037] In this embodiment, controlling the moisture content of the first powder within the above-mentioned range helps to improve the spray drying effect and obtain highly dispersed, small-sized primary particles.

[0038] In some embodiments, the first sintering step includes: first heating to 300℃~450℃ and holding for 3h~6h, then heating to 500℃~700℃ and holding for 3h~6h, and finally cooling to 20℃~40℃.

[0039] In this embodiment, the first sintering process adopts a segmented sintering method, which can better control the reaction process and construct a first precursor with fewer impurities and a more complete and ordered crystal structure.

[0040] In some embodiments, a surfactant is added to the step of mixing the first precursor, the second carbon source, and the solvent. The surfactant includes at least one of diethylene glycol, cetyltrimethylammonium bromide, polypropylene glycol, oleic acid, polyvinylpyrrolidone, polypyrrole, polyethylene oxide, polypropylene oxide, Tween, and Span.

[0041] In this embodiment, by adding a surfactant, the surface tension of the solvent is changed to obtain a highly conductive and highly dispersed structure, thereby reducing the specific surface area of ​​the lithium manganese iron phosphate material. The surfactant is inexpensive and readily available, and has good compatibility with the second precursor liquid, which is beneficial for good dispersion and effective function.

[0042] In some embodiments, the solvent is a mixture of water and an organic solvent, wherein the organic solvent includes at least one of anhydrous ethanol, ethylene glycol, propylene glycol, isopropanol, N,N-dimethylformamide, and dimethyl sulfoxide, and the water content in the mixture is 50% to 95% by mass.

[0043] In this embodiment, a mixed solvent is used to disperse the second carbon source and the first precursor. Compared with water alone, the mixed solvent composed of water and organic solvent in a specific ratio has a lower boiling point and lower surface tension, which helps to better disperse the second carbon source and the first precursor, resulting in a highly dispersed structure.

[0044] In some embodiments, the mass ratio of the first precursor to the second carbon source is 1:(0.04~0.1).

[0045] In this embodiment, by controlling the mass ratio of the first precursor and the second carbon source within the above-mentioned range, the proportion of carbon layer in the lithium manganese iron phosphate material can be adjusted within a suitable range. This promotes uniform coating of the core by the carbon layer, fully improves the conductivity and electrochemical stability of the material, and avoids hindering the diffusion of lithium ions during the insertion / extraction process due to excessive carbon layer thickness.

[0046] In some embodiments, the D50 particle size of the solid particles in the second precursor liquid is less than or equal to 800 nm.

[0047] In this embodiment, when mixing and grinding the second precursor liquid, controlling the particle size after grinding to be within the above-mentioned range helps to ensure that the second carbon source and the first precursor are fully and uniformly mixed, improve the uniformity of carbon layer coating, reduce the particle size of the mixed system, and control the particle size of lithium manganese iron phosphate material to be within a suitable range.

[0048] In some embodiments, the water content in the second powder is less than or equal to 2% by mass.

[0049] In this embodiment, controlling the moisture content of the second powder within the above-mentioned range helps to improve the spray drying effect and obtain highly dispersed, small-sized lithium manganese iron phosphate particles.

[0050] In some embodiments, the second sintering step includes: first heating to 350°C~450°C and holding for 3h~5h, then heating to 550°C~800°C and holding for 3h~10h, and finally cooling to 20°C~40°C.

[0051] In this embodiment, the second sintering process adopts a segmented sintering method to better control the carbonization reaction and crystal structure reconstruction process, thereby constructing lithium manganese iron phosphate particles with more uniform carbon layer coating and more complete and ordered crystal structure.

[0052] In some embodiments, the D50 particle size of the lithium manganese iron phosphate particles is 0.5 μm to 1 μm.

[0053] In this embodiment, after the second powder is sintered for the second time, it can be further crushed and sieved to optimize the particle size distribution and obtain lithium manganese iron phosphate particles with suitable particle size and specific surface area.

[0054] In some embodiments, the mass ratio of the first precursor to the surfactant is 1:(0.005~0.2).

[0055] In this embodiment, controlling the amount of surfactant added within the above range can improve the dispersibility of the material structure and reduce its specific surface area, while avoiding abnormal slurry viscosity due to excessive surfactant addition.

[0056] In some embodiments, during the first spray drying, the inlet air temperature is 200℃~290℃, the outlet air temperature is 90℃~120℃, and the feed rate is 5mL / min~50mL / min.

[0057] In this embodiment, the water content of the product can be indirectly controlled to be less than or equal to 1% by adjusting the conditions of spray drying, thereby controlling the morphology and dispersibility of the product.

[0058] In some embodiments, during the second spray drying, the inlet air temperature is 200℃~250℃, the outlet air temperature is 100℃~110℃, and the feed rate is 10mL / min~20mL / min.

[0059] In this embodiment, the water content of the product can be indirectly controlled to be less than or equal to 2% by adjusting the conditions of spray drying, thereby controlling the morphology and dispersibility of the product.

[0060] In some embodiments, the amounts of the lithium source, the phosphorus source, the manganese source, and the iron source added satisfy the following: the molar ratio of lithium, phosphorus, manganese, and iron is (1~1.03):(1~1.01):x:(1-x), where 0 <x<1。

[0061] In this embodiment, by feeding materials according to the above-mentioned element ratio, the element ratio in the product can be controlled to obtain lithium manganese iron phosphate material with a suitable element ratio, which helps to improve the electrochemical performance of the material.

[0062] In some embodiments, the lithium source is at least one of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, lithium phosphate, and lithium hydroxide; the phosphorus source is at least one of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, lithium phosphate, iron phosphate, and ferrous phosphate; the manganese source is at least one of manganese dioxide, manganese carbonate, manganese tetroxide, manganese trioxide, manganese acetate, manganese sulfate, and manganese nitrate; the iron source is at least one of ferrous oxalate, iron oxide, iron phosphate, ferrous phosphate, ferrous sulfate, and ferric nitrate; and the first carbon source and the second carbon source each independently include at least one of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl alcohol, and phenolic resin.

[0063] In this embodiment, the compounds of the lithium source, phosphorus source, iron source, manganese source, and the first carbon source are widely available and have good compatibility with other components, allowing them to react and fuse well as raw materials. The compounds of the second carbon source are also widely available, can be carbonized effectively to form a carbon layer, and have a good coating effect.

[0064] In some embodiments, the pH value of the first precursor solution is 4 to 8.

[0065] In this embodiment, controlling the pH value of the first precursor liquid within the above-mentioned range helps to regulate the stability of the slurry, while protecting the equipment and reducing metal contamination.

[0066] In some embodiments, at least one of the lithium source, the phosphorus source, and the reaction product of the lithium source and the phosphorus source is soluble in water.

[0067] In this embodiment, lithium and / or phosphorus sources that are soluble in themselves or react with each other are used, which allows the raw materials to react and disperse better, which is beneficial to obtaining a structure with high conductivity and high dispersion.

[0068] Thirdly, embodiments of this application provide a positive electrode sheet, comprising lithium manganese iron phosphate material as described above, or lithium manganese iron phosphate material prepared by the preparation method described above.

[0069] In this embodiment, the positive electrode sheet contains the aforementioned lithium manganese iron phosphate material, thus possessing the advantages of high charge / discharge capacity, high rate performance, and high cycle performance.

[0070] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet described above.

[0071] In this embodiment, the secondary battery includes the aforementioned positive electrode plate, thus possessing the advantages of high charge / discharge capacity, high rate performance, and high cycle performance.

[0072] Fifthly, embodiments of this application provide an electrical device, including the secondary battery described above.

[0073] In this embodiment, the electrical device includes the aforementioned secondary battery, thus possessing advantages such as high charge / discharge capacity, high rate performance, and high cycle performance.

[0074] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0075] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0076] Figure 1 This is a schematic flowchart of a method for preparing lithium manganese iron phosphate material according to an embodiment of this application; Figure 2 This is a schematic flowchart illustrating a method for preparing lithium manganese iron phosphate material according to another embodiment of this application; Figure 3 The image shows the SEM image of the lithium manganese iron phosphate material prepared in Example 1 at a magnification of 20KX. Figure 4 The image shows the SEM image of the lithium manganese iron phosphate material prepared in Example 1 at a magnification of 100 KX. Figure 5 The image shows the SEM image of the lithium manganese iron phosphate material prepared in Comparative Example 1 at a magnification of 20KX. Figure 6 The image shows the SEM image of the lithium iron phosphate material prepared in Comparative Example 1 at a magnification of 100 KX. Figure 7 The image shows the XRD pattern of the lithium manganese iron phosphate material prepared in Example 1. Figure 8 The image shows the XRD pattern of the lithium manganese iron phosphate material prepared in Example 14. Figure 9 This is a TEM image of the lithium manganese iron phosphate material prepared in Example 1; Figure 10 The image shows a TEM image of the lithium manganese iron phosphate material prepared in Comparative Example 1. Figure 11 The charge-discharge curves of the secondary battery prepared using lithium manganese iron phosphate material in Example 1 are shown at 0.1C, 0.2C, 1C, and 5C. Figure 12 The graph shows the 1C cycle performance of the secondary battery made using the lithium manganese iron phosphate material from Example 1. Figure 13 The image shows the 01C charge-discharge curves of the secondary battery made using the lithium manganese iron phosphate material from Example 2. Detailed Implementation

[0077] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0078] 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.

