Carbon-coated modified lithium manganese iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of batteries, and discloses a carbon-coated modified lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, and the carbon-coated modified lithium manganese iron phosphate positive electrode material comprises carbon-coated modified lithium manganese iron phosphate positive electrode material particles, the carbon-coated modified lithium manganese iron phosphate positive electrode material particle comprises a lithium manganese iron phosphate core, a first carbon coating layer and a second carbon coating layer, wherein the first carbon coating layer is positioned between the lithium manganese iron phosphate and the second carbon coating layer; the graphitization degree of the first carbon coating layer is greater than that of the second carbon coating layer. The carbon-coated modified lithium manganese iron phosphate positive electrode material has relatively high conductivity, compaction density and the like, and a battery using the carbon-coated modified lithium manganese iron phosphate positive electrode material has relatively high charge-discharge rate, excellent energy density, excellent capacity and the like.

Description

Carbon-coated modified lithium manganese iron phosphate cathode material, its preparation method and application Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to carbon-coated modified lithium manganese iron phosphate cathode material and its preparation method and application, and more specifically relating to carbon-coated modified lithium manganese iron phosphate cathode material and its preparation method, cathode sheet, battery, and electrical device. Background Technology

[0002] With the rapid development of markets such as electronic devices, electric vehicles, smart homes, power tools, and intelligent transportation, the demand for batteries is also constantly increasing. Lithium-ion batteries have gradually become the most mainstream power batteries due to their excellent energy density, high safety performance, low cost, and long lifespan.

[0003] Lithium manganese iron phosphate (LMP) is considered one of the most promising cathode active materials for power lithium-ion batteries due to its high voltage platform and energy density. However, LMP also has some drawbacks as a cathode material, such as low electronic conductivity and low compaction density, which affect the energy density of the battery. Carbon coating of LMP can effectively improve these drawbacks. Therefore, developing a carbon-coated modified LMP cathode material is one of the current challenges. Summary of the Invention

[0004] This application aims to at least partially solve one of the technical problems in the related art. To this end, this application proposes a carbon-coated modified lithium manganese iron phosphate cathode material, its preparation method, and its application. This carbon-coated modified lithium manganese iron phosphate cathode material has high electrical conductivity, high compaction density, etc., and batteries using this carbon-coated modified lithium manganese iron phosphate cathode material have high charge / discharge rates, high energy density, or high capacity.

[0005] The first aspect of this application proposes a carbon-coated modified lithium manganese iron phosphate (LMP) cathode material, comprising carbon-coated modified LMP cathode material particles. The carbon-coated modified LMP cathode material particles include a LMP core, a first carbon coating layer, and a second carbon coating layer, with the first carbon coating layer located between the LMP and the second carbon coating layer. The graphitization degree of the first carbon coating layer is greater than that of the second carbon coating layer. In this application, gradient carbon coating and multifunctional coating are achieved by forming carbon coating layers with different graphitization degrees on the surface of the LMP core. Specifically, the first and second carbon coating layers can work synergistically to better construct and optimize the conductive network of the LMP core, forming a continuous conductive pathway inside and on the surface of the LMP, enhancing electron transport capability. Simultaneously, the double-layer carbon coating can more precisely control the microstructure of the LMP, thereby effectively controlling the particle size, compaction density, etc., of the cathode material, effectively improving various performance characteristics of the cathode material.

[0006] In some embodiments, the carbon-coated modified lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: the first carbon coating layer I D / I G =0.60~0.95; I of the second carbon coating layer D / I G =0.90~1.40; where, I D / I G I represents the ratio of the Raman D peak to the G peak. D / I G The smaller the ratio, the higher the degree of graphitization.

[0007] This facilitates gradient and multifunctional carbon coating.

[0008] In some embodiments, the carbon-coated modified lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: the thickness of the first carbon coating layer is 1 nm to 5 nm; the thickness of the second carbon coating layer is 5 nm to 10 nm; and the chemical formula of the lithium manganese iron phosphate core is Li. a Fe x Mn 1-x PO4, where 0.4≤x≤0.50, 0.9≤a≤1.2. This helps the second and first carbon coating layers work synergistically to maintain the overall structural stability of lithium iron manganese phosphate, while effectively controlling parameters such as the compaction density of the cathode material, thus improving its various properties.

[0009] In some embodiments, the carbon-coated modified lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: the particle size of the carbon-coated modified lithium manganese iron phosphate cathode material is 0.35 μm-0.60 μm; the compaction density of the carbon-coated modified lithium manganese iron phosphate cathode material is 2.25 g / cm³. 3 ~2.35 g / cm 3 This helps to improve the various electrochemical performance characteristics of the battery.

[0010] A second aspect of this application provides a method for preparing the aforementioned carbon-coated modified lithium manganese iron phosphate cathode material, comprising: mixing and ball-milling a manganese iron phosphate precursor and a lithium source to obtain a raw material mixture; mixing the raw material mixture with a first carbon source and performing a first ball milling to obtain a first mixture; sequentially subjecting the first mixture to a first sand milling, a first spray drying, and a first sintering to obtain an intermediate product; mixing the intermediate product with a second carbon source and performing a second ball milling to obtain a second mixture; and sequentially subjecting the second mixture to a second sand milling, a second spray drying, a second sintering, and pulverization to obtain the carbon-coated modified lithium manganese iron phosphate cathode material; wherein the first carbon source includes one of the functional groups such as hydroxyl and aldehyde; and the second carbon source includes at least one of the functional groups such as carboxyl, ether, epoxy, and hydroxyl. Thus, the carbon-coated modified lithium manganese iron phosphate cathode material of this application is prepared.

[0011] In some embodiments, at least one of the following conditions is met: the first ball milling time is 2 min to 10 min; the second ball milling time is 2 min to 10 min; the solid content of the second mixture is 15% to 65%; and the particle size of the product from the second sand milling is 0.35 μm to 0.45 μm. This helps to further improve the structural stability of the cathode material, thereby improving its various properties.

