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

Abstract

This application discloses a lithium manganese iron phosphate (LFP) cathode material, its preparation method, and its applications. The LFP cathode material comprises LFP particles, which, based on the iron content distribution, are sequentially divided into a core region, a middle region, and a surface region from the center of the LFP particles towards the surface. The middle region covers the core region, and the surface region covers the middle region. From the core region to the middle region, the iron content in the first composite region (forming the core and middle regions) decreases gradually; from the middle region to the outer surface of the surface region, the iron content in the second composite region (forming the middle and surface regions) increases gradually. The LFP cathode material of this application exhibits high compaction density, high energy density, and a high voltage plateau, as well as good cycle performance and low-temperature performance. Its preparation ensures the stability of the electrochemical performance of the prepared LFP cathode material.

Description

Technical Field

[0001] This application belongs to the technical field of electrode materials, and particularly relates to a lithium iron manganese phosphate cathode material, its preparation method and application. Background Art

[0002] Lithium-ion batteries are widely used in many fields such as 3C electronic products, electric vehicles and energy storage power stations due to their high energy density, low self-discharge, no memory effect and long cycle life, and are currently a research hotspot in new energy storage and conversion systems.

[0003] The lithium iron manganese phosphate material (LiMn 1-x Fe x PO4 (0 < x < 1)) has the characteristics of rich raw materials, low cost, relatively high specific capacity, good thermal stability, etc. Compared with lithium iron phosphate (LiFePO4), it has a higher discharge voltage (3.8V vs 3.3V), which can increase the energy density of the battery by about 15%. It is one of the cathode materials for the new generation of industrialized lithium-ion batteries. However, existing lithium iron manganese phosphate also has some defects, such as low electronic conductivity, insufficient tap density, two voltage platforms (4.1V and 3.4V), etc., and its rate performance, cycle performance and energy density and other performances are not ideal.

[0004] Existing reports attempt to modify lithium iron manganese phosphate to overcome some of its defects. For example, in a disclosed graphene in-situ composite lithium iron manganese phosphate cathode material, graphene is used to coat lithium iron manganese phosphate. Although it effectively controls the growth of grains, the grains inside the material are arranged orderly, improves the packing density performance, improves its structural stability and speeds up the electron migration rate. However, this modified lithium iron manganese phosphate only significantly improves its conductivity, and does not improve its voltage platform, rate performance, cycle performance and energy density and other performances. Summary of the Invention

[0005] The purpose of this application is to overcome the above deficiencies of the prior art, and provide a lithium iron manganese phosphate cathode material and its preparation method to solve the technical problems that existing lithium iron manganese phosphate has unsatisfactory performances such as voltage platform, rate performance, cycle performance and energy density.

[0006] Another purpose of this application is to provide a cathode and a secondary battery containing this cathode to solve the technical problems that existing lithium iron manganese phosphate secondary batteries have low voltage platform, unsatisfactory cycle performance and specific capacity.

[0007] To achieve the aforementioned objectives, the first aspect of this application provides a lithium manganese iron phosphate cathode material. This lithium manganese iron phosphate cathode material comprises lithium manganese iron phosphate particles, which, based on the iron content distribution, are sequentially divided into a core region, an intermediate region, and a surface region from the center of the lithium manganese iron phosphate particles to the surface. The intermediate region covers the core region, and the surface region covers the intermediate region. Furthermore, from the core region to the intermediate region, the iron content in the first composite region formed by the core region and the intermediate region decreases in a gradient; from the outer surface of the intermediate region to the surface region, the iron content in the second composite region formed by the intermediate region and the surface region increases in a gradient.

[0008] A second aspect of this application provides a method for preparing a lithium manganese iron phosphate cathode material. The method for preparing the lithium manganese iron phosphate cathode material of this application includes the following steps:

[0009] The lithium source, phosphorus source, first iron source, and manganese source are mixed in the proportions required for preparing lithium manganese iron phosphate to obtain a precursor.

[0010] In a protective atmosphere, the precursor is subjected to a first sintering treatment to obtain lithium manganese iron phosphate particles, wherein the iron content first decreases in a gradient from the interior to the surface of the lithium manganese iron phosphate particles, and then increases in a gradient.

[0011] A third aspect of this application provides a positive electrode. The positive electrode of this application includes a current collector and a positive electrode active layer bonded to the surface of the current collector. The positive electrode active layer includes a positive electrode active material, a binder, and a conductive agent; wherein the positive electrode active material is the lithium manganese iron phosphate positive electrode material of this application.

[0012] A fourth aspect of this application provides a secondary battery. This application includes a positive electrode, which is the positive electrode of this application.

[0013] Compared with the prior art, this application has the following technical effects:

[0014] The lithium manganese iron phosphate (LFP) cathode material of this application features an iron-rich core region within the LFP particles, effectively increasing particle size and compaction density. The middle region within the LFP particles is designed as a low-iron content region, effectively improving energy density. The surface region of the LFP particles is also designed as an iron-rich region, protecting the middle region and modifying conductivity, thus improving the structural mechanical properties and cycle performance. Therefore, the LFP cathode material of this application features a gradient distribution of iron content from the interior to the surface of the LFP particles, initially decreasing and then increasing. This results in high compaction density, high energy density, high rate performance, and good cycle performance, thereby endowing the LFP cathode material with high compaction density, high energy density, good cycle performance, and low-temperature performance, while also improving its voltage plateau.

[0015] The method for preparing lithium manganese iron phosphate (LFP) cathode material in this application controls the mixing and sintering of LFP precursors, resulting in LFP cathode material with high compaction density, energy density, rate performance, voltage plateau, good cycle performance, and excellent low-temperature performance. Furthermore, the method ensures stable electrochemical performance and high efficiency of the prepared LFP cathode material, thus saving production costs.

[0016] Because the cathode of this application contains the lithium manganese iron phosphate cathode material of this application, the cathode of this application has high energy density, rate performance and voltage plateau, and excellent cycle performance and low temperature performance.

[0017] Because the secondary battery of this application contains the positive electrode of this application, the secondary battery of this application has high energy density, rate performance and voltage plateau, and excellent cycle performance and low temperature performance. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the lithium manganese iron phosphate particle structure contained in the lithium manganese iron phosphate cathode material of the present application embodiment;

[0020] Figure 2 This is a schematic diagram of the structure of the functional coating layer contained in the lithium manganese iron phosphate cathode material of this application embodiment;

[0021] Figure 3 This is a schematic flowchart of the method for preparing lithium manganese iron phosphate cathode material according to an embodiment of this application. Detailed Implementation

[0022] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0023] In this application, the term "and / or" describes 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, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0024] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0025] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0026] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0027] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0028] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0029] Firstly, this application provides a lithium manganese iron phosphate cathode material. The lithium manganese iron phosphate cathode material of this application includes lithium manganese iron phosphate particles. Within these lithium manganese iron phosphate particles, based on the iron content distribution, from the center of the particle to the surface, it can be sequentially divided into a core region, a middle region, and a surface region, with the middle region covering the core region and the surface region covering the middle region. Specifically, it can be as follows: Figure 1 As shown, from the center of the lithium manganese iron phosphate particle 01 to the surface, it can be divided into a core region 11, an intermediate region 12, and a surface region 13 according to the iron content distribution. Specifically, the intermediate region 12 covers the core region 11, and the surface region 13 covers the intermediate region 12. Since the regions are defined based on the iron content distribution, and iron and other elements are distributed according to the gradient described below, there may be no clear boundary between the interfaces of two adjacent regions of the core region 11, the intermediate region 12, and the surface region 13.

