Manganese iron lithium phosphate positive electrode material, its manufacturing method, and lithium ion battery

Abstract

The lithium manganese iron phosphate positive electrode material can solve the technical problems of conventional lithium manganese iron phosphate, such as low kinetic diffusion capacity and low high-rate charge / discharge capacity. The present invention relates to the technical field of lithium manganese iron phosphate, and more particularly to a lithium manganese iron phosphate positive electrode material, a method for manufacturing the same, and a lithium ion battery. The crystallite size Dx of the (020) characteristic peak of the positive electrode material measured by XRD and the single particle size Ds of the positive electrode material measured by SEM electron microscope satisfy the relationship 2.0≦Ds / Dx≦4.0.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to a patent application filed in China on December 19, 2023 (application number 202311864311.8), the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to the technical field of lithium ion batteries, and in particular to a lithium manganese iron phosphate positive electrode material and a method for producing the same, and a lithium ion battery. [Background technology]

[0003] As the number of electric vehicles increases and the energy storage market expands, people are gradually focusing on safety and cost in the field of lithium-ion batteries. In addition to improving battery design technology, research and development of battery materials is becoming increasingly important. Lithium iron phosphate (LFP) materials are gradually being used as the main cathode material for electric vehicles and storage batteries due to their high safety and low cost. However, LFP materials have very limited room for development in terms of their energy density. Lithium manganese iron phosphate (LMFP) materials have a similar crystalline structure to LFP materials, and offer advantages such as stable chemical properties and excellent safety performance. Compared to the 3.4 V charge / discharge voltage level of the Fe element in LFP, the Mn element in LMFP materials has a higher charge / discharge voltage level (4.1 V), which could increase the theoretical energy density of LMFP materials by 15–20% compared to LFP materials. Therefore, LMFP is expected to become a next-generation high-energy-density, highly safe, and low-cost lithium-ion battery cathode material of choice.

[0004] However, when Mn is introduced into the lithium manganese iron phosphate system, the electron conduction ability of the material is reduced, and the conductivity of lithium iron phosphate is reduced to 10 -9 S / cm, whereas the conductivity of lithium manganese iron phosphate is only 10 -13S / cm. Structurally, the lithium manganese iron phosphate system does not have a continuous network structure where the edges of FeO6 (MnO6) octahedra are connected, but rather has a structure where PO4 tetrahedra are connected, which makes it impossible to form a continuous metal-oxygen bond structure and restricts the movement of lithium within a one-dimensional path, resulting in very poor conductivity of the material and therefore very poor high-power charge / discharge characteristics.

[0005] Coating the surface of lithium manganese iron phosphate materials with highly conductive carbon materials is a common method for enhancing the material's electronic conductivity. In CN106887586A, a conductive network is formed using a carbon agglomerate gel, which is then filled with a lithium manganese iron phosphate precursor solution. High-temperature sintering is then performed to simultaneously achieve high conductivity and uniform particle size. This significantly reduces the material's powder resistivity and significantly improves its charge / discharge performance. However, using a carbon agglomerate gel to form the network is complex and expensive. In CN116314762A, amino-containing carbon quantum dots are introduced into the coating layer, and the mass ratio of manganese ions to amino-containing carbon quantum dots is controlled to improve conductivity and inhibit Mn ion leaching, thereby simultaneously improving the material's conductivity and lifespan.

[0006] Another idea is to design the shape of lithium manganese iron phosphate materials, reduce the size of primary particles, and shorten the ion diffusion path, thereby improving charge / discharge capacity. CN115636402A uses a solvothermal method to control directional crystal growth during high-temperature nucleation of lithium manganese iron phosphate through a thiol-vinyl click chemistry reaction, thereby forming a two-dimensional structure and increasing reaction kinetics. However, this method requires the use of a hydrophobic ligand solvent, making it difficult to achieve large-scale production. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention proposes a lithium manganese iron phosphate cathode material and a manufacturing method thereof, and a lithium ion battery to overcome the shortcomings of the current technology, thereby solving the technical problems of the conventional lithium manganese iron phosphate, such as low dynamic diffusion capacity and low high-rate charge / discharge capacity. [Means for solving the problem]

[0008] To achieve the above object, a first aspect of the present invention provides a lithium manganese iron phosphate cathode material, in which the crystallite size Dx of the cathode material at the (020) characteristic peak measured by XRD and the single particle size Ds of the cathode material measured by SEM satisfy the relationship 2.0≦Ds / Dx≦4.0.

[0009] In a second aspect of the present invention, a method for producing a lithium iron manganese phosphate cathode material is proposed, the method comprising the steps of: (1) dispersing a manganese iron phosphate, a lithium source, a first carbon source, and an additive M′ in a solvent, performing a first polishing, drying, and then performing a first sintering in a nitrogen atmosphere to obtain a manganese iron lithium phosphate material; (2) dispersing the first lithium manganese iron phosphate material and the second carbon source in a solvent, polishing the first lithium manganese iron phosphate material for a second time, drying the first lithium manganese iron phosphate material for a second time, and then crushing and sieving the resulting material to obtain a lithium manganese iron phosphate positive electrode material.

[0010] The size of the granules polished by the second polishing is 70-160 nm. In a third aspect, the present invention provides a lithium-ion battery, which comprises the lithium iron manganese phosphate cathode material described above. [Effects of the Invention]

[0011] Through the above technical means, the lithium manganese iron phosphate positive electrode material and its manufacturing method, and the lithium ion battery proposed in the present invention have the following beneficial effects:

[0012] In the present invention, the specific relationship between the crystallite size Dx and the single particle size Ds in the (020) characteristic peak of the lithium manganese iron phosphate positive electrode material indicates that the number of crystallite grain boundaries in the body phase of the positive electrode material is within an appropriate range, thereby preventing lithium ions from rapidly intercalating and deintercalating within the large single crystal granules in the positive electrode material due to diffusion through the crystallite grain boundaries and avoiding the risk of excessive defects within the single crystal granules resulting in insufficient crystallinity and granule strength, thereby allowing the lithium manganese iron phosphate positive electrode material to have both a high compaction density and high multiplication capacity.