[0079] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0080] 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.

[0081] 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 generally indicates that the preceding and following related objects have an "or" relationship.

[0082] 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).

[0083] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0084] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0085] In this application, "water" may include, but is not limited to, at least one of deionized water, distilled water, pure water, and ultrapure water.

[0086] To improve the cycle stability of lithium manganese iron phosphate (LMP) materials, a carbon layer is typically coated onto the surface of the LMP material. For ease of description, this material is named carbon-coated LMP. Carbon-coated LMP is mainly prepared by the following method: first, LMP precursors are mixed and calcined to obtain LMP; then, LMP and a carbon source are mixed and sintered at high temperature to obtain carbon-coated LMP. However, most current carbon-coated LMP materials suffer from uneven carbon layer coverage or excessively thick carbon layers, which affect the material's interfacial electron transport capacity and lithium-ion diffusion, thus limiting the improvement of the material's conductivity.

[0087] In view of this, this application provides a lithium manganese iron phosphate material and its preparation method, a positive electrode sheet, and a secondary battery. By forming Fe-P nanocrystal domains on the core surface of the lithium manganese iron phosphate material, the conductivity is enhanced, thereby helping to improve the capacity and rate performance of the positive electrode sheet, the secondary battery, and the power device.

[0088] In a first aspect, embodiments of this application provide a lithium manganese iron phosphate material, the lithium manganese iron phosphate material comprising a core and a carbon layer covering the core, wherein the core material comprises LiMn. x Fe 1-x PO4, wherein the value of x satisfies the charge balance within the chemical formula, and x can be any value between 0 and 1 (excluding 0 and 1), for example, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99, and any value between any two of the above. Furthermore, in some embodiments, the surface of the core is formed with nanocrystalline domains, and the nanocrystalline domains contain iron and phosphorus elements.

[0089] In the technical solution of this application embodiment, nanocrystalline domains are formed on the surface of the material. These nanocrystalline domains are Fe-P systems with high conductivity, which helps to enhance the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of lithium manganese iron phosphate material. When using this lithium manganese iron phosphate material to prepare secondary batteries, it helps to improve the capacity and rate performance of the secondary batteries.

[0090] In some embodiments, the nanocrystal domains are distributed in a discontinuous cluster.

[0091] Furthermore, in some embodiments, the value of x is preferably 0.6 to 0.9. By controlling x within this range, the core can construct a better phase, which helps to improve the electrochemical performance of lithium manganese iron phosphate materials.

[0092] Furthermore, in some embodiments, the material of the nanocrystal domains includes one or more of FeP, Fe2P, Fe3P, and FeP2.

[0093] In this embodiment, the above-mentioned compound has high conductivity, and the components of the nanocrystal domains are selected from the above-mentioned compound, which is more conducive to enhancing the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of lithium manganese iron phosphate material.

[0094] Furthermore, in some embodiments, the average size of the nanocrystal domains is 5nm to 30nm, and the maximum geometric size is less than or equal to 30nm; for example, the average size can be 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 12nm, 13nm, 15nm, 18nm, 20nm, 22nm, 25nm, 27nm, 29nm, 30nm, or any value between any two of the above. It can be understood that the average size refers to the equivalent circle diameter of the nanocrystal domain; the maximum geometric size can be understood as the maximum diameter, that is, the length of the line segment passing through the center point of the nanocrystal domain and connecting two points on its periphery.

[0095] In this embodiment, the size of the nanocrystal domains is controlled within the above-mentioned range, which can give full play to the high conductivity of the nanocrystal domains while avoiding the obstruction of lithium ion diffusion due to excessively large nanocrystal domains.

[0096] Furthermore, in some embodiments, the D50 particle size of the lithium manganese iron phosphate material is 0.5μm to 1μm; for example, it can be 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, or any value between any two of the above.

[0097] In this embodiment, the lithium manganese iron phosphate material within the above-mentioned range has a suitable particle size, which can simultaneously obtain better reactivity, structural stability and dispersibility, thereby comprehensively improving the electrochemical performance and dispersibility of the lithium manganese iron phosphate material, which is beneficial to improving the charge and discharge efficiency and rate performance of secondary batteries.

[0098] Furthermore, in some embodiments, the specific surface area of ​​the lithium manganese iron phosphate material is 12 m². 2 / g~32m 2 / g.

[0099] In this embodiment, the lithium manganese iron phosphate material within the above-mentioned range has a suitable specific surface area, which can simultaneously obtain better reactivity, structural stability and dispersibility, thereby comprehensively improving the electrochemical performance and dispersibility of the lithium manganese iron phosphate material, which is beneficial to improving the charge and discharge efficiency and rate performance of secondary batteries.

[0100] Furthermore, in some embodiments, the carbon layer in the lithium manganese iron phosphate material has a mass percentage content of 1.1% to 2.5%.

[0101] In this embodiment, the mass percentage of the carbon layer is controlled within the above-mentioned range, which can ensure that the core is fully coated to fully improve the conductivity and electrochemical stability of the lithium manganese iron phosphate material, prevent the core from being oxidized by external influences, improve the crystal phase purity and cycle stability, and at the same time avoid hindering the diffusion of lithium ions during the insertion and extraction process due to excessive carbon layer thickness.

[0102] Furthermore, in some embodiments, the compaction density of the lithium manganese iron phosphate material under 3T pressure is 1.98 g / cm³. 3 ~2.35g / cm 3 .

[0103] In this embodiment, the high compaction density of the lithium manganese iron phosphate material is beneficial for improving the energy density of the secondary battery.

[0104] Secondly, this application provides a method for preparing lithium manganese iron phosphate material. Please refer to [link to relevant documentation]. Figure 1 The preparation method includes the following steps: S10 provides a carbon-coated component and lithium manganese iron phosphate particles, wherein the lithium manganese iron phosphate particles include a core and a carbon layer covering the core, and the core material includes LiMn. x Fe 1-x PO4, 0 <x<1; S20, the carbonaceous coating comes into contact with the lithium manganese iron phosphate particles to form an assembly; S30, in an environment with an oxygen content of less than 15 ppm, an electric field is applied to the assembly to perform pulse heating, thereby obtaining lithium manganese iron phosphate material.

[0105] In the technical solution of this application embodiment, a carbon-coated component is brought into contact with lithium manganese iron phosphate particles. Then, an electric current is applied to the assembly to induce Joule heating, thereby creating a high-temperature environment to heat the lithium manganese iron phosphate particles. Based on this, by controlling the current, the lithium manganese iron phosphate particles can be rapidly heated and cooled, promoting the in-situ synthesis of highly conductive Fe-P nanocrystalline domains on their surface. This helps enhance the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of the lithium manganese iron phosphate material. When using this lithium manganese iron phosphate material to prepare secondary batteries, it helps to improve the capacity and rate performance of the secondary batteries. The preparation method used in this application is simple, stable, controllable, and easy to mass-produce.

[0106] It is understood that "contact with the lithium manganese iron phosphate particles" means that there is contact between the carbon-coated component and the surface of the lithium manganese iron phosphate particles, which can effectively conduct current to achieve heating. Under this premise, the carbon-coated component can contact the lithium manganese iron phosphate particles in any way, such as supporting, pressing against, partially covering (i.e., wrapping part of the surface of the lithium manganese iron phosphate particles), or completely covering them. As a preferred embodiment, the lithium manganese iron phosphate particles can be wrapped with a carbon-coated component, which helps to achieve more uniform and efficient high-temperature pulse heating.

[0107] In the phrase "environment with oxygen content below 15 ppm", "ppm" refers to the volume number of parts per million of oxygen in the ambient gas.

[0108] Furthermore, in some embodiments, in step S10, the material of the carbonaceous coating includes graphite.

[0109] In this embodiment, graphite has high conductivity. Using a carbon-coated part made of graphite to contact lithium manganese iron phosphate particles can effectively achieve current conduction between the two, improve the heat transfer effect of the carbon-coated part on the material, and at the same time avoid the carbon-coated part reacting with the lithium manganese iron phosphate particles and introducing other impurities.

[0110] Furthermore, in some embodiments, in step S10, the carbonaceous coating includes one of carbon sheet, carbon paper, carbon felt, and carbon crucible.

[0111] In this embodiment, the contact between the carbon-coated component and the lithium manganese iron phosphate particles can be as follows: the carbon-coated component can completely encapsulate the lithium manganese iron phosphate particles, or it can encapsulate a portion of the surface of the lithium manganese iron phosphate particles, or it can provide a surface for holding the lithium manganese iron phosphate particles. Correspondingly, the carbon-coated component can be a material that can directly hold the particles, such as a carbon sheet, carbon paper, carbon felt, or carbon crucible, or it can be a material that is easily processed into an encapsulation shape, such as a carbon sheet, carbon paper, or carbon felt.

[0112] In step S10, the lithium manganese iron phosphate particles can be commercially available finished particles or self-prepared products.