[0030] The iron content in the lithium manganese iron phosphate particle 01 exhibits a gradient distribution. Specifically, from the center to the surface, the iron content initially decreases and then increases gradually until reaching the surface of the particle. Combined with... Figure 1As shown, the iron content gradient distribution in the lithium manganese iron phosphate particle 01 is as follows: from the core region 11 to the middle region 12, the core region 11 and the middle region 12 constitute a first composite region, and the iron content decreases gradually in the first composite region. Simultaneously, from the middle region 12 to the outer surface of the surface region 13, the middle region 12 and the surface region 13 constitute a second composite region, and the iron content increases gradually in the first composite region. Thus, the core region 11, the internal center of the lithium manganese iron phosphate particle 01, is set as an iron-rich region, which can effectively increase the particle size and compaction density of the lithium manganese iron phosphate particle 01. The middle region 12, the internal center of the lithium manganese iron phosphate particle 01, is set as a low-iron content region, which can effectively improve the energy density of the lithium manganese iron phosphate particle 01. Specifically, the iron content can be gradient-decreasing from the interface between the core region 11 and the interface between the core region 11 and the interface between the core region 11 and the interface between the core region 11 and the interface between the core region 12 and the surface region 13, and gradient-increasing from the interface between the core region 12 and the surface region 13. The surface region 13 of this lithium manganese iron phosphate particle 01 is also set as an iron-rich region, which can protect the core region 12 and modify its conductivity, thereby improving the structural mechanical properties and cycle performance of the lithium manganese iron phosphate particle. The protective effect of the iron-rich surface region 13 on the core region 12 includes preventing the electrolyte from corroding the core region 12 material, such as lithium manganese iron phosphate, and improving cycle performance.

[0031] In the embodiments, as described above and Figure 1As shown, in the core region 11, the intermediate region 12, and the surface region 13, the molar ratio of iron, manganese, and phosphorus in these regions satisfies the following condition: total molar content of iron + manganese : molar content of phosphorus = 1 : (0.95-1.05). The iron element gradient distribution in the first composite region formed by the core region 11 and the intermediate region 12 can be controlled by adjusting the molar ratio of iron to phosphorus in this first composite region from (0.6-0.8) : (0.95-1.05), and further from 0.7 : (0.95-1.05) to 0 : (0.95-1.05). At this point, the molar content of iron and phosphorus in the core region 11, which is the center of the lithium manganese iron phosphate particle 01, is the highest (0.6-0.8):(0.95-1.05), which can be further 0.7:(0.95-1.05). It then decreases from (0.6-0.8):(0.95-1.05) to 0.7:(0.95-1.05) until the molar content of iron in the middle region 12 is the lowest, which is 0:(0.95-1.05) (at which point the iron content is 0). In other embodiments, the molar content of iron in the first composite region formed by the core region 11 and the intermediate region 12 decreases at a rate of 0.00175-0.0032 mol / nm. For example, the molar ratio of iron to phosphorus in the core region 11 decreases at the rate of 0.7:(0.95-1.05), which is the highest (0.6-0.8):(0.95-1.05), and further, 0.7:(0.95-1.05), until the molar ratio of iron to phosphorus in the intermediate region 12 is the lowest at 0:(0.95-1.05). In this case, the material at the lowest point where the molar content of iron to phosphorus in the intermediate region 12 is 0 should be lithium manganese phosphate; the material at the non-zero points of the molar content of iron to phosphorus in the first composite region is lithium iron manganese phosphate.

[0032] In the embodiments, as described above and Figure 1As shown, the iron element gradient distribution in the second composite region composed of the intermediate region 12 and the surface region 13 can be controlled by increasing the molar ratio of iron to phosphorus in the second composite region from 0:(0.95-1.05) (that is, the iron content is 0) to (0.6-0.8):(0.95-1.05), and further can be 0.7:(0.95-1.05). At this point, the highest molar content of iron in the surface region 13 of the lithium manganese iron phosphate particle 01 is (0.6-0.8):(0.95-1.05), which can be further 0.7:(0.95-1.05). The ratio of iron to phosphorus molar content increases gradually from the lowest point of 0 in the intermediate region 12 until the highest molar content ratio of iron to phosphorus on the surface region 13 is (0.6-0.8):(0.95-1.05), which can be further 0.7:(0.95-1.05). In other embodiments, the molar content of iron in the second composite region formed by the intermediate region 12 and the surface region 13 increases at a rate of 0.00175-0.0032 mol / nm. For example, the molar content of iron in the intermediate region 12 increases at this rate from its lowest point (0) to the surface of the surface region 13, where the molar ratio of iron to phosphorus is (0.6-0.8):(0.95-1.05), or more specifically, 0.7:(0.95-1.05). In this case, the material on the surface of the surface region 13 where the molar content of iron in the intermediate region 12 is not zero is lithium manganese iron phosphate. Furthermore, by controlling and adjusting the molar content or concentration rate of the iron gradient distribution in the first and second composite regions, the compaction density, energy density, protective effect on the intermediate region 12, and conductive modification effect of the lithium manganese iron phosphate particles 01 are further improved.

[0033] Furthermore, based on the aforementioned gradient distribution of iron content in the lithium manganese iron phosphate particles 01, in this embodiment, the manganese content in the first composite region formed by the core region 11 and the intermediate region 12 increases gradient from the core region 11 to the intermediate region 12. In other embodiments, the manganese content in the second composite region formed by the intermediate region 12 and the surface region 13 decreases gradient from the outer surface of the intermediate region 12 to the outer surface of the surface region 13. By controlling the distribution of manganese content in the lithium manganese iron phosphate particles 01, specifically by ensuring that the manganese content in the intermediate region is higher than that at the particle center and surface, the intermediate region 12 becomes a manganese-rich region, allowing the intermediate region 12 to fully utilize its energy density function and thus increasing the energy density of the lithium manganese iron phosphate particles 01.

[0034] When the manganese content distribution in the lithium manganese iron phosphate particles 01 exhibits the aforementioned gradient distribution, as in the examples above, and... Figure 1As shown, in the core region 11, the intermediate region 12, and the surface region 13, the molar ratio of iron, manganese, and phosphorus in the core region 11, the intermediate region 12, and the surface region 12 satisfies: total molar content of iron + manganese : molar content of phosphorus = 1 : (0.95-1.05). Specifically, the gradient distribution of manganese in the first composite region formed by the core region 11 and the intermediate region 12 can be controlled by adjusting the molar ratio of manganese to phosphorus in this first composite region from (0.2-0.4):(0.95-1.05), and further, from 0.3:(0.95-1.05) to 1:(0.95-1.05). At this point, the molar ratio of manganese to phosphorus in the core region 11, which is the inner center of the lithium manganese iron phosphate particle 01, can be as low as 0.2:(0.95-1.05), and can be further increased to 0.3:(0.95-1.05), and gradually increased from the lowest 0.2:(0.95-1.05) until the molar ratio of manganese to phosphorus in the middle region 12 is as high as 1:(0.95-1.05). In other embodiments, the molar content of manganese in the first composite region formed by the core region 11 and the intermediate region 12 increases at a rate of 0.00175-0.0032 mol / nm. For example, starting from a minimum molar ratio of manganese to phosphorus of 0.2:(0.95-1.05) in the core region 11, and further possibly 0.3:(0.95-1.05), the ratio gradually increases at this rate until the maximum molar ratio of manganese to phosphorus in the intermediate region 12 reaches 1:(0.95-1.05). In this case, in the first composite region, the material where the molar ratio of manganese to phosphorus in the intermediate region 12 is at its maximum of 1:(0.95-1.05) should be lithium manganese phosphate; the material where the molar content of manganese in the first composite region is not 1:(0.95-1.05) is lithium manganese iron phosphate.

