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

Abstract

This application relates to the field of lithium battery technology, and in particular to a coated lithium manganese iron phosphate cathode material, its preparation method, and its application. The coated lithium manganese iron phosphate cathode material includes: a first coating layer, a second coating layer, and a third coating layer of lithium manganese iron phosphate material; the first coating layer is coated on the surface of the lithium manganese iron phosphate material, and the material of the first coating layer is carbon derived from organic matter; the second coating layer is coated on the surface of the first coating layer; the third coating layer is coated on the surface of the second coating layer; the materials of the second and third coating layers independently include magnesium and / or tellurium. The coated lithium manganese iron phosphate cathode material provided by this application has high electrical conductivity and excellent cycle performance.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a coated lithium manganese iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] With increasing environmental awareness and the booming development of new energy products such as electric vehicles, the demand for high-performance lithium batteries (low internal resistance, high energy density, high safety, and long cycle stability) is becoming more urgent. Cathode materials are a decisive factor affecting lithium battery performance; therefore, improving the electrochemical performance of cathode materials has a significant impact on improving the energy density and cycle stability of lithium batteries.

[0003] However, traditionally used cathode materials, such as lithium manganese iron phosphate (LFP), have poor conductivity and cycle performance, failing to meet market demands. To address this, researchers have attempted to improve these issues by forming a coating layer on its surface. However, current methods typically involve first preparing the LFP core and then depositing the coating layer on its surface. This method makes it difficult for the coating layer to uniformly and completely coat the LFP core surface, resulting in LFP cathode materials with conductivity and cycle performance that still fall short of practical requirements, thus limiting their application. Summary of the Invention

[0004] Therefore, it is necessary to provide a coated lithium manganese iron phosphate cathode material, its preparation method, and its applications. The coated lithium manganese iron phosphate cathode material provided in this application has high conductivity and excellent cycle performance.

[0005] In a first aspect, this application provides a coated lithium manganese iron phosphate cathode material, comprising:

[0006] Lithium iron manganese phosphate materials;

[0007] A first coating layer is applied to the surface of the lithium manganese iron phosphate material. The material of the first coating layer is carbon, and the carbon is derived from organic matter.

[0008] A second coating layer is applied to the surface of the first coating layer.

[0009] A third coating layer is applied to the surface of the second coating layer.

[0010] The materials of the second and third coating layers each independently comprise magnesium and / or tellurium.

[0011] In some embodiments, the organic compound is a polysaccharide, which includes one or more of glucose and starch.

[0012] In some embodiments, the material of the second coating layer includes magnesium, and the material of the third coating layer includes tellurium.

[0013] In some embodiments, the thicknesses of the first coating layer, the second coating layer, and the third coating layer are each independently 1.0 nm to 10.0 nm.

[0014] In some embodiments, the particle size D50 of the lithium manganese iron phosphate material is 800 nm to 1000 nm;

[0015] And / or, the particle size D50 of the coated lithium manganese iron phosphate cathode material is 800nm ​​to 1030nm.

[0016] Secondly, this application provides a method for preparing a coated lithium manganese iron phosphate cathode material as described in the first aspect, comprising the following steps:

[0017] The lithium manganese iron phosphate material is mixed with the organic matter and then subjected to a first sintering to melt the organic matter and form the first coating layer on the surface of the lithium manganese iron phosphate material.

[0018] The lithium manganese iron phosphate material with the first coating layer is mixed with the first metal and then sintered a second time to form a second coating layer on the surface of the first coating layer.

[0019] The lithium manganese iron phosphate material with the second coating layer is mixed with the second metal and then sintered for a third time to form a third coating layer on the surface of the second coating layer.

[0020] The first metal and the second metal each independently comprise magnesium and / or tellurium.

[0021] In some embodiments, the mass ratio of the lithium manganese iron phosphate material to the organic matter, the first metal, and the second metal is (95-99):(0.1-2):(0.3-3):(0.6-3).

[0022] In some embodiments, the atmosphere of the first sintering is a protective gas atmosphere, the temperature is 400℃~1200℃, and the time is 4h~30h;

[0023] And / or, the atmosphere of the second sintering is a protective gas atmosphere, the temperature is 400℃~1200℃, and the time is 4h~30h;

[0024] And / or, the atmosphere for the third sintering is a protective gas atmosphere, the temperature is 400℃~1200℃, and the time is 4h~30h.