[0012] In some embodiments, the first sintering includes: heating to 400°C to 500°C at a rate of 3°C / min to 6°C / min and sintering for 3 to 5 hours; then heating to 650°C to 750°C at a rate of 3°C / min to 6°C / min and sintering for 6 to 12 hours. This helps to precisely control the formation of the first carbon coating layer and the formation of the lithium manganese iron phosphate crystal phase, ensuring that the first carbon coating layer is tightly adhered to the surface of the lithium manganese iron phosphate particles with an olivine structure. This enhances conductivity, protects the material structure, and improves the electrochemical performance of the cathode material.

[0013] In some embodiments, the second sintering includes: heating to 400°C to 500°C at a rate of 3°C to 6°C and sintering for 3 to 8 hours; then heating to 550°C to 680°C at a rate of 3°C to 6°C and sintering for 6 to 10 hours. This helps to rearrange and recombine carbon atoms to form a disordered second carbon coating layer.

[0014] In some embodiments, based on the theoretical mass of lithium manganese iron phosphate, the amount of the first carbon source added is 1wt% to 5wt%, and / or based on the theoretical mass of lithium manganese iron phosphate, the mass ratio of the amount of the second carbon source added to the amount of the first carbon source added is 2:1. This helps to form a second carbon coating layer of the target thickness, thereby contributing to its synergistic effect with the first carbon coating layer in maintaining the overall structural stability of the lithium iron manganese phosphate.

[0015] In some embodiments, at least one of the following conditions is met: the first carbon source includes at least one selected from sucrose, glucose, polyethylene, and polypropylene; the second carbon source includes at least one selected from polyethylene glycol, cellulose, citric acid, tartaric acid, and epoxy resin. This helps to obtain a carbon coating layer with the target degree of graphitization.

[0016] In some embodiments, the molar ratio of the lithium source to the manganese iron phosphate precursor is 1.01 to 1.09:1.

[0017] In some embodiments, the ball milling of the iron-manganese phosphate precursor and the lithium source includes: mixing the lithium source and water and performing a third ball milling to obtain a third mixture; mixing the third mixture and the iron-manganese phosphate precursor and performing a fourth ball milling to obtain the raw material mixture.

[0018] In some embodiments, at least one of the following conditions is met: the third ball milling time is 1 min to 10 min; the fourth ball milling time is 10 min to 30 min; the molar concentration of the lithium source in the third mixture is 1 mol / L to 3 mol / L; the particle size of the lithium source is 1 μm to 5 μm; and the lithium source includes at least one of lithium carbonate, lithium sulfate, lithium bicarbonate, and lithium dihydrogen carbonate. This helps to ensure that the prepared lithium manganese iron phosphate precursor has an accurate chemical composition and stable structure, thereby facilitating the subsequent preparation of high-performance cathode materials.

[0019] A third aspect of this application provides a positive electrode sheet comprising the aforementioned carbon-coated modified lithium manganese iron phosphate positive electrode material. Therefore, this positive electrode sheet exhibits excellent conductivity.

[0020] A fourth aspect of this application discloses a battery comprising the aforementioned carbon-coated modified lithium manganese iron phosphate cathode material or the aforementioned cathode sheet. This battery exhibits high charge / discharge rates, excellent energy density, and capacity.

[0021] A fifth aspect of this application provides an electrical device comprising the aforementioned carbon-coated modified lithium manganese iron phosphate cathode material, the aforementioned cathode sheet, or the aforementioned battery. Therefore, the electrical device has high capacity and energy density. Detailed Implementation

[0022] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0023] As mentioned earlier, lithium manganese iron phosphate has some drawbacks as a cathode material, specifically: 1. The conductivity of lithium manganese iron phosphate is only 10. -11 S / cm, lithium-ion diffusion rate is 10 -9 cm 2 / s, 1 lower than lithium iron phosphate 1. Two orders of magnitude lower electronic conductivity and lower lithium-ion diffusion rate; 2. Smaller lithium manganese iron phosphate material particles result in lower compaction density, affecting the overall energy density performance of the material; 3. Dissolution of trivalent manganese ions leads to reduced capacity. If they migrate to the negative electrode, they can damage the SEI film and accelerate the degradation of cycle performance.

[0024] To address the aforementioned drawbacks, related technologies have employed a carbon coating layer designed for lithium manganese iron phosphate materials. Specifically, the carbon coating layer and lithium manganese iron phosphate can form a conductive network and conductive channels, thereby increasing the diffusion rate of lithium ions and improving electron conductivity. The carbon coating layer can also increase the compaction density of the cathode material to a certain extent, further enhancing the battery's energy density. In addition, carbon itself can act as a reducing agent, effectively suppressing Fe... 2+ Mn 2+ Oxidation reduces the dissolution of trivalent manganese ions. However, most carbon coatings in related technologies are single-layered. While this addresses the aforementioned issues to some extent, single-layer carbon coatings have limited functionality. Specifically, the conductive network constructed by a single-layer carbon coating is limited, resulting in only a slight improvement in conductivity. Furthermore, its poor structural stability makes it difficult to effectively suppress manganese precipitation and lattice distortion. Based on these findings, the inventors considered developing a new carbon coating method, specifically designing a cathode material with a double-layer carbon coating.

[0025] In view of this, the first aspect of this application proposes a carbon-coated modified lithium manganese iron phosphate cathode material, comprising carbon-coated modified lithium manganese iron phosphate cathode material particles. The carbon-coated modified lithium manganese iron phosphate cathode material particles include a lithium manganese iron phosphate core, a first carbon coating layer, and a second carbon coating layer, wherein the first carbon coating layer is located between the lithium manganese iron phosphate and the second carbon coating layer, and the graphitization degree of the first carbon coating layer is greater than that of the second carbon coating layer. In this application, by forming a first carbon coating layer and a second carbon coating layer with different graphitization degrees on the surface of the lithium manganese iron phosphate core, gradient carbon coating and multifunctional coating are achieved. Thus, the first and second carbon coating layers can work synergistically to better construct and optimize the conductive network of the lithium manganese iron phosphate core, forming a continuous conductive pathway inside and on the surface of the lithium manganese iron phosphate, enhancing electron transport capability. Simultaneously, the double-layer carbon coating can more precisely control the microstructure of the lithium manganese iron phosphate, thereby effectively controlling the particle size, compaction density, etc., of the cathode material, effectively improving various performance characteristics of the cathode material.