[0013] Furthermore, the lithium manganese iron phosphate positive electrode material proposed in the present invention has a substrate, and the surface and / or interior of the substrate contain carbon layers, and the carbon layers within the substrate of the positive electrode material are mainly distributed within the crystallite grain boundaries. The present invention not only controls the number of crystallite boundaries, but also restricts the content of the carbon layers within the crystallite grain boundaries within a specific range, and uses the carbon layers to adjust the crystal size and crystallinity of the solid phase, improve lithium diffusion in the solid phase, and ensure rapid insertion and extraction of lithium ions.

[0014] Furthermore, in the present invention, the appropriate amount of carbon layers between the microcrystalline grain boundaries of the positive electrode material eliminates the adverse effects caused by excessively large single-crystal grains, and the high-pressure density is achieved by appropriately increasing the single grain size, thereby resolving the contradiction between the capacity, especially the magnification characteristics, of the lithium manganese iron phosphate material and the improvement of the grain size and compaction density.

[0015] In the present invention, the manufacturing method of the lithium manganese iron phosphate positive electrode material includes a crystallization-crushing-recrystallization process, and controls the granule size after crushing, so that the crystallite size Dx and the single particle size Ds of the (020) characteristic peak of the positive electrode material satisfy a specific relationship, thereby improving the capacity and magnification performance of the positive electrode material.

[0016] Furthermore, the manufacturing method proposed in the present invention can adjust the relationship between the crystallite size Dx and the single particle size Ds, and the content of the carbon layer inside the positive electrode material substrate, by controlling the second sintering temperature, thereby further improving the capacity and sintering rate performance of the positive electrode material.

[0017] The manufacturing method proposed in the present invention is highly compatible with existing production lines and processes, does not require the introduction of expensive additives, processes, or equipment, and retains the low cost advantages of lithium manganese iron phosphate, making it suitable for large-scale industrial production. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is an XRD pattern of a lithium iron manganese phosphate material in Example 1. [Figure 2] 1 is an SEM image of a positive electrode material in Example 1. [Figure 3] 1 is an XRD pattern of a positive electrode material in Example 1. [Figure 4] FIG. 1 is a comparison diagram of 0.2 C charge / discharge curves in Example 1 and Comparative Example 1. [Figure 5] FIG. 1 is a comparison diagram of 1C charge / discharge curves in Example 1 and Comparative Example 1. [Figure 6] 1 shows the state of the positive electrode material after sintering it in an air atmosphere at 400° C. for 3 hours in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0019] The endpoints of any ranges described herein and any value should not be understood to be limiting to the range or value, but rather to include ranges or values ​​close to the range or value. In the case of numerical ranges, the endpoints of each range may be combined, or each range endpoint may be combined with an individual point value, or each individual point value may be combined to obtain one or more new numerical ranges, and these numerical ranges are considered to be disclosed herein. In a first aspect of the present invention, a lithium manganese iron phosphate positive electrode material is proposed, in which the crystallite size Dx of the positive electrode material at the (020) characteristic peak measured by XRD and the single particle size Ds of the positive electrode material measured by SEM satisfy the relationship 2.0≦Ds / Dx≦4.0.

[0020] In the present invention, the specific relationship between the crystallite size Dx and the single particle size Ds in the (020) characteristic peak of the lithium manganese iron phosphate positive electrode material indicates that the number of crystallite grain boundaries in the body phase of the positive electrode material is within an appropriate range, thereby preventing lithium ions from rapidly intercalating and deintercalating within the large single crystal granules in the positive electrode material due to diffusion through the crystallite grain boundaries and avoiding the risk of excessive defects within the single crystal granules resulting in insufficient crystallinity and granule strength, thereby allowing the lithium manganese iron phosphate positive electrode material to have both a high compaction density and high multiplication capacity.

[0021] In the present invention, through research, we have found that the microcrystalline grain boundaries present in the bulk phase of the positive electrode material can significantly improve the electron transmission capacity and ion transmission capacity of the bulk phase of the positive electrode material, providing support for the lithium manganese iron phosphate product to achieve high compaction and high conductivity, thereby meeting the usage requirements of high energy density and fast charging, and avoiding the physicochemical properties of high specific surface area and low compaction density caused by the product design of small particle size single crystal granules, as well as the associated problems of difficult processing, high interfacial side reactions, low electrode load, etc. In the present invention, the microcrystalline grain boundary refers to a region within a single crystal granule where the direction of the atoms changes or the atoms are not connected due to differences in the crystal growth direction between different regions within the single crystal granule of the lithium manganese iron phosphate positive electrode material, resulting in a transition from one atomic arrangement to another, or the atoms are covered with part of carbon during the fusion process of the crystal granules, causing the atomic arrangement to be disconnected.

[0022] In the present invention, the ratio Ds / Dx between the crystallite size Dx at the (020) characteristic peak of the positive electrode material and the single particle size Ds of the positive electrode material measured by SEM electron microscope can indicate the number of crystallite grain boundaries in the positive electrode material.

