Lithium iron manganese phosphate composite material and preparation method therefor as well as secondary battery

Abstract

The present application discloses a lithium iron manganese phosphate composite material, a preparation method therefor as well as a secondary battery. The lithium iron manganese phosphate composite material sequentially comprises a core, a first coating layer and a second coating layer, wherein the core comprises LixMnyFe1-y-zDzPO4 material, 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; the first coating layer comprises LiaAlbMcOd material, 0<a≤1, 0<b<1, 0<c<1, 0<d<3, M comprises at least one of Y element, Zr element and Mg element, and the second coating layer comprises carbon. The method for preparing the lithium manganese iron phosphate composite material comprises: providing a core, which comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4; preparing a first coating layer on the outer surface of the core utilizing an atomic layer deposition technology, the first coating layer comprises a material having a chemical general formula LiaAlbMcOd; and preparing a second coating layer on the outer surface of the first coating layer, the second coating layer comprises carbon.

Description

[0001]LITHIUM IRON MANGANESE PHOSPHATE COMPOSITE MATERIAL AND PREPARATION METHOD THEREFOR AS WELL AS SECONDARY BATTERY FIELD OF THE INVENTION The present application relates to the technical field of cathode active material, in particular to a lithium iron manganese phosphate composite material and a preparation method therefor, as well as a secondary battery. BACKGROUND OF THE INVENTION Compared with lithium iron phosphate (LFP) electrode materials, lithium iron manganese phosphate (LMFP) has a higher voltage plateau (4.1V vs. 3.2V). Theoretically, the energy density of LMFP is about 15% higher than that of LFP. Therefore, LMFP can balance high safety and high energy density, and has great application prospects. However, due to the lower electronic / ionic conductivity of LMFP, its capacity and rate capability are relatively poor, and its cycling performance is also relatively poor because manganese is affected by the Jahn-Teller effect. In order to solve the above problems, the related technology proposes to use surface coating, elemental doping, particle nanocrystallization and the like to modify LMFP, but the modification effect is limited. SUMMARY OF THE INVENTION The present application provides a lithium iron manganese phosphate composite material to solve the problems of lower capacity, poor rate capability and cycling performance of lithium iron manganese phosphate secondary batteries in the prior art. The present application also provides a method for preparing a lithium iron manganese phosphate composite material to solve the problem that the coating layer formed by the existing method is too thick or inhomogeneous and affects the performance of the LMFP. In a first aspect, the present application provides a lithium iron manganese phosphate composite material comprising a core, a first coating layer and a second coating layer, wherein the first coating layer is uniformly coated on the outer surface of the core, and the second coating layer is coated on the outer surface of the first coating layer; the core comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4, wherein 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; the first coating layer comprises a material having a chemical general formula LiaAlbMcOd (abbreviated as LAMO), 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element; and the second coating layer comprises carbon. In an optional embodiment, the content of the first coating layer is in a range from 0.1 wt% to 3.0 wt% based on the total weight of the lithium iron manganese phosphate composite material. In an optional embodiment, the first coating layer has a thickness ranging from 1 nm to 10 nm. In an optional embodiment, the content of the second coating layer is a range from 1.0 wt % to 3.0 wt % based on the total weight of the lithium iron manganese phosphate composite material. In an optional embodiment, 0.6≤y<0.8. In an optional embodiment, 0≤z<0.2. In an optional embodiment, 0.1≤b≤0.9. In an optional embodiment, 0.1≤c≤0.9. In an optional embodiment, 1.55≤d≤2.45. In an optional embodiment, D comprises at least one of Ti element, Zr element, V element, Nb element and Mg element. In a second aspect, the present application provides a method for preparing a lithium iron manganese phosphate composite material, comprising the following steps: providing a core, which comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4; preparing a first coating layer on the outer surface of the core utilizing an atomic layer deposition technology, the first coating layer comprises a material having a chemical general formula LiaAlbMcOd; and preparing a second coating layer on the outer surface of the first coating layer, wherein the second coating layer comprises carbon; wherein, 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; and 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element. In an optional embodiment, the atomic layer deposition technology comprises: placing the material of the core in a deposition chamber and heating it to a temperature ranging from 200°C to 600°C, and then passing an inert carrier gas carrying with a precursor of lithium element, a precursor of aluminum element, a precursor of M element, and a precursor of oxygen element therein, and setting the pulse time to be 0.2s to 2s and the number of deposition cycles to be 10 to 150. In an optional embodiment, D comprises at least one of Ti element, Zr element, V element, Nb element and Mg element. In an optional embodiment, the precursor of lithium element comprises at least one of lithium