[0026] In some embodiments, the first carbon coating layer is a carbon layer with a high degree of graphitization, wherein I D / I G =0.60~0.95, where, I D / I G I represents the ratio of the Raman D peak to the G peak. D / I G The smaller the ratio, the higher the degree of graphitization. For example, the I of the first carbon coating layer... D / I G The possible values ​​are 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, and 0.95. Due to its high degree of graphitization, it possesses excellent electronic conductivity. The first carbon coating layer, located between the lithium manganese iron phosphate and the second carbon coating layer, can initially construct efficient electron transport channels on the surface of lithium manganese iron phosphate, allowing electrons to transport more quickly across the surface and laying the foundation for building a continuous conductive network.

[0027] Graphitization degree is a core indicator for measuring the degree to which carbon materials transform into a graphite crystal structure at high temperatures. Essentially, it reflects the level of orderliness in the arrangement of carbon atoms within the material, that is, how close the microstructure of the carbon material is to the ideal graphite crystal structure. For example, carbon atoms have a hexagonal planar structure, but if a carbon layer with a high degree of graphitization is formed, it indicates a highly ordered layered structure between the carbon atoms. Conversely, if a carbon layer with a low degree of graphitization is formed, it indicates a predominantly random layered structure with no fixed stacking pattern.

[0028] Specifically, the degree of graphitization can be measured using Raman spectroscopy, by analyzing a 1350 cm⁻¹ graph. -1The left and right D peaks (representing structural defects) and 1580cm -1 The intensity ratio of the left and right G peaks (representing the degree of graphitization) (I) D / I G The graphitization degree is assessed by a ratio of 1 / 2 to 2 / 3. The smaller the ratio, the higher the graphitization degree.

[0029] In some embodiments, the thickness of the first carbon coating layer is 1 nm to 5 nm. For example, the thickness of the first carbon coating layer can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, etc. The thickness of the first carbon coating layer within the above range can basically form a sufficient conductive path to ensure electron transport capability. This can basically avoid the problems that may occur if the thickness is too thin, which may result in the inability to form a continuous and effective conductive layer, and the problems that may occur if the thickness is too thick, which may increase the internal resistance of the material and increase the unnecessary carbon content.

[0030] Specifically, the thickness of the first carbon coating layer can be measured using transmission electron microscopy (TEM). The sample is ultrasonically dispersed in ethanol, and after preparing the sample using a micro-grid copper mesh, the morphology of the coating layer is measured using TEM. This allows direct observation of the microstructure of the carbon coating layer and measurement of its thickness. Alternatively, for materials such as lithium manganese iron phosphate, a resin embedding method can be used to prepare the sample. Through mechanical thinning and double-sided ion thinning, the sample forms a structure with a distinct interface, with the coated material as the core and an ultrathin coating material as the outer shell. The sample is then continuously tilted in the TEM, and an appropriate tilt angle is determined based on the Kikuchi pattern. The electron beam is then incident along the low-index zone axis of the carbon-coated lithium manganese iron phosphate, allowing for clear and accurate observation of the interface between the carbon coating layer and the internal material, and precise measurement of the thickness of the ultrathin coating layer.

[0031] In some embodiments, the second carbon coating layer is a carbon layer with a low degree of graphitization, wherein I D / I G =0.90~1.40, where, I D / I G I represents the ratio of the Raman D peak to the G peak. D / I G The smaller the ratio, the higher the degree of graphitization. For example, the second carbon coating layer I... D / I G The values ​​can be 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, etc. The second carbon coating layer can be amorphous carbon, and its main function is to further improve the conductive network. Working synergistically with the first carbon coating layer, it forms a continuous conductive path inside and on the surface of lithium manganese iron phosphate, further enhancing the electron transport capability. In addition, the disordered carbon layer can, to some extent, buffer and protect the material, mitigating the damage to the material structure caused by factors such as volume changes during battery charging and discharging.

[0032] In some embodiments, the thickness of the second carbon coating layer is 5 nm to 10 nm. For example, the thickness of the second carbon coating layer can be 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, etc. The thickness of the second carbon coating layer within the above range helps to work synergistically with the first carbon coating layer to maintain the stability of the overall structure of lithium iron manganese phosphate, while effectively controlling parameters such as the compaction density of the cathode material and improving the various properties of the cathode material.

[0033] Specifically, the test method for the thickness of the second carbon layer is the same as the test method for the first carbon coating layer mentioned above.

[0034] In some embodiments, the general chemical formula of the lithium manganese iron phosphate core is Li. a Fe x Mn 1-x PO4, where 0.4≤x≤0.50, 0.9≤a≤1.2, thus, the lithium manganese iron phosphate core can effectively balance the effects of Fe and Mn, thereby improving the electrochemical performance of the cathode material, such as charge and discharge capacity and cycle stability. In addition, the lithium ion content within the above range can basically ensure that the battery can smoothly insert and extract during charge and discharge, thereby optimizing the rate performance of the battery.

[0035] In some embodiments, the particle size of the carbon-coated modified lithium manganese iron phosphate cathode material is 0.35 μm to 0.60 μm, specifically, it can be 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, etc. The particle size of the cathode material within the above range can basically ensure that lithium ions can be inserted and extracted more quickly, thereby helping to improve the charge and discharge rate and rate performance of the battery. At the same time, the particle size of the cathode material within the above range is also conducive to ensuring the contact area between the cathode material and the electrolyte, promoting the electrochemical reaction, and improving the battery capacity.

[0036] Specifically, the particle size mentioned above is the D50 particle size, meaning that 50% of the particles in the carbon-coated modified lithium manganese iron phosphate cathode material are smaller than this size. This can be tested using a Malvern particle size analyzer. During testing, the instrument's opacity should be set to 5%-12%. Select the appropriate material type and dispersant from the material library, then add the sample drop by drop and observe.