[0035] In the embodiments, as described above and Figure 1As shown, in the core region 11, the intermediate region 12, and the surface region 13, the molar ratio of iron, manganese, and phosphorus in these regions satisfies the following condition: total molar content of iron + manganese : molar content of phosphorus = 1 : (0.95-1.05). The gradient decreasing distribution of manganese in the second composite region formed by the intermediate region 12 and the surface region 13 can be achieved by controlling the molar ratio of manganese to phosphorus in this second composite region to decrease from 1 : (0.95-1.05) to (0.2-0.4) : (0.95-1.05), or more specifically, to 0.3 : (0.95-1.05). At this point, the highest molar ratio of manganese to phosphorus in the middle region 12 of the lithium manganese iron phosphate particle 01 is 1:(0.95-1.05), and it gradually decreases from the highest molar ratio of manganese to phosphorus in the middle region 12 of 1:(0.95-1.05) until the lowest molar ratio of manganese to phosphorus on the surface of the surface region 13 is 0.2:(0.95-1.05), and further can be 0.3:(0.95-1.05). In other embodiments, the molar content of manganese in the second composite region formed by the intermediate region 12 and the surface region 13 decreases at a rate of 0.00175-0.0032 mol / nm. For example, the molar content ratio of manganese to phosphorus in the intermediate region 12, which is the highest at 1:(0.95-1.05), decreases at this rate until the molar content ratio of manganese to phosphorus on the surface of the surface region 13, which can be as low as 0.2:(0.95-1.05), or even 0.3:(0.95-1.05. In this case, in the second composite region, when the molar content ratio of manganese to phosphorus in the intermediate region 12 is the highest at 1:(0.95-1.05), the material here is lithium manganese phosphate; when the molar content of manganese in the second composite region is not 1:(0.95-1.05), the material is lithium manganese iron phosphate.

[0036] Furthermore, by controlling and adjusting the molar content or concentration rate of the gradient distribution of manganese element in the first and second composite zones, the energy density of lithium manganese iron phosphate particles 01 can be further improved, and it can also have a synergistic effect with the gradient distribution of iron element in the first and second composite zones, thereby further improving the energy density, compaction density, rate performance, and cycle performance of lithium manganese iron phosphate particles 01.

[0037] In a specific embodiment, the molar ratio of the four elements iron, manganese, phosphorus and lithium contained in the core region 11, the intermediate region 12 and the surface region 13 satisfies the following: total molar content of iron + manganese: molar content of phosphorus: lithium = 1:(0.95-1.05):(0.95-1.10).

[0038] In addition, in the above embodiments, in the core region 11, the intermediate region 12, and the surface region 13, when the molar ratio of the total molar content of iron + manganese to elements such as phosphorus is 1:(0.95-1.05), since the molar ratio of the sum of the molar contents of iron and manganese to phosphorus is 1:(0.95-1.05), when the iron content increases in a gradient, the manganese content decreases in a gradient; when the iron content decreases in a gradient, the manganese content increases in a gradient, so as to satisfy the requirement that the molar ratio of the total molar content of iron + manganese to elements such as phosphorus is 1:(0.95-1.05).

[0039] Based on the above embodiments, as an embodiment of this application, the lithium manganese iron phosphate particle 01 also contains dopant elements. In the embodiments, from the core region 11 to the intermediate region 12, the content of dopant elements in the first composite region formed by the core region 11 and the intermediate region 12 decreases in a gradient, such as the molar ratio of dopant elements to phosphorus elements contained in the lithium manganese iron phosphate particle 01 decreasing from (0.01-0.02):(0.95-1.05) to 0:(0.95-1.05). In other embodiments, from the intermediate region 12 to the outer surface of the surface region 13, the content of dopant elements in the second composite region formed by the intermediate region 12 and the surface region 13 increases in a gradient, such as the molar ratio of dopant elements to phosphorus elements contained in the lithium manganese iron phosphate particle 01 increasing from 0:(0.95-1.05) to (0.01-0.02):(0.95-1.05). In other embodiments, the molar ratio of the dopant element to the iron element contained in the lithium manganese iron phosphate particle 01 is (0.0125-0.033):1. In specific embodiments, the dopant element includes at least one of Mg, Ti, Cr, Co, Ni, Ca, Mn, S, and B. By doping the lithium manganese iron phosphate particle 01 with elements, or by further controlling the gradient distribution of these dopant elements in the lithium manganese iron phosphate particle 01 and controlling and selecting the types of dopant elements, the degradation of cycle performance caused by structural distortion of the lithium manganese iron phosphate particle 01 can be alleviated, thereby further improving the voltage plateau, rate performance, and cycle performance of the lithium manganese iron phosphate particle 01.

[0040] In the embodiments, the molar ratio of iron, manganese, phosphorus, and dopant elements in lithium manganese iron phosphate particles satisfies the following ratio: total molar content of iron + manganese : molar content of dopant elements : molar content of phosphorus = 1 : (0.01-0.02) : (0.95-1.05). By controlling and optimizing the molar ratio of each element in lithium manganese iron phosphate particles, the electrochemical performance, such as energy density, of the lithium manganese iron phosphate particles can be improved.

[0041] Based on the above embodiments, as an embodiment of this application, the lithium manganese iron phosphate cathode material of this application embodiment further includes a functional coating layer containing elemental iron and / or iron compounds, such as... Figure 2The functional coating layer 02 shown coats the lithium manganese iron phosphate particles 01. By adding an iron-containing compound and / or elemental iron to the surface of the lithium manganese iron phosphate particles 01, a protective layer can be effectively provided, such as preventing the electrolyte from corroding the lithium manganese iron phosphate contained in the particles 01, thus improving the cycle performance of the lithium manganese iron phosphate cathode material in this embodiment. Simultaneously, since the functional coating layer 02 contains iron compounds and / or elemental iron, it has excellent conductivity and can modify the conductivity between itself and the iron-rich surface region 13, significantly improving the electronic conductivity of the lithium manganese iron phosphate cathode material and increasing its rate performance. In a specific embodiment, the iron compound may include at least one of lithium iron phosphate, lithium ferrite, and iron oxide.

[0042] In the embodiments, the thickness of the functional coating layer 02 can be 5-20 nm; or in a further embodiment, the iron compound and / or elemental iron in the functional coating layer 02 accounts for 1-10% of the total weight of the lithium manganese iron phosphate cathode material. By controlling and optimizing the thickness of the functional coating layer 02 or the content of iron compound and / or elemental iron, the above-mentioned effects of the functional coating layer 02 are improved, thereby further improving the rate performance, voltage plateau, and cycle performance of the lithium manganese iron phosphate cathode material.