tert-butoxide, tert-butyllithium, isobutyllithium and n-butyllithium. In an optional embodiment, the precursor of aluminum element comprises at least one of trimethylaluminum and dimethylaluminum isopropoxide. In an optional embodiment, the precursor of M element is an organometallic compound containing M element. In an optional embodiment, the precursor of oxygen element comprises at least one of deionized water, alcohol compounds and ketone compounds. In an optional embodiment, the preparing a second coating layer on the outer surface of the first coating layer comprises: mixing an intermediate with a carbon source, heating it to a temperature ranging from 600°C to 800°C at a heating rate ranging from 3°C / min to 5°C / min under an inert atmosphere, and holding the temperature for a time period ranging from 5h to 10h; or placing an intermediate in a vapor deposition apparatus, heating it to a temperature ranging from 700°C to 800°C, passing a gas containing a precursor of carbon element therein, adjusting the apparatus to have a pressure ranging from 0.1 MPa to 0.5 MPa, and holding the temperature and pressure for a time period ranging from 5h to 12h; and wherein the intermediate is LixMnyFe1-y-zDzPO4 having a LiaAlbMcOd coating layer. In an optional embodiment, the weight ratio of the intermediate to the carbon source is in a range from 1:0.06 to 1:0.12, and the carbon source comprises at least one of glucose, sucrose, polyethylene glycol, phenolic resin, polyvinyl alcohol and citric acid. In an optional embodiment, the precursor of carbon element comprises at least one of acetylene, ethylene and methane. In a third aspect, the present application provides a positive electrode plate, comprising: a positive electrode current collector, and a cathode active material layer provided on at least one side of the positive electrode current collector; wherein, the cathode active material layer comprises the lithium iron manganese phosphate composite material as described in the first aspect of the present application or a lithium iron manganese phosphate composite material prepared by the method as described in the second aspect of the present application. In a fourth aspect, the present application also provides a secondary battery, comprising the positive electrode plate as described in the third aspect of the present application. In a fifth aspect, the present application also provides a powered device, comprising the secondary battery as described in the fourth aspect of the present application. The technical solution of the present application has the following advantages. 1. The present application provides a lithium iron manganese phosphate composite material comprising a core, a first coating layer and a second coating layer, wherein the first coating layer is uniformly coated on the outer surface of the core, and the second coating layer is coated on the outer surface of the first coating layer; the core comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4, wherein 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; the first coating layer comprises a material having a chemical general formula LiaAlbMcOd, 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element; and the second coating layer comprises carbon. In the presence of the carbon coating layer as the outermost layer, the co-doping of Al and M elements in the first coating layer can produce a synergistic effect, which can effectively reduce the contact area between LMFP and electrolyte solution and the occurrence of side reactions, and solve the problems of low conductivity and serious cycling performance degradation due to manganese dissolution during repeated lithiation / de-lithiation of LMFP materials, so as to further improve the structural stability as well as the electrochemical performances of the LMFP, such as capacity, rate capability and cycling performance. 2. The method for preparing a lithium iron manganese phosphate composite material provided in the present application utilizes atomic layer deposition technology, which can form an ultra-thin and homogeneous LAMO coating layer on the surface of the LMFP as well as co-doped zones of Al and M elements near the surface. Compared with the traditional dry or wet coating technology, it is capable of avoiding the influence of the over-thick or inhomogeneous coating layer on the performance of the LMFP. Additional aspects and advantages of the examples of the present application will be partially described and shown in the subsequent description or explained by the implementation of the examples of the present application. BRIEF DESCRIPTION OF THE DRAWINGS In order to more clearly illustrate the technical solutions in the specific embodiments of the present application or prior art, the following will briefly introduce the accompanying drawings that need to be used in the description of specific embodiments or prior art, and it is obvious that the accompanying drawings in the following description are some of the embodiments of the present application, and that for those skilled in the art, other accompanying drawings can be obtained based on these drawings without creative work. FIG. 1 is a SEM image of the lithium iron manganese phosphate composite material prepared in Example 1 of the present application. FIG. 2 is a STEM-EDX image of the lithium iron manganese phosphate composite material prepared in Example 1 of the present application. FIG. 3 is a comparison of a measured XRD spectrum of the material in the core of Example 1 of the present application with a standard spectrum. DETAILED DESCRIPTION OF THE INVENTION Reference will be made clearly and completely to the technical solutions in the embodiments of the present application with accompanying drawings. The embodiments described here are only part of the embodiments of the present application and are not all embodiments of the present application. Based on the embodiments of the present application, other embodiments obtained by those skilled in the art without creative work are within the scope of the present application. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs. The terms used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. The terms “includes” and “comprises” and any variation thereof in the description and claims of the present application are intended to indicate a non-exclusive inclusion. In the description of the embodiments of the present application, the technical terms “first”, “second” and the like are only used for distinction between different objects and are not to be understood as indicating or implying relative importance or implicitly indicating a number, a particular order or a primary or secondary relationship of the technical features. In the description of the embodiments of the present application, “a plurality of” means two or more, unless specified otherwise. Reference to an “embodiment” herein means that a feature, structure or characteristic described in connection with the embodiment may be comprised in at least one embodiment of the present application. The “embodiment” in various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. One skilled in the art explicitly and implicitly understands that an embodiment described herein may be combined with other embodiments. Term “range” disclosed in the present application is defined in the form of a lower limit and an upper limit, a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The range defined in this way can be inclusive or exclusive, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that ranges of 60-110 and 80-120 are also obtained. In addition, if the listed minimum values are 1 and 2, and if the listed maximum values are 3, 4 and 5, the ranges of 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5 may be obtained. In the present application, unless otherwise specified, the numerical range “a-b” means the abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range “0-5” means that all the real numbers between “0-5” have been listed, and “0-5” is only the abbreviated representation of these numerical combinations. In addition, when a parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. In the description of the present application, the term “and / or”, which describes an associated relationship of associated objects, means that there may be three relationships, for example, A and / or B, which may mean that A exists alone, A and B exist at the same time, and B exists alone. A character “ / ” generally indicates that contextual objects are in an “or” relationship. Traditional coating techniques, such as dry coating by mechanical mixing or wet coating by coating the coating material in the form of solution on the surface of the material, have their own limitations, which may lead to relatively thick or inhomogeneous coating layer and affect the coating effect; and the existing materials used for coating LMFP also need to be strengthened for improving the performance of the battery. In order to solve the above problems existing in the related art, according to the first aspect of the present application, there is provided a lithium iron manganese phosphate composite material comprising a core, a first coating layer and a second coating layer, wherein the first coating layer is uniformly coated on the outer surface of the core, and the second coating layer is coated on the outer surface of the first coating layer; the core comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4, wherein 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; the first coating layer comprises a material having a chemical general formula LiaAlbMcOd, 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element; and the second coating layer comprises carbon. The electronic conductivity of LMFP material is relatively low. By coating the conductive material uniformly on the surface of LMFP, the conductive resistance can be reduced to some extent, and the electrochemical performance of the material can be improved. Carbon material is the preferred material to improve the electronic conductivity of LMFP. The improvement of the properties of LMFP materials by carbon coating is mainly reflected in the following aspects: firstly, it can effectively prevent the growth of crystal particles; secondly, it can prevent agglomeration among particles; thirdly, it can improve the electronic conductivity of the material and provide an effective lithium ion diffusion channel; fourthly, it can be used as a reducing agent to prevent oxidation of materials. Under the condition that the outermost carbon coating layer exists, in the present application, a layer of LiaAlbMcOd material is uniformly coated on the outer surface of the core LMFP, thereby forming a bulk Al element and M element co-doped region near the surface of the LMFP, wherein Al element is beneficial to stabilizing the structure, relieving the dissolution of Mn3+and improving the cycling performance; the ion radius of Y element is relatively large, which can increase the diffusion channel of lithium ions and is beneficial to the transmission of lithium ions; Zr element doping can play a "pillar" role, reduce lattice distortion, stabilize crystal structure and improve conductivity; Mg element ions have low electronegativity and low adsorption force to electrolyte solution, which can alleviate the excessive decomposition of electrolyte solution. Compared with single metal element doping, the co-doping of Al element and M element (Y element, Zr element, Mg element) can produce a synergistic effect, which can effectively reduce the contact area between LMFP and