[0023] In lithium manganese iron phosphate materials, lithium ions diffuse along a one-dimensional path along the b-axis (perpendicular to the (020) crystal plane). The shorter the diffusion distance, the smaller the single particle size in that direction, which is advantageous for achieving high fold efficiency. However, to ensure the compaction density of the lithium manganese iron phosphate positive electrode material, the primary particle size must be large. Research has shown that by limiting the number of microcrystalline grain boundaries in the lithium manganese iron phosphate positive electrode material to a specific range, the risk of lithium ions in the large primary particles in the positive electrode material rapidly becoming trapped and desorbed due to the diffusion of microcrystalline grain boundaries can be avoided, and excessive defects within the primary particles can lead to insufficient crystallinity and granular strength. This allows the lithium manganese iron phosphate positive electrode material to achieve both high compaction density and high fold efficiency.

[0024] In the present invention, the crystallite size Dx is calculated based on the half-peak width of (020) in the XRD pattern of the positive electrode material and the Scherrer equation.

[0025] In the present invention, for a monocrystalline lithium manganese iron phosphate positive electrode material, the single particle size refers to the size of a single crystal granule, and for a polycrystalline lithium manganese iron phosphate positive electrode material, the single particle size refers to the size of a primary particle forming a secondary particle in the polycrystalline lithium manganese iron phosphate positive electrode material.

[0026] In the present invention, the half-peak width of the (020) characteristic peak of the lithium iron manganese phosphate positive electrode material is 0.10-0.25°.

[0027] In the present invention, Ds is a statistical result obtained by randomly selecting approximately 100 single crystal granules from an SEM image as samples, measuring the longest and shortest diagonals of each single crystal granule, and then calculating the average value to determine the granule size, with the shortest diagonal perpendicular to the longest diagonal. In the SEM test, primary particles must be randomly sampled once and an area must be randomly selected, and the resulting SEM image represents the average level of the positive electrode material.

[0028] In the present invention, the relationship between the crystallite size Dx of the (020) characteristic peak of the positive electrode material measured by XRD and the single particle size Ds of the positive electrode material measured by SEM electron microscope satisfies 2.0≦Ds / Dx≦4.0. For example, Ds / Dx is 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or any value within a range between any two values, and preferably 2.0≦Ds / Dx≦3.5.

[0029] According to the present invention, the crystallite size Dx is 30-70 nm. In the present invention, when the crystallite size Dx of the lithium manganese iron phosphate positive electrode material satisfies the above range, the diffusion distance of lithium ions along that direction is short, which is favorable for rapid ion insertion and desorption, and thus has a high charge-discharge capacity. By controlling the crystallite size so that it is not too small, it is possible to maintain the high crystallinity of the material and avoid the ion diffusion path being too curved or even blocked due to excessively disordered atomic arrangement and excessive defects, which would result in poor electrochemical activity of the material.

[0030] In the present invention, the crystallite size Dx is 30-70 nm. For example, Dx is 30 nm, 32 nm, 34 nm, 36 nm, 38 nm, 40 nm, 42 nm, 44 nm, 46 nm, 48 nm, 50 nm, 52 nm, 54 nm, 56 nm, 58 nm, 60 nm, 62 nm, 64 nm, 66 nm, 68 nm, 70 nm, and any value within the range between any two values, preferably 40-60 nm.

[0031] According to the present invention, the single particle size Ds of the lithium iron manganese phosphate positive electrode material is 80-200 nm.

[0032] In the present invention, when the single-particle size Ds of the lithium iron manganese phosphate cathode material satisfies the above range, the distance for lithium ions to diffuse from the bulk phase of a single particle through the interior of the microcrystals and the grain boundaries of the microcrystals to the surface can be shortened, which helps to achieve high-rate charge and discharge. By controlling so that Ds is not too small, the number and volume of pores generated by the deposition between single crystal grains can be reduced, enabling the realization of the high-pressure density characteristics of the material.

[0033] In the present invention, the single-particle size Ds is 80 to 200 nm. For example, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, and any value within the range between any two values. Preferably, the single-particle size Ds of the lithium iron manganese phosphate cathode material is 40 - 60 nm.

[0034] Based on the present invention, the lithium iron manganese phosphate cathode material includes a substrate and a carbon layer present on the surface and / or inside of the substrate.

[0035] In the present invention, according to research, when a carbon layer exists inside the substrate, especially at the grain boundaries of the microcrystals, the electrical conductivity of the bulk phase of the cathode material can be significantly improved. Under the action of carbon enriched at the grain boundaries of the microcrystals, the fusion growth of the crystals can be suppressed during high-temperature heat treatment, and the bulk phase defects can be further repaired by high-temperature treatment to form good crystalline microcrystals, ensuring the rapid insertion and extraction of lithium ions, thereby obtaining a lithium iron manganese phosphate cathode material with high-rate charge and discharge capabilities. Based on the present invention, the substrate has the composition shown in Formula I. Li 1+a Mn x Fe y M’ z PO4 (Formula I) In Formula I, 0 ≦ a ≦ 0.2, 0.3 ≦ x < 1, 0 < y ≦ 0.7, 0 < z ≦ 0.05, and 0.8 ≦ x + y + z ≦ 1, and M’ is selected from at least one element of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

[0036] In the present invention, for formula I, 0 ≤ a ≤ 0.2, for example, 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, and any value within the range between any two values, 0.3 ≤ x < 1, for example, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and any value within the range between any two values, 0 < y ≤ 0.7, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and any value within the range between any two values, 0 < z ≤ 0.05, for example, 0.01, 0.02, 0.03, 0.04, 0.05, and any value within the range between any two values, 0.8 ≤ x + y + z ≤ 1, for example, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, and any value within the range between any two values.

[0037] Furthermore, for formula I, 0.01 ≤ a ≤ 0.1, 0.4 ≤ x ≤ 0.9, 0.1 ≤ y ≤ 0.6, 0 < z ≤ 0.03, and 0.9 ≤ x + y + z ≤ 0.99, and M' is selected from at least one element of Al, Ti, V, Co, Nb, and W.