[0043] In the embodiments, the D50 particle size of the lithium manganese iron phosphate particles 01 contained in the lithium manganese iron phosphate cathode material in the above embodiments can be 200-500 nm; or in a further embodiment, the particle size of the lithium manganese iron phosphate particles 01 and the thickness of the functional coating layer 02 are controlled and adjusted, and the D50 particle size of the lithium manganese iron phosphate cathode material in the above embodiments is 400-800 nm. Therefore, the lithium manganese iron phosphate cathode material of the present application embodiments has high compaction density and energy density.

[0044] As can be seen from the lithium manganese iron phosphate particles 01 or the functional coating layer 02 contained in the lithium manganese iron phosphate cathode material of this application embodiment in the above embodiments, the lithium manganese iron phosphate cathode material of this application embodiment has high compaction density, energy density, good cycle performance and low temperature performance, and also has a high voltage platform.

[0045] Secondly, this application also provides a method for preparing the lithium manganese iron phosphate cathode material described above. The process flow for preparing the lithium manganese iron phosphate cathode material in this application is as follows: Figure 3 As shown, it includes the following steps:

[0046] S01: The lithium source, phosphorus source, first iron source and manganese source are mixed in the proportions required for preparing lithium manganese iron phosphate to obtain a precursor.

[0047] S02: Under a protective atmosphere, the precursor is subjected to a first sintering treatment to obtain lithium manganese iron phosphate particles.

[0048] In step S01, the precursor prepared can be lithium manganese iron phosphate particles contained in the lithium manganese iron phosphate cathode material described above. This is done to improve the gradient distribution of metal elements such as iron and manganese, or further doping elements, contained in the lithium manganese iron phosphate particles as described above.

[0049] In this embodiment, the precursor in step S01 is prepared by a method comprising the following steps:

[0050] S011: Take a portion including lithium source, phosphorus source, first iron source and manganese source to prepare precursor core;

[0051] S012: Multiple coating layers are sequentially formed on the surface of the precursor core prepared in step S011 to form a precursor; wherein, during the formation of each coating layer, the mixing ratio of the first iron source and the manganese source is controlled to achieve a distribution in the precursor in which the iron element first decreases and then increases in the gradient from the precursor core to the precursor surface; or the iron element first decreases and then increases in the gradient distribution while the manganese element first increases in the gradient distribution and then decreases in the gradient distribution.

[0052] In a further embodiment, during the preparation of the coating layer contained in the precursor using the layer-by-layer coating method, the dopant source of the dopant element contained in the lithium manganese iron phosphate particles mentioned above can be further added and mixed with the first iron source, manganese source, etc. to form the corresponding coating layer, thereby achieving a gradient distribution of the dopant element in the coating layer, specifically as the gradient distribution of the dopant element contained in the lithium manganese iron phosphate particles mentioned above.

[0053] In this embodiment, the dopant element provided by the dopant source can be one of the dopant elements contained in the lithium manganese iron phosphate cathode material mentioned above, such as at least one of Mg, Ti, Cr, Co, Ni, Ca, Mn, S, and B. In a further embodiment, the dopant source is added according to the gradient distribution of the dopant elements in the lithium manganese iron phosphate particles mentioned above, or further according to a molar ratio of iron to dopant elements in the lithium manganese iron phosphate particles of 1:(0.001-0.02). By adding the dopant source, the precursor contains dopant elements, specifically the aforementioned gradient-distributed dopant elements.

[0054] In a specific embodiment, the precursor core prepared in step S011, including lithium source, phosphorus source, first iron source and manganese source, can be added according to the proportion of elements in the core region of the lithium manganese iron phosphate particles mentioned above. The method for preparing the precursor core can be solvent precipitation method or solid phase method.

[0055] In step S012, the mixing ratio of each coating layer, including the lithium source, phosphorus source, first iron source, and manganese source, and the thickness of each coating layer, should meet the requirements of the element ratio and distribution in the intermediate and surface regions of the lithium manganese iron phosphate particles formed after the first sintering treatment. The coating layers can also be formed by solvent precipitation for in-situ deposition on the precursor core surface or by solid-phase layer-by-layer coating.

[0056] After the first mixing process in step S01, the various source compounds, including lithium source, phosphorus source, first iron source, and manganese source, can form the precursor to further form the precursor with the above-mentioned iron, manganese or further doped element gradient distribution coating layer, which is the precursor to form the lithium manganese iron phosphate particles 01 contained in the lithium manganese iron phosphate cathode material of the above application embodiment. In this embodiment, the lithium source, phosphorus source, first iron source, and manganese source in step S01 can be mixed with fillers according to the above-described layer-by-layer coating method to form a precursor with a gradient distribution of corresponding elements. The molar ratio of lithium element, phosphorus element, iron element, and manganese element provided by the first iron source is (0.95-1.10):(0.95-1.05):(0-0.8):(0.2-1), which can be further 0.95-1.05:(0.95-1.05):(0.1-0.8):(0.2-0.9), and even further 0.95-1.05:(0.95-1.05):(0.1-0.7):(0.2-0.7). By controlling the mixing ratio of compounds such as lithium source, phosphorus source, first iron source, and manganese source, the proportion of elements such as lithium, phosphorus, iron, and manganese can be controlled, thereby enabling the precursor to form lithium manganese iron phosphate particles 01 containing iron elements or further including manganese elements in a gradient distribution in the first sintering process of the lithium manganese iron phosphate cathode material of the above application embodiment.

[0057] In a specific embodiment, the lithium source, phosphorus source, first iron source, and manganese source in step S01 are the lithium source, phosphorus source, first iron source, and manganese source used in the preparation of lithium manganese iron phosphate. As in the embodiment, the lithium source may include one of metallic lithium, lithium phosphate, lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium nitrite, lithium acetate, lithium oxide, and lithium oxalate; the first iron source may include one of ferric sulfate, ferrous sulfate, ferric nitrate, ferrous nitrate, ferric oxide, ferrous oxide, ferrous chloride, ferrous chloride, ferric carbonate, and ferrous carbonate; the phosphorus source may include any one of phosphoric acid, monoammonium phosphate, diammonium phosphate, and triammonium phosphate; and the manganese source may include any one of manganese carbonate, manganese phosphate, manganese hydroxide, manganese oxide, and manganese sulfate.

[0058] In addition, the first mixing process in step S01 is to ensure that the source compounds are mixed evenly. Therefore, any mixing process that can ensure that the source compounds are mixed evenly is within the scope of the specification of this application, such as but not limited to stirring.

[0059] In step S02, after the first sintering treatment, the precursor prepared in step S01 is sintered to form the lithium manganese iron phosphate particles 01 contained in the lithium manganese iron phosphate cathode material described above. When the precursor containing an elementally gradient-distributed coating layer is prepared using the layer-by-layer coating method described above, the precursor containing the coating layer undergoes thermal diffusion during the first sintering treatment. That is, when the ambient temperature rises, atoms vibrate near the equilibrium lattice point. Some of them gain enough energy to leave the equilibrium lattice point and become interstitial atoms, while creating vacancies on the original lattice point. When other nearby atoms or matrix atoms migrate to the vacancy, vacancy diffusion occurs. Therefore, in the sintered lithium manganese iron phosphate particles, the iron element or further manganese element and / or doping elements contained therein exhibit the gradient distribution described above. For example, from the interior to the surface of the lithium manganese iron phosphate particle, the iron element content first decreases in a gradient and then increases in a gradient.