electrolyte solution and the occurrence of side reactions, solve the problems of low conductivity and serious cycling performance degradation due to manganese dissolution during repeated lithiation / de-lithiation of LMFP materials, and further improve the structural stability as well as the electrochemical performances of the LMFP, such as capacity, rate capability and cycling performance. Those skilled in the art can understand that the coating layer should not be too thick, otherwise it will affect the performance of LMFP. At the same time, the coating layer thickness should be as uniform as possible, that is, coating should be uniform, so as to achieve the coating effect and achieve the purpose of coating. In an optional embodiment, the content of the first coating layer is in a range from 0.1 wt% to 3.0 wt% based on the total weight of the lithium iron manganese phosphate composite material. Thus, an ultra-thin and uniform LAMO coating layer can be formed on the outer surface of LMFP, which can achieve the purpose of improving the conductivity and inhibiting the dissolution of manganese, without affecting the capacity release of LMFP. As an example, the content of the first coating layer can be, for example, 0.1 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, or within a range composed of any two of the above values. In an optional embodiment, the thickness of the first coating layer is in a range from 1nm to 10nm. The ultra-thin LAMO coating layer can play a role of coating that improves the electrical conductivity and inhibits the dissolution of manganese without affecting the capacity release of LMFP. However, when the first coating layer is too thin, the LMFP core will inevitably contact the electrolyte solution directly, resulting in the dissolution of manganese ions. As an example, the thickness of the first coating layer may be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or within a range composed of any two of the above values. In an optional embodiment of the present application, the content of carbon is in a range from 1.0 wt% to 3.0 wt% based on the total weight of the lithium iron manganese phosphate composite material; as an example, the content of carbon can be, for example, 1.0 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, or within a range composed of any two of above values. The applicant's research found that if the carbon content is too high, the compaction density and energy density of the material will be reduced, and because the carbon material itself has no electrochemical activity, too high carbon content will also lead to the decrease of the overall electrochemical capacity of the material; on the contrary, if the carbon content is too low, it cannot play the role to improve the electronic conductivity, at the same time, it is not conducive to alleviating the manganese dissolution problem of the core material LMFP. The research shows that doping metal ions in LMFP is an effective means to improve its performance. For example, doping vanadium and magnesium can obviously improve the charging-discharging performance and cycling performance of LMFP. The introduction of titanium element can inhibit the melting of particles, thus inhibiting the generation of large particles, reducing the size of the particles and shortening the lithium ion transmission path, which is beneficial to the electrochemical performance under the condition of charging and discharging at high current. The introduction of zirconium element can strengthen the crystal structure and make the structure stable and not collapse when the material is charged and discharged, which is then beneficial to the cycling performance. Doping niobium element can improve the conductivity of crystal, make the transmission speed of electrons faster, and be beneficial to the performance of energy efficiency and the like. Therefore, in an optional embodiment, the core material LMFP of the present application can contain doping element D, and the element D comprises at least one of Ti element, Zr element, V element, Nb element and Mg element. In a second aspect of the present application, there is provided a method for preparing a lithium iron manganese phosphate composite material, comprising the following steps: providing a core, which comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4; preparing a first coating layer on the outer surface of the core utilizing an atomic layer deposition technology, the first coating layer comprises a material having a chemical general formula LiaAlbMcOd; and preparing a second coating layer on the outer surface of the first coating layer, wherein the second coating layer comprises carbon; wherein, 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; and 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element. Compared with the traditional dry or wet coating technology, the atomic layer deposition technology used in the present application can form an ultra-thin and uniform LAMO coating layer on the surface of LMFP and Al and M co-doped regions near the surface, so as to avoid the influence of the coating layer on the performance of LMFP due to being too thick or uneven, and the co-doping of Al and M can produce a synergistic effect, which can effectively reduce the contact area between LMFP and electrolyte solution and the occurrence of side reactions, solve the problems of low conductivity and serious cycling performance degradation due to manganese dissolution during repeated lithiation / de-lithiation of LMFP materials, and then further improve the structural stability as well as the electrochemical performances of the LMFP, such as capacity, rate capability and cycling performance. Those skilled in the art can understand that the atomic layer deposition process conditions will affect