Lithium manganese iron phosphate positive electrode material, preparation method therefor, and lithium-ion battery

Abstract

A lithium manganese iron phosphate positive electrode material, a preparation method therefor, and a lithium-ion battery. The crystallite size Dx at a characteristic peak (020) of the positive electrode material measured by means of XRD and the single particle size Ds of the positive electrode material measured by means of an SEM electron microscope satisfy: 2.0≤Ds / Dx≤4.0. The lithium manganese iron phosphate positive electrode material can solve technical problems such as poor dynamic diffusion capabilities and poor high-rate charging and discharging capabilities, which occur for existing lithium manganese iron phosphate.

Description

Lithium manganese iron phosphate positive electrode material and preparation method thereof, and lithium ion battery

[0001] Priority information

[0002] This application claims priority to and the benefits of patent application 202311864311.8 filed on December 29, 2023 with the State Intellectual Property Office of China, and incorporates the entire text of that application herein by reference. Technical Field

[0003] The present invention relates to the field of lithium-ion batteries, and in particular to a lithium iron manganese phosphate positive electrode material and a preparation method thereof, and a lithium-ion battery. Background Art

[0004] With the increasing number of electric vehicles and the gradual expansion of the energy storage market, the focus on safety and cost in the lithium-ion battery field has gradually increased. In addition to improvements in battery design technology, the research and development of battery materials is particularly critical. Lithium iron phosphate (LFP) has gradually become the primary cathode material of choice for electric vehicles and energy storage batteries due to its high safety and low cost. However, there is little room for improvement in the energy density of LFP. Lithium manganese iron phosphate (LMFP) has a similar crystal structure to LFP and also offers chemical stability and excellent safety. Compared to the 3.4V charge and discharge voltage platform of the Fe element in LFP, the Mn element in LMFP has a higher charge and discharge voltage platform (4.1V), which can increase the theoretical energy density of LMFP by 15-20% compared to LFP. Therefore, LMFP is expected to become the cathode material of choice for the next generation of high-energy-density, high-safety, and low-cost lithium-ion batteries.

[0005] However, the introduction of Mn element in the lithium manganese iron phosphate system also leads to a decrease in the electron transmission ability of the material. The conductivity of lithium iron phosphate is 10 -9 S / cm, while lithium manganese iron phosphate is only 10 -13 Structurally, lithium manganese iron phosphate does not have a continuous FeO6 (MnO6) octahedral network, but is connected by PO4 tetrahedra. Therefore, it cannot form a continuous metal-oxygen bond structure, which restricts the movement of lithium in the one-dimensional channel, resulting in poor conductivity of the material and, in turn, poor high-rate charge and discharge performance.

[0006] Coating the surface of lithium manganese iron phosphate materials with carbon materials having good electrical conductivity is a common method for improving the material's electronic conduction ability. CN106887586A uses carbon aerogel to construct a conductive network, which is then filled with a lithium manganese iron phosphate precursor material solution and then sintered at high temperature. This achieves both high conductivity and uniform particle size control, significantly reducing the resistivity of the material powder and significantly improving the charge and discharge capacity. However, the construction of the carbon aerogel network complicates the process and increases the cost. CN116314762A introduces carbon quantum dots containing amino groups into the coating layer, controls the mass ratio of manganese ion content to the carbon quantum dots containing amino groups, and suppresses the dissolution of Mn ions while improving the conductivity, achieving a simultaneous improvement in the material's conductivity and lifespan.

[0007] Another improvement strategy involves reshaping the lithium manganese iron phosphate material's morphology, reducing primary particle size and shortening ion diffusion paths to enhance charge and discharge capabilities. CN115636402A employs a solvothermal method, using a thiol-ene click chemistry reaction to control the directional crystal growth of lithium manganese iron phosphate during high-temperature nucleation, thereby forming a two-dimensional structure and improving reaction kinetics. However, this approach uses a hydrophobic ligand solvent, making large-scale production unfeasible.

[0008] Summary of the Invention

[0009] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a lithium iron manganese phosphate positive electrode material and a preparation method thereof, as well as a lithium ion battery, so as to solve the technical problems of the existing lithium iron manganese phosphate, such as poor kinetic diffusion ability and poor high-rate charge and discharge ability.

[0010] In order to achieve the above-mentioned purpose, the first aspect of the present invention provides a lithium manganese iron phosphate positive electrode material, wherein the crystallite size Dx at 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 microscopy satisfy the following relationship: 2.0≤Ds / Dx≤4.0.

[0011] A second aspect of the present invention provides a method for preparing the above-mentioned lithium manganese iron phosphate positive electrode material, characterized in that the preparation method comprises the following steps:

[0012] (1) dispersing ferromanganese phosphate, a lithium source, a first carbon source, and an additive M' in a solvent, performing a first grinding and drying process, and then performing a first sintering process in a nitrogen atmosphere to obtain a first lithium ferromanganese phosphate material;

[0013] (2) dispersing the first lithium manganese iron phosphate material and the second carbon source in a solvent, performing a second grinding and drying, and then performing a second sintering in a nitrogen atmosphere, and obtaining the lithium manganese iron phosphate positive electrode material after crushing and screening;

[0014] The second grinding is performed so that the particle size after grinding is 70-160 nm.