[0060] In this embodiment, the temperature of the first sintering treatment is 400-500℃. Furthermore, the sintering time at this temperature should at least ensure the formation of lithium manganese iron phosphate particles from each source compound. In this embodiment, the time of the first sintering treatment can also be controlled so that the iron element, or further manganese element, and / or doping elements in the generated lithium manganese iron phosphate particles exhibit the gradient distribution described above. For example, in this embodiment, the time of the first sintering treatment can be 1-3 hours.

[0061] In addition, the protective atmosphere in step S02 can be the protective atmosphere of the lithium manganese iron phosphate preparation method, such as inert atmospheres including nitrogen, argon, and helium.

[0062] When the lithium manganese iron phosphate cathode material in the embodiments of this application is as described above and Figure 2 When the functional coating layer 02 is also included, the preparation method of the lithium manganese iron phosphate cathode material in this application embodiment further includes the following step S03 after the first sintering treatment step in step S02:

[0063] S031: Lithium manganese iron phosphate particles are subjected to a second mixing treatment with a second iron source or the second iron source and a reducing agent to obtain a mixture;

[0064] S032: The mixture is subjected to a second sintering treatment to form a functional coating layer containing iron compounds and / or elemental iron on the surface of lithium manganese iron phosphate particles.

[0065] In step S031, the second iron source is a precursor of iron compound and / or elemental iron, and the reducing agent reduces the iron in the second iron source to elemental iron. Therefore, by controlling the mixing ratio of the second iron source and lithium manganese iron phosphate particles, the amount of iron compound generated from the second iron source can be controlled, thereby controlling the thickness or content of the functional coating layer, thus adjusting the function of the functional coating layer and improving the cycle performance and rate performance of the prepared lithium manganese iron phosphate cathode material. In the embodiments, the lithium manganese iron phosphate particles, reducing agent, and second iron source can be mixed in a mass ratio of (3-10):(0.03-0.3):(0.1-0.5), or the lithium manganese iron phosphate particles and the second iron source can be mixed in a mass ratio of (3-10):(0.1-0.5).

[0066] In a specific embodiment, when the second iron source is reduced to elemental iron, the second iron source can be at least one of ferric oxide, ferric sulfate, ferrous sulfate, ferric nitrate, ferrous nitrate, ferrous oxide, ferric chloride, ferrous chloride, ferric carbonate, and ferrous carbonate. The reducing agent can include at least one of a carbon source, H2, and S. The carbon source can be at least one of glucose, sucrose, etc.

[0067] When the second iron source forms an iron compound, the second iron source may include at least one of the following: iron phosphate, iron hydroxide, iron nitrate, lithium ferrite, iron nitrate, iron oxalate, iron hydroxide, and iron citrate. Specifically, the second iron source may be a mixture of iron phosphate and lithium carbonate (the sintering product of both is lithium iron phosphate), or a mixture of iron hydroxide and / or iron nitrate and lithium phosphate (the sintering product of both is lithium iron phosphate), or a mixture of iron phosphate and lithium nitrate (the sintering product of both is lithium iron phosphate). Of course, iron nitrate, iron oxalate, iron hydroxide, and iron citrate may be added individually and decomposed into iron oxide (iron compound).

[0068] In step S032, after the second sintering treatment, the second iron source in the mixture, under the action of the reducing agent, generates iron compounds and / or elemental iron, which in situ coat the surface of the lithium manganese iron phosphate particles generated in step S02 and form a functional coating layer, as described above. Figure 2 The functional coating layer 02 is used. In this embodiment, the temperature of the second sintering treatment is 500-800°C. Furthermore, the sintering time at this temperature should at least ensure the formation of compounds and / or elemental iron from the second iron source. For example, in this embodiment, the time of the second sintering treatment at this temperature can be 3-8 hours.

[0069] In addition, the second sintering process should be carried out in a protective atmosphere, such as an inert atmosphere including nitrogen, argon, helium, etc.

[0070] Therefore, the methods for preparing lithium manganese iron phosphate cathode materials in the above embodiments, through controlled mixing and sintering of the lithium manganese iron phosphate precursor, or further in-situ coating with iron-containing compounds and / or elemental iron, result in lithium manganese iron phosphate cathode materials with high compaction density, energy density, rate performance, voltage plateau, good cycle performance, and excellent low-temperature performance. Furthermore, the electrochemical performance of the lithium manganese iron phosphate cathode material can be further improved by adjusting relevant process conditions in the corresponding steps, such as material ratio, sintering temperature, and time. In addition, the methods for preparing lithium manganese iron phosphate cathode materials in this application ensure stable electrochemical performance of the prepared lithium manganese iron phosphate cathode material, are highly efficient, and save production costs.

[0071] Thirdly, embodiments of this application also provide a positive electrode. The positive electrode of this application includes a positive current collector and a positive active layer bonded to the surface of the positive current collector.

[0072] Among them, the positive current collector of the positive electrode can be, but is not limited to, any of the copper foil and aluminum foil.

[0073] The positive electrode active layer includes components such as positive electrode active material, binder and conductive agent.

[0074] The positive electrode active material in the positive electrode active layer is the lithium manganese iron phosphate positive electrode material of the above-described embodiments. Therefore, the positive electrode of this embodiment has high energy density, rate performance, and voltage plateau, as well as excellent cycle performance and low-temperature performance. In the embodiments, the mass percentage of the lithium manganese iron phosphate positive electrode material in the positive electrode active layer can be controlled to be 40%-60%.

[0075] In the embodiments, the content of the binder in the positive electrode active layer can be 1%-3% by mass. In specific embodiments, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0076] In the embodiments, the content of the conductive agent in the positive electrode active layer can be 1wt%-2wt%. In specific embodiments, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 and carbon nanotubes.

[0077] In the embodiments, the preparation process of the positive electrode can be as follows: mixing the positive electrode active material, conductive agent and binder to obtain an electrode slurry, coating the electrode slurry on the current collector, and preparing the positive electrode through steps such as drying, rolling and die cutting.

[0078] Fourthly, this application also provides a secondary battery. The secondary battery of this application includes necessary components such as a positive electrode, a negative electrode, a separator, and an electrolyte, as well as other necessary or auxiliary components. The positive electrode is the positive electrode described in this application, meaning that the positive electrode active layer contains the lithium manganese iron phosphate positive electrode material described in this application. Because the secondary battery of this application contains the lithium manganese iron phosphate positive electrode material described in this application, it exhibits high energy density, high rate performance, high voltage plateau, excellent cycle performance, and excellent low-temperature performance.

[0079] The following examples illustrate the lithium manganese iron phosphate cathode material and its preparation method in this application.

[0080] 1. Examples of lithium manganese iron phosphate cathode materials and their preparation methods:

[0081] Example A1

[0082] This embodiment provides a lithium manganese iron phosphate cathode material and its preparation method. The lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate particle core and an elemental iron coating layer covering the core. The lithium manganese iron phosphate (LFP) particles contain magnesium as a dopant. From the center of the LFP particle core to the outer surface, the molar ratio of iron to phosphorus gradually decreases from 0.8:1 to 0 (i.e., 0:1) and then gradually increases back to 0.8:1. The rate of decrease in iron content is 0.0032 mol / nm, and the rate of increase is 0.0032 mol / nm. Similarly, the molar ratio of magnesium to phosphorus gradually decreases from 0.02:1 to 0 (i.e., 0:1) and then gradually increases back to 0.02:1. Likewise, the molar ratio of manganese to phosphorus gradually increases from 0.2:1 to 1:1 and then gradually decreases back to 0.2:1. The rate of increase in manganese content is 0.0032 mol / nm, and the rate of decrease is 0.0032 mol / nm.