[0113] For further details, please refer to Figure 2 In some embodiments, before step S10, a step of preparing lithium manganese iron phosphate particles is further included, as follows: S101, a first precursor solution is obtained by mixing a lithium source, a phosphorus source, a manganese source, an iron source, a first carbon source, and water and grinding them. S102, the first precursor liquid is subjected to first spray drying to obtain the first powder; S103, the first powder is subjected to a first sintering to obtain a first precursor, wherein the mass percentage of carbon in the first precursor is less than or equal to 0.2%; S104, the first precursor, the second carbon source and the solvent are mixed and ground to obtain the second precursor liquid; S105, the second precursor liquid is subjected to a second spray drying to obtain a second powder; S106, the second powder is subjected to a second sintering and then pulverized to obtain lithium manganese iron phosphate particles.

[0114] In this embodiment, the preparation process of lithium manganese iron phosphate particles is optimized to synthesize lithium manganese iron phosphate particles with good density, suitable particle size, good dispersibility, uniform particle distribution and uniform carbon layer coating. Combined with the high temperature pulse heating process, it helps to comprehensively improve the compaction density, capacity and rate performance of lithium manganese iron phosphate materials. Specifically: (1) Adding a small amount of carbon source when mixing raw materials such as lithium source can prevent the oxidation of divalent manganese and divalent iron in the raw materials and reduce the trivalent manganese and trivalent iron in the raw materials to divalent, while preventing primary particle agglomeration and improving the dispersibility of lithium manganese iron phosphate. If too much carbon source is added in step S101, resulting in a carbon content higher than 0.2% in step S103, it is easy to cause residual carbon on the surface of the first precursor, which is not conducive to secondary coating. (2) By adopting a secondary spray drying and secondary sintering method, the particle size distribution of the material can be effectively controlled. During the secondary spraying, only the first precursor and the coating source are present, which makes it easier to improve the uniformity of the coating. The secondary sintering helps to further promote the fusion between particles, thereby helping to reduce the specific surface area of ​​lithium manganese iron phosphate particles, increase the compaction density, and achieve uniform carbon coating.

[0115] Furthermore, in some embodiments, in step S101, the lithium source is at least one of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, lithium phosphate, and lithium hydroxide; the phosphorus source is at least one of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, lithium phosphate, ferric phosphate, and ferrous phosphate; the iron source is at least one of ferrous oxalate, iron oxide, ferric phosphate, ferrous phosphate, ferrous sulfate, and ferric nitrate; the manganese source is at least one of manganese dioxide, manganese carbonate, manganese tetroxide, manganese trioxide, manganese acetate, manganese sulfate, and manganese nitrate; and the first carbon source includes at least one of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl alcohol, and phenolic resin.

[0116] In this embodiment, the above-mentioned compound is widely available and has good compatibility with other components, so it can react and fuse well as a raw material.

[0117] Furthermore, in some embodiments, in step S101, among the optional iron-containing compounds, the iron source is preferably an iron source containing ferric iron, such as iron oxide or ferric phosphate. Ferrous iron reacts more readily with reducing agents such as carbon and hydrogen to generate highly conductive compounds containing Fe and P.

[0118] Furthermore, in some embodiments, in step S101, at least one of the lithium source, the phosphorus source, and the reaction product of the lithium source and the phosphorus source is soluble in water.

[0119] In this embodiment, lithium and / or phosphorus sources that are soluble in themselves or react with each other are used, which allows the raw materials to react and disperse better, which is beneficial to obtaining a structure with high conductivity and high dispersion.

[0120] Further, in some embodiments, in step S101, the pH value of the first precursor solution is 4-8; for example, it can be 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, and the values between any two of the above. In some other embodiments, the pH value of the first precursor solution is preferably 4.5-7.5.

[0121] In this embodiment, controlling the pH value of the first precursor solution within the above range helps to regulate the stability of the slurry, while protecting the equipment and reducing metal contamination. It can be understood that when the pH value is not within the above range, acids or alkaline substances can be used to adjust the acidity and alkalinity of the first precursor solution. Specifically, the acids that can be used include, but are not limited to, at least one of oxalic acid, acetic acid, citric acid, and phosphoric acid; the alkaline substances that can be used include, but are not limited to, at least one of lithium hydroxide, lithium carbonate, ammonium carbonate, and ammonium bicarbonate.

[0122] Further, in some embodiments, in step S101, the addition amounts of the lithium source, the phosphorus source, the manganese source, and the iron source satisfy: the molar ratio of lithium element, phosphorus element, manganese element, and iron element is (1-1.03):(1-1.01):x:(1-x), where 0 < x < 1, and x is preferably 0.6-0.9.

[0123] In this embodiment, feeding according to the above element ratio can regulate the element ratio in the product and obtain a lithium iron manganese phosphate material with an appropriate element ratio, which helps to improve the electrochemical performance of the material. It can be understood that in actual production, the raw materials used may not be pure substances. Therefore, when feeding, the purity of the raw materials needs to be considered, and the actual feeding amount needs to be calculated comprehensively.

[0124] Further, in some embodiments, in step S101, the mass percentage of the first carbon source in the total mass of the first carbon source, the lithium source, the phosphorus source, the manganese source, and the iron source is greater than 0 and less than or equal to 6%; for example, it can be 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, etc. In some other embodiments, the mass percentage of the first carbon source is preferably 3.5%-4.5%.

[0125] In this embodiment, the main function of the first carbon source is to act as a reducing agent to inhibit the oxidation of divalent manganese and divalent iron and to prevent the aggregation of primary particles. Therefore, the addition amount of the first carbon source should not be too much or too little. If it is too little, it is not enough to effectively inhibit oxidation and aggregation. If it is too much, it is easy to cause residual carbon on the surface of the first precursor, and when performing secondary spraying, an effective coating cannot be formed on the surface of the material.

[0126] Furthermore, in some embodiments, in step S101, the D50 particle size of the solid particles in the first precursor liquid is less than or equal to 200 nm.

[0127] In this embodiment, when mixing and grinding the first precursor liquid, the particle size after grinding is controlled within the aforementioned range. This ensures that the raw material particle size is sufficiently small and the mixing is thorough, thereby helping to ensure uniform mixing of the raw materials, increasing the contact and reaction between multiple raw material components, which is beneficial for obtaining lithium manganese iron phosphate with a better phase structure and reducing the particle size of the lithium manganese iron phosphate material. In actual operation, this particle size range can be used as the standard for the grinding step. Under this premise, this application does not limit the specific grinding method, and it can be ball milling, air jet milling, sand milling, etc. In some embodiments, step S101 can be implemented through the following steps: mixing lithium source, phosphorus source, manganese source, iron source, first carbon source, and water to obtain a first mixture; coarsely grinding the first mixture to a D50 particle size of 1μm~3μm; and then sand milling to refine the solid particles to a D50 particle size less than or equal to 200nm to obtain the first precursor liquid. By first coarsely grinding and then finely grinding, the grinding can be more uniform, which helps to improve the grinding effect.

[0128] Furthermore, in some embodiments, in step S101, the solid content of the first precursor liquid is 0.5% to 6%; for example, it can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, or any value between any two of the above, and is more preferably 1% to 4%. It can be understood that, in this document, solid content refers to the mass percentage of solid components in the mixed system.

[0129] In this embodiment, controlling the solid content of the first precursor liquid within the aforementioned range can regulate the spray drying effect, contributing to obtaining highly dispersed, small-sized primary particles. In actual operation, the solid content of the first precursor liquid can be sampled and tested after grinding, and deionized water can be added to adjust the concentration based on the test results.

[0130] Furthermore, in some embodiments, in step S102, the mass percentage of moisture in the first powder is less than or equal to 1%.

[0131] In this embodiment, controlling the moisture content of the first powder within the above-mentioned range helps to improve the spray drying effect and obtain highly dispersed, small-sized primary particles.

[0132] Furthermore, in some embodiments, in step S102, during the first spray drying, the inlet air temperature, outlet air temperature, and feed rate are controlled as follows: the inlet air temperature is 200℃~290℃, for example, it can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, or any value between any two of the above, and more preferably 210℃~250℃; the outlet air temperature is 90℃~120℃, for example, it can be 90℃, 100℃, 110℃, 120℃, or any value between any two of the above, and more preferably 95℃~105℃; the feed rate is 5mL / min~50mL / min, for example, it can be 5mL / min, 10mL / min, 20mL / min, 30mL / min, 40mL / min, 50mL / min, or any value between any two of the above, and more preferably 8mL / min~15mL / min.

[0133] In this embodiment, the water content of the product can be indirectly controlled to be less than or equal to 1% by adjusting the conditions of spray drying, thereby controlling the morphology and dispersibility of the product.

[0134] Furthermore, in some embodiments, step S103, the first sintering step includes: first heating to a first temperature of 300℃~450℃, holding at that temperature for 3h~6h, then heating to a second temperature of 500℃~700℃, holding at that temperature for 3h~6h, and finally cooling to a third temperature of 20℃~40℃. The first temperature can be 300℃, 350℃, 400℃, 450℃, or any value between any two of these values, more preferably 350℃~400℃; the second temperature can be 500℃, 550℃, 600℃, 650℃, 700℃, or any value between any two of these values.

[0135] In this embodiment, the first sintering process adopts a segmented sintering method, which can better control the reaction process and construct a first precursor with fewer impurities and a more complete and ordered crystal structure.

[0136] Furthermore, in specific implementation, heating / cooling can be performed using a programmed heating / cooling method. For example: first, heat to a first temperature of 300℃~450℃ at a first heating rate of 3℃ / min~5℃ / min, hold for 3h~6h, then heat to a second temperature of 500℃~700℃ at a second heating rate of 3℃ / min~5℃ / min, hold for 3h~6h, and finally cool to a third temperature of 20℃~40℃ at a first cooling rate of 3℃ / min~5℃ / min. The first heating rate, second heating rate, and first cooling rate can each be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any two of these values; they can be the same or different.