[0015] A third aspect of the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises the above-mentioned lithium iron manganese phosphate positive electrode material.

[0016] Through the above technical solution, the lithium manganese iron phosphate positive electrode material and its preparation method, and the lithium ion battery provided by the present invention achieve the following beneficial effects:

[0017] In the present invention, the crystallite size Dx at the (020) characteristic peak of the lithium manganese iron phosphate positive electrode material satisfies a specific relationship with the single particle size Ds, indicating that the number of micrograin boundaries in the bulk phase of the positive electrode material is within an appropriate range, so that the lithium ions inside the larger single particles in the positive electrode material can be quickly deintercalated by diffusion at the micrograin boundaries, while avoiding the risk of insufficient crystallinity and particle strength due to excessive defects inside the single particles, so that the lithium manganese iron phosphate positive electrode material can have both high compaction density and high rate performance.

[0018] Furthermore, the lithium manganese iron phosphate cathode material provided by the present invention includes a matrix having a carbon layer on its surface and / or interior, with the carbon layer within the matrix primarily distributed within the microcrystalline boundaries. This method not only controls the number of microcrystalline boundaries but also regulates the carbon layer content within the microcrystalline boundaries within a specific range. This carbon layer is used to adjust crystal size and bulk crystallinity, improving bulk lithium diffusion and ensuring rapid lithium ion deintercalation.

[0019] Furthermore, the appropriate amount of carbon layer content between the microcrystalline boundaries of the positive electrode material in the present invention can eliminate the disadvantages brought about by the excessive size of individual particles. By appropriately increasing the size of individual particles, a high compaction density is achieved, which solves the contradiction between the capacity of the lithium manganese iron phosphate material, especially the rate characteristics, and the increase in particle size and compaction density.

[0020] In the present invention, in the preparation method of the lithium manganese iron phosphate positive electrode material, through the process of crystallization-crushing-recrystallization and controlling the particle size after crushing, it is possible to achieve a specific relationship between the crystallite size Dx at the (020) characteristic peak of the positive electrode material and the single particle size Ds, thereby improving the capacity and rate performance of the positive electrode material.

[0021] Furthermore, in the preparation method provided by the present invention, by controlling the second sintering temperature, the relationship between the crystallite size Dx and the single particle size Ds and the carbon layer content inside the positive electrode material matrix can be adjusted, so that the capacity and rate performance of the positive electrode material are further improved.

[0022] The preparation method provided by the present invention is highly compatible with existing production lines, processes, etc., does not require the introduction of expensive additives, processes, equipment, etc., maintains the low-cost advantage of lithium manganese iron phosphate materials, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG1 is an XRD spectrum of the first lithium iron manganese phosphate material in Example 1;

[0024] FIG2 is a SEM image of the positive electrode material of Example 1;

[0025] FIG3 is an XRD spectrum of the positive electrode material of Example 1;

[0026] FIG4 is a comparison of 0.2C charge and discharge curves of Example 1 and Comparative Example 1;

[0027] FIG5 is a comparison of 1C charge-discharge curves of Example 1 and Comparative Example 1;

[0028] FIG6 shows the material state of the positive electrode material of Example 1 after sintering at 400° C. in air atmosphere for 3 hours.

[0029] Detailed Description of the Invention

[0030] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0031] A first aspect of the present invention provides a lithium manganese iron phosphate positive electrode material, wherein the crystallite size Dx at 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 microscopy satisfy the following relationship: 2.0≤Ds / Dx≤4.0.

[0032] In the present invention, the crystallite size Dx at the (020) characteristic peak of the lithium manganese iron phosphate positive electrode material satisfies a specific relationship with the single particle size Ds, indicating that the number of micrograin boundaries in the bulk phase of the positive electrode material is within an appropriate range, so that the lithium ions inside the larger single particles in the positive electrode material can be quickly deintercalated by diffusion at the micrograin boundaries, while avoiding the risk of insufficient crystallinity and particle strength due to excessive defects inside the single particles, so that the lithium manganese iron phosphate positive electrode material can have both high compaction density and high rate performance.

[0033] In the present invention, the inventors have discovered that the microcrystalline boundaries existing in the bulk phase of the positive electrode material can significantly improve the bulk electronic conductivity and ion transmission capacity of the positive electrode material, and can support the lithium manganese iron phosphate products to achieve high compaction and high conductivity characteristics, meet the use scenarios such as high energy density and fast charging, and avoid the physical and chemical properties of high specific surface area and low compaction density brought about by the design of small-particle single particles, as well as the resulting processing difficulty, high interface side reactions, low electrode load and other problems.

[0034] In the present invention, the microcrystalline boundary refers to the difference in crystal growth direction between different regions within a single particle of lithium manganese iron phosphate positive electrode material, that is, the transition from one atomic arrangement to another atomic arrangement; or due to the inclusion of part of the carbon in the grain fusion process, resulting in a discontinuity in the atomic arrangement; the area where the atomic arrangement direction changes or is discontinuous within the single particle caused by the above factors is called a microcrystalline boundary.

[0035] 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 microscopy can indicate the number of crystal boundaries in the positive electrode material.