[0083] The preparation method of the lithium iron manganese phosphate cathode material in this embodiment includes the following steps:

[0084] S1: Prepare a precursor using lithium hydroxide, monoammonium phosphate, iron oxide, magnesium oxide, and manganese sulfate. Adjust the raw material ratio so that the lithium to phosphorus molar ratio of the lithium manganese iron phosphate particles prepared in step S2 is 0.95:1.05. Keep the ratio of lithium hydroxide and monoammonium phosphate constant, gradually decrease the amount of iron oxide added, and gradually increase the amount of manganese sulfate added. The reduction in the molar amount of iron oxide is compensated by manganese sulfate. Multiple coating layers are sequentially applied to the surface of the precursor core through in-situ coating, and the multiple coating layers sequentially coat the precursor core. This causes the molar ratio of phosphorus to iron in the particles to gradually decrease from 1:0.8 to 1:0 and then gradually increase to 1:0.8 from the inside out; the molar ratio of phosphorus to magnesium to gradually decrease from 1:0.02 to 1:0.01 and then gradually increase to 1:0.02; and the molar ratio of phosphorus to manganese to gradually increase from 1:0.2 to 1:1 and then gradually decrease to 1:0.2.

[0085] S2: The prepared precursor is sintered at 500℃ for 3 hours, cooled, and crushed to obtain lithium iron manganese phosphate particles.

[0086] S3: Take the sintered lithium manganese iron phosphate particles, add sucrose and iron oxide and mix them, then continue sintering at 700℃ for 5 hours to form an in-situ iron functional coating layer on the surface of the lithium manganese iron phosphate particles. Cool and pulverize to obtain the lithium manganese iron phosphate cathode material. The amount of sucrose and iron oxide added accounts for 2% of the lithium manganese iron phosphate, and the molar ratio of sucrose to iron oxide is 0.3:0.5.

[0087] Testing revealed that the D50 particle size of the lithium manganese iron phosphate cathode material in this embodiment is 500 nm, and the D50 particle size of the lithium manganese iron phosphate particle core it contains is 400 nm.

[0088] Example A2

[0089] This embodiment provides a lithium manganese iron phosphate cathode material and its preparation method. The lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate particle core and an elemental iron coating layer covering the core. The lithium manganese iron phosphate (LFP) particles contain chromium dopant. From the center of the LFP particle core to the outer surface, the molar ratio of iron to phosphorus gradually decreases from 0.7:1 to 0 (i.e., 0:1) and then gradually increases back to 0.7:1. The rate of decrease in iron content is 0.00175 mol / nm, and the rate of increase is also 0.00175 mol / nm. Similarly, the molar ratio of chromium dopant to phosphorus gradually decreases from 0.01:1 to 0 and then gradually increases back to 0.01:1. Likewise, the molar ratio of manganese to phosphorus gradually increases from 0.3:1 to 1:1 and then gradually decreases back to 0.3:1. The rate of increase in manganese content is 0.00175 mol / nm, and the rate of decrease is also 0.00175 mol / nm.

[0090] The preparation method of the lithium iron manganese phosphate cathode material in this embodiment includes the following steps:

[0091] S1: Lithium carbonate, diammonium phosphate, ferric chloride, manganese oxide, and chromium oxide are prepared according to the layer-by-layer coating method in Example A1 to obtain a precursor. The amount of raw materials added is controlled so that the lithium-phosphorus molar ratio of the lithium manganese iron phosphate particles prepared in step S2 is 1:0.95. The molar ratio of iron to phosphorus content in the particles gradually decreases from 0.7:1 to 0 and then gradually increases to 0.7:1 from the inside to the outside. The content of chromium impurity element to phosphorus element gradually decreases from 0.01:1 to 0 and then gradually increases to 0.01:1. The molar ratio of manganese to phosphorus content gradually increases from 0.3:1 to 1:1 and then gradually decreases to 0.3:1.

[0092] S2: The prepared precursor is sintered at 400℃ for 2 hours, cooled, and crushed to obtain lithium iron manganese phosphate particles.

[0093] S3: Take the above-mentioned sintered lithium manganese iron phosphate particles, add glucose and ferric chloride, and continue sintering at 800℃ for 4 hours. Cool and pulverize to obtain lithium manganese iron phosphate. The amount of glucose and ferric chloride added accounts for 1.5% of the lithium manganese iron phosphate, and the molar ratio of glucose to ferric chloride is 0.3:0.3.

[0094] Testing revealed that the D50 particle size of the lithium manganese iron phosphate cathode material in this embodiment is 800 nm, and the D50 particle size of the lithium manganese iron phosphate particle core it contains is 500 nm.

[0095] Example A3

[0096] This embodiment provides a lithium manganese iron phosphate cathode material and its preparation method. The lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate particle core and an elemental iron coating layer covering the core. The lithium manganese iron phosphate (LFP) particles contain nickel dopant. From the center of the LFP particle core to the outer surface, the molar ratio of iron to phosphorus gradually decreases from 0.6:1 to 0 and then gradually increases back to 0.6:1, with a decreasing rate of 0.003 mol / nm and a increasing rate of 0.003 mol / nm for iron content. The molar ratio of nickel to phosphorus dopant gradually decreases from 0.015:1 to 0 and then gradually increases back to 0.015:1 from the center of the LFP particle core to the outer surface. The molar ratio of manganese to phosphorus gradually increases from 0.4:1 to 1:1 and then gradually decreases back to 0.4:1 from the center of the LFP particle core to the outer surface, with a increasing rate of 0.045 mol / nm and a decreasing rate of 0.045 mol / nm for manganese content.

[0097] The preparation method of the lithium iron manganese phosphate cathode material in this embodiment includes the following steps:

[0098] S1: Lithium oxalate, phosphoric acid, iron carbonate, manganese hydroxide, and nickel oxide are prepared according to the discrete in-situ layer-by-layer coating method in Example A1 to obtain a precursor. The amount of raw materials added is controlled so that the lithium-phosphorus molar ratio of the lithium manganese iron phosphate particles prepared in step S2 is 1.05:1. The molar ratio of iron to phosphorus content in the particles gradually decreases from 0.6:1 to 0 and then gradually increases to 0.6:1 from the inside to the outside. The content of nickel impurity element to iron element also gradually decreases from 0.015 to 0 and then gradually increases to 0.015. The molar ratio of manganese to phosphorus content gradually increases from 0.4:1 to 1:1 and then gradually decreases to 0.4:1.

[0099] S2: (2) The prepared precursor was sintered at 450℃ for 1 hour, cooled, and crushed to obtain lithium manganese iron phosphate particles.

[0100] S3: Take the sintered lithium manganese iron phosphate particles, add citric acid and iron carbonate, and continue sintering at 500℃ for 5 hours. Cool and pulverize to obtain lithium manganese iron phosphate cathode material. The amount of citric acid and iron carbonate added accounts for 5% of the lithium manganese iron phosphate, and the molar ratio of citric acid to iron carbonate is 0.2:0.5.

[0101] Testing revealed that the D50 particle size of the lithium manganese iron phosphate cathode material in this embodiment is 500 nm, and the D50 particle size of the lithium manganese iron phosphate particle core it contains is 400 nm.