[0037] In some embodiments, the compaction density of the carbon-coated modified lithium manganese iron phosphate cathode material is 2.25 g / cm³. 3 ~2.35g / cm 3 Specifically, it can be 2.25 g / cm³. 3 2.30 g / cm 3 2.35 g / cm 3The compaction density within the above range can basically ensure sufficient contact between the cathode material particles, thereby ensuring the electronic conductivity of the electrode, reducing the internal resistance of the electrode, promoting the electrochemical reaction, and improving the power performance of the battery.

[0038] Specifically, compaction density is a physical parameter that measures the mass of a unit volume of a loose solid material (such as the carbon-coated modified lithium manganese iron phosphate cathode material particles mentioned above) after being compacted under external pressure. Essentially, it reflects the degree to which the voids between the material particles are compressed. The compaction density of particles can be calculated using ρ=m / v, where m is the mass of the particle (in grams) and v is the total volume of the compacted particles (in centimeters). 3 .

[0039] In a second aspect of this application, a method for preparing carbon-coated modified lithium manganese iron phosphate cathode material is proposed, comprising: S10: mixing and ball milling a manganese iron phosphate precursor and a lithium source to obtain a raw material mixture.

[0040] In this step, the lithium source and water are first ball-milled a third time to obtain a third mixture, and then the third mixture and the iron-manganese phosphate precursor are ball-milled a fourth time to obtain the raw material mixture.

[0041] In some embodiments, the third ball milling time is 1 min to 10 min, for example, it can be 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, etc. The third ball milling time within the above range can basically ensure that the lithium source is evenly dispersed in water, resulting in a uniform third mixture; in addition, the fourth ball milling time is 10 min to 30 min, for example, it can be 10 min, 15 min, 20 min, 25 min, 30 min, etc. The ball milling time within the above range can basically ensure that the third mixture and the iron manganese phosphate precursor are evenly mixed, resulting in a uniform raw material mixture.

[0042] In some embodiments, an exemplary method for preparing the iron-manganese phosphate precursor is as follows: manganese source, iron source, and phosphorus source are mixed in a mass ratio of 6:4:10, and a certain amount of ammonia is added to adjust the pH of the reaction system to 4-4.5, causing a co-precipitation reaction. The precursor is then obtained by filtration, drying, and sintering. 0.4 Mn 0.6 (PO4). This preparation method forms an atomically uniformly dispersed manganese iron phosphate precursor through a liquid-phase reaction, which helps to improve the structural stability of the cathode material and thus improve its various properties.

[0043] In some embodiments, the molar concentration of the lithium source in the third mixture is 1 mol / L to 3 mol / L. Specifically, the molar concentration of the lithium source in the third mixture can be 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, etc. The molar concentration of the lithium source within the above range can basically ensure that the chemical composition of the prepared lithium manganese iron phosphate precursor is accurate and the structure is stable, which is beneficial to the subsequent preparation of a cathode material with excellent performance.

[0044] In some embodiments, the molar ratio of the lithium source to the iron-manganese phosphate precursor is 1.01 to 1.09:1. Specifically, the molar ratio of the lithium source to the iron-manganese phosphate precursor can be 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, 1.09:1, etc. In the reaction system for synthesizing lithium iron-manganese phosphate precursor, an excess lithium source can provide sufficient lithium ions for the reaction, allowing the iron, manganese, and other elements in the iron-manganese phosphate precursor to combine more fully with the lithium ions, promoting the reaction towards the formation of lithium iron-manganese phosphate, and improving the conversion rate of the reaction.

[0045] In some embodiments, the particle size of the lithium source is 1 micrometer to 5 micrometers, specifically, it can be 1 micrometer, 2 micrometers, 3 micrometers, 4 micrometers, 5 micrometers, etc. The particle size of the lithium source within the above range can basically ensure that it is mixed uniformly with other components during ball milling, so that the lithium source is more fully dispersed in the system, which is conducive to achieving a more uniform distribution in subsequent reactions. At the same time, the particle size of the lithium source within the above range can basically ensure that the specific surface area of ​​the lithium source is large, thereby improving the reactivity of the lithium source, making the subsequent reaction more complete and faster, and helping to form a more stable and ideal lithium iron manganese phosphate precursor structure.

[0046] Specifically, the particle size of the lithium source mentioned above is D50, which can be tested using a Malvern particle size analyzer.

[0047] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium sulfate, lithium monohydrogen carbonate, and lithium dihydrogen carbonate. The lithium sources mentioned above have high solubility in water, which helps to uniformly disperse them in the aqueous solution to obtain a homogeneous second mixture. Furthermore, the lithium sources mentioned above have high chemical stability at room temperature, which facilitates storage and transportation.

[0048] In some embodiments, the manganese source includes at least one of manganese carbonate, manganese sulfate, manganese dioxide, and manganese oxide; the iron source includes at least one of ferrous sulfate, ferric carbonate, and ferrous oxide; and the phosphorus source includes at least one of sodium dihydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, and phosphoric acid.

[0049] S20: The raw material mixture and the first carbon source are mixed and subjected to a first ball milling to obtain a first mixture.

[0050] In this step, the aforementioned raw material mixture and the first carbon source are placed in a ball mill for a first ball milling to obtain a first mixture. The first ball milling time is 2 min to 10 min, specifically, it can be 2 min, 4 min, 6 min, 8 min, 10 min, etc. The first ball milling time within the above range can basically ensure that the raw material mixture and the first carbon source are mixed evenly, which helps to uniformly coat the surface of the first carbon coating layer on the lithium manganese iron phosphate through subsequent sintering, while ensuring the structural stability of the lithium manganese iron phosphate core obtained after sintering. In some embodiments, the first carbon source includes at least one of sucrose, glucose, polyethylene, and polypropylene. The molecular structure of the carbon source is relatively simple. Specifically, the carbon source includes only one of the functional groups such as hydroxyl and aldehyde groups. The carbon atoms are arranged in a relatively ordered manner, and the carbonization process is relatively easy to control. It can quickly deposit and form a first carbon layer on the material surface, providing a basis for subsequent carbon coating. At the same time, since the carbon atoms in the carbon source are arranged in a relatively ordered manner, the graphitization degree of the first carbon coating layer formed after carbonization will be relatively high, which can provide a good electronic conductivity basis for lithium iron manganese phosphate, initially build an electron transport channel, and help improve the conductivity of lithium iron manganese phosphate.