[0025] Thirdly, this application provides a positive electrode sheet, including a positive current collector and a positive active material layer formed on at least one side of the positive current collector, wherein the positive active material layer includes the coated lithium manganese iron phosphate positive electrode material described in the first aspect.

[0026] Fourthly, this application provides a lithium battery, including the positive electrode sheet described in the third aspect.

[0027] The coated lithium manganese iron phosphate cathode material provided in this application is first coated with a carbon layer, then with a metal layer. The carbon layer improves the conductivity of the material itself, while the metal layer stabilizes the cathode material structure and enhances its conductivity. Through the synergistic effect of the multi-layered coating structure, the conductivity and structural stability of the coated lithium manganese iron phosphate cathode material can be improved, thereby increasing its cycle life and conductivity. Moreover, both the carbon layer and the metal layer can be uniformly and completely coated on the surface of the lithium manganese iron phosphate material. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of a method for preparing a coated lithium manganese iron phosphate cathode material in one embodiment;

[0030] Figure 2 This is a scanning electron microscope image of the coated lithium manganese iron phosphate cathode material prepared in Example 1;

[0031] Figure 3 This is a scanning electron microscope image of the coated lithium manganese iron phosphate cathode material prepared in Example 1. Detailed Implementation

[0032] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0034] the term:

[0035] As used herein, the term "and / or" encompasses any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" includes three parallel options: A, B, and "a combination of A and B".

[0036] In this document, unless otherwise stated, "one or more" means any one of the listed items or any combination of the listed items. Similarly, "one or more" and other instances of "one or more" are to be understood in the same way unless otherwise stated.

[0037] In this document, terms such as "further," "even further," "especially," "for example," "as," "example," and "exemplary" are used for descriptive purposes to indicate a connection in the coverage of different technical solutions presented earlier and later. However, they should not be construed as limitations on the preceding technical solution or on the scope of protection of this document. Unless otherwise specified, A (as in B) indicates that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0038] In this document, "optionally," "optionally," and "optional" mean that something is optional, that is, it is selected from either "present" or "absent." If multiple "options" appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "option" is independent. In this application, descriptions such as "optionally contains" and "optionally includes" indicate "contains or does not contain." "Optional component X" indicates whether component X exists or does not exist, or whether component X is contained or not.

[0039] In this document, the terms "first aspect," "second aspect," "third aspect," and "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," and "fourth" serve only as a non-exhaustive enumeration and should be understood as not constituting a closed limitation on quantity.

[0040] In this article, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions that include the listed features.

[0041] In this document, when referring to numerical intervals (i.e., numerical ranges), unless otherwise specified, the distribution of selectable values ​​within a numerical interval is considered continuous, and includes the two endpoints (i.e., the minimum and maximum values) of the numerical interval, as well as every value between these two endpoints. Unless otherwise specified, when a numerical interval refers only to integers within that interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints, which is equivalent to directly listing every integer. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be merged. In other words, unless otherwise specified, the numerical ranges disclosed herein should be understood to include any and all subranges included therein. The "numerical value" in this numerical interval can be any quantitative value, such as a number, percentage, ratio, etc. The term "numerical interval" can be broadly included to include percentage intervals, ratio intervals, proportion intervals, and other numerical interval types.

[0042] In this document, the terms "room temperature" or "normal temperature" generally refer to 4°C to 35°C, for example, 20°C ± 5°C. In some embodiments of this document, "room temperature" or "normal temperature" refers to 10°C to 30°C. In some embodiments of this document, "room temperature" or "normal temperature" refers to 20°C to 30°C.

[0043] In this document, for methods involving multiple steps, unless otherwise explicitly stated herein, there is no strict order constraint on the execution of these steps; they may be executed in any order other than those described. Moreover, any step may include multiple sub-steps or multiple stages, which are not necessarily completed at the same time, but may be executed at different times, and their execution order is not necessarily sequential, but may be executed in turn, alternately, or simultaneously with other steps or parts of the sub-steps or stages of other steps.

[0044] Traditionally prepared coated lithium manganese iron phosphate cathode materials suffer from uneven and incomplete coating layers, resulting in poor conductivity and limited cycle life. Therefore, this application provides a coated lithium manganese iron phosphate cathode material to improve its conductivity and cycle performance.