[0102] Example A4

[0103] This embodiment provides a lithium manganese iron phosphate cathode material and its preparation method. The lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate particle core and an iron compound coating layer covering the core. The lithium manganese iron phosphate particle core is the same as that in Example A2, and the iron compound coating layer is made of lithium iron phosphate.

[0104] The preparation method of the lithium iron manganese phosphate cathode material in this embodiment includes the following steps:

[0105] S1: Refer to step S1 of embodiment A2;

[0106] S2: Refer to step S2 of embodiment A2;

[0107] S3: Take the above sintered lithium manganese iron phosphate particles, add a mixture of iron phosphate and lithium nitrate, and continue sintering at 700℃ for 3 hours. After cooling and crushing, lithium manganese iron phosphate is obtained.

[0108] Testing revealed that the D50 particle size of the lithium manganese iron phosphate cathode material in this embodiment is 800 nm, and the D50 particle size of the lithium manganese iron phosphate particle core it contains is 500 nm.

[0109] Example A5

[0110] This embodiment provides a lithium manganese iron phosphate cathode material and its preparation method. The lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate particle core and an iron oxide coating layer covering the core. The lithium manganese iron phosphate particle core is the same as that in Example A2.

[0111] The preparation method of the lithium iron manganese phosphate cathode material in this embodiment includes the following steps:

[0112] S1: Refer to step S1 of embodiment A2;

[0113] S2: Refer to step S2 of embodiment A2;

[0114] S3: Take the above-mentioned sintered lithium manganese iron phosphate particles, add ferric citrate, and continue sintering at 500℃ for 8 hours. Cool and pulverize to obtain lithium manganese iron phosphate. The amount of ferric citrate added accounts for 1.5% of the lithium manganese iron phosphate particles.

[0115] Testing revealed that the D50 particle size of the lithium manganese iron phosphate cathode material in this embodiment is 800 nm, and the D50 particle size of the lithium manganese iron phosphate particle core it contains is 500 nm.

[0116] Comparative Example A1

[0117] This embodiment provides a lithium manganese iron phosphate cathode material and its preparation method. The lithium manganese iron phosphate cathode material is prepared according to the following method:

[0118] S1. Refinement and activation of iron oxide raw materials: Micron-sized iron oxide raw materials and carbon source are mixed at a mass ratio of 1:0.6 and ball-milled in an organic solvent system for 3 hours. After drying, they are conditioned and activated in a controlled atmosphere furnace. The specific steps of activation and sintering are as follows: activation holding temperature is 500℃, room temperature is increased to activation holding temperature at a rate of 5℃ / min, holding time is 6 hours, and at the end of the holding time, the sintering atmosphere is changed from nitrogen protection to oxygen protection. After cooling to room temperature, activated iron oxide raw materials are obtained.

[0119] S2. Precursor synthesis: The lithium source, magnesium source, activated iron oxide raw material, manganese source, phosphorus source and carbon source are mixed according to the molar ratio of each source in Example A1, and ground and refined in an organic solvent system for 2 hours. The dried precursor mixed powder is reserved for pre-calcination.

[0120] S3. Pre-calcination: Control the heating rate at 2℃ / min, and keep the dried precursor mixture powder at 450℃ for 6h under nitrogen protection. Then, cool it down to room temperature with the furnace to obtain lithium manganese iron phosphate precursor.

[0121] S4. Coarse grinding: The pre-calcined lithium manganese iron phosphate precursor is coarsely ground to control the particle size D100 of the coarsely ground lithium manganese iron phosphate precursor to be <100um.

[0122] S5. Sintering: Under nitrogen protection, the coarsely crushed lithium iron phosphate particles were sintered at 700°C with a heating rate of 5°C / min, held at that temperature for 8 hours, and then cooled to room temperature to obtain carbon-coated lithium iron phosphate particles.

[0123] S6: Take the sintered lithium manganese iron phosphate particles and process them according to step S3 in Example A1 to form a lithium manganese iron phosphate cathode material with iron-coated lithium manganese iron phosphate particles.

[0124] 2. Example of a lithium-ion battery:

[0125] Examples B1 to B5 and Comparative Example B1 each provide a lithium-ion battery. Each lithium-ion battery is assembled according to the following method:

[0126] 1) Positive electrode plate:

[0127] The lithium manganese iron phosphate cathode materials provided in Examples A1 to A5 and Comparative Example A1 were used as the positive electrode active materials in Examples B1 to B5 and Comparative Example B1, respectively. Under the same conditions, NMP: lithium manganese iron phosphate cathode material: Super P: PVDF were mixed in a mass ratio of 100:93:2:3. The mixing method was ball milling, and the ball milling time was 60 min. The speed was set to 30 Hz. After homogenization-coating-drying-cutting operations, positive electrode sheets were prepared. The positive electrode sheets were baked in a vacuum oven at 100°C to remove trace amounts of water.

[0128] 2) Negative electrode: Lithium metal sheet.

[0129] 3) Diaphragm: Polyethylene (PE) diaphragm is used.

[0130] 4) Electrolyte: The electrolyte is a 1 mol / L LiPF6 solution, and the solvent is composed of EC (ethylene carbonate) and DEC (diethyl carbonate) in a volume ratio of 1:1.

[0131] 5) Assembly of secondary batteries:

[0132] The above-mentioned positive electrode, negative electrode, electrolyte and separator are assembled into a lithium-ion pouch battery according to the lithium-ion battery assembly requirements.

[0133] 3. Electrochemical performance of lithium-ion batteries:

[0134] The electrochemical performance of the lithium secondary batteries containing Examples B1 to B5 and Comparative Example B1 was tested according to the relevant performance tests shown in Table 1 below. The test conditions were determined according to industry standard test methods.

[0135] The relevant electrochemical performance test results of lithium secondary batteries are shown in Table 1 below.

[0136] Table 1

[0137]

[0138] As can be seen from Table 1, compared with the lithium manganese iron phosphate battery in Comparative Example B1, the lithium manganese iron phosphate battery prepared in this application has a higher voltage plateau, higher energy density, and better cycle performance, and these improvements are significant. This indicates that, compared with traditional lithium manganese iron phosphate cathode materials with homogeneous element distribution, the lithium manganese iron phosphate cathode material in this application sets the corresponding elements into a gradient distribution structure, improving the compaction density, energy density, voltage plateau, and structural mechanical properties of the lithium manganese iron phosphate cathode material, resulting in high cycle performance.

[0139] Further comparisons with Examples B1 to B5, i.e., with Examples A1 to A5, reveal that in the lithium manganese iron phosphate cathode material of this application, when the iron content in the core and surface layers is high, the energy density and voltage platform of the lithium manganese iron phosphate battery in this application are relatively high. However, higher is not always better. Specifically, in Example B2 containing the lithium manganese iron phosphate cathode material provided in Example A2, the energy density of the lithium manganese iron phosphate battery is higher than that of the lithium manganese iron phosphate batteries in Examples B1 and B3 containing the lithium manganese iron phosphate cathode material provided in Example A1 and Example A3, respectively. The voltage platform of the lithium manganese iron phosphate battery in Example B2 is also higher than that of the lithium manganese iron phosphate batteries in Examples B1 and B2.