[0051] For example, glucose is a monosaccharide with a six-carbon aldose structure. Its molecular structure is simple and well-defined, and its carbon atoms are arranged in an orderly manner. When treated at high temperatures, non-carbon elements (such as hydrogen and oxygen) in the molecule can be removed relatively uniformly and stably in the form of small molecules (such as water and carbon dioxide), leaving a relatively regular carbon skeleton, thereby forming a carbon layer with a high degree of graphitization.

[0052] In some embodiments, based on the theoretical mass of lithium manganese iron phosphate, the amount of the first carbon source added is 1wt% to 5wt%, specifically, it can be 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, etc. Within this range, the amount of the first carbon source added can generally form a first carbon coating layer of moderate thickness on the surface of lithium manganese iron phosphate, effectively enhancing the conductivity of lithium manganese iron phosphate while ensuring a high proportion of active material, i.e., high energy density. This largely avoids problems such as a reduced proportion of active material due to excessive addition of the first carbon source, affecting the performance of the cathode material, and a thin first carbon coating layer due to insufficient addition of the first carbon source, which cannot effectively construct electron transport channels.

[0053] For example, if the target lithium manganese iron phosphate has the chemical formula LiFe 0.5 Mn 0.5PO4, during the reaction, 1 mol of lithium source (based on lithium atoms) and 1 mol of iron-manganese phosphate precursor were actually added to generate 1 mol of LiFe. 0.5 Mn 0.5 The theoretical mass of PO4 is m = 0.5 × (M Fe +M Mn )+M P +4×M o +M Li Where M is the relative atomic mass, the amount of the first carbon source added can be calculated based on the theoretical weight mentioned above.

[0054] S30: The first mixture is mixed and subjected to first sand milling, first spray drying and first sintering to obtain an intermediate product.

[0055] In this step, for example, the first mixture is placed in a sand mill for first sand milling to further mix the first mixture. The mixture after sand milling is spray dried at 100℃~105℃ for 30~90 minutes and then sintered for the first time to obtain an intermediate product, which is lithium manganese iron phosphate with a first carbon coating layer.

[0056] In some embodiments, the temperature of the spray drying inlet is 100℃~105℃, specifically 100℃, 101℃, 102℃, 103℃, 104℃, 105℃, etc. This temperature range provides sufficient heat to rapidly evaporate the moisture in the mixture, achieving efficient drying. Effective removal of moisture ensures the stability of the material composition during subsequent high-temperature sintering, preventing material agglomeration and porosity caused by residual moisture during sintering, which would otherwise affect the quality and structure of the intermediate product.

[0057] In some embodiments, the first sintering includes: heating to 400°C~500°C at a rate of 3°C / min~6°C / min and sintering for 3h~5h, then heating to 650°C~750°C at a rate of 3°C / min~6°C / min and sintering for 6h~12h. Specifically, the heating rate can be 3°C / min, 4°C / min, 5°C / min, 6°C / min, etc. Heating rates within the above range can basically ensure uniform heating of the mixture, which helps to achieve a uniform structure in the final intermediate product. It can largely avoid uneven heating of the mixture caused by excessively rapid heating rates, which can easily damage the structural stability of the intermediate product due to localized overheating.

[0058] Furthermore, in the first sintering process, the temperature is first raised to 400℃~500℃, for example, it can be 400℃, 420℃, 440℃, 460℃, 480℃, 500℃, etc. Within the above temperature range, the carbon source begins to initially carbonize and form some small carbon particles. At the same time, the lithium iron phosphate precursor forms lithium manganese iron phosphate with an olivine structure within the above temperature range. Sintering for 3h~5h can basically ensure that the carbon source is completely carbonized into carbon particles. Then the sintering temperature is raised to 650℃~750℃, specifically, it can be 650℃, 680℃, 700℃, 720℃, 750℃, etc. Within the above temperature range, the carbon particles further grow and fuse to form a continuous, uniform first carbon coating layer with a high degree of graphitization.

[0059] The aforementioned gradient heating process can precisely control the formation of the first carbon coating layer and the formation of the lithium manganese iron phosphate crystal phase, so that the first carbon coating layer is tightly attached to the surface of the lithium manganese iron phosphate particles with olivine structure, thereby enhancing conductivity, protecting the material structure, and improving the electrochemical performance of the cathode material.

[0060] S30: The intermediate product and the second carbon source are mixed and subjected to a second ball milling to obtain a second mixture.

[0061] In this step, for example, the intermediate product and the second carbon source obtained above are placed in a ball mill and ball milled for 2 to 10 minutes. The ball milling time within the above range can basically ensure that the intermediate product and the second carbon source are evenly dispersed, thereby obtaining a uniform second mixture.

[0062] In some embodiments, the mass ratio of the second carbon source added to the first carbon source added is 2:1, which helps to form a second carbon coating layer of the target thickness, thereby helping to work synergistically with the first carbon coating layer to maintain the stability of the overall structure of lithium iron manganese phosphate.

[0063] In some embodiments, the second carbon source includes at least one of polyethylene glycol, cellulose, citric acid, tartaric acid, and epoxy resin. The structure of the carbon source is more complex than that of the first carbon source. Specifically, the carbon source includes at least one of the functional groups such as carboxyl, ether, epoxy, and hydroxyl groups. The arrangement of carbon atoms is relatively irregular. As mentioned above, the carbon skeleton left after high temperature is also irregular, thus forming a disordered second carbon coating layer. This helps to further improve the conductive network and works synergistically with the first carbon coating layer to form a continuous conductive path inside and on the surface of lithium manganese iron phosphate, further enhancing the electron transport capability. At the same time, the disordered second carbon coating layer can buffer and protect the cathode material to a certain extent, mitigating the damage to the cathode material structure caused by factors such as changes in the volume of the cathode material during battery charging and discharging.