[0045] In a first aspect, this application provides a coated lithium manganese iron phosphate cathode material, comprising lithium manganese iron phosphate (LMFP) material, a first coating layer, a second coating layer, and a third coating layer; wherein, the first coating layer coats the surface of the lithium manganese iron phosphate material, the second coating layer coats the surface of the first coating layer, and the third coating layer coats the surface of the second coating layer; the material of the first coating layer comprises carbon, which is derived from organic matter; the materials of the second coating layer and the third coating layer each independently comprise magnesium and / or tellurium.

[0046] The coated lithium manganese iron phosphate cathode material provided in this application is first coated with a carbon layer, then with a metal layer. The carbon layer improves the conductivity of the material itself, while the metal layer stabilizes the cathode material structure and enhances its conductivity. Through the synergistic effect of the multi-layered coating structure, the conductivity and structural stability of the coated lithium manganese iron phosphate cathode material can be improved, thereby increasing its cycle life and conductivity. Moreover, both the carbon layer and the metal layer can be uniformly and completely coated on the surface of the lithium manganese iron phosphate material.

[0047] It is understandable that organic matter is used as a carbon source to form the carbon material coating layer. The organic matter mainly includes polysaccharides; that is, the "carbon" material of the first coating layer is a carbon-containing compound formed by the carbonization of polysaccharides. Specifically, during high-temperature sintering, polysaccharides can be converted into gaseous hydrocarbons, thereby depositing a carbon-based first coating layer on the surface of the lithium manganese iron phosphate material. In some embodiments, the polysaccharides include one or more of glucose and starch. Coating with a carbon coating layer before the metal coating layer can improve the conductivity of the coated lithium manganese iron phosphate cathode material.

[0048] It is understood that the material of the second coating layer can be magnesium and / or tellurium; the material of the third coating layer can also be magnesium and / or tellurium. The materials of the second and third coating layers can be the same or different, preferably different. In some embodiments, the material of the second coating layer includes magnesium, and the material of the third coating layer includes tellurium. By coating with the second and third coating layers, the conductivity and stability of the coated lithium manganese iron phosphate cathode material can be further improved.

[0049] In some implementations, the thickness of the first coating layer is 1.0 nm to 10.0 nm, for example, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, or 9 nm.

[0050] In some embodiments, the thickness of the second coating layer is 1.0 nm to 10.0 nm, for example, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, or 9 nm.

[0051] In some implementations, the thickness of the third coating layer is 1.0 nm to 10.0 nm, for example, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, and 9 nm.

[0052] By controlling the thickness of the first, second, and third coating layers within the aforementioned range, the conductivity of the coated lithium manganese iron phosphate cathode material can be effectively improved. If the coating layer is too thin, its conductivity cannot be effectively improved; if the coating layer is too thick, the proportion of lithium manganese iron phosphate material will be reduced, resulting in poor electrochemical performance of the coated lithium manganese iron phosphate cathode material.

[0053] In some embodiments, the particle size D50 of the lithium manganese iron phosphate material is 800 nm to 1000 nm, for example, 850 nm, 900 nm, and 950 nm. Controlling the particle size of the lithium manganese iron phosphate material within this range can shorten the transport path of the material, improve transport efficiency, and the generated carbon coating layer can both improve conductivity and prevent the aggregation of the lithium manganese iron phosphate material, thus preventing its continuous growth in particle size. This is beneficial for controlling the particle size of the coated lithium manganese iron phosphate cathode material.

[0054] In some embodiments, the particle size D50 of the coated lithium manganese iron phosphate is 800 nm to 1030 nm.

[0055] See Figure 1 Secondly, this application provides a method for preparing a coated lithium manganese iron phosphate cathode material as described in the first aspect, comprising steps S100 to S300.

[0056] Step S100: The lithium manganese iron phosphate material is mixed with organic matter and then sintered for the first time to melt the organic matter and form a first coating layer on the surface of the lithium manganese iron phosphate material. By performing the sintering treatment, not only can the stability of lithium manganese iron phosphate be improved, but a carbon coating layer can also be formed in situ on the surface of the lithium manganese iron phosphate material, resulting in lithium manganese iron phosphate material coated with the first coating layer.