[0036] In lithium iron manganese phosphate materials, lithium ions diffuse along a one-dimensional channel in the b-axis direction (perpendicular to the (020) crystal plane). The shorter the diffusion distance, that is, the smaller the size of the individual crystals in this direction, the more conducive it is to achieving high-rate performance. To ensure the compaction density of the lithium iron manganese phosphate positive electrode material, the primary particle size needs to be increased. The inventors have found that by controlling the number of micrograin boundaries in the lithium iron manganese phosphate positive electrode material to meet a specific range, the lithium ions inside the larger primary particles in the positive electrode material can be rapidly deintercalated by diffusion at the micrograin boundaries, while avoiding the risk of insufficient crystallinity and particle strength caused by excessive defects inside the primary particles. This allows the lithium iron manganese phosphate positive electrode material to have both high compaction density and high-rate performance.

[0037] In the present invention, the crystallite size Dx is calculated based on the half-peak width (020) and the Scherrer formula in the XRD spectrum of the positive electrode material.

[0038] In the present invention, for single-crystal lithium manganese iron phosphate positive electrode materials, the single particle size refers to the size of single-crystal particles; for polycrystalline lithium manganese iron phosphate positive electrode materials, the single particle size refers to the size of primary particles that form secondary particles in the polycrystalline lithium manganese iron phosphate positive electrode materials.

[0039] In the present invention, the half-peak width of the (020) characteristic peak of the lithium manganese iron phosphate positive electrode material is 0.10-0.25°.

[0040] In the present invention, Ds is the statistical result obtained by randomly selecting approximately 100 single crystal particles from the SEM image as a sample. The longest and shortest diagonals of each single crystal particle are measured and the average value is taken as the particle size, where the shortest diagonal is perpendicular to the longest diagonal. SEM testing requires random sampling of particles and randomly selected areas. The resulting SEM image is representative of the average level of the positive electrode material.

[0041] In the present invention, the crystallite size Dx at 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 microscopy satisfy: 2.0≤Ds / Dx≤4.0, for example, it can be 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, and the range between any two values, preferably, 2.0≤Ds / Dx≤3.5.

[0042] According to the present invention, the crystallite size Dx is 30-70 nm.

[0043] In the present invention, when the crystallite size Dx of the lithium manganese iron phosphate positive electrode material meets the above range, the diffusion distance of lithium ions along this direction is short, which is conducive to achieving rapid ion deintercalation, that is, high-rate charge and discharge capability; controlling the crystallite size not to be too small can maintain the high crystallinity of the material, avoid excessively disordered atomic arrangement, excessive defects causing the ion diffusion path to be too tortuous or even blocked, and thus deteriorating the electrochemical activity of the material.

[0044] In the present invention, the crystallite size Dx is 30-70nm, for example, it can be 30nm, 32nm, 34nm, 36nm, 38nm, 40nm, 42nm, 44nm, 46nm, 48nm, 50nm, 52nm, 54nm, 56nm, 58nm, 60nm, 62nm, 64nm, 66nm, 68nm, 70nm, and the range between any two values. Preferably, the crystallite size Dx is 40-60nm.

[0045] According to the present invention, the single particle size Ds of the lithium manganese iron phosphate positive electrode material is 80-200 nm.

[0046] In the present invention, when the single particle size Ds of the lithium manganese iron phosphate positive electrode material meets the above range, the distance for lithium ions to diffuse from the single particle bulk through the interior of the microcrystal and the microcrystalline boundary to the surface can be shortened, thereby facilitating high-rate charge and discharge. By controlling Ds not to be too small, the number and volume of pores generated by the accumulation between single particles can be reduced, thereby achieving the high compaction density characteristics of the material.

[0047] In the present invention, the single particle size Ds is 80-200 nm, for example, it can be 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 a range between any two values. Preferably, the single particle size Ds of the lithium manganese iron phosphate positive electrode material is 100-160 nm.

[0048] According to the present invention, the lithium manganese iron phosphate positive electrode material includes a matrix and a carbon layer present on the surface of the matrix and / or inside the matrix.

[0049] In the present invention, the inventors have found that when a carbon layer exists inside the matrix, especially at the grain boundaries, the bulk conductivity of the positive electrode material can be significantly improved. The carbon enriched at the grain boundaries can inhibit the crystal fusion and growth during high-temperature heat treatment. The high-temperature treatment can further repair the bulk defects and form well-crystallized crystallites, ensuring the rapid deintercalation of lithium ions, thereby obtaining a lithium manganese iron phosphate positive electrode material with high-rate charge and discharge capabilities.

[0050] According to the present invention, the matrix has a composition shown in Formula I: Li 1+a Mn x Fe y M' z PO4 Formula I;

[0051] Wherein, 0≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0<z≤0.05, and 0.8≤x+y+z≤1, wherein M' is selected from at least one element of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo and W.

[0052] In the present invention, in Formula I, 0≤a≤0.2, for example, it can be 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 the range between any two values; 0.3≤x<1, for example, it can be 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and the range between any two values; 0<y≤0.7, for example, it can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and the range between any two values; 0<z≤ 0.05, for example, it can be 0.01, 0.02, 0.03, 0.04, 0.05, and the range between any two values; 0.8≤x+y+z≤1, for example, it can be 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 the range between any two values.

[0053] 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, wherein M′ is at least one element selected from Al, Ti, V, Co, Nb and W.