[0038] Based on the present invention, based on the total weight of the lithium manganese iron phosphate cathode material, the content of the carbon layer is 1 to 2.5 wt%.

[0039] In the present invention, when the carbon layer content satisfies the above-mentioned range, the granule surface of the material is sufficiently coated, a complete conductive carbon network is formed, and electrons can be quickly and uniformly transported into the powder material, resulting in high power generation characteristics. However, because the density of carbon is lower than that of lithium manganese iron phosphate, an excessive carbon layer content is detrimental to the compaction degree of the material. In addition, carbon is not electrochemically active within the voltage range of lithium intercalation and deintercalation of lithium manganese iron phosphate, and therefore does not contribute to capacity. Therefore, excessive carbon reduces the capacity per mass of the lithium manganese iron phosphate / carbon composite material. Therefore, by limiting the carbon layer content within the above-mentioned range, the lithium manganese iron phosphate positive electrode material can combine the utilization of electrochemical performance with the compaction degree of the material powder.

[0040] In the present invention, the content of the carbon layer is 1 to 2.5 wt%, for example, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, and any value within a range between any two values. Furthermore, the content of the carbon layer is 1.4 to 2.1 wt % based on the total weight of the lithium iron manganese phosphate positive electrode material.

[0041] According to the present invention, the content of the carbon layer present inside the substrate is 0.05 to 0.20 wt % based on the total weight of the lithium iron manganese phosphate positive electrode material.

[0042] In the present invention, the carbon layer present inside the substrate is mainly distributed within the microcrystalline grain boundaries of the substrate, and the content of the carbon layer present inside the substrate specifically refers to the content of the carbon layer measured using a carbon-fluoride analyzer after placing the lithium manganese iron phosphate positive electrode material in a muffle furnace and sintering it in an air atmosphere at 400°C for 3 hours.

[0043] In the present invention, when the content of the carbon layer present inside the substrate satisfies the above range, the capacity and the multiplier performance of the positive electrode material can be significantly improved.

[0044] In one preferred embodiment of the present invention, the content of the specific carbon layers present within the substrate is balanced with the number of crystallite boundaries (Ds / Dx), thereby controlling not only the number of crystallite boundaries but also the content of the carbon layers within the crystallite boundaries. The appropriate content of the carbon layers present within the substrate can significantly improve the electrical conductivity of the body phase of the positive electrode material, inhibit the fusion growth of crystallites during high-temperature heat treatment, repair body-phase defects, and form well-defined crystallites. This avoids the risk of excessive defects within the single-crystal granules resulting in insufficient crystallinity and granule strength. This ensures rapid intercalation and deintercalation of lithium ions through the diffusion of the crystallite boundaries, appropriately increasing the size of the single particles and achieving a high degree of compaction. This resolves the contradiction between the capacity, especially the multiplication characteristics, of the lithium manganese iron phosphate material and the improvement of the granule size and compaction.

[0045] In the present invention, the content of the carbon layer present inside the substrate is 0.05 to 0.20 wt%, for example, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.1 wt%, 0.11 wt%, 0.12 wt%, 0.13 wt%, 0.14 wt%, 0.15 wt%, 0.16 wt%, 0.17 wt%, 0.18 wt%, 0.19 wt%, 0.20 wt%, and any value within a range between any two values.

[0046] Furthermore, the content of the carbon layer present inside the substrate is 0.08 to 0.18 wt %, preferably 0.08 to 0.15 wt %, based on the total weight of the lithium manganese iron phosphate positive electrode material.

[0047] According to the present invention, the powder compaction density of the positive electrode material is 2.1-2.6 g / cm 3 For example, 2.1 g / cm 3 , 2.2g / cm 3 , 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm3 , 2.6g / cm 3 , and any value in the range between any two values, preferably 2.1 to 2.5 g / cm 3 is.

[0048] In a second aspect of the present invention, a method for producing a lithium iron manganese phosphate cathode material is proposed, the method comprising the steps of: (1) dispersing a manganese iron phosphate, a lithium source, a first carbon source, and an additive M′ in a solvent, performing a first polishing, drying, and then performing a first sintering in a nitrogen atmosphere to obtain a manganese iron lithium phosphate material; (2) dispersing the first lithium manganese iron phosphate material and the second carbon source in a solvent, polishing the first lithium manganese iron phosphate material for a second time, drying the first lithium manganese iron phosphate material for a second time, and then crushing and sieving the resulting material to obtain a lithium manganese iron phosphate positive electrode material.

[0049] The size of the granules polished by the second polishing is 70-160 nm. In the present invention, the manufacturing method of the lithium manganese iron phosphate positive electrode material includes a crystallization-crushing-recrystallization process, and controls the granule size after crushing, so that the crystallite size Dx and the single particle size Ds of the (020) characteristic peak of the positive electrode material satisfy a specific relationship, thereby improving the capacity and magnification performance of the positive electrode material.

[0050] Specifically, in the present invention, a first sintering process is performed to obtain a first lithium manganese iron phosphate material, and then a second sintering process is performed on a mixture containing the first lithium manganese iron phosphate material and a second carbon source, and the second sintering process is performed to obtain the first lithium manganese iron phosphate material according to the first embodiment of the present invention. Through the processes of crushing and recrystallizing the crystals and controlling the granule size after the second polishing, new grain boundaries are exposed and integrated to various degrees, and fine grain boundaries of the positive electrode material are controllably formed.