[0137] Furthermore, in some embodiments, in step S104, the solvent is a mixture of water and an organic solvent, wherein the organic solvent includes at least one of anhydrous ethanol, ethylene glycol, propylene glycol, isopropanol, N,N-dimethylformamide, and dimethyl sulfoxide, and the water content in the mixture is 50% to 95% by mass; for example, it can be 50%, 60%, 70%, 80%, 90%, 95%, or any value between any two of the above.

[0138] In this embodiment, a mixed solvent is used to disperse the second carbon source and the first precursor. Compared to deionized water alone, this solvent not only has lower surface tension, enabling it to effectively wet both hydrophilic and hydrophobic raw materials, laying a solid foundation for uniform dispersion, but also has lower viscosity and higher fluidity, making it easier to circulate the slurry formed by the mixture of the first precursor, the second carbon source, and the solvent during grinding. Furthermore, the lower boiling point and better volatility of the mixed solvent contribute to the formation of a more stable colloidal structure in the slurry, preventing particles from settling too quickly. In some embodiments, the mass ratio of the first precursor to the surfactant is 1:(10~20); for example, it can be 1:10, 1:12, 1:15, 1:18, 1:20, or any value between two of the above.

[0139] Furthermore, in some embodiments, in step S104, the second carbon source includes at least one selected from glucose, sucrose, citric acid, polyethylene glycol, polyvinyl alcohol, and phenolic resin. The first carbon source and the second carbon source may be the same or different.

[0140] In this embodiment, the above-mentioned compounds are widely available, can be carbonized well to form a carbon layer, and have a good coating effect.

[0141] Furthermore, in some embodiments, in step S104, the mass ratio of the first precursor to the second carbon source is 1:(0.04~0.1); for example, it can be 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1, and any value between any two of the above.

[0142] In this embodiment, by controlling the mass ratio of the first precursor and the second carbon source within the above-mentioned range, the proportion of carbon layer in the lithium manganese iron phosphate material can be adjusted within a suitable range. This promotes uniform coating of the core by the carbon layer, fully improves the conductivity and electrochemical stability of the material, prevents the core from being oxidized by external influences, and improves the crystal phase purity and cycle stability. At the same time, it avoids hindering the diffusion of lithium ions during the insertion and extraction process due to excessive carbon layer thickness.

[0143] Furthermore, in some embodiments, in step S104, a surfactant is added during the step of mixing the first precursor, the second carbon source, and the solvent. Accordingly, the step of mixing the first precursor, the second carbon source, and the solvent may specifically include: mixing the first precursor, the second carbon source, the surfactant, and the solvent.

[0144] In this embodiment, by adding a surfactant, the surface tension of the solvent can be altered to obtain a highly conductive and highly dispersed structure, thereby reducing the specific surface area of ​​the lithium manganese iron phosphate material. As a preferred embodiment, a surfactant and a mixed solvent can be used simultaneously. Using the mixed solvent in conjunction with the surfactant helps to improve the dispersion effect. Specifically, the use of a mixed solvent enhances the "spreading" ability of the entire solvent system, creating a more favorable macroscopic environment for the effective operation of the surfactant. This allows the surfactant to be adsorbed onto the particle surface more quickly and effectively, exerting its wetting and penetrating effects.

[0145] Furthermore, in some embodiments, the surfactant may include, but is not limited to, at least one of diethylene glycol, cetyltrimethylammonium bromide, polypropylene glycol, oleic acid, polyvinylpyrrolidone, polypyrrole, polyethylene oxide, polypropylene oxide, Tween, and Span.

[0146] In this embodiment, the surfactant is inexpensive and readily available, and has good compatibility with the second precursor liquid, allowing it to disperse well and function effectively.

[0147] Furthermore, in some embodiments, in step S1041, the mass ratio of the first precursor to the surfactant is 1:(0.005~0.2); for example, it can be 1:0.005, 1:0.01, 1:0.05, 1:0.1, 1:0.15, 1:0.2, and any value between any two of the above.

[0148] In this embodiment, controlling the amount of surfactant added within the above range can improve the dispersibility of the material structure and reduce its specific surface area, while avoiding abnormal slurry viscosity due to excessive surfactant addition.

[0149] Furthermore, in some embodiments, in step S104, the D50 particle size of the solid particles in the second precursor liquid is less than or equal to 800 nm, preferably 200 nm to 800 nm.

[0150] In this embodiment, controlling the particle size of the second precursor liquid within the aforementioned range during mixing and grinding helps ensure that the second carbon source and the first precursor are fully and uniformly mixed, improves the uniformity of carbon layer coating, reduces the particle size of the mixed system, and controls the particle size of the lithium manganese iron phosphate material within a suitable range. In actual operation, this particle size range can be used as the standard for the grinding step. Under this premise, this application does not limit the specific grinding method, which can be ball milling, air jet milling, sand milling, etc. In some embodiments, step S104 can be implemented through the following steps: mixing the first precursor, the second carbon source, and the solvent to obtain a suspension; coarsely grinding the suspension first, and then sand milling it to refine the solid particles to a D50 particle size of less than or equal to 800 nm, to obtain the second precursor liquid. By coarsely grinding first and then finely grinding, the grinding can be more uniform, which helps to improve the grinding effect.

[0151] Furthermore, in some embodiments, in step S104, the solid content of the second precursor liquid is 1% to 5%.

[0152] In this embodiment, controlling the solid content of the second precursor liquid within the aforementioned range can regulate the effect of the second spray drying, contributing to obtaining a highly dispersed, small-sized second powder. In actual operation, the solid content of the second precursor liquid can be sampled and tested after grinding, and deionized water can be added to adjust the concentration based on the test results.

[0153] Furthermore, in some embodiments, in step S105, the water content in the second powder is less than or equal to 2% by mass.

[0154] In this embodiment, controlling the moisture content of the second powder within the above-mentioned range helps to improve the spray drying effect and obtain highly dispersed, small-sized lithium manganese iron phosphate particles.

[0155] Furthermore, in some embodiments, during step S105, the inlet air temperature, outlet air temperature, and feed rate are controlled as follows: the inlet air temperature is 200℃~250℃, for example, it can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, or any two of the above values, and more preferably 210℃~240℃; the outlet air temperature is 90℃~110℃, for example, it can be 90℃, 100℃, 110℃, 120℃, or any two of the above values, and more preferably 95℃~105℃; the feed rate is 10mL / min~20mL / min, for example, it can be 10mL / min, 13mL / min, 15mL / min, 18mL / min, 20mL / min, or any two of the above values, and more preferably 10mL / min~15mL / min.

[0156] In this embodiment, the water content of the product can be indirectly controlled to be less than or equal to 2% by adjusting the conditions of spray drying, thereby controlling the morphology and dispersibility of the product.

[0157] Furthermore, in some embodiments, step S106, the second sintering step includes: first heating to a fourth temperature of 350℃~450℃, holding at that temperature for 3h~5h, then heating to a fifth temperature of 550℃~800℃, holding at that temperature for 3h~10h, and finally cooling to a sixth temperature of 20℃~40℃. The fourth temperature can be 350℃, 400℃, 450℃, or any value between any two of the above, more preferably 350℃~400℃; the fifth temperature can be 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, or any value between any two of the above.

[0158] In this embodiment, the second sintering process adopts a segmented sintering method, which can better control the carbonization reaction and crystal structure reconstruction process, and construct lithium manganese iron phosphate particles with more uniform carbon layer coating and more complete and ordered crystal structure.

[0159] Furthermore, in specific implementation, heating / cooling can be carried out using a programmed heating / cooling method. For example: first, heat to a fourth temperature of 350℃~450℃ at a third heating rate of 3℃ / min~5℃ / min, hold for 3h~5h, then heat to a fifth temperature of 550℃~800℃ at a fourth heating rate of 3℃ / min~5℃ / min, hold for 3h~10h, and finally cool to a sixth temperature of 20℃~40℃ at a second cooling rate of 3℃ / min~5℃ / min. The third heating rate, fourth heating rate, and second cooling rate can each be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any two of these values; they can be the same or different.

[0160] Furthermore, in some embodiments, in step S106, the D50 particle size of the lithium manganese iron phosphate particles is 0.5μm~1μm.

[0161] In this embodiment, after the second sintering of the second powder, it can be further pulverized and sieved to optimize the particle size distribution and obtain lithium manganese iron phosphate particles with suitable particle size and specific surface area. In actual operation, after pulverization, the powder can be sieved using a 200-400 mesh screen. The specifications of the screen can refer to GBT5330.1-2012 Industrial Metal Wire Mesh Screens and Metal Wire Braided Mesh: Combination of Mesh Size and Wire Diameter.

[0162] In steps S103 and S106, the sintering steps are carried out under a protective atmosphere, which includes, but is not limited to, nitrogen, argon, nitrogen / argon mixture, hydrogen / nitrogen mixture, argon / hydrogen mixture, or hydrogen / nitrogen / argon mixture.

[0163] Furthermore, in some embodiments, in step S30, the pulse current of the electric field varies in the range of 0A to 250A, preferably 50A to 200A.