[0064] In some embodiments, the solid content of the second mixture is 15% to 65%, specifically, it can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, etc. The solid content within the above range helps to balance the flowability of the second mixture with the particle dispersibility therein, thereby further improving the uniformity of the cathode material. It can basically avoid particle agglomeration caused by excessively high solid content of the second mixture, as well as the reduction in subsequent drying and sintering efficiency caused by excessively low solid content.

[0065] S40: The second mixture is subjected to a second sand milling, spray drying, second sintering, and pulverization in sequence to obtain carbon-coated modified lithium manganese iron phosphate cathode material.

[0066] In this step, for example, the second mixture is placed in a sand mill for further mixing. The milled mixture is then spray-dried at 100℃~105℃ for 30~90 minutes, followed by a second sintering and pulverization to obtain carbon-coated modified lithium manganese iron phosphate cathode material. The particle size of the product after the second sand milling is controlled to be 0.35 micrometers~0.45 micrometers, specifically 0.35 micrometers, 0.37 micrometers, 0.39 micrometers, 0.40 micrometers, 0.42 micrometers, 0.44 micrometers, 0.45 micrometers, etc. The particle size of the product after the second sand milling is within the above range, which can basically ensure that the specific surface area of ​​the product is suitable. This helps to accelerate the evaporation of moisture and improve the drying efficiency during subsequent drying, while also making the drying more uniform and facilitating more complete subsequent sintering.

[0067] In some embodiments, the second sintering includes: heating to 400°C-500°C at a rate of 3°C-6°C and sintering for 3-8 hours, then heating to 550°C-680°C at a rate of 3°C-6°C and sintering for 6-10 hours. Specifically, the heating rate within the above range can basically ensure that the second mixture is heated uniformly, which helps to obtain a uniform structure of the final product. The final sintering temperature is 550°C-680°C, specifically 550°C, 570°C, 590°C, 600°C, 620°C, 640°C, 650°C, 680°C, etc. The bond energy of the second carbon source is relatively low, and it can break and recombine at a lower temperature. At the same time, the second sintering temperature is lower than the first sintering temperature, which helps to rearrange and recombine carbon atoms to form a disordered second carbon coating layer.

[0068] In some embodiments, the product after the second sintering is pulverized to obtain a refined carbon-coated modified lithium manganese iron phosphate cathode material. Specifically, the pulverization frequency is 1 min / time to 3 min / time, for example, it can be 1 min / time, 2 min / time, 3 min / time, etc., and the number of pulverizations is 2 to 5 times. This helps to ensure that the particle size of the obtained carbon-coated modified lithium manganese iron phosphate cathode material is within the target range, thereby further improving the compaction density of the carbon-coated modified lithium manganese iron phosphate cathode material and increasing the energy density of the cathode material.

[0069] In a third aspect of this application, a positive electrode sheet is provided, comprising the aforementioned carbon-coated modified lithium manganese iron phosphate positive electrode material. Specifically, the positive electrode sheet comprises a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer comprising the carbon-coated modified lithium manganese iron phosphate positive electrode material.

[0070] In some embodiments, the cathode material layer includes the aforementioned carbon-coated modified lithium manganese iron phosphate cathode material and auxiliary materials, including conductive agents, binders, dispersants, etc., thereby helping to obtain a cathode material layer with superior conductivity.

[0071] In a fourth aspect of this application, a battery is proposed. This battery includes the aforementioned carbon-coated modified lithium manganese iron phosphate cathode material or the aforementioned cathode sheet, thereby exhibiting high charge / discharge rates, excellent energy density, and capacity.

[0072] In some embodiments, the battery can be an all-solid-state battery, including the above-mentioned positive electrode, negative electrode and separator, etc. During the charging and discharging process of the battery, active ions are inserted and extracted back and forth between the positive electrode and the negative electrode. The separator is disposed between the positive electrode and the negative electrode, mainly to prevent short circuit between the positive electrode and the negative electrode, while allowing active ions to pass through.

[0073] In some embodiments, the negative electrode sheet may include a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector. As an example, the negative electrode active material layer may include a negative electrode material, a thickener, a conductive agent, and a binder.

[0074] Specifically, the negative electrode current collector can be a metal foil, for example, copper foil. The negative electrode material can include carbon-based materials, silicon-based materials, tin-based materials, etc. The binder in the negative electrode material layer can include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), and carboxymethyl chitosan (CMCS). The conductive agent in the negative electrode material layer can include, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0075] In some embodiments, the separator may be a separator known in the art that can be used in lithium-ion batteries and is stable to the electrolyte used, such as a polyethylene separator, a polypropylene separator, a polyethylene / polypropylene composite separator, etc.

[0076] In a fifth aspect of this application, an electrical device is provided, comprising the aforementioned positive electrode or the aforementioned battery. Therefore, the electrical device has high capacity and energy density.

[0077] In some embodiments, the specific type of electrical device is not particularly limited and can be any device that uses an all-solid-state battery as a power source or energy storage unit. Examples of electrical devices include, but are not limited to, electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), mobile terminals (e.g., mobile phones, laptops, game consoles, wearable devices, etc.), drones, aerospace equipment, satellites, ships, energy storage systems, and so on.

[0078] It is understandable that, in addition to the all-solid-state battery mentioned above, the electrical device also includes other necessary structures and components, all of which can be made with reference to conventional technologies, such as electric vehicles, which may include the body, chassis, tires, navigation system, radar system, steering system, braking system, lubrication system, cooling system, driving system, etc., which will not be described in detail here.

[0079] The present application will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present application in any way. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in the art or in accordance with the product manual.