[0051] Furthermore, during this process, the carbon sources (the first and second carbon sources, especially the second carbon source) adhere to the new crystal boundary surfaces formed by the crushing and undergo thermal decomposition. As the crystals fuse, some of the carbon remains at the crystal grain boundaries to form a carbon layer, improving the conductivity of the volume phase of the material. Under the action of the carbon layer enriched at the crystal grain boundaries, the fusion growth of the crystals can be inhibited during high-temperature heat treatment, and the defects in the volume phase can be repaired by high-temperature treatment, forming well-defined crystallites and ensuring the rapid insertion and desorption of lithium ions, thereby obtaining a lithium manganese iron phosphate cathode material with high charge / discharge capacity.

[0052] In the present invention, the size of the granules polished by the second polishing is the same as the granule size of the mixture of the monolithium manganese iron phosphate material and the second carbon source. In the present invention, there is no particular limitation on the drying method in steps (1) and (2), and any drying method commonly used in the art, such as spray drying, may be used.

[0053] In the present invention, the size of the granules polished by the second polishing is 70 to 160 nm, for example, 70 nm, 72 nm, 74 nm, 76 nm, 78 nm, 80 nm, 82 nm, 84 nm, 86 nm, 88 nm, 90 nm, 92 nm, 94 nm, 96 nm, 98 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 150 nm, 155 nm, 160 nm, and any value within a range between any two values.

[0054] In the present invention, no special restrictions are imposed on the first polishing and the second polishing, and polishing equipment generally used in this technical field may be used, as long as the granule size after polishing meets the requirements of the present invention.

[0055] According to the present invention, the temperature of the second sintering is 600-800°C, for example, 600°C, 650°C, 700°C, 750°C, 800°C, and any value within the range between any two values.

[0056] In the present invention, by controlling the second sintering temperature, it is possible to adjust the relationship between the crystallite size Dx and the single particle size Ds, and the content of the carbon layer inside the positive electrode material substrate, thereby further improving the capacity and sintering rate performance of the positive electrode material.

[0057] In the present invention, the time for the second sintering is 5 to 20 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, and any value within a range between any two values, and preferably 8 to 15 hours.

[0058] In the present invention, the temperature of the first sintering is 400 to 600°C, for example, 400°C, 450°C, 500°C, 550°C, 600°C, and any value within a range between any two values.

[0059] In the present invention, the time for the first sintering is 1 to 10 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any value within a range between any two values, and preferably 2 to 6 hours.

[0060] In the present invention, the first carbon source and the second carbon source are each independently selected from at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol, and dopamine.

[0061] In the present invention, the molecular weight of the first carbon source and the second carbon source is independently 100-10,000 g / mol, for example, 100 g / mol, 140 g / mol, 180 g / mol, 220 g / mol, 260 g / mol, 300 g / mol, 400 g / mol, 500 g / mol, 600 g / mol, 700 g / mol, 800 g / mol, 900 g / mol, 1000 g / mol, 1100 g / mol, 1200 g / mol, 1300 g / mol, 1400 g / mol, mol, 1500 g / mol, 2000 g / mol, 2500 g / mol, 3000 g / mol, 3500 g / mol, 4000 g / mol, 4500 g / mol, 5000 g / mol, 5500 g / mol, 6000 g / mol, 6500 g / mol, 7000 g / mol, 7500 g / mol, 8000 g / mol, 8500 g / mol, 9000 g / mol, 9500 g / mol, 10000 g / mol, and any value in the range between any two values.

[0062] In the present invention, for small molecule compounds such as glucose, the molecular weight refers to the relative molecular mass, and for high molecular weight polymers such as polyacrylic acid, the molecular weight refers to the mass average molecular weight.

[0063] In the present invention, when the above-mentioned specific type of carbon source or a carbon source having a specific molecular weight is used, the surface of the lithium manganese iron phosphate positive electrode material can be uniformly coated and carbonized with high efficiency, thereby reducing the resistance of the material and taking into consideration the cost of various carbon sources, making it suitable for large-scale industrial production.

[0064] Furthermore, the first carbon source and the second carbon source are each independently selected from at least one of glucose, sucrose, cellulose, citric acid, and polyethylene glycol.

[0065] According to the present invention, the amounts of the iron manganese phosphate, the first carbon source and the second carbon source are adjusted so that the content of the carbon layer is 1 to 2.5 wt % based on the total weight of the lithium iron manganese phosphate positive electrode material.

[0066] In the present invention, the content of the carbon layer is 1 to 2.5 wt%, for example, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, and any value in the range between any two values.

[0067] Furthermore, the amounts of the manganese iron phosphate, the first carbon source, and the second carbon source are adjusted so that the content of the carbon layer is 1.4 to 2.1 wt % based on the total weight of the lithium manganese iron phosphate positive electrode material.

[0068] In the present invention, the mass ratio of the first carbon source to the second carbon source is 1:1.5-5, for example, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:3, 1:3.1, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9, 1:4, 1:4.1, 1:4.2, 1:4.3, 1:4.4, 1:4.5, 1:4.6, 1:4.7, 1:4.8, 1:4.9, 1:5, and any value within a range between any two values.

[0069] In the present invention, the dosage ratio of the first carbon source to the second carbon source is controlled within the above range to ensure that the necessary reducing atmosphere is generated for the monolithium manganese iron phosphate material during the first sintering process, and to prevent excess pyrolysis carbon from being left behind during the first sintering process, and to form free carbon through peeling during the second polishing process. The second carbon source is the main source of the coating carbon layer, which can sufficiently coat the crystallized monolithium manganese iron phosphate material and reduce free carbon, thereby effectively reducing the electrical resistance of the material and suppressing side reactions of excessive electrolyte, ensuring the utilization of electrical performance.

[0070] In the present invention, the additive M' contains an element selected from at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

[0071] In the present invention, no special limitation is imposed on the specific type of the additive M'. It may include at least one of an oxide containing M', a hydroxide containing M', a hydroxy-oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'.

[0072] In the present invention, no special restriction is imposed on the type of the lithium source, and a lithium source commonly used in the technical field may be utilized. For example, it may be selected from at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate.