the structure and properties of LAMO coating layer. In an optional embodiment, the atomic layer deposition technology comprises the following process conditions: placing the core material in a deposition chamber, heating to 200℃-600℃, and then introducing an inert carrier gas carrying with a precursor of lithium element, a precursor of aluminum element, a precursor of M element and a precursor of oxygen element, wherein the pulse time is set as 0.2s to 2s, and the number of deposition cycles is 10 to 150. Therefore, an ultra-thin and uniform LAMO coating layer can be formed on the surface of LMFP and Al and M co-doped regions can be formed near the surface, thus further improving the structural stability and electrochemical properties of LMFP, such as capacity, rate capability, cycling performance and the like. As an example, the deposition temperature can be 200℃, 300℃, 400℃, 500℃, 600℃, or within a range composed of any two of above values; the pulse time can be, for example, 0.2s, 0.5s, 0.8s, 1s, 1.2s, 1.5s, 1.8s, 2s, or within a range composed of any two of above values; and the number of deposition cycles is, for example, 10, 30, 50, 75, 100, 125, 150, or within a range composed of any two of above values. The applicant’s research found that, if the deposition temperature is too low, LAMO cannot be formed, while if the deposition temperature is too high, it will lead to volatilization of Li element; too long pulse time will lead to uneven coating, and too short pulse time will lead to too long coating period; and if the number of deposition cycles is too many, the coating layer will be too thick, and if the number of deposition cycles is too few, the coating layer will be too thin and uneven. In an optional embodiment, D element comprises at least one of Ti element, Zr element, V element, Nb element and Mg element. Vanadium and magnesium doping can obviously improve the charging-discharging performance and cycling performance of LMFP, titanium can inhibit particle melting, zirconium can strengthen crystal structure, and niobium can improve crystal conductivity. In an optional embodiment, the precursor of lithium element comprises at least one of lithium tert-butoxide, tert-butyllithium, isobutyllithium and n-butyllithium. In an optional embodiment, the precursor of aluminum element comprises at least one of trimethylaluminum and dimethylaluminum isopropoxide. In an optional embodiment, the precursor of M element is an organometallic compound containing M element. As an example, the precursor of M element can be at least one of ethyl magnesium iodide, bis(methylcyclopentadienyl)magnesium, magnesocene, dimethylmagnesocene, zirconium(IV) acetylacetonate and yttrium tris(2-methoxyethanolate). In an optional embodiment, the precursor of oxygen element comprises at least one of deionized water, alcohol compounds and ketone compounds. As an example, the alcohol compounds may be methanol or ethanol, and the ketone compounds may be acetone. It can be understood by those skilled in the art that, common carbon coating methods, such as vapor deposition, liquid-phase deposition, hydrothermal method, pyrolysis method, etc., can be applied to the carbon coating step in the present application, so as to form a carbon coating layer on the outer surface of LiaAlbMcOd coating layer. In an optional embodiment, the intermediate (i.e. LixMnyFe1-y-zDzPO4 with LiaAlbMcOd coating layer) can be mixed with a carbon source, heated to 600-800℃ at a heating rate of 3℃ / min-5℃ / min in an inert atmosphere, and held at the temperature for 5h to 10h, thus forming a carbon coating layer outside the intermediate. As an example, the carbon source comprises at least one of glucose, sucrose, polyethylene glycol, phenolic resin, polyvinyl alcohol and citric acid. The heating rate can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or within a range composed of any two of above values. The pyrolysis temperature is, for example, 600℃, 650℃, 700℃, 750℃, 800℃, or within a range composed of any two of above values. The holding time can be 5h, 6h, 7h, 8h, 9h, 10h, or within a range composed of any two of above values. In an optional embodiment, the intermediate (i.e. LixMnyFe1-y-zDzPO4 with LiaAlbMcOd coating layer) can also be placed in a vapor deposition apparatus, and the temperature is raised to 700℃ to 800℃, and a gas containing a precursor of carbon element is introduced, and the pressure of the apparatus is adjusted to 0.1MPa to 0.5MPa for 5 h to 12 h; the intermediate is LixMnyFe1-y-zDzPO4 with LiaAlbMcOd coating layer; as an example, the precursor of carbon element comprises at least one of acetylene, ethylene and methane, and the deposition temperature can be 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, or within a range composed of any two of above values; the deposition pressure can be 0.1MPa, 0.2MPa, 0.25MPa, 0.3MPa, 0.4MPa, 0.5MPa, or within a range composed of any two of above values; and the holding time can be 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, or within a range composed of any two of above values. In an optional embodiment, the weight ratio of the intermediate to the carbon source is 1:(0.06-0.12), so that the content of the carbon coating layer in the finally formed LMFP composite material can be between 1.0% and 3.0%, thereby effectively improving the electronic conductivity. As an example, the weight ratio of the intermediate to the carbon source can be, for example, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.10, 1:0.11, 1:0.12, or within a range composed of any two of above values. The content of carbon coating layer in LMFP composite material can be 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or within a range composed of any two of above values. It should be noted that the core material LixMnyFe1-y-zDzPO4 