[0057] In some embodiments, the atmosphere for the first sintering is a protective gas atmosphere, the temperature is 400℃ to 1200℃, and the time is 4h to 30h. The protective gas atmosphere may include one or more of nitrogen, argon, and helium. Preferably, the temperature for the first sintering is 600℃ to 1000℃; more preferably, the temperature for the first sintering is 700℃ to 900℃. Preferably, the time for the first sintering is 10h to 24h. By controlling the sintering temperature and sintering time, the formation of the carbon coating layer can be promoted while maintaining the particle size and stability of the lithium manganese iron phosphate material.

[0058] In this application, there are no restrictions on the method of mixing lithium manganese iron phosphate material with organic matter, as long as it can be mixed evenly. For example, a mixer can be used for mixing, and the mixing time can be 0.5h to 1h.

[0059] Step S200: The lithium manganese iron phosphate material with the first coating layer is mixed with the first metal and then sintered for a second time to form a second coating layer on the surface of the first coating layer. The first metal includes magnesium and / or tellurium.

[0060] In this application, there are no restrictions on the method of mixing the lithium manganese iron phosphate material with the first coating layer with the first metal, as long as it can be mixed evenly. For example, a mixer can be used for mixing, and the mixing time can be 0.5h to 1h.

[0061] In some embodiments, the atmosphere for the second sintering is a protective gas atmosphere, the temperature is 400℃ to 1200℃, and the time is 4h to 30h. The protective gas atmosphere may include one or more of nitrogen, argon, and helium. Preferably, the temperature for the second sintering is 600℃ to 1000℃; more preferably, the temperature for the second sintering is 700℃ to 900℃. Preferably, the time for the second sintering is 10h to 24h. By controlling the temperature and time of the second sintering, the formation of the second coating layer can be promoted while maintaining the particle size and stability of the lithium manganese iron phosphate material.

[0062] Step S300: The lithium manganese iron phosphate material with the second coating layer is mixed with a second metal and then sintered for a third time to form a third coating layer on the surface of the second coating layer. The second metal includes magnesium and / or tellurium. By controlling the temperature and time of the third sintering, the formation of the second coating layer can be promoted while maintaining the particle size and stability of the lithium manganese iron phosphate material.

[0063] It is understandable that the first coating layer, the second coating layer, and the third coating layer are formed sequentially on the surface of the lithium manganese iron phosphate material.

[0064] In some embodiments, the atmosphere for the third sintering is a protective gas atmosphere, the temperature is 400℃ to 1200℃, and the time is 4h to 30h. The protective gas atmosphere may include one or more of nitrogen, argon, and helium. Preferably, the temperature for the third sintering is 600℃ to 1000℃; more preferably, the temperature for the third sintering is 700℃ to 900℃. Preferably, the time for the third sintering is 10h to 24h.

[0065] In some embodiments, the mass ratio of lithium manganese iron phosphate material to organic matter, the first metal, and the second metal is (95-99):(0.5-3):(0.5-3):(0.5-3). It should be noted that the sum of the masses of the lithium manganese iron phosphate cathode material to organic matter, the first metal, and the second metal is 100. By controlling the mass ratio of lithium manganese iron phosphate material to organic matter, the first metal, and the second metal within the above range, it is possible to ensure that the content of lithium manganese iron phosphate material is within a suitable range, avoiding adverse effects on conductivity; on the other hand, it is possible to ensure the formation of a first coating layer, a second coating layer, and a third coating layer of suitable thickness, further improving the conductivity and stability of the coated lithium manganese iron phosphate cathode material.

[0066] Thirdly, this application provides a positive electrode sheet, including a positive current collector and a positive active material layer formed on at least one side of the positive current collector, wherein the positive active material layer includes the coated lithium manganese iron phosphate positive electrode material described in the first aspect.

[0067] It is understood that the material of the positive electrode current collector is not subject to many restrictions; any positive electrode current collector commonly used in the lithium battery field can be selected, such as aluminum foil. In addition to the coated lithium manganese iron phosphate positive electrode material described in the first aspect, the positive electrode active material layer may also include necessary components such as conductive agents, binders, and solvents, and the types of these components are not limited in this application.

[0068] Fourthly, this application provides a lithium battery, including the positive electrode sheet described in the third aspect.

[0069] In this application, lithium battery mainly refers to lithium secondary battery. Besides the positive electrode, it may also include necessary components such as negative electrode, separator, and electrolyte. The materials of the negative electrode, separator, and electrolyte are not limited in this application; any material known in the field of lithium batteries may be used.