[0073] In the present invention, no special restriction is imposed on the type of manganese iron phosphate, and a manganese iron phosphate material commonly used in the technical field may be used.

[0074] In the present invention, no special restriction is imposed on the type of the solvent, and it is only necessary that each component can be sufficiently mixed and uniformly dispersed. For example, it may be pure water.

[0075] In the present invention, the dosages of the manganese iron phosphate, the lithium source, and the additive M' satisfy n(Li):n(Mn):n(Fe):n(M') = 1 + a:x:y:z. In the formula, 0 ≦ a ≦ 0.2, 0.3 ≦ x < 1, 0 < y ≦ 0.7, 0 < z ≦ 0.05, and 0.8 ≦ x + y + z ≦ 1.

[0076] Furthermore, 0.01 ≦ a ≦ 0.1, 0.4 ≦ x ≦ 0.9, 0.1 ≦ y ≦ 0.6, 0 < z ≦ 0.03, and 0.9 ≦ x + y + z ≦ 0.99.

[0077] In a third aspect, the present invention provides a lithium-ion battery, which comprises the lithium iron manganese phosphate cathode material described above.

[0078] According to the present invention, the lithium ion battery has a ratio of discharge capacity at 1C to discharge capacity at 0.2C of ≧89%.

[0079] The present invention will now be described in detail with reference to examples. XRD is used to measure the crystalline properties of the lithium manganese iron phosphate positive electrode material, and the specific test method is as follows: In the XRD pattern of the lithium manganese iron phosphate positive electrode material, the peak position and half-peak width of the (020) peak within the 2θ=16-19° range are read, and according to the Scherrer equation D=Kλ / (β·cosθ), K is set to 0.89, λ is set to 0.154, and β is set to the (020) half-peak width, to calculate the crystallite size Dx of the lithium manganese iron phosphate positive electrode material.

[0080] The morphology and single particle size of the lithium manganese iron phosphate cathode material were measured using an SEM. The specific test method was as follows: a Hitachi S-4800 scanning electron microscope was used to take a 30Kx magnification image of the powder material, 100 single crystal particles were randomly selected from the image, and the longest and shortest diagonals of each single crystal particle were measured and the average value was calculated to obtain the single particle size Ds.

[0081] The composition of the lithium manganese iron phosphate positive electrode material is measured using an atomic emission spectrometer (ICP). The specific test method is as follows: 0.2 g of lithium manganese iron phosphate sample is accurately measured, and acid is added to carry out thermal decomposition to a constant volume. The contents of Li, Mn, Fe, P, and additive elements in the material are measured using the standard curve method.

[0082] The carbon content in the lithium manganese iron phosphate positive electrode material is measured using a carbon dioxide analyzer, and the specific measurement method is as follows: 0.2 g of lithium manganese phosphate material is accurately measured and mixed with 2 g of combustion improver, then placed in a high-temperature crucible, and transferred to a carbon dioxide analyzer to sinter by passing oxygen through it, and the amount of carbon dioxide gas generated during sintering is measured to obtain the carbon content in the material.

[0083] The carbon content of the lithium manganese iron phosphate positive electrode material substrate is measured using a carbon dioxide analyzer, and the specific measurement method is as follows: the lithium manganese iron phosphate positive electrode material is placed in a muffle furnace and sintered in an air atmosphere at 400°C for 3 hours, and after observing that the appearance of the material changes from black or lead gray to orange, the carbon dioxide analyzer is used to measure the carbon content.

[0084] The powder compaction density of the lithium iron manganese phosphate positive electrode material is measured under a pressure of 3 tons. All of the raw materials used in the examples and comparative examples are commercially available products.

[0085] Example 1 (1) Manganese iron phosphate, lithium carbonate, TiO2 additive and primary carbon source are dispersed in pure water, and the dosage of manganese iron phosphate, lithium carbonate and TiO2 additive meets the condition of n(Li):n(Mn):n(Fe):n(Ti):n(P)=1.02:0.57:0.38:0.02:1. After that, the material is first polished, spray dried and transferred to a nitrogen atmosphere furnace, and first sintered at 500°C for 4 hours to obtain primary lithium manganese iron phosphate material. (2) The first lithium manganese iron phosphate material and the second carbon source are dispersed in pure water and polished a second time to make the granule size 90 nm after polishing. Then, the material is spray dried and transferred to a nitrogen atmosphere furnace, and sintered a second time at 700°C for 12 hours. After crushing and sieving, lithium manganese iron phosphate positive electrode material A1 is obtained.

[0086] The first carbon source was glucose (molecular weight 180 g / mol), and the second carbon source was polyethylene glycol (mass average molecular weight 4000 g / mol). The amounts of the manganese iron phosphate, the first carbon source, and the second carbon source were adjusted so that the carbon layer content was 1.7 wt% based on the total weight of the lithium manganese iron phosphate positive electrode material. The mass ratio of the first carbon source to the second carbon source was 1:3.

[0087] 1 is an XRD pattern of the monolithium manganese iron phosphate material in Example 1. As can be seen from FIG. 1, the monolithium manganese iron phosphate material obtained after the initial sintering already formed a good crystalline material of monolithium manganese iron phosphate.

[0088] Fig. 2 is an SEM image of the positive electrode material in Example 1. According to Fig. 2, the granules of the lithium iron manganese phosphate positive electrode material A1 are spherical or near-spherical.

[0089] 3 is an XRD pattern of the lithium manganese iron phosphate positive electrode material A1 in Example 1. As shown in FIG. 3, the lithium manganese iron phosphate positive electrode material A1 has a pure phase structure of lithium manganese iron phosphate.