[0164] In this embodiment, by controlling the pulse current of the electric field to vary within the above-mentioned range, the Joule heat generated by the material during the energization process can be precisely adjusted, thereby achieving heat treatment of the material at different temperatures.

[0165] Furthermore, in some embodiments, in step S30, the pulse heating step includes a cyclic heating program, which includes first heating to T1 at a heating rate, holding at that temperature for a first time, then cooling to T2 at a cooling rate, and holding at that temperature for a second time. The heating rate is 500℃~1000℃ / s, the cooling rate is 200℃ / s~500℃ / s, T1 is 1000℃~2000℃, the first time is 1s~3s, T2 is 500℃~800℃, the second time is 1s~5s, and the number of cycles is 3~20.

[0166] In this embodiment, the lithium manganese iron phosphate particles are subjected to multiple pulse heating cycles. Within each pulse cycle, the temperature is rapidly increased and then rapidly decreased. The rapid heating improves processing efficiency, while the rapid cooling allows for multiple pulses without pulse accumulation. These multiple pulses facilitate the rapid formation of Fe-P nanocrystalline domains. Optimizing T1 and controlling it within the range of 1000℃ to 2000℃ accelerates the reaction between the material and the carbon layer. Optimizing T2 and controlling it within the range of 500℃ to 800℃ shortens the time to reach the high-temperature pulse. Controlling the number of cycles to 3 to 20 promotes the dispersion and distribution of nanocrystalline domains on the material surface and prevents overheating. In some embodiments, the number of cycles is preferably 5 to 15.

[0167] Furthermore, in some embodiments, in step S30, the environment with an oxygen content below 15 ppm is a vacuum environment or a protective gas atmosphere environment with an oxygen content below 15 ppm, wherein the gas in the protective gas atmosphere environment includes at least one of nitrogen and argon, or a mixture of at least one of nitrogen and argon with hydrogen. It is understood that an ambient oxygen content below 15 ppm is preferably less than or equal to 10 ppm, more preferably less than or equal to 5 ppm, and even more preferably less than or equal to 1 ppm, etc.

[0168] In this embodiment, high-temperature pulse heating in an oxygen-free or extremely low-oxygen environment can prevent material oxidation and the introduction of impurity phases, thereby reducing the electrochemical performance of lithium manganese iron phosphate material.

[0169] Thirdly, embodiments of this application provide a positive electrode sheet, comprising lithium manganese iron phosphate material as described above, or lithium manganese iron phosphate material prepared by the preparation method described above.

[0170] In this embodiment, the positive electrode sheet contains the aforementioned lithium manganese iron phosphate material, thus possessing the advantages of high charge / discharge capacity, high rate performance, and high cycle performance.

[0171] Fourthly, embodiments of this application provide a secondary battery, which includes the positive electrode sheet described above.

[0172] In this embodiment, the secondary battery includes the aforementioned positive electrode plate, thus possessing the advantages of high charge / discharge capacity, high rate performance, and high cycle performance.

[0173] Fifthly, embodiments of this application also provide an electrical device, including the secondary battery described above.

[0174] The electrical devices provided in this application embodiment can be, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0175] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0176] I. Preparation Method Example 1 S1. Lithium dihydrogen phosphate, lithium carbonate, iron phosphate, and manganese carbonate (in a molar ratio of Li:P:Mn:Fe = 1.02:1:0.6:0.4) were dispersed in deionized water containing 0.36 g of glucose (equivalent to the first carbon source accounting for 1.5% of the total mass of the raw materials). The mixture was stirred for 20 minutes to obtain a suspension. The suspension was ball-milled for 2 hours, and the D50 particle size of the ball-milled product was 2.3 μm. Then, it was sand-milled for 2 hours at a speed of 2000 rpm to obtain a first precursor solution with a pH of 5 and a D50 particle size of 128 nm.

[0177] S2. Adjust the solid content of the first precursor liquid to 3% (by mass fraction), perform spray drying, control the inlet air temperature to 250℃, the outlet air temperature to 100℃, and the feed rate to 15mL / min to obtain the first powder, the moisture content of the first powder being 0.95% (by mass fraction).

[0178] S3. The first powder is placed in a quartz crucible and heated to 450°C at 5°C / min under a nitrogen atmosphere and held for 4 hours. Then, the temperature is increased to 600°C at 5°C / min and held for 4 hours. Finally, the temperature is decreased to room temperature (25°C) at 5°C / min to obtain the first precursor LMFP. The carbon content in the first precursor is 0.1% (by mass fraction).

[0179] S4. Mix 119g of deionized water and 119g of anhydrous ethanol to form a mixed solvent (equivalent to 50% by mass of deionized water), and set aside. Add 0.6g of glucose and 1.1g of polypyrrole to the mixed solvent, stir magnetically for 30 minutes, then add 15g of the first precursor LMFP, mix thoroughly to obtain a suspension (equivalent to a mass ratio of first precursor, second carbon source glucose, surfactant polypyrrole, and mixed solvent of 1:0.04:0.073:15.9). Ball mill the suspension for 2 hours, then transfer it to a sand mill for 2 hours at 1800 rpm to obtain a second precursor solution with a D50 particle size of 650nm.

[0180] S5. Adjust the solid content of the second precursor liquid to 4% (by mass fraction), perform spray drying, control the inlet air temperature to 240℃, the outlet air temperature to 100℃, and the feed rate to 10mL / min to obtain the second powder, the moisture content of the second powder being 1.6% (by mass fraction).

[0181] S6. The second powder is placed in a quartz crucible and heated to 450℃ at 5℃ / min under a nitrogen atmosphere, and held for 4 hours. Then, the temperature is increased to 600℃ at 5℃ / min and held for 6 hours. Finally, the temperature is decreased to room temperature at 5℃ / min to obtain LMFP / C material. The LMFP / C material is pulverized and then sieved using a 325-mesh sieve to control the D50 particle size to 560nm, thus obtaining lithium manganese iron phosphate particles.

[0182] S7. Lithium manganese iron phosphate particles are wrapped in carbon paper to form an assembly. The two ends of the assembly are fixed with conductive clips and placed in a vacuum environment (oxygen content below 15ppm). The assembly is then energized and pulsed heated to obtain lithium manganese iron phosphate material. The pulse current is 180A, the number of pulse cycles is 8, and each pulse cycle is set as follows: heating to T1 at a heating rate of 1000℃ / min, T1 is 1300℃, held for 1s, then cooling to T2 at a cooling rate of 300℃ / min, T2 is 700℃, held for 2s.

[0183] Example 2 S1. Lithium dihydrogen phosphate, ferrous oxalate, and manganese carbonate (in the ratio of Li:P:Mn:Fe = 1:1:0.6:0.4) were dispersed in deionized water containing 0.2 g of glucose (equivalent to 0.8% of the total mass of the first carbon source). The mixture was stirred for 20 minutes to obtain a suspension. The suspension was ball-milled for 2 hours, and the D50 particle size of the ball-milled product was 2 μm. Then, it was sand-milled for 2 hours at 2000 rpm to obtain a first precursor solution with a pH of 5.3 and a D50 particle size of 120 nm.

[0184] S2. Adjust the solid content of the first precursor liquid to 3% (by mass fraction), perform spray drying, control the inlet air temperature to 260℃, the outlet air temperature to 105℃, and the feed rate to 15mL / min to obtain the first powder, the moisture content of the first powder being 0.91% (by mass fraction).

[0185] S3. The first powder is placed in a quartz crucible and heated to 400°C at 5°C / min under a nitrogen atmosphere and held for 4 hours. Then, the temperature is increased to 650°C at 5°C / min and held for 4 hours. Finally, the temperature is decreased to room temperature (25°C) at 5°C / min to obtain the first precursor LMFP. The carbon content in the first precursor is 0.15% (by mass fraction).

[0186] S4. Mix 215g of deionized water and 24g of anhydrous ethanol to form a mixed solvent (equivalent to 90% by mass of deionized water), and set aside. Add 0.85g of glucose and 0.3g of diethylene glycol to the mixed solvent, stir magnetically for 30 minutes, then add 15g of the first precursor LMFP, mix thoroughly to obtain a suspension (equivalent to a mass ratio of first precursor, second carbon source glucose, surfactant diethylene glycol, and mixed solvent of 1:0.057:0.02:15.9). Ball mill the suspension for 2 hours, then transfer it to a sand mill for 2 hours at 2000 rpm to obtain a second precursor solution with a D50 particle size of 600nm.

[0187] S5. Adjust the solid content of the second precursor liquid to 3% (by mass fraction), perform spray drying, control the inlet air temperature to 220℃, the outlet air temperature to 105℃, and the feed rate to 10mL / min to obtain the second powder, the moisture content of the second powder being 1.8% (by mass fraction).

[0188] S6. The second powder is placed in a quartz crucible and heated to 400℃ at 5℃ / min under a nitrogen atmosphere, and held for 4 hours. Then, the temperature is increased to 770℃ at 5℃ / min and held for 8 hours. Finally, the temperature is lowered to room temperature at 5℃ / min to obtain LMFP / C material. The LMFP / C material is pulverized and then sieved using a 325-mesh sieve to control the D50 particle size to 800nm, thus obtaining lithium manganese iron phosphate particles.