[0070] The present application will be further described in detail below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application. For experimental methods in the following embodiments where specific conditions are not specified, please refer to the guidelines given in this application, or follow experimental manuals or conventional conditions in the art, or follow the conditions recommended by the manufacturer, or refer to experimental methods known in the art.

[0071] In the specific embodiments described below, the measurement parameters involving raw material components may have slight deviations within the weighing accuracy range unless otherwise specified. Temperature and time parameters are subject to acceptable deviations due to instrument testing accuracy or operational precision. "Room temperature" refers to 25°C.

[0072] Example 1

[0073] 1) Mix 96g of lithium manganese iron phosphate and 2g of glucose in a mixer for 30 minutes, discharge the mixture, and transfer it to a coating reactor. Then, under a nitrogen atmosphere, heat the mixture to 700℃ at a heating rate of 2℃ / min and treat it for 20 hours to melt the glucose and form a glucose coating layer with a thickness of 1.2nm on the surface of the lithium manganese iron phosphate.

[0074] 2) The glucose-coated lithium manganese iron phosphate obtained in step 1) was mixed with 1g of metallic magnesium in a mixer for 30 minutes, then transferred to an atmosphere furnace and heated to 1150℃ at a heating rate of 2℃ / min under a nitrogen atmosphere. The mixture was kept at this temperature for 4 hours to form a 1.0nm thick metallic magnesium coating on the outer surface of the glucose coating (the side away from the lithium manganese iron phosphate). The mixture was then cooled to room temperature.

[0075] 3) The magnesium-coated lithium manganese iron phosphate obtained in step 2) is mixed with 1g of tellurium in a mixer for 30 minutes, then transferred to an atmosphere furnace and heated to 1100℃ at a heating rate of 2℃ / min under a nitrogen atmosphere. The mixture is then kept at this temperature for 4 hours to form a 1.0nm thick tellurium coating on the outer surface of the magnesium coating (the side opposite to the glucose coating), thus obtaining the coated lithium manganese iron phosphate cathode material.

[0076] Example 2

[0077] The preparation method of Example 2 is basically the same as that of Example 1, except that the amount of each component added is different. The specific steps are as follows:

[0078] 1) Mix 98g of lithium manganese iron phosphate and 1g of glucose in a mixer for 30 minutes, discharge the mixture, and transfer it to a coating reactor. Then, under a nitrogen atmosphere, heat the mixture to 700℃ at a heating rate of 2℃ / min and treat it for 20 hours to melt the glucose and form a glucose coating layer with a thickness of 1.0nm on the surface of the lithium manganese iron phosphate.

[0079] 2) The glucose-coated lithium manganese iron phosphate obtained in step 1) was mixed with 0.5 g of metallic magnesium in a mixer for 30 min, then transferred to an atmosphere furnace and heated to 1150 °C at a heating rate of 2 °C / min under a nitrogen atmosphere. The mixture was kept at this temperature for 4 h to form a 1.0 nm thick metallic magnesium coating on the outer surface of the glucose coating (the side away from the lithium manganese iron phosphate). The mixture was then cooled to room temperature.

[0080] 3) The magnesium-coated lithium manganese iron phosphate obtained in step 2) is mixed with 0.5g of tellurium in a mixer for 30 minutes, then transferred to an atmosphere furnace and heated to 1100℃ at a heating rate of 2℃ / min under a nitrogen atmosphere. The mixture is then kept at this temperature for 4 hours to form a 1.0nm thick tellurium coating on the outer surface of the magnesium coating (the side opposite to the glucose coating), thus obtaining the coated lithium manganese iron phosphate cathode material.

[0081] Example 3

[0082] The preparation method of Example 3 is basically the same as that of Example 1, except that the amount of each component added and the material of the coating layer are different. The specific steps are as follows:

[0083] 1) Mix 95g of lithium manganese iron phosphate and 2.5g of glucose in a mixer for 30 minutes, discharge the mixture, and transfer it to a coating reactor. Then, under a nitrogen atmosphere, heat the mixture to 700℃ at a heating rate of 2℃ / min and treat it for 20 hours to melt the glucose and form a glucose coating layer with a thickness of 1.5nm on the surface of the lithium manganese iron phosphate.