[0054] According to the present invention, based on the total weight of the lithium manganese iron phosphate positive electrode material, the content of the carbon layer is 1-2.5 wt %.

[0055] In the present invention, when the content of the carbon layer meets the above range, it can fully cover the surface of the material particles, build a complete conductive carbon network, and ensure that electrons reach the interior of the powder material quickly and evenly to achieve high-rate characteristics; and the material density of carbon is lower than that of lithium iron manganese phosphate. Excessive carbon layer content is not conducive to the compaction density of the material, and because carbon has no electrochemical activity within the lithium iron manganese phosphate lithium intercalation and deintercalation voltage range and does not contribute to capacity, excessive carbon will lead to a decrease in the capacity of the lithium iron manganese phosphate / carbon composite material per unit mass; therefore, controlling the carbon layer content within this range can take into account both the electrochemical performance and the compaction density of the material powder.

[0056] In the present invention, the content of the carbon layer is 1-2.5wt%, for example, it can be 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, and the range between any two values.

[0057] Furthermore, based on the total weight of the lithium manganese iron phosphate positive electrode material, the content of the carbon layer is 1.4-2.1 wt%.

[0058] According to the present invention, based on the total weight of the lithium manganese iron phosphate positive electrode material, the content of the carbon layer present in the substrate is 0.05-0.20 wt%.

[0059] In the present invention, the carbon layer inside the matrix is ​​mainly distributed in the microcrystalline boundaries of the matrix. The carbon layer content inside the matrix specifically refers to placing the lithium manganese iron phosphate positive electrode material in a muffle furnace, sintering it in air at 400°C for 3 hours, and then using a carbon-sulfur meter to test the carbon layer content in the positive electrode material after sintering.

[0060] In the present invention, when the content of the carbon layer present in the matrix of the positive electrode material satisfies the above range, the capacity and rate performance of the positive electrode material can be significantly improved.

[0061] In a preferred embodiment of the present invention, the specific carbon layer content within the matrix matches the number of micrograin boundaries (Ds / Dx). While controlling the number of micrograin boundaries, the carbon layer content within the micrograin boundaries is also regulated. An appropriate carbon layer content within the matrix can significantly improve the bulk conductivity of the cathode material. It can also effectively inhibit the fusion growth of crystals during high-temperature heat treatment. While repairing bulk defects and forming well-crystallized microcrystals, it also eliminates the risk of insufficient crystallinity and particle strength due to excessive defects within larger individual particles. This ensures that lithium ions can rapidly deintercalate via micrograin boundary diffusion, enabling a moderate increase in individual particle size and achieving a high compaction density. This resolves the conflict between the capacity of lithium manganese iron phosphate materials, particularly their rate performance, and the increase in particle size and compaction density.

[0062] In the present invention, the content of the carbon layer present in the substrate is 0.05-0.20wt%, for example, it can be 0.05wt%, 0.06wt%, 0.07wt%, 0.08wt%, 0.09wt%, 0.1wt%, 0.11wt%, 0.12wt%, 0.13wt%, 0.14wt%, 0.15wt%, 0.16wt%, 0.17wt%, 0.18wt%, 0.19wt%, 0.20wt%, and the range between any two values.

[0063] Furthermore, based on the total weight of the lithium manganese iron phosphate positive electrode material, the content of the carbon layer present in the matrix is ​​0.08-0.18 wt %, more preferably 0.08-0.15 wt %.

[0064] According to the present invention, the powder compaction density of the positive electrode material is 2.1-2.6 g / cm 3 , for example, it can be 2.1 g / cm 3 , 2.2g / cm 3 , 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , and the range between any two values, preferably 2.1-2.5 g / cm 3 .

[0065] A second aspect of the present invention provides a method for preparing the above-mentioned lithium manganese iron phosphate positive electrode material, characterized in that the preparation method comprises the following steps:

[0066] (1) dispersing ferromanganese phosphate, a lithium source, a first carbon source, and an additive M' in a solvent, performing a first grinding and drying process, and then performing a first sintering process in a nitrogen atmosphere to obtain a first lithium ferromanganese phosphate material;

[0067] (2) dispersing the first lithium manganese iron phosphate material and the second carbon source in a solvent, performing a second grinding and drying, and then performing a second sintering in a nitrogen atmosphere, and obtaining the lithium manganese iron phosphate positive electrode material after crushing and screening;

[0068] The second grinding is performed so that the particle size after grinding is 70-160 nm.

[0069] In the present invention, in the preparation method of the lithium manganese iron phosphate positive electrode material, through the process of crystallization-crushing-recrystallization and controlling the particle size after crushing, it is possible to achieve a specific relationship between the crystallite size Dx at the (020) characteristic peak of the positive electrode material and the single particle size Ds, thereby improving the capacity and rate performance of the positive electrode material.

[0070] Specifically, in the present invention, a first lithium iron manganese phosphate material is first obtained through a first sintering process. A second grinding and crushing process is then performed on the mixture comprising the first lithium iron manganese phosphate material and the second carbon source. After the second sintering process, the lithium iron manganese phosphate cathode material described in the first aspect of the present invention is obtained. By means of the crystal crushing and recrystallization processes and by controlling the particle size after the second grinding process, varying degrees of new crystal interface exposure and fusion are achieved, thereby achieving a controlled synthesis of the number of micrograin boundaries in the cathode material.