[0189] S7. Lithium manganese iron phosphate particles are wrapped in carbon paper to form an assembly. The two ends of the assembly are fixed with conductive clips and placed in a vacuum environment. The assembly is energized and pulsed heated to obtain lithium manganese iron phosphate material. The pulse current is 100A, the number of pulse cycles is 8, and each pulse cycle is set as follows: heating to T1 at a heating rate of 800℃ / min, T1 is 1800℃, held for 1.5s, then cooling to T2 at a cooling rate of 400℃ / min, T2 is 800℃, held for 1s.

[0190] Example 3 S1. Lithium oxalate, diammonium hydrogen phosphate, manganese trioxide, and iron oxide (equivalent to Li:P:Mn:Fe = 1.02:1.0:0.7:0.3) are dispersed in deionized water containing glucose (equivalent to the first carbon source accounting for 6% of the total mass of the raw materials). The mixture is stirred for 20 minutes to obtain a suspension. The suspension is ball-milled for 2 hours, and the D50 particle size of the ball-milled product is 3 μm. Then, it is sand-milled for 2 hours at a speed of 2000 rpm to obtain a first precursor solution with a pH of 8 and a D50 particle size of 135 nm.

[0191] S2. Adjust the solid content of the first precursor liquid to 0.5% (by mass fraction), perform spray drying, control the inlet air temperature to 200℃, the outlet air temperature to 90℃, and the feed rate to 5mL / min to obtain the first powder, the moisture content of the first powder being 0.98% (by mass fraction).

[0192] S3. The first powder is placed in a quartz crucible and heated to 300°C at 3°C / min under a nitrogen atmosphere and held for 6 hours. Then, the temperature is increased to 700°C at 3°C / min and held for 3 hours. Finally, the temperature is decreased to room temperature (25°C) at 3°C / min to obtain the first precursor LMFP. The carbon content in the first precursor is 0.18% (by mass fraction).

[0193] S4. Mix 119g of deionized water and 119g of dimethyl sulfoxide to form a mixed solvent (equivalent to 50% by mass of deionized water), and set aside. Add 0.3g of sucrose, 0.3g of polyvinyl alcohol, and 0.075g of oleic acid to the mixed solvent, stir magnetically for 30 minutes, then add 15g of the first precursor LMFP, mix thoroughly to obtain a suspension (equivalent to a mass ratio of 1:0.04:0.005:15.9 of the first precursor, second carbon source sucrose, surfactant polyvinyl alcohol, and mixed solvent). Ball mill the suspension for 2 hours, then transfer it to a sand mill for 2 hours at 1800 rpm to obtain a second precursor solution with a D50 particle size of 650nm.

[0194] S5. Adjust the solid content of the second precursor liquid to 4% (by mass fraction), perform spray drying, control the inlet air temperature to 250℃, the outlet air temperature to 110℃, and the feed rate to 15mL / min to obtain the second powder, the moisture content of the second powder being 1.6% (by mass fraction).

[0195] S6. Place the second powder into a quartz crucible, heat it to 450℃ at 3℃ / min and hold for 3 hours under a nitrogen atmosphere, then heat it to 800℃ at 3℃ / min and hold for 3 hours, and then cool it to room temperature at 3℃ / min to obtain LMFP / C material. Crush the LMFP / C material and then sieve it using a 325-mesh sieve to control the D50 particle size to 635nm, thus obtaining lithium manganese iron phosphate particles.

[0196] S7. Lithium manganese iron phosphate particles are wrapped in carbon paper to form an assembly. The two ends of the assembly are fixed with conductive clips and placed in a vacuum environment. The assembly is then energized and pulsed heated to obtain lithium manganese iron phosphate material. The pulse current is 180A, the number of pulse cycles is 8, and each pulse cycle is set as follows: heating at a rate of 500℃ / min to T1 (1300℃), holding for 3 seconds, then cooling at a rate of 500℃ / min to T2 (500℃), holding for 5 seconds.

[0197] Example 4 S1. Lithium phosphate, phosphoric acid, ammonium dihydrogen phosphate, manganese dioxide, and ferrous sulfate (equivalent to Li:P:Mn:Fe = 1.03:1.01:0.9:0.1) are dispersed in deionized water containing glucose (equivalent to the first carbon source accounting for 4% of the total mass of the raw materials). The mixture is stirred for 20 minutes to obtain a suspension. The suspension is ball-milled for 2 hours, and the D50 particle size of the ball-milled product is 1 μm. Then, it is sand-milled for 2 hours at a speed of 2000 rpm to obtain a first precursor solution with a pH of 4 and a D50 particle size of 115 nm.

[0198] S2. Adjust the solid content of the first precursor liquid to 6% (by mass fraction), perform spray drying, control the inlet air temperature to 290℃, the outlet air temperature to 120℃, and the feed rate to 50mL / min to obtain the first powder, the moisture content of the first powder being 0.89% (by mass fraction).

[0199] S3. The first powder is placed in a quartz crucible and heated to 350°C at 4°C / min under a nitrogen atmosphere and held for 3 hours. Then, the temperature is increased to 500°C at 4°C / min and held for 6 hours. Finally, the temperature is decreased to room temperature (25°C) at 4°C / min to obtain the first precursor LMFP. The carbon content in the first precursor is 0.05% (by mass fraction).

[0200] S4. Mix 143g of deionized water and 95g of ethylene glycol to form a mixed solvent (equivalent to 60% by mass of deionized water), and set aside. Add 0.15g of phenolic resin, 1g of diethylene glycol, and 2g of polyvinylpyrrolidone to the mixed solvent, and stir magnetically for 30 minutes. Then add 15g of the first precursor LMFP and mix thoroughly to obtain a suspension (equivalent to a mass ratio of 1:0.01:0.2:15.9 of the first precursor, the second carbon source phenolic resin, the surfactant diethylene glycol and polyvinylpyrrolidone, and the mixed solvent). Ball mill the suspension for 2 hours, and then transfer it to a sand mill for 2 hours at 1800 rpm to obtain a second precursor liquid with a D50 particle size of 650nm.

[0201] S5. Adjust the solid content of the second precursor liquid to 4% (by mass fraction), perform spray drying, control the inlet air temperature to 200℃, the outlet air temperature to 100℃, and the feed rate to 20mL / min to obtain the second powder, the moisture content of the second powder being 1.6% (by mass fraction).

[0202] S6. The second powder is placed in a quartz crucible and heated to 350℃ at 4℃ / min under a nitrogen atmosphere, and held for 4 hours. Then, the temperature is increased to 550℃ at 4℃ / min and held for 10 hours. Finally, the temperature is decreased to room temperature at 4℃ / min to obtain LMFP / C material. The LMFP / C material is pulverized and then sieved using a 325-mesh sieve to control the D50 particle size to 512nm, thus obtaining lithium manganese iron phosphate particles.

[0203] S7. Lithium manganese iron phosphate particles are wrapped in carbon paper to form an assembly. The two ends of the assembly are fixed with conductive clips and placed in a nitrogen environment. The assembly is then energized and pulsed heated to obtain lithium manganese iron phosphate material. The pulse current is 250A, the number of pulse cycles is 8, and each pulse cycle is set as follows: heating at a rate of 800℃ / min to T1 (T1 = 1300℃), holding for 1 second, then cooling at a rate of 200℃ / min to T2 (T2 = 800℃), holding for 3 seconds.

[0204] Example 5 This embodiment is basically the same as Embodiment 1, except that in step S7, the pulse heating environment is changed to a hydrogen / nitrogen mixed gas environment, with a hydrogen to nitrogen volume ratio of 5:95. All other steps and conditions remain unchanged.

[0205] Example 6 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, T1 is 900℃. All other steps and conditions remain unchanged.

[0206] Example 7 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, T1 is 1000℃. All other steps and conditions remain unchanged.

[0207] Example 8 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, T1 is 2000℃. All other steps and conditions remain unchanged.

[0208] Example 9 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, T1 is 2100℃. All other steps and conditions remain unchanged.

[0209] Example 10 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, the loop is repeated once. All other steps and conditions remain unchanged.

[0210] Example 11 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, the number of iterations is 3. All other steps and conditions remain unchanged.

[0211] Example 12 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, the number of iterations is 13. All other steps and conditions remain unchanged.

[0212] Example 13 This embodiment is basically the same as Embodiment 1, except that in step S7 of this embodiment, the number of iterations is 20. All other steps and conditions remain unchanged.

[0213] Example 14 This embodiment is basically the same as Embodiment 1, except that in step S1 of this embodiment, the first carbon source glucose is not added, and correspondingly, the carbon content in the first precursor is 0. All other steps and conditions remain unchanged.

[0214] Example 15 This embodiment is basically the same as Embodiment 1, except that in step S1 of this embodiment, the mass of the first carbon source accounts for 5% of the total mass of the raw materials, and correspondingly, the carbon content in the first precursor is 0.3%. Apart from this, all other steps and conditions remain unchanged.

[0215] Example 16 This embodiment is basically the same as Embodiment 1, except that no surfactant is added in step S4. All other steps and conditions remain unchanged.

[0216] Example 17 This embodiment is basically the same as Embodiment 1, except that in step S4, the mixed solvent is replaced with an equal mass of deionized water. All other steps and conditions remain unchanged.