[0084] 2) The glucose-coated lithium manganese iron phosphate obtained in step 1) was mixed with 1g of metallic magnesium and 0.5g of metallic tellurium in a mixer for 30 minutes, then transferred to an atmosphere furnace and heated to 1150°C at a heating rate of 2°C / min under a nitrogen atmosphere. The mixture was kept at this temperature for 4 hours to form a 1.0nm thick composite coating of metallic magnesium and metallic tellurium on the outer surface of the glucose coating layer (the side away from the lithium manganese iron phosphate). The mixture was then cooled to room temperature.

[0085] 3) The lithium manganese iron phosphate coated with magnesium and tellurium obtained in step 2) was mixed with 1g of tellurium in a mixer for 30min, then transferred to an atmosphere furnace and heated to 1100℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, and kept at the temperature for 4h to form a 1.2nm thick tellurium coating layer on the outer surface of the magnesium and tellurium composite coating layer (the side opposite to the glucose coating layer), thus obtaining the coated lithium manganese iron phosphate cathode material.

[0086] Example 4

[0087] The preparation method of Example 4 is basically the same as that of Example 1, except that the types of organic materials and the materials of the metal coating layer are different. The specific steps are as follows:

[0088] 1) Mix 96g of lithium manganese iron phosphate and 2g of starch in a mixer for 30 minutes, discharge the mixture, and transfer it to a coating reactor. Then, under a nitrogen atmosphere, heat the mixture to 700℃ at a heating rate of 2℃ / min and treat it for 20 hours to melt the glucose and form a starch coating layer with a thickness of 1.2nm on the surface of the lithium manganese iron phosphate.

[0089] 2) The starch-coated lithium manganese iron phosphate obtained in step 1) was mixed with 1g of metal tellurium in a mixer for 30min, then transferred to an atmosphere furnace and heated to 1150℃ at a heating rate of 2℃ / min under a nitrogen atmosphere. The mixture was kept at this temperature for 4h to form a 1.5nm thick metal tellurium coating on the outer surface of the starch coating (the side away from the lithium manganese iron phosphate). The mixture was then cooled to room temperature.

[0090] 3) The lithium manganese iron phosphate coated with tellurium obtained in step 2) is mixed with 1g of magnesium in a mixer for 30min, then transferred to an atmosphere furnace and heated to 1100℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, and kept at the temperature for 4h to form a magnesium coating layer with a thickness of 1.0nm on the outer surface of the tellurium coating layer (the side opposite to the starch coating layer), thus obtaining the coated lithium manganese iron phosphate cathode material.

[0091] Comparative Example 1

[0092] Comparative Example 1 is a commercially available lithium manganese iron phosphate cathode material that has not undergone coating modification.

[0093] The raw material ratios used in the preparation methods of Examples 1-4 are shown in Table 1:

[0094] Table 1

[0095] Example 1 96g 2g glucose 1g of metallic magnesium 1g metallic tellurium Example 2 98g 1g glucose 0.5g metallic magnesium 0.5g metallic tellurium Example 3 95g 2.5g glucose 1g of metallic magnesium and 0.5g of metallic tellurium 1g metallic tellurium Example 4 96g 2g starch 1g metallic tellurium 1g of metallic magnesium

[0096] The scanning electron microscope (SEM) images of the coated lithium manganese iron phosphate cathode material prepared in Example 1 are shown below. Figure 2 and 3 As shown. Where "1" represents lithium manganese iron phosphate material; "2" represents carbon coating layer; "3" represents first metal coating layer; and "4" represents second metal coating layer. Figure 2 and Figure 3 It is known that the coated lithium manganese iron phosphate cathode material prepared in this application has a uniform and complete coating layer.

[0097] The relevant performance of lithium batteries prepared using the coated lithium manganese iron phosphate cathode material obtained in Example 1 and the lithium manganese iron phosphate cathode material in Comparative Example 1 as raw materials was tested. The test results are shown in Table 2 below.

[0098] The testing steps for each performance test item are as follows:

[0099] 1) Preparation of positive electrode sheet: According to the mass percentage, 97% of the coated lithium manganese iron phosphate positive electrode material prepared in Example 1 and the lithium manganese iron phosphate positive electrode material in Comparative Example 1 are respectively mixed with 1% conductive agent and 2% binder PVDF to form a positive electrode slurry; then the positive electrode slurry is coated on aluminum foil to form a positive electrode sheet.