[0071] Furthermore, in this process, the carbon source (the first carbon source and the second carbon source, especially the second carbon source) adheres to and pyrolyzes on the new crystal interface formed by the crushing. As the crystals fuse, part of the carbon remains at the microcrystalline boundaries to form a carbon layer, thereby improving the bulk electrical conductivity of the material. The carbon layer enriched at the microcrystalline boundaries can inhibit the crystal fusion and growth during high-temperature heat treatment. The high-temperature treatment can repair the bulk defects and form well-crystallized microcrystals, thereby ensuring the rapid deintercalation of lithium ions, thereby obtaining a lithium manganese iron phosphate material with high-rate charge and discharge capabilities.

[0072] In the present invention, the second grinding is performed so that the particle size after grinding refers to the particle size of the mixture including the first lithium manganese iron phosphate material and the second carbon source.

[0073] In the present invention, there is no particular limitation on the drying method in step (1) and step (2), and conventional drying methods in the art, such as spray drying, can be used.

[0074] In the present invention, the second grinding is performed so that the particle size after grinding is 70-160nm, for example, it can be 70nm, 72nm, 74nm, 76nm, 78nm, 80nm, 82nm, 84nm, 86nm, 88nm, 90nm, 92nm, 94nm, 96nm, 98nm, 100nm, 105nm, 110nm, 115nm, 120nm, 125nm, 130nm, 135nm, 140nm, 145nm, 150nm, 155nm, 160nm, and the range between any two values.

[0075] In the present invention, there is no particular limitation on the methods of the first grinding and the second grinding, and conventional grinding equipment in the art can be used for grinding, as long as the particle size after grinding can meet the requirements of the present invention.

[0076] According to the present invention, the temperature of the second sintering is 600-800°C, for example, it can be 600°C, 650°C, 700°C, 750°C, 800°C, and a range between any two values.

[0077] In the present invention, by controlling the second sintering temperature, the relationship between the crystallite size Dx and the single particle size Ds and the carbon layer content inside the positive electrode material matrix can be adjusted, so that the capacity and rate performance of the positive electrode material are further improved.

[0078] According to the present invention, the second sintering time is 5-20h, for example, it can be 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, and the range between any two values, preferably 8-15h.

[0079] According to the present invention, the first sintering temperature is 400-600°C, for example, it can be 400°C, 450°C, 500°C, 550°C, 600°C, and a range between any two values.

[0080] According to the present invention, the first sintering time is 1-10 hours, for example, it can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, and a range between any two values, preferably 2-6 hours.

[0081] According to 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.

[0082] According to the present invention, the molecular weight of the first carbon source and the second carbon source is independently 100-10000 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, g / 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 ranges between any two values.

[0083] In the present invention, for small molecular compounds such as glucose, the molecular weight refers to the relative molecular mass, and for high molecular polymers such as polyacrylic acid, the molecular weight refers to the weight average molecular weight.

[0084] In the present invention, when the above-mentioned specific type of carbon source or a carbon source with a specific molecular weight is used, uniform coating and efficient carbonization of the surface of the lithium manganese iron phosphate material can be achieved, the material resistance can be reduced, and the cost of different carbon sources can be taken into account, which is suitable for large-scale industrial production.

[0085] 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.

[0086] According to the present invention, the amounts of the manganese iron phosphate, the first carbon source and the second carbon source are such that, based on the total weight of the manganese iron phosphate lithium positive electrode material, the content of the carbon layer is 1-2.5 wt%.

[0087] In the present invention, the content of the carbon layer is 1-2.5wt%, for example, it can be 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, and the range between any two values.

[0088] Furthermore, the amounts of the manganese iron phosphate, the first carbon source and the second carbon source are such that, based on the total weight of the manganese iron phosphate lithium positive electrode material, the content of the carbon layer is 1.4-2.1 wt%.

[0089] According to the present invention, the mass ratio of the first carbon source to the second carbon source satisfies 1:1.5-5, for example, it can be 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 the range between any two values.

[0090] In the present invention, when the amount relationship between the first carbon source and the second carbon source is controlled to meet the above-mentioned range, the reducing atmosphere required for the generation of the first lithium manganese iron phosphate material in the first sintering process can be met, and excessive pyrolytic carbon can be avoided from remaining in the first sintering process and being stripped off to form free invalid carbon in the second grinding process; the second carbon source is the main source of the coating carbon layer, which can fully coat the first lithium manganese iron phosphate material that has formed crystals, reduce invalid carbon, thereby effectively reducing the material resistance and inhibiting excessive electrolyte side reactions, thereby ensuring the electrical performance.

[0091] According to the present invention, 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.

[0092] In the present invention, there is no particular limitation on the specific type of the additive M', which may be at least one of an oxide containing M', a hydroxide containing M', an oxyhydroxyl containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'.

[0093] In the present invention, there is no particular limitation on the type of lithium source, and conventional lithium sources in the art can be used, such as at least one selected from but not limited to 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.

[0094] In the present invention, there is no particular limitation on the type of ferromanganese phosphate, and conventional ferromanganese phosphate materials in the art can be used.

[0095] In the present invention, there is no particular limitation on the type of solvent, as long as it can fully mix and evenly disperse the components, such as pure water.