[0140] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A lithium iron phosphate cathode material, characterized in that: It includes lithium manganese iron phosphate particles and a functional coating layer, wherein the functional coating layer coats the lithium manganese iron phosphate particles; The lithium manganese iron phosphate particles are divided into a core region, a middle region, and a surface region from the center to the surface, based on the iron content distribution. The middle region covers the core region, and the surface region covers the middle region. From the core region to the middle region, the iron content in the first composite region (formed by the core region and the middle region) decreases gradually. From the middle region to the outer surface of the surface region, the iron content in the second composite region (formed by the middle region and the surface region) increases gradually. The rate at which the molar iron content in the first composite region decreases according to the gradient, and the rate at which the molar iron content in the second composite region increases according to the gradient, are independently 0.00175-0.0032 mol / nm. From the core region to the intermediate region, the manganese content in the first composite region increases gradually; from the intermediate region to the outer surface of the surface region, the manganese content in the second composite region decreases gradually, making the intermediate region a manganese-rich region; the rate at which the molar content of manganese in the first composite region increases gradually and the rate at which the molar content of manganese in the second composite region decreases gradually are independently 0.00175-0.0032 mol / nm. The material of the functional coating layer contains at least one of elemental iron and iron compounds, wherein the iron compounds include at least one of lithium iron phosphate, lithium ferrite, and iron oxide.

2. The lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The molar ratio of iron, manganese, and phosphorus in the core region, intermediate region, and surface region satisfies the following condition: total molar content of iron and manganese : molar content of phosphorus = 1 : (0.95-1.05); In the first composite region, the molar ratio of iron to phosphorus decreases from (0.6-0.8):(0.95-1.05) to 0:(0.95-1.05) in the aforementioned gradient; and / or In the second composite region, the molar ratio of iron to phosphorus increases from 0:(0.95-1.05) to (0.6-0.8):(0.95-1.05) in the aforementioned gradient.

3. The lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the core region, intermediate region, and surface region, the molar ratio of iron, manganese, and phosphorus satisfies the following condition: total molar content of iron and manganese : molar content of phosphorus = 1 : (0.95-1.05); where, In the first composite region, the molar ratio of manganese to phosphorus increases from (0.2-0.4): (0.95-1.05) to 1: (0.95-1.05) in the aforementioned gradient.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that: In the core region, intermediate region, and surface region, the molar ratio of iron, manganese, and phosphorus satisfies the following condition: total molar content of iron and manganese : molar content of phosphorus = 1 : (0.95-1.05); where, In the second composite region, the molar ratio of manganese to phosphorus decreases from 1:(0.95-1.05) to (0.2-0.4):(0.95-1.05) in the aforementioned gradient.

5. The lithium manganese iron phosphate cathode material according to any one of claims 1-4, characterized in that: In the core region, intermediate region, and surface region, the molar ratios of the four elements iron, manganese, phosphorus, and lithium satisfy the following: total molar content of iron and manganese : molar content of phosphorus : molar content of lithium = 1 : (0.95-1.05) : (0.95-1.10); and / or The lithium manganese iron phosphate particles also contain doping elements.

6. The lithium manganese iron phosphate cathode material according to claim 5, characterized in that: From the core region to the intermediate region, the content of the dopant element in the first recombination region decreases in a gradient; and / or From the outer surface of the intermediate region to the surface region, the content of the dopant element in the second composite region increases in the gradient; and / or In the lithium manganese iron phosphate particles, the molar ratio of iron, manganese, phosphorus, and dopant elements satisfies: total molar content of iron + manganese : molar content of dopant elements : molar content of phosphorus = 1 : (0.01-0.02) : (0.95-1.05); and / or The doping element includes at least one of Mg, Ti, Cr, Co, Ni, Ca, Mn, S, and B; and / or The molar ratio of the dopant element to the iron element contained in the lithium manganese iron phosphate particles is (0.0125-0.033):

1.

7. The lithium manganese iron phosphate cathode material according to claim 5, characterized in that: The thickness of the functional coating layer is 5-20 nm; and / or The elemental iron and / or iron compounds in the functional coating layer account for 1-10% of the total weight of the lithium iron manganese phosphate cathode material; and / or In the first composite region, the molar ratio of the dopant element to phosphorus element decreases from (0.01-0.02):(0.95-1.05) to 0 in the aforementioned gradient; and / or In the second composite region, the molar ratio of the dopant element to phosphorus element increases from 0 to (0.01-0.02):(0.95-1.05) in the gradient.

8. The lithium manganese iron phosphate cathode material according to any one of claims 1-4, 6, and 7, characterized in that: The D50 particle size of the lithium manganese iron phosphate particles is 200-500 nm; and / or The D50 particle size of the lithium manganese iron phosphate cathode material is 400-800 nm.

9. The method for preparing lithium manganese iron phosphate cathode material according to any one of claims 1-8, characterized in that, Includes the following steps: The lithium source, phosphorus source, first iron source, and manganese source are mixed in the proportions required for preparing lithium manganese iron phosphate to obtain a precursor. In a protective atmosphere, the precursor is subjected to a first sintering treatment to obtain lithium manganese iron phosphate particles, wherein the iron content of the lithium manganese iron phosphate particles first decreases in a gradient from the interior to the surface, and then increases in a gradient.

10. The preparation method according to claim 9, characterized in that: The precursor is prepared by a method comprising the following steps: A precursor nucleus was prepared by taking a portion of the source, including a lithium source, a phosphorus source, a first iron source, and a manganese source; Multiple coating layers are sequentially formed on the surface of the precursor core to form the precursor; wherein, during the formation of each coating layer, the mixing ratio of the first iron source and the manganese source is controlled to achieve a distribution in the precursor in which the iron element first decreases and then increases in a gradient from the precursor core to the precursor surface; or the iron element first decreases and then increases in a gradient distribution while the manganese element first increases in a gradient distribution and then decreases in a gradient distribution. and / or The lithium source, phosphorus source, first iron source, and manganese source are subjected to the first mixing treatment according to a molar ratio of lithium, phosphorus, and iron and manganese provided by the first iron source of (0.95-1.10):(0.95-1.05):(0-0.8):(0.2-1); and / or The temperature of the first sintering treatment is 400-500℃; and / or In the first mixing process, a dopant compound is also added, wherein the dopant compound is added at a molar ratio of iron source to dopant compound of 1:(0.001-0.02); and / or Following the first sintering process, the following steps are also included: The lithium manganese iron phosphate particles are subjected to a second mixing treatment with a second iron source or the second iron source and a reducing agent to obtain a mixture. The mixture is subjected to a second sintering treatment to form a functional coating layer containing elemental iron and / or iron compounds on the surface of the lithium manganese iron phosphate particles.

11. The preparation method according to claim 10, characterized in that: The reducing agent includes at least one of carbon source, H2, and S; The temperature for the second sintering treatment is 500-800℃; The lithium manganese iron phosphate particles, the reducing agent, and the second iron source are mixed in the second mixing process at a mass ratio of (3-10):(0.03-0.3):(0.1-0.5).

12. A positive electrode, characterized in that: The device includes a current collector and a positive electrode active layer bonded to the surface of the current collector. The positive electrode active layer includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material is the lithium manganese iron phosphate positive electrode material according to any one of claims 1-8 or the lithium manganese iron phosphate positive electrode material prepared by the preparation method according to any one of claims 9-11.

13. A secondary battery, comprising a positive electrode, characterized in that: The positive electrode is the positive electrode according to claim 12.

Images