[0080] Example 1: Carbon-coated modified manganese iron phosphate cathode material was prepared according to the preparation method of this application, specifically as follows: 1. Preparation of manganese iron phosphate precursor: Manganese carbonate, ferrous sulfate, and phosphoric acid were mixed in a mass ratio of 6:4:10. A certain amount of ammonia was added to adjust the pH of the reaction system to 4.5, causing a co-precipitation reaction. The manganese iron phosphate precursor (Fe) was obtained through filtration, drying, and sintering. 0.4 Mn 0.6 PO4); 2. Preparation of raw material mixture: Place 284.67g of lithium carbonate and 1800g of water in a ball mill and ball mill for 5 minutes, then add 1000g of Fe 0.4 Mn 0.6 After adding PO4, continue ball milling for 10 minutes; 3. Add 30g of sucrose to the above raw material mixture and ball mill for the first time. After ball milling for 6 minutes, the first mixture is obtained; place the first mixture in a sand mill and sand mill for 30 minutes. Spray dry the sand-milled mixture at 100℃ for 40 minutes. After drying, heat up to 450℃ at a rate of 3℃ / min and sinter for 3 hours. Then heat up to 720℃ at a rate of 3℃ / min and sinter for 8 hours to obtain an intermediate product, which is manganese iron phosphate with a first carbon coating layer; 4 The intermediate product obtained above and 60g of polyethylene glycol were placed in a ball mill and ball milled for 6 minutes to obtain a homogeneous second mixture. The second mixture was then placed in a sand mill and sand milled for 30 minutes. The sand-milled mixture was spray-dried at 100°C for 40 minutes, then heated to 400°C at a rate of 3°C / min and sintered for 4 hours. The temperature was then increased to 650°C at a rate of 3°C / min and sintered for 10 hours. After sintering, the mixture was placed in a mechanical pulverizer and pulverized 5 times at a frequency of 1 minute / time to obtain carbon-coated modified lithium manganese iron phosphate cathode material.

[0081] Preparation of the positive electrode sheet: The positive electrode material, conductive agent, binder, and dispersant were mixed in a mass ratio of 96.8:1.5:1.5:0.2. The mixture was first dry-mixed, then wet-mixed with an appropriate amount of solvent to obtain a positive electrode slurry. The solid content of the positive electrode slurry was controlled to be 65%. Further, the positive electrode slurry was coated onto a 14μm thick carbon-coated aluminum foil (where the aluminum foil thickness was 12 micrometers, and each side of the aluminum foil was coated with a 1-micrometer thick carbon layer), resulting in a double-sided areal density of 40 mg / cm³ for the positive electrode sheet. 2 After drying at 100℃~120℃, the dried positive electrode sheet is obtained. Finally, the dried positive electrode sheet is rolled to achieve a compaction density of 2.30 g / cm³. 3The prepared positive electrode sheet was die-cut according to the cell size and placed in a nitrogen oven at 100℃ for later use. The negative electrode sheet was prepared by homogenizing a mixture of hard carbon, conductive agent, and binder in a mass ratio of 96.5:1.0:2.5. The mixture was first dry-mixed, then wet-mixed with an appropriate amount of solvent to obtain the negative electrode slurry, with a solid content controlled at 55%. The negative electrode slurry was then coated onto a 6μm thick copper foil, resulting in a double-sided areal density of 13mg / cm². 2 The negative electrode sheet is dried at 80℃. Finally, the dried negative electrode sheet is rolled to achieve a compaction density of 1.50 g / cm³. 3 Then, the rolled negative electrode sheet is die-cut according to the cell size and placed in a nitrogen oven at 90℃ for later use. Lithium-ion battery assembly: a stacking process is used, and one more negative electrode sheet is needed than the positive electrode sheet. The number of electrode sheets is calculated based on a design capacity of 1Ah. The stacking process is performed as follows: separator, negative electrode sheet, separator, positive electrode sheet, separator, negative electrode sheet. After hot pressing, welding of tabs, and encapsulation, the moisture content is controlled below 400ppm. Electrolyte is injected according to the design capacity, with an injection coefficient of 4.0g / Ah.

[0082] Examples 2-13 are the same as Example 1, with the main differences shown in Table 1.

[0083] Comparative Example 1 is the same as Example 1, except that it does not have a second carbon coating layer.

[0084] The test results for the particle size, compaction density, carbon coating thickness, and graphitization degree of the carbon-coated modified lithium manganese iron phosphate cathode materials obtained through Examples 1-13 and Comparative Example 1 are shown in Table 2.

[0085] The electrochemical performance of the batteries using the cathode materials prepared in Examples 1-13 and Comparative Example 1 is shown in Table 3.

[0086] Detection method: Graphitization degree of the carbon coating layer was determined by Raman spectroscopy, measuring the Raman shifts D and G peaks, and calculating I. D / I G The smaller the ratio, the greater the degree of graphitization.

[0087] The dissolution values ​​of Mn and Fe are determined by mixing lithium manganese iron phosphate powder with electrolyte and allowing it to stand or stir at a specific temperature for a period of time (usually 24 hours) to simulate the storage or cycling conditions of a battery at high temperatures. The mixed electrolyte is then separated from the solid powder by centrifugation or filtration to obtain a clear electrolyte containing dissolved metal ions. The separated electrolyte is then diluted to a suitable concentration range with reagents such as nitric acid. Finally, the elements Mn and Fe are tested using the ICP-MS method.

[0088] Powder resistivity test: The powder sample is loaded into a mold and pressed into a disc under a fixed pressure; four equally spaced probes are pressed vertically onto the sample surface; current is passed through the two outer probes and the voltage drop between the two inner probes is measured; the volume resistivity or surface resistivity of the sample is calculated according to the formula. The test results are affected by pressure, so they need to be performed under standard pressure, usually 25 MPa.

[0089] Specific surface area: At liquid nitrogen temperature, an inert gas (such as nitrogen) is physically adsorbed on the sample surface; the adsorption amount is measured under different pressures by changing the gas pressure; the adsorption data is processed according to the BET formula to calculate the monolayer saturated adsorption amount; combined with the cross-sectional area of ​​the adsorbed molecules, the total specific surface area of ​​the sample is calculated.