[0096] According to the present invention, the amounts of the ferromanganese phosphate, the lithium source, and the additive M' are such that n(Li):n(Mn):n(Fe):n(M') is 1+a:x:y:z, wherein 0≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0<z≤0.05, and 0.8≤x+y+z≤1.

[0097] Further, 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.

[0098] A third aspect of the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises the above-mentioned lithium iron manganese phosphate positive electrode material.

[0099] According to the present invention, the ratio of the discharge capacity of the lithium-ion battery at 1C to the discharge capacity at 0.2C is ≥89%.

[0100] The present invention will be described in detail below by way of examples.

[0101] The crystallization characteristics of the lithium manganese iron phosphate positive electrode material are measured by XRD. Specifically, the test method is: in the XRD spectrum of the lithium manganese iron phosphate positive electrode material, the peak position and half-height width data of the (020) peak in the range of 2θ=16-19° are read, and according to the Scherrer formula D=Kλ / (β·cosθ), K is taken as 0.89, λ is taken as 0.154, and β is the half-height width of the (020) peak, the crystallite size Dx of the lithium manganese iron phosphate material is calculated.

[0102] The morphology and individual particle size of the lithium manganese iron phosphate positive electrode material were measured using SEM. Specifically, the test method was as follows: a scanning electron microscope S-4800 model from Hitachi HITACHI of Japan was used to take a 30K magnification image of the powder material, 100 individual particles in the image were randomly selected, the longest diagonal and the shortest diagonal of each particle were measured, and the average value was taken to obtain the individual particle size Ds.

[0103] The composition of the lithium manganese iron phosphate positive electrode material was measured using an atomic emission spectrometer (ICP). Specifically, the test method was as follows: 0.2 g of lithium manganese iron phosphate sample was accurately weighed, thermally digested with acid, and then the volume was fixed. The content of Li, Mn, Fe, P and additive elements in the material was tested using a standard curve method.

[0104] The content of the carbon layer in the lithium manganese iron phosphate positive electrode material is measured using a carbon-sulfur meter. Specifically, the test method is: accurately weigh 0.2g of lithium manganese iron phosphate material, mix it with 2g of combustion aid, place it in a high-temperature crucible, transfer it to a carbon-sulfur meter, introduce oxygen for sintering, and detect the amount of carbon dioxide gas generated by sintering to obtain the carbon content ratio in the material.

[0105] The content of the carbon layer inside the matrix of the lithium manganese iron phosphate positive electrode material is measured using a carbon-sulfur meter. Specifically, the test method is: place the lithium manganese iron phosphate positive electrode material in a muffle furnace, sinter it at 400°C in air for 3 hours, observe the appearance of the material change from black or lead gray to orange, and then use a carbon-sulfur meter to test the carbon layer content.

[0106] The powder compaction density of lithium manganese iron phosphate positive electrode material is obtained by testing under a pressure of 3t.

[0107] The raw materials used in the examples and comparative examples are all commercially available products.

[0108] Example 1

[0109] (1) dispersing ferromanganese phosphate, lithium carbonate, TiO2 additive and a first carbon source in pure water, wherein the amounts of ferromanganese phosphate, lithium carbonate and TiO2 additive are such that n(Li):n(Mn):n(Fe):n(Ti):n(P)=1.02:0.57:0.38:0.02:1; then performing a first grinding, and then transferring the mixture into a nitrogen atmosphere furnace after spray drying, and performing a first sintering at 500° C. for 4 hours to obtain a first lithium ferromanganese phosphate material;

[0110] (2) The first lithium manganese iron phosphate material and the second carbon source were dispersed in pure water, and a second grinding was performed to make the particle size after grinding 90 nm. After spray drying, the mixture was transferred to a nitrogen atmosphere furnace and a second sintering was performed at 700° C. for 12 h. After crushing and screening, the lithium manganese iron phosphate positive electrode material A1 was obtained;

[0111] The first carbon source is glucose (molecular weight 180 g / mol), the second carbon source is polyethylene glycol (weight average molecular weight 4000 g / mol), and the amounts of ferromanganese phosphate, the first carbon source, and the second carbon source are such that, based on the total weight of the lithium ferromanganese phosphate positive electrode material, the content of the carbon layer is 1.7 wt %. The mass ratio of the first carbon source to the second carbon source is 1:3.

[0112] FIG1 is an XRD spectrum of the first lithium iron manganese phosphate material in Example 1. As shown in FIG1 , the first lithium iron manganese phosphate material obtained after the first sintering has formed a relatively good lithium iron manganese phosphate crystalline material.

[0113] FIG2 is a SEM image of the lithium manganese iron phosphate positive electrode material A1. As can be seen from FIG2 , the particles of the lithium manganese iron phosphate positive electrode material A1 are spherical or quasi-spherical.

[0114] FIG3 is an XRD diagram of the lithium iron manganese phosphate positive electrode material A1. As can be seen from FIG3 , the lithium iron manganese phosphate positive electrode material A1 is a pure phase structure of lithium iron manganese phosphate.

[0115] Examples 2-4

[0116] The positive electrode material was prepared according to the method of Example 1. The raw material ratio and specific process conditions are shown in Table 1. Other conditions were the same as in Example 1, and lithium manganese iron phosphate positive electrode materials A2-A4 were prepared respectively.