[0090] Examples 2-4 The cathode materials were prepared according to the method of Example 1, with the raw material proportions and specific manufacturing methods as shown in Table 1, and other conditions being the same as those of Example 1, to obtain lithium manganese iron phosphate cathode materials A2-A4, respectively.

[0091] Comparative Example 1-2 The cathode materials were prepared according to the method of Example 1, with the raw material proportions and specific manufacturing methods as shown in Table 1, and other conditions being the same as those of Example 1. Lithium manganese iron phosphate cathode materials D1-D2 were obtained respectively.

[0092] [Table 1]

[0093] Based on the dosage of a,b-manganese iron phosphate. JPEG2026502428000003.jpg123170

[0094] Based on the dosage of a,b-manganese iron phosphate. The compositions and physicochemical parameters of the positive electrode materials obtained in the examples and comparative examples were tested, and the results are shown in Table 2.

[0095] [Table 2]

[0096] JPEG2026502428000005.jpg98170

[0097] Test Example Button batteries were assembled using the manganese iron lithium phosphate positive electrode materials of the examples and comparative examples. The specific assembly method was as follows: A composite manganese iron lithium phosphate positive electrode active material for non-aqueous electrolyte secondary batteries, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90%:5%:5%, coated on aluminum foil, dried, and pressed under a pressure of 100 MPa to form a positive electrode piece with a diameter of 12 mm and a thickness of 120 μm. The positive electrode piece was then placed in a vacuum drying box and dried at 120°C for 12 hours.

[0098] A 17 mm diameter, 1 mm thick Li metal piece is used as the negative electrode. A 25 μm thick polyethylene porous membrane is used as the diaphragm. A mixture of equal parts of 1.0 mol / L LiPF6, ethylene carbonate (EC), and diethyl carbonate (DEC) is used as the electrolyte.

[0099] In an argon-filled glove box with a moisture content and oxygen content of less than 5 ppm, a positive electrode, a diaphragm, a negative electrode, and an electrolyte are assembled into a 2025 button cell battery. This battery is used as an unactivated battery.

[0100] The electrochemical characteristics of the button cell batteries were tested using the following specific test method: After fabrication, the button cell batteries were allowed to stand for two hours. After the open-circuit voltage stabilized, they were charged to a cutoff voltage of 4.35V at a current density of 0.1C on the positive electrode, then charged at a constant voltage for 30 minutes, and subsequently discharged to a cutoff voltage of 2.5V at the same current density. This procedure was repeated once, and the battery at this stage was used as the activated battery. The activated battery was then subjected to charge-discharge tests at different charge rates: 0.2C / 0.2C, 0.5C / 0.5C, and 1C / 1C. The charge-discharge capacities at each charge rate were recorded, and the ratio of the discharge capacities at 1C and 0.2C was used as an index for evaluating charging performance. The test results are shown in Table 3.

[0101] [Table 3]

[0102] According to Tables 1-3, the preparation method proposed in the present invention is applicable to different doping means (Examples 1, 2 and 11), as well as to lithium manganese iron phosphate materials with different Mn / Fe ratios (Example 10).

[0103] Compared with the first-time sintered material (Comparative Example 1), the positive electrode materials obtained by the manufacturing method proposed in the present invention (Examples 1, 3, 4, 7, 8 and 9) have a narrower (020) half-peak width, a significantly improved crystallinity, a larger crystallite size, a moderate number of crystallite grain boundaries, and a larger granule size and a higher compaction degree, which can achieve better capacity utilization and multiplier performance.

[0104] According to Example 3, when the granule size of the abrasive is approached, lowering the second sintering temperature weakens the fusion between granules, reduces the number of microcrystalline grain boundaries and internal residual carbon, and in turn reduces the granule size, which is unfavorable for the compaction degree.

[0105] According to Example 7, increasing the granule size of the abrasive increases the ratio of the second carbon source to the first carbon source, suppressing intergranular fusion growth and significantly reducing the internal carbon content, which in turn slightly increases the granule size of the positive electrode material of the product without affecting the crystallinity of the material, resulting in a slight deterioration in dynamic performance.In contrast, in Example 9, the dosage of the first carbon source and the sintering temperature are increased to improve the crystalline perfection of the material during the initial sintering process, thereby increasing the crystallite size of the product and appropriately increasing the internal residual carbon, resulting in an improvement in dynamic performance compared to Example 7, but not as good as Example 1.

[0106] According to Examples 8 and 4, reducing the granule size of the abrasive and increasing the second sintering temperature strengthens the crystallization of the granule phase, increases the interfacial fusion between the granules, and forms more microcrystalline grain boundaries and internal residual carbon, which in turn increases the granule size and is beneficial to the degree of compaction. However, as can be seen, under the premise of larger granule size, by benefiting from high crystallinity, microcrystalline grain boundaries, and internal residual carbon, the positive electrode materials of Examples 8 and 4 still exhibit ideal capacity and power characteristics.

[0107] According to Example 5, after adjusting the types and proportions of the first carbon source and the second carbon source, the amount of the second carbon source added is small, which results in insufficient uniformity of the surface coating of the lithium manganese iron phosphate material, which is likely to lead to direct fusion and growth of adjacent lithium manganese iron phosphate crystals, which results in a significantly larger single particle size of the product and is also unfavorable for the residual carbon remaining inside the crystals, resulting in a deterioration of the magnification performance.

[0108] According to Example 6, if the second sintering temperature is too low, the degree of fusion between the crystallites of the manganese iron lithium phosphate will be significantly reduced, the granule size will not be large, the compaction degree will be low, and the crystallinity will be insufficient due to the low processing temperature, so the magnification performance will not be as good as that of Example 1.