[0217] Example 18 This embodiment is basically the same as embodiment 1, except that the lithium manganese iron phosphate particles in this embodiment are made by spray drying and sintering in one step. Accordingly, steps S1 to S3 are removed, and the 15g of the first precursor LMFP in step S4 is replaced with "10.5g of lithium dihydrogen phosphate, 0.08g of lithium carbonate, 6.03g of iron phosphate, and 6.9g of manganese carbonate".

[0218] Comparative Example 1 This comparative example is basically the same as Example 1, except that step S7 is omitted. All other steps and conditions remain unchanged.

[0219] Comparative Example 2 This comparative example is basically the same as Example 18, except that step S7 is omitted. All other steps and conditions remain unchanged.

[0220] II. Testing Methods (1) Performance testing of lithium manganese iron phosphate materials 1. Element content: The lithium manganese iron phosphate material was tested using an inductively coupled plasma optical emission spectrometer (ICP-OES) to detect the content of each element in the material, and the carbon content was tested using an infrared carbon-sulfur analyzer. The test results are shown in Table 1.

[0221] 2. Microstructure characterization of lithium manganese iron phosphate materials: The lithium manganese iron phosphate materials synthesized in Example 1 and Comparative Example 1 were characterized using a MERLIN Compact field emission scanning electron microscope (SEM) manufactured by Zeiss. The test results are as follows: Figure 3 , Figure 4 , Figure 5 and Figure 6 As shown.

[0222] 3. Impurity phase content: The lithium manganese iron phosphate materials prepared in Examples 1 and 14 were tested using a DX-2700 X-ray diffractometer (XRD) manufactured by Dandong Haoyuan Instrument Co., Ltd. The test results are as follows: Figure 7 and Figure 8As shown in the figure, the horizontal axis 2-theta represents the diffraction angle 2θ, and the vertical axis intensity represents the diffraction intensity.

[0223] 4. Nanocrystal domain characterization: The lithium manganese iron phosphate materials synthesized in Example 1 and Comparative Example 1 were characterized using a Talos F200X transmission electron microscope (TEM) manufactured by Thermo Fisher Scientific. The detection results are as follows: Figure 9 and Figure 10 As shown.

[0224] 5. D50 particle size detection: The test was conducted using a Malvern laser particle size analyzer, and the test results are shown in Table 2.

[0225] 6. Specific surface area (BET) test: The specific surface area was tested using a BELSORP MaxII specific surface area analyzer manufactured by Machikebaier of Japan. The test results are shown in Table 2.

[0226] 7. Compacted density (PD): The compacted density was tested using a UTM7305 battery powder compaction density tester provided by Shenzhen Sansi Zongheng Technology Co., Ltd. The test pressure was 3T and the pressing time was 30s. The test results are shown in Table 2.

[0227] (2) Performance testing of secondary batteries The lithium manganese iron phosphate materials prepared in the various embodiments and comparative examples were mixed with conductive carbon black and binder (PVDF5130) at a mass ratio of 90:5:5, respectively. The PVDF was dissolved in N-methylpyrrolidone (NMP) to form a slurry. The slurry was coated onto a 12 μm thick aluminum foil to obtain an electrode. The electrode was dried in a 100°C vacuum oven. Subsequently, based on the electrode compaction density, the electrode was rolled to a certain thickness. The electrode was then punched to a diameter of 15 μm, weighed, and the mass of the active material was calculated. After obtaining the dried electrode, CR2032 button half-cells were assembled using an LG2400 / 1000TS glove box manufactured by Wiegand Gas Purification Technology (Suzhou) Co., Ltd.

[0228] The CR2032 button cell was tested using a battery performance testing system (model: CT2001A) manufactured by Wuhan Landian Electronics Technology Co., Ltd. The test temperature was 25±1℃, and the voltage range was 2-4.35V. The test results are shown in Table 3. Figures 11 to 13 As shown.

[0229] III. Analysis of Test Results for Each Embodiment and Comparative Example Table 1 Elemental content in lithium manganese iron phosphate materials

[0230] Table 2 Performance test results of lithium manganese iron phosphate materials

[0231] Table 3 Performance test results of secondary batteries

[0232] Results analysis: Figure 3 and Figure 4 The results show that the lithium manganese iron phosphate material obtained in Example 1 has high density and good dispersibility, small particle size and no agglomeration; Figure 5 and Figure 6 The results show that the lithium manganese iron phosphate material obtained in Comparative Example 1 has many pores on its surface, and it is obvious that the secondary particles are composed of primary nanoparticles with a particle size D50 < 180 nm.

[0233] Figure 7 and Figure 8 show, Figure 8 The spectrum of Example 14 shown contains an FePO4 impurity phase, while Figure 7 The spectrum of Example 1 shown conforms to the characteristic peaks of lithium manganese iron phosphate, and the material has high crystallinity and purity; this indicates that in the presence of trivalent iron and trivalent manganese, the addition of a small amount of carbon source can effectively inhibit the oxidation of manganese and iron, and improve the phase purity of the material.

[0234] Figure 7 The spectrum of Example 1 shown indicates the formation of an Fe2P conductive phase in the material, indicating the formation of Fe-P nanocrystalline domains. Simultaneously, combined with... Figure 9 and Figure 10 Observation, among which, Figure 9 In the TEM image of Example 1 shown, it can be observed that the material bulk (region B, lithium manganese iron phosphate phase) contains Fe-P highly conductive nanocrystalline domains on the surface (region A, Fe2P phase), while... Figure 10 The TEM image of Comparative Example 1 shows that no corresponding crystal planes of Fe-P nanocrystal domains are visible, indicating that highly conductive Fe-P nanocrystal domains can be formed by pulse heating of lithium manganese iron phosphate particles.

[0235] Table 2 shows that the BET specific surface area of ​​the lithium manganese iron phosphate materials provided in Examples 1-18 is 15 m². 2 / g~32m 2 / g, compacted density is 1.98g / cm³ 3 ~2.30g / cm 3Within the specified range, it is evident that the lithium manganese iron phosphate material proposed in this application has a suitable specific surface area and a high compaction density. Furthermore, the surface of the lithium manganese iron phosphate material contains nanocrystalline domains with an average size of 12 nm to 27 nm. Meanwhile, Table 3 shows that the secondary batteries based on the lithium manganese iron phosphate materials provided in Examples 1 to 18 have a charging specific capacity in the range of 152.6 to 169.6 mAh / g and a discharging specific capacity in the range of 150.1 to 157.4 mAh / g at 0.1C, indicating that the lithium manganese iron phosphate material proposed in this application has a high capacity. Figure 11 The charging and discharging curves of the secondary battery prepared using the lithium manganese iron phosphate material provided in Example 1 as the positive electrode active material are shown at 0.1C, 0.2C, 1C, and 5C: the charging specific capacity at 0.1C is 164.9 mAh / g, and the discharging specific capacity at 0.2C is 155.9 mAh / g; the charging specific capacity at 1C is 152.4 mAh / g, and the discharging specific capacity at 1C is 145.6 mAh / g; and the charging specific capacity at 5C is 129.5 mAh / g, and the discharging specific capacity at 119.3 mAh / g. Combined with the performance data of other examples and comparative examples shown in Table 3, it can be seen that each example has excellent rate performance, indicating that the lithium manganese iron phosphate material proposed in this application has high rate performance. Figure 12 The secondary battery in Example 1 showed a specific capacity of 129.7 mAh / g after 200 cycles at 1C, with a capacity retention rate of 99.3%, demonstrating good cycle performance. Combined with the capacity retention rate data (85.3~99.3%) of other examples and comparative examples shown in Table 3, it can be seen that the lithium manganese iron phosphate material proposed in this application has good cycle performance.

[0236] Furthermore, comparing Examples 1 to 13 with Comparative Example 1, and comparing Example 18 with Comparative Example 2, it can be seen that, compared to the lithium manganese iron phosphate material provided in Comparative Example 1, the lithium manganese iron phosphate materials provided in Examples 1 to 13 have a lower BET specific surface area and a higher compaction density. Compared to the lithium manganese iron phosphate material provided in Comparative Example 2, the lithium manganese iron phosphate material provided in Example 18 has a lower BET specific surface area and a higher compaction density. This indicates that by pulse heating the lithium manganese iron phosphate particles, the structure of the lithium manganese iron phosphate material can be optimized secondaryly, reducing the specific surface area, increasing particle density, and thus improving the powder compaction density.

[0237] Furthermore, Figure 11 and Figure 13As shown in Table 3, the secondary batteries prepared using lithium manganese iron phosphate material as the positive electrode active material in Examples 1 to 13 exhibit a charging specific capacity in the range of 157.5 mAh / g to 169.6 mAh / g and a discharging specific capacity in the range of 150.1 mAh / g to 157.4 mAh / g at 0.1C, which is significantly higher than the specific capacity of the secondary battery prepared using lithium manganese iron phosphate material as the positive electrode active material in Comparative Example 1. The secondary battery prepared using lithium manganese iron phosphate material as the positive electrode active material in Example 18 also exhibits a charging specific capacity and discharging specific capacity at 0.1C that are significantly higher than those of the secondary battery prepared using lithium manganese iron phosphate material as the positive electrode active material in Comparative Example 2. Meanwhile, in terms of rate performance and capacity retention, the lithium manganese iron phosphate materials provided in Examples 1 to 13 and Example 18 also showed better performance than the lithium manganese iron phosphate materials provided in Comparative Examples 1 and 2, respectively. This indicates that pulse heating of lithium manganese iron phosphate particles helps to enhance the conductivity between particles and the interfacial electron transport rate, thereby improving the conductivity of lithium manganese iron phosphate materials. When using this lithium manganese iron phosphate material to prepare secondary batteries, it helps to improve the capacity and rate performance of secondary batteries.