[0117] Comparative Example 1-2

[0118] The positive electrode material was prepared according to the method of Example 1. The raw material ratio and specific process conditions are shown in Table 1. Other conditions were the same as in Example 1, and lithium manganese iron phosphate positive electrode materials D1-D2 were prepared respectively.

[0119] Table 1 a,b -Based on the amount of ferromanganese phosphate.

[0120] Table 1 continued a,b -Based on the amount of ferromanganese phosphate.

[0121] The compositions and physicochemical parameters of the cathode materials prepared in the examples and comparative examples were tested, and the results are shown in Table 2.

[0122] Table 2

[0123] Table 2 continued

[0124] Test Case

[0125] The lithium iron manganese phosphate positive electrode materials of the embodiment and the comparative example are assembled into button batteries. Specifically, the assembly method is as follows: first, a composite lithium iron manganese phosphate positive electrode active material for non-aqueous electrolyte secondary batteries, acetylene black and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 90%:5%:5%, coated on aluminum foil and dried, and stamped into a positive electrode sheet with a diameter of 12 mm and a thickness of 120 μm using a pressure of 100 MPa. Then, the positive electrode sheet is placed in a vacuum drying oven and dried at 120°C for 12 hours.

[0126] The negative electrode uses a Li metal sheet with a diameter of 17 mm and a thickness of 1 mm; the separator uses a polyethylene porous membrane with a thickness of 25 μm; and the electrolyte uses an equal mixture of 1 mol / L LiPF6, ethylene carbonate (EC) and diethyl carbonate (DEC).

[0127] The positive electrode sheet, separator, negative electrode sheet and electrolyte were assembled into a 2025 button cell in an Ar gas glove box with a water content and an oxygen content of less than 5 ppm. The cell at this time was regarded as an unactivated cell.

[0128] The electrochemical performance of the button cell was tested. Specifically, the test method was as follows: After the button cell was prepared, it was allowed to stand for 2 hours. After the open-circuit voltage stabilized, the positive electrode was charged at a current density of 0.1C to a cutoff voltage of 4.35V. Then, constant voltage charging was performed for 30 minutes, and then discharge was performed at the same current density to a cutoff voltage of 2.5V. This process was repeated once, and the battery at this point was considered an activated battery. The activated battery was then subjected to charge and discharge tests at different rates, 0.2C / 0.2C, 0.5C / 0.5C, and 1C / 1C. The charge and discharge capacity at each rate was recorded, and the ratio of the discharge capacity at 1C to 0.2C was used as an evaluation indicator of rate performance. The test results are shown in Table 3.

[0129] Table 3

[0130] It can be seen from Tables 1 to 3 above that the preparation method proposed in the present invention is applicable to different doping schemes (Examples 1, 2, and 11), and is also applicable to lithium manganese iron phosphate materials with different Mn / Fe ratios (Example 10).

[0131] Compared with the once-fired material (Comparative Example 1), the positive electrode materials (Examples 1, 3, 4, 7, 8, and 9) obtained by the preparation method proposed in the present invention have narrower (020) half-width, significantly improved crystallinity, larger crystallite size, and a moderate number of crystallite boundaries after the second sintering to repair local defects. They can achieve better capacity utilization and rate performance on the basis of achieving larger particle size and higher compaction.

[0132] Example 3 shows that when the abrasive particles have similar sizes, lowering the second sintering temperature and weakening the fusion between particles can reduce the number of microcrystalline boundaries and internal carbon residues, but the particle size will be reduced, which is not conducive to compaction.

[0133] Example 7 demonstrates that increasing the abrasive particle size, coupled with an increased ratio of the second carbon source to the first carbon source, inhibits inter-particle fusion growth and significantly reduces internal carbon content. However, the particle size of the finished positive electrode material increases only slightly, and the material's crystallinity remains unaffected, resulting in a slight deterioration in kinetic performance. In contrast, in Example 9, increasing the first carbon source dosage and sintering temperature, improving the material's crystallization integrity during the first sintering step, increases the finished crystallite size. Combined with an appropriate increase in internal residual carbon, kinetic performance improves compared to Example 7, but still not as good as Example 1.

[0134] Examples 8 and 4 illustrate that reducing the size of the abrasive particles and increasing the second sintering temperature can strengthen the bulk crystallization of the particles, increase inter-particle interface fusion, and form more microcrystalline boundaries and internal residual carbon, but the particle size also increases, which is beneficial to compaction. However, it can be seen that under the premise of a larger particle size, thanks to the high crystallinity, microcrystalline boundaries, and residual carbon, the positive electrode materials of Examples 8 and 4 still exhibit more ideal capacity and rate characteristics.

[0135] Example 5 shows that after adjusting the types and ratios of the first and second carbon sources, due to the low amount of the second carbon source added, its coating uniformity on the surface of the lithium manganese iron phosphate material is insufficient, which easily leads to the direct fusion and growth of adjacent lithium manganese iron phosphate crystals. Therefore, the size of the individual particles of the finished product is greatly increased, and it is not conducive to the residual carbon remaining inside the crystal, so the rate performance is poor.

[0136] Example 6 shows that if the second sintering temperature is too low, the degree of fusion between lithium manganese iron phosphate crystals is significantly reduced, the particle size does not grow, and the compaction is low; and because the processing temperature is low and the crystallinity is insufficient, the rate performance is not as good as the preferred condition Example 1.