[0090] 0.1C initial efficiency / 0.1C charging capacity / 0.1C discharging capacity: For coin cells, the prepared carbon-coated modified lithium manganese iron phosphate material, acetylene black, and polytetrafluoroethylene were mixed in a mass ratio of 97.9:0.9:1.2 in a vacuum mixer. Then, NMP solvent was added to the mixture, and the mixture was stirred until homogeneous under vacuum to obtain the positive electrode slurry of this embodiment. The above positive electrode slurry was uniformly coated on both surfaces of the positive electrode current collector aluminum foil, air-dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet was obtained. The semi-finished positive electrode sheet was then cold-pressed and cut to obtain the positive electrode sheet to be assembled. A button cell was then assembled using an R2032 coin cell casing, with a lithium sheet as the negative electrode and a PE separator. 80 mL of electrolyte (using Xinzhoubang lithium-ion battery electrolyte, model LBC3401A60) was added.

[0091]

[0092]

[0093] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the indicated orientation or positional relationship, and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0094] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0095] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0096] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0097] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0098] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A carbon-coated modified lithium manganese iron phosphate cathode material, characterized in that, The material includes carbon-coated modified lithium manganese iron phosphate cathode material particles, wherein the carbon-coated modified lithium manganese iron phosphate cathode material particles include a lithium manganese iron phosphate core, a first carbon coating layer and a second carbon coating layer, and the first carbon coating layer is located between the lithium manganese iron phosphate core and the second carbon coating layer; the graphitization degree of the first carbon coating layer is greater than that of the second carbon coating layer.

2. The carbon-coated modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, At least one of the following conditions must be met: the I of the first carbon coating layer D / I G =0.60~0.95; I of the second carbon coating layer D / I G =0.90~1.40; where, I D / I G I represents the ratio of the Raman D peak to the G peak. D / I G The smaller the ratio, the higher the degree of graphitization.

3. The carbon-coated modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, At least one of the following conditions must be met: the thickness of the first carbon coating layer is 1 nm to 5 nm; the thickness of the second carbon coating layer is 5 nm to 10 nm; and the chemical formula of the lithium manganese iron phosphate core is Li. a Fe x Mn 1-x PO4, where 0.4≤x≤0.50, 0.9≤a≤1.

2.

4. The carbon-coated modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The carbon-coated modified lithium manganese iron phosphate cathode material must meet at least one of the following conditions: the particle size is 0.35 μm to 0.60 μm; the compaction density of the carbon-coated modified lithium manganese iron phosphate cathode material is 2.25 g / cm³. 3 ~2.35 g / cm 3 .

5. A method for preparing a carbon-coated modified lithium manganese iron phosphate cathode material as described in any one of claims 1 to 4, characterized in that, include: The iron-manganese phosphate precursor and the lithium source are mixed and ball-milled to obtain a raw material mixture; the raw material mixture is mixed with a first carbon source and then ball-milled to obtain a first mixture; The first mixture is subjected to a first sand milling, a first spray drying, and a first sintering in sequence to obtain an intermediate product; the intermediate product and a second carbon source are mixed and subjected to a second ball milling to obtain a second mixture; the second mixture is subjected to a second sand milling, a second spray drying, a second sintering, and pulverization in sequence to obtain a carbon-coated modified lithium manganese iron phosphate cathode material; wherein, the first carbon source includes one of hydroxyl and aldehyde functional groups; the second carbon source includes at least one of carboxyl, ether, epoxy, and hydroxyl functional groups.

6. The preparation method according to claim 5, characterized in that, The mixture meets at least one of the following conditions: the first ball milling time is 2 min to 10 min; the second ball milling time is 2 min to 10 min; the solid content of the second mixture is 15% to 65%; and the particle size of the product from the second sand milling is 0.35 micrometers to 0.45 micrometers.

7. The preparation method according to claim 5, characterized in that, The first sintering includes: heating to 400℃~500℃ at a rate of 3℃ / min~6℃ / min and sintering for 3h~5h; then heating to 650℃~750℃ at a rate of 3℃ / min~6℃ / min and sintering for 6h~12h.

8. The preparation method according to claim 5, characterized in that, The second sintering process includes: heating to 400℃~500℃ at a rate of 3℃~6℃ and sintering for 3h~8h; then heating to 550℃~680℃ at a rate of 3℃~6℃ and sintering for 6h~10h.

9. The preparation method according to claim 5, characterized in that, Based on the theoretical mass of lithium manganese iron phosphate, the amount of the first carbon source added is 1wt%~5wt%, and / or based on the theoretical mass of lithium manganese iron phosphate, the mass ratio of the amount of the second carbon source added to the amount of the first carbon source added is 2:

1.

10. The preparation method according to claim 5, characterized in that, The first carbon source includes at least one of sucrose, glucose, polyethylene, and polypropylene; the second carbon source includes at least one of polyethylene glycol, cellulose, citric acid, tartaric acid, and epoxy resin.

11. The preparation method according to claim 5, characterized in that, The molar ratio of the lithium source to the iron-manganese phosphate precursor is 1.01~1.09:

1.

12. The preparation method according to claim 5, characterized in that, The process of ball milling the iron-manganese phosphate precursor and the lithium source includes: mixing the lithium source and water and performing a third ball milling to obtain a third mixture; mixing the third mixture with the iron-manganese phosphate precursor and performing a fourth ball milling to obtain the raw material mixture.

13. The preparation method according to claim 12, characterized in that, The following conditions must be met: the third ball milling time is 1 min to 10 min; the fourth ball milling time is 10 min to 30 min; the molar concentration of the lithium source in the third mixture is 1 mol / L to 3 mol / L; the particle size of the lithium source is 1 μm to 5 μm; and the lithium source includes at least one of lithium carbonate, lithium sulfate, lithium bicarbonate, and lithium dihydrogen carbonate.

14. A positive electrode plate, characterized in that, Including the carbon-coated modified lithium manganese iron phosphate cathode material as described in any one of claims 1 to 4.

15. A battery, characterized in that, Includes the carbon-coated modified lithium manganese iron phosphate cathode material according to any one of claims 1 to 4 or the cathode sheet according to claim 14.

16. An electrical appliance, characterized in that, Includes the carbon-coated modified lithium manganese iron phosphate cathode material according to any one of claims 1 to 4, the cathode sheet according to claim 14, or the battery according to claim 15.

Images