[0238] Furthermore, compared to the lithium manganese iron phosphate materials provided in Examples 14 and 15, the lithium manganese iron phosphate material provided in Example 1 has a lower BET specific surface area, and the secondary battery prepared with it as the positive electrode active material has higher capacity and rate performance. This indicates that adding an appropriate amount of the first carbon source is beneficial to prevent the oxidation of divalent manganese and divalent iron in the raw materials, and to reduce the trivalent manganese and trivalent iron in the raw materials to divalent, while preventing primary particle agglomeration and improving the dispersibility of lithium manganese iron phosphate. At the same time, it controls the carbon content in the first precursor within a suitable range, avoiding the impact of excessive residual carbon on secondary coating, thereby improving the overall capacity and rate performance of the secondary battery using the lithium manganese iron phosphate material.

[0239] Compared to the lithium manganese iron phosphate materials provided in Examples 16 and 17, the lithium manganese iron phosphate material provided in Example 1 has a significantly smaller D50 particle size, a lower BET specific surface area, and better electrochemical performance, indicating that the simultaneous addition of surfactants and mixed solvents helps to improve the dispersion effect.

[0240] Furthermore, the lithium manganese iron phosphate material provided in Example 1 has lower BET specific surface area, higher compaction density, and higher capacity than the lithium manganese iron phosphate material provided in Example 18. This indicates that the process of two spray drying and two sintering is beneficial for more effectively controlling the particle size distribution of lithium manganese iron phosphate particles, thereby helping to reduce the specific surface area of ​​lithium manganese iron phosphate material, increase compaction density, and achieve uniform carbon coating of lithium manganese iron phosphate particles. This is more conducive to improving the capacity and rate performance of secondary batteries using lithium manganese iron phosphate material as the positive electrode active material.

[0241] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium manganese iron phosphate material, characterized in that, It includes a core and a carbon layer covering the core, the core being made of LiMn. x Fe 1-x PO4, wherein the surface of the core is formed with nanocrystalline domains, and the nanocrystalline domains contain iron and phosphorus elements, wherein 0 <x<1。 2. The lithium manganese iron phosphate material according to claim 1, characterized in that, The materials of the nanocrystalline domains include one or more of FeP, Fe2P, Fe3P, and FeP2; and / or, The average size of the nanocrystal domains is 5nm~30nm, and the maximum geometric size is less than or equal to 30nm.

3. The lithium manganese iron phosphate material according to claim 1, characterized in that, The D50 particle size of the lithium manganese iron phosphate material is 0.5 μm to 1 μm; and / or, In the lithium manganese iron phosphate material, the carbon layer has a mass percentage content of 1.1% to 2.5%; and / or, The compaction density of the lithium manganese iron phosphate material under 3T pressure is 1.98 g / cm³. 3 ~2.35g / cm 3 ; and / or, The specific surface area of ​​the lithium manganese iron phosphate material is 12m². 2 / g~32m 2 / g.

4. A method for preparing lithium manganese iron phosphate material, characterized in that, Includes the following steps: A carbon-coated component and lithium manganese iron phosphate particles are provided. The lithium manganese iron phosphate particles include a core and a carbon layer covering the core. The core material includes LiMn. x Fe 1-x PO4, 0 <x<1; The carbon-coated component comes into contact with the lithium manganese iron phosphate particles to form an assembly; In an environment with an oxygen content of less than 15 ppm, an electric field is applied to the composite material for pulse heating to obtain lithium manganese iron phosphate material.

5. The preparation method according to claim 4, characterized in that, In the step of applying an electric field to the assembly and pulse heating it in an environment with an oxygen content of less than 15 ppm to obtain lithium manganese iron phosphate material, The pulsed current of the electric field varies in the range of 0A to 250A; and / or, The pulse heating step includes a cyclic heating program, which includes first heating to T1 at a heating rate, holding at that temperature for a first time, then cooling to T2 at a cooling rate, and holding at that temperature for a second time. The heating rate is 500℃~1000℃ / s, the cooling rate is 200℃ / s~500℃ / s, T1 is 1000℃~2000℃, the first time is 1s~3s, T2 is 500℃~800℃, the second time is 1s~5s, and the cycle is repeated 3 to 20 times; and / or, The environment with an oxygen content of less than 15 ppm is a vacuum environment or a protective gas atmosphere environment, wherein the gas in the protective gas atmosphere environment includes at least one of nitrogen and argon, or a mixture of at least one of nitrogen and argon with hydrogen.

6. The preparation method according to claim 4, characterized in that, The carbonaceous coating includes one of the following: carbon sheet, carbon paper, carbon felt, and carbon crucible; and / or, The material of the carbon-coated component includes graphite.

7. The preparation method according to claim 4, characterized in that, Prior to the steps of providing the carbon-coated component and lithium manganese iron phosphate particles, the following are also included: A first precursor solution is obtained by mixing lithium source, phosphorus source, manganese source, iron source, first carbon source and water and grinding them. The first precursor liquid is subjected to a first spray drying process to obtain a first powder; The first powder is subjected to a first sintering to obtain a first precursor, wherein the mass percentage of carbon in the first precursor is less than or equal to 0.2%; The first precursor, the second carbon source, and the solvent are mixed and ground to obtain the second precursor liquid. The second precursor liquid is subjected to a second spray drying to obtain a second powder; The second powder is subjected to a second sintering process, followed by pulverization to obtain lithium manganese iron phosphate particles.

8. The preparation method according to claim 7, characterized in that, The D50 particle size of the solid particles in the first precursor solution is less than or equal to 200 nm; and / or, The percentage of the mass of the first carbon source relative to the total mass of the first carbon source, the lithium source, the phosphorus source, the manganese source, and the iron source is greater than 0% and less than or equal to 6%; and / or, In the first powder, the water content by mass is less than or equal to 1%; and / or, The steps of the first sintering include: first heating to 300°C to 450°C, holding for 3 h to 6 h, then heating to 500°C to 700°C, holding for 3 h to 6 h, and finally cooling to 20°C to 40°C; and / or, In the step of mixing the first precursor, the second carbon source and the solvent, a surfactant is further added. The surfactant includes at least one of diethylene glycol, cetyltrimethylammonium bromide, polypropylene glycol, oleic acid, polyvinylpyrrolidone, polypyrrole, polyethylene oxide, polypropylene oxide, Tween, and Span; and / or, The solvent is a mixed solvent of water and an organic solvent. Among them, the organic solvent includes at least one of absolute ethanol, ethylene glycol, propylene glycol, isopropyl alcohol, N,N-dimethylformamide, and dimethyl sulfoxide. And in the mixed solvent, the mass percentage content of water is 50% to 95%; and / or, The mass ratio of the first precursor to the second carbon source is 1:(0.04 to 0.1); and / or, The D50 particle size of the solid particles in the second precursor solution is less than or equal to 800 nm; and / or, In the second powder, the mass percentage content of moisture is less than or equal to 2%; and / or, The steps of the second sintering include: first heating to 350°C to 450°C, holding for 3 h to 5 h, then heating to 550°C to 800°C, holding for 3 h to 10 h, and finally cooling to 20°C to 40°C; and / or, The D50 particle size of the lithium iron phosphate manganese particles is 0.5 μm to 1 μm.

9. The preparation method according to claim 8, characterized in that, The mass ratio of the first precursor to the surfactant is 1:(0.005 to 0.2); and / or, When the first spray drying is carried out, the inlet air temperature is 200°C to 290°C, the outlet air temperature is 90°C to 120°C, and the feeding rate is 5 mL / min to 50 mL / min; and / or, When the second spray drying is carried out, the inlet air temperature is 200°C to 250°C, the outlet air temperature is 100°C to 110°C, and the feeding rate is 10 mL / min to 20 mL / min.

10. The preparation method according to claim 7, characterized in that, The addition amounts of the lithium source, the phosphorus source, the manganese source and the iron source satisfy that the molar ratio of lithium element, phosphorus element, manganese element and iron element is (1 to 1.03):(1 to 1.01):x:(1 - x), where 0 < x < 1; and / or, The lithium source is at least one of lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, lithium phosphate, and lithium hydroxide; and / or, The phosphorus source is at least one of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, lithium phosphate, iron phosphate, and ferrous phosphate; and / or, The manganese source is at least one of manganese dioxide, manganese carbonate, manganese tetroxide, manganese sesquioxide, manganese acetate, manganese sulfate, and manganese nitrate; and / or, The iron source is at least one of ferrous oxalate, iron oxide, iron phosphate, ferrous phosphate, ferrous sulfate, and iron nitrate; and / or, The first carbon source and the second carbon source each independently include at least one of glucose, sucrose, citric acid, polyethylene glycol, polyvinyl alcohol, and phenolic resin; and / or, The pH value of the first precursor solution is 4 to 8.

11. A positive electrode plate, characterized in that, It includes the lithium manganese iron phosphate material according to any one of claims 1 to 3, or the lithium manganese iron phosphate material prepared by the preparation method according to any one of claims 4 to 10.

12. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 11.

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