[0100] 2) Preparation of negative electrode sheet: According to the mass percentage, take 96% graphite, 1.7% SBR, 1% conductive carbon black and 1.3% CMC and dissolve them in water. Stir and disperse them evenly in a mixer and vacuum to obtain a uniform negative electrode slurry without bubbles. Then coat the negative electrode slurry onto copper foil to make a negative electrode sheet.

[0101] 3) Packaging and Formation: The prepared positive and negative electrode sheets are stacked with the separator to form a battery cell, with tabs on the same side. The tabs are then welded to the current collector using an ultrasonic welding machine and encapsulated using an aluminum-plastic film. After baking the cell, electrolyte is injected into it, and after formation and capacity testing, a battery is manufactured.

[0102] The battery cells were cycle-tested using the following method:

[0103] a) At 25℃±3℃, the prepared battery cell was charged at 0.5C with constant current and constant voltage to a value of 4.2V, and the cutoff current was 0.02C.

[0104] b) Let stand for 30 minutes.

[0105] c) Subsequently, at 25℃±3℃, the cell was discharged to 2.0V using a constant current of 1C, and the capacity D1 at this time was recorded.

[0106] d) Let stand for 30 minutes.

[0107] e) Repeat steps a) to d) N times, and record the capacity D2 after N cycles. Calculate the capacity retention rate according to the following formula.

[0108] Capacity retention rate: SOH = (D2 / D1) × 100%.

[0109] Table 2

[0110] Example 1 0.31 3000 85.6％ Comparative Example 1 0.82 1500 69.5％

[0111] As shown in Table 2 above, the coated lithium manganese iron phosphate cathode material provided in this application has excellent cycle performance and low internal resistance, indicating that it has high conductivity, and the coated layer of lithium manganese iron phosphate is uniform and complete.

[0112] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0113] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims, and the specification and drawings can be used to interpret the scope of the claims.

Claims

1. A coated lithium manganese iron phosphate cathode material, characterized in that, include: Lithium iron manganese phosphate materials; A first coating layer is applied to the surface of the lithium manganese iron phosphate material. The material of the first coating layer is carbon, and the carbon is derived from organic matter. A second coating layer is applied to the surface of the first coating layer. A third coating layer is applied to the surface of the second coating layer. The materials of the second and third coating layers each independently comprise magnesium and / or tellurium.

2. The coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, The organic substance is a polysaccharide, which includes one or more of glucose and starch.

3. The coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, The material of the second coating layer includes magnesium, and the material of the third coating layer includes tellurium.

4. The coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, The thicknesses of the first coating layer, the second coating layer, and the third coating layer are each independently 1.0 nm to 10.0 nm.

5. The coated lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, The particle size D50 of the lithium manganese iron phosphate material is 800nm ​​to 1000nm; And / or, the particle size D50 of the coated lithium manganese iron phosphate cathode material is 800nm ​​to 1030nm.

6. A method for preparing a coated lithium manganese iron phosphate cathode material as described in any one of claims 1 to 5, characterized in that, Includes the following steps: The lithium manganese iron phosphate material is mixed with the organic matter and then subjected to a first sintering to melt the organic matter and form the first coating layer on the surface of the lithium manganese iron phosphate material. The lithium manganese iron phosphate material with the first coating layer is mixed with the first metal and then sintered a second time to form a second coating layer on the surface of the first coating layer. The lithium manganese iron phosphate material with the second coating layer is mixed with the second metal and then sintered for a third time to form a third coating layer on the surface of the second coating layer. The first metal and the second metal each independently comprise magnesium and / or tellurium.

7. The preparation method according to claim 6, characterized in that, The mass ratio of the lithium manganese iron phosphate material to the organic matter, the first metal, and the second metal is (95-99):(0.1-2):(0.3-3):(0.6-3).

8. The preparation method according to claim 6 or 7, characterized in that, The atmosphere for the first sintering is a protective gas atmosphere, the temperature is 400℃~1200℃, and the time is 4h~30h; And / or, the atmosphere of the second sintering is a protective gas atmosphere, the temperature is 400℃~1200℃, and the time is 4h~30h; And / or, the atmosphere for the third sintering is a protective gas atmosphere, the temperature is 400℃~1200℃, and the time is 4h~30h.

9. A positive electrode plate, characterized in that, It includes a positive current collector and a positive active material layer formed on at least one side of the positive current collector, wherein the positive active material layer includes the coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 5.

10. A lithium battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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