[0137] Comparative Example 2 shows that when only the abrasive particle size is reduced and the second sintering temperature is low, it is impossible to achieve an increase in bulk crystallinity and fusion between particles to form microcrystalline boundaries and internal residual carbon, resulting in poor rate characteristics, small particle size, and low compaction.

[0138] Figure 4 compares the 0.2C charge and discharge curves of button cells assembled with the positive electrode materials of Example 1 and Comparative Example 1. As can be seen from Figure 4, Comparative Example 1 did not undergo secondary sintering and recrystallization to repair internal defects, and there were too many micrograin boundaries inside the particles, resulting in poor crystallinity. Even at a 0.2C rate, the capacity was insufficient, and the discharge platform retention was worse than that of Example 1.

[0139] FIG5 is a comparison of the 1C charge and discharge curves of button cells assembled with the positive electrode materials of Example 1 and Comparative Example 1. As can be seen from FIG5 , due to the above-mentioned defects, the lithium diffusion kinetics of Comparative Example 1 is severely limited. At a high rate of 1C, the constant current charging capacity ratio is greatly reduced, the discharge platform almost completely disappears, and the capacity performance is far less than that of Example 1.

[0140] FIG6 shows the state of the lithium manganese iron phosphate positive electrode material of Example 1 after sintering in an air atmosphere at 400° C. for 3 hours. FIG6 shows that the surface color of the material has shifted to orange, indicating that the surface carbon layer has been burned off. The carbon content of the material was tested, and the test result corresponds to the residual carbon inside the material.

[0141] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A lithium iron manganese phosphate cathode material, characterized in that, The microcrystalline size Dx at the (020) characteristic peak of the cathode material measured by XRD and the single particle size Ds of the cathode material measured by SEM satisfy: 2.0 ≤ Ds / Dx ≤ 4.

0.

2. The lithium iron manganese phosphate cathode material according to claim 1, wherein, 2.0 ≤ Ds / Dx ≤ 3.

5.

3. The lithium iron manganese phosphate cathode material according to claim 1 or 2, wherein The microcrystalline size Dx is 30 - 70 nm, preferably 40 - 60 nm.

4. The lithium iron manganese phosphate cathode material according to any one of claims 1-3, wherein, The single particle size Ds is 80 - 200 nm, preferably 100 - 160 nm.

5. The lithium iron manganese phosphate cathode material according to any one of claims 1-4, wherein, The full width at half maximum of the (020) characteristic peak of the cathode material is 0.10 - 0.25°.

6. The lithium iron manganese phosphate cathode material according to any one of claims 1-5, wherein, The lithium iron manganese phosphate cathode material includes a matrix and a carbon layer present on the surface and / or inside the matrix.

7. The lithium iron manganese phosphate cathode material according to claim 6, wherein The matrix has the composition shown in Formula I: Li 1+a Mn x Fe y M’ z PO4 Formula I; 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 lithium iron manganese phosphate cathode material according to claim 6 or 7, wherein, Based on the total weight of the lithium iron manganese phosphate cathode material, the content of the carbon layer is 1.0 - 2.5 wt%. Preferably, based on the total weight of the lithium iron manganese phosphate cathode material, the content of the carbon layer present inside the matrix is 0.05 - 0.20 wt%; preferably 0.08 - 0.18 wt%.

9. The lithium iron manganese phosphate cathode material according to any one of claims 1-8, wherein, The tap density of the powder of the positive electrode material is 2.1-2.6 g / cm 3 .

10. A method for preparing the lithium iron manganese phosphate cathode material according to any one of claims 1-9, characterized in that, The preparation method includes the following steps: (1) Disperse lithium iron manganese phosphate, a lithium source, a first carbon source, and an additive M' in a solvent, perform first grinding and drying, and then perform first sintering in a nitrogen atmosphere to obtain a first lithium iron manganese phosphate material; (2) Disperse the first lithium iron manganese phosphate material and a second carbon source in a solvent, perform second grinding and drying, and then perform second sintering in a nitrogen atmosphere, and obtain the lithium iron manganese phosphate cathode material after crushing and sieving; Wherein, the second grinding makes the particle size after grinding be 70 - 160 nm.

11. According to the preparation method described in claim 10, wherein, The temperature of the second sintering is 600 - 800 °C.

12. The preparation method according to claim 10 or 11, wherein, The time of the second sintering is 5 - 20 h.

13. The preparation method according to any one of claims 10-12, wherein, The temperature of the first sintering is 400 - 600 °C.

14. The preparation method according to any one of claims 10-13, wherein, The time of the first sintering is 1 - 10 h.

15. The preparation method according to any one of claims 10-14, wherein, 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 weights of the first carbon source and the second carbon source are each independently 100 - 10000 g / mol.

16. The preparation method according to any one of claims 10-15, wherein, The dosages of the lithium iron manganese phosphate, the first carbon source, and the second carbon source are such that, based on the total weight of the lithium iron manganese phosphate cathode material, the content of the carbon layer is 1 - 2.5 wt%.

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

18. The preparation method according to any one of claims 10-17, wherein, 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.

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

20. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium iron manganese phosphate cathode material described in any one of claims 1-9.

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

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