[0109] According to Comparative Example 2, when the granule size of the abrasive is reduced but the sintering temperature is low, the improvement of the bulk crystallinity and the microcrystalline grain boundaries formed by fusion between granules and internal residual carbon cannot be realized, resulting in poor magnification characteristics, small granule size, and low compaction degree.

[0110] 4 is a comparison diagram of the 0.2C charge / discharge curves of the button batteries of Example 1 and Comparative Example 1. As can be seen from FIG. 4, in Comparative Example 1, the internal defects were not repaired through recrystallization in the second sintering, the microcrystalline grain boundaries inside the granules were too large, and the crystallinity was poor. Even at a rate of 0.2C, the capacity utilization was insufficient, and the discharge level maintenance was worse than in Example 1.

[0111] 5 is a comparison diagram of the 1C charge / discharge curves of the button batteries in Example 1 and Comparative Example 1. From FIG. 5, it can be seen that the above defects in Comparative Example 1 severely limited the lithium diffusion kinetics, resulting in a significant decrease in the charge capacity share at a constant current of 1C at a large rate, almost complete loss of discharge level, and capacity utilization far below that of Example 1.

[0112] Figure 6 shows the state of the cathode material in Example 1 after sintering at 400°C in an air atmosphere for 3 hours. Figure 6 shows that the surface of the material is orange, indicating that the carbon layer on the surface has been burned off. When this material is subjected to a carbon content test, the detection result corresponds to the residual carbon inside the material.

[0113] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any embodiment that has the same configuration as the essence of the technical concept and achieves the same effect within the scope of the technical means of the present invention is included in the technical scope of the present invention. Furthermore, various modifications that can be conceived by those skilled in the art without departing from the spirit of the present invention, and other embodiments formed by combining part of the components of the embodiments, are also included in the scope of the present invention.

Claims

1. the relationship between the crystallite size Dx at the (020) characteristic peak measured by XRD and the single particle size Ds of the positive electrode material measured by SEM electron microscope satisfies 2.0≦Ds / Dx≦4.0; A lithium manganese iron phosphate positive electrode material.

2. 2. The lithium manganese iron phosphate positive electrode material according to claim 1, wherein Ds / Dx satisfies 2.0≦Ds / Dx≦3.

5.

3. 3. The lithium manganese iron phosphate positive electrode material according to claim 1, wherein the crystallite size Dx is 30-70 nm, preferably 40-60 nm.

4. 4. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that the single particle size Ds is 80-200 nm, preferably 100-160 nm.

5. 5. The lithium manganese iron phosphate positive electrode material according to claim 1, wherein the half-peak width of the (020) characteristic peak is 0.10-0.25°.

6. 6. The lithium manganese iron phosphate positive electrode material according to claim 1, comprising a substrate and a carbon layer present on the surface of the substrate and / or inside the substrate.

7. the substrate has a composition shown in Formula I; Li １＋ａ Mn ｘ Fe ｙ M' ｚ PO ４ (Formula I) 7. The lithium manganese iron phosphate positive electrode material of claim 6, wherein 0≦a≦0.2, 0.3≦x<1, 0<y≦0.7, 0<z≦0.05, and 0.8≦x+y+z≦1; and M′ is selected from at least one element of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

8. The content of the carbon layer is 1 to 2.5 wt % based on the total weight, Preferably, the content of the carbon layer present inside the substrate is 0.05 to 0.20 wt %, and more preferably 0.08 to 0.18 wt %, based on the total weight.

8. The lithium manganese iron phosphate positive electrode material according to claim 6 or 7.

9. Powder compaction density is 2.1-2.6 g / cm ３ The positive electrode material according to any one of claims 1 to 8, characterized in that

10. (1) dispersing a manganese iron phosphate, a lithium source, a first carbon source, and an additive M′ in a solvent, performing a first polishing, drying, and then performing a first sintering in a nitrogen atmosphere to obtain a manganese iron lithium phosphate material; (2) dispersing the first lithium manganese iron phosphate material and the second carbon source in a solvent, polishing the first lithium manganese iron phosphate material for a second time, drying the first lithium manganese iron phosphate material for a second time, and then crushing and sieving the first lithium manganese iron phosphate material for a second time to obtain the positive electrode material; The size of the granules polished by the second polishing is 70-160 nm. A method for producing a lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 9.

11. The manufacturing method according to claim 10, wherein the temperature of the second sintering is 600 to 800°C.

12. The manufacturing method according to claim 10 or 11, characterized in that the second sintering time is 5 to 20 hours.

13. The manufacturing method according to any one of claims 10 to 12, characterized in that the temperature of the first sintering is 400 to 600°C.

14. The manufacturing method according to any one of claims 10 to 13, characterized in that the first sintering time is 1 to 10 hours.

15. the first carbon source and the second carbon source are each independently selected from at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol, and dopamine; Preferably, the molecular weight of the first carbon source and the second carbon source is independently 100-10,000 g / mol. The method according to any one of claims 10 to 14.

16. The amount of the manganese iron phosphate, the first carbon source, and the second carbon source is such that the content of the carbon layer is 1 to 2.5 wt % based on the total weight of the lithium manganese iron phosphate positive electrode material.

16. The method according to any one of claims 10 to 15.

17. The method according to any one of claims 10 to 16, wherein the mass ratio of the first carbon source to the second carbon source is 1:1.5-5.

18. the additive M' is a compound containing at least one element selected from B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W; 18. The method according to any one of claims 10 to 17.

19. the dosages of the iron manganese phosphate, the lithium source, and the additive M' are n(Li):n(Mn):n(Fe):n(M')=1+a:x:y:z; 19. The method according to any one of claims 10 to 18.

20. A lithium ion battery comprising the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 9.

21. 21. The battery of claim 20, wherein the ratio of the discharge capacity at 1 C to the discharge capacity at 0.2 C is ≥ 89%.

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