can be prepared by weighing, mixing and sintering a lithium source, a manganese source, an iron source, a D source (oxides, hydrochlorides, sulfates, nitrates and / or acetates of at least one of Ti element, Zr element, V element, Nb element and Mg element), a phosphorus source and a carbon source according to stoichiometric ratio. The main function of adding the carbon source is to prevent the oxidation of the iron source, and the amount of carbon source is relatively small, which makes the carbon content in the prepared core material negligible. It can be understood that LMFP materials doped with different D elements will be obtained by using different D sources. These are the conventional technical means for preparing LMFP materials in the art, so they will not be described in detail in the present application. In a third aspect, the present application also provides a positive electrode plate, comprising: a positive electrode current collector, and a cathode active material layer provided on at least one side of the positive electrode current collector; wherein the cathode active material layer comprises the lithium iron manganese phosphate composite material as described in the first aspect of the present application or a lithium iron manganese phosphate composite material prepared by the method as described in the second aspect of the present application. The present application provides a positive electrode plate comprising the lithium iron manganese phosphate composite material as described herein, thus having the advantage of stable structure and good electrochemical performance. In a fourth aspect, the present application also provides a secondary battery, comprising the positive electrode plate as described in the third aspect of the present application. Typically, a secondary battery comprises a positive electrode plate, a negative electrode plate, an electrolyte, and a separator. During the charging and discharging process of the battery, active ions are intercalated and deintercalated back and forth between the positive electrode plate and the negative electrode plate. The electrolyte serves to transmit ions between the positive electrode plate and the negative electrode plate. The separator is set between the positive electrode plate and the negative electrode plate, mainly plays the role of preventing the positive and negative electrodes from short-circuiting, and at the same time allows the ions to pass through. The following uses a lithium ion battery as an example to illustrate the secondary battery of the present application. [Positive electrode plate] The positive electrode plate comprises a positive electrode current collector and a cathode active material layer disposed on at least one surface of the positive electrode current collector. As an example, the positive electrode current collector has two surfaces facing in opposite directions along the thickness direction itself, and the cathode active material layer is provided on either or both of the two surfaces of the positive electrode current collector facing in opposite directions. The cathode active material layer comprises the cathode active material. The cathode active material may be selected from materials capable of absorbing and releasing lithium. The specific types of the cathode active material are not particularly limited and may be selected according to requirements. The cathode active material described in the present application is selected from the lithium iron phosphate (LiFePO4) type. As an example, in addition thereto, the cathode active material suitable for the battery system may also comprise at least one of the following materials: lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), iron pyrophosphate (Li2FeP2O7), lithium cobaltate (LiCoO2), spinel-type lithium manganate (LiMn2O4), spinel-type lithium nickel manganate (LiNi0.5Mn1.5O4), layered lithium manganate (LiMnO2), lithium nickelate (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium magnesiate (LiMgO2), lithium calciate (LiCaO2), lithium cuprate (LiCuO2), lithium zincate (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxides (LiNixCoyAl1-x-yO2, 0<x<1, 0<y<1, 0 <x + y<1, e.g. LiNi0.8Co0.15Al0.05O2), lithium nickel cobalt manganese oxides (LiNixCoyMn1-x-yO2, 0<x<1, 0<y<1, 0<x + y<1, e.g., LiNi1 / 3Co1 / 3Mn1 / 3O2, LiNi0.5Co0.2Mn0.3O2, LiNi0.6Co0.2Mn0.2O2, LiNi0.8Co0.1Mn0.1O2, etc.), lithium-rich materials (e.g. lithium-rich nickel cobalt manganese oxides), manganese oxides (MnO2), vanadium oxides, sulfur oxides, silicate oxides, and at least one of their respective modified compounds. These materials may be used separately or in combination, for example two or more kinds of materials are used together. The modification of each of the above cathode active materials may comprise a doping modification, a surface coating modification, or a doping-coating simultaneous modification of the cathode active material, and the like. In an optional embodiment, the positive electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil may be used. The composite current collector may comprise a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metallic material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a substrate of a high molecular material such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc. In an optional embodiment, the cathode active material layer can also comprise a binder. As an example, the binder may comprise at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin. In an optional embodiment, the cathode active material layer can also comprise a conductive agent. As an example, the conductive agent may comprise at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In an optional embodiment, the positive electrode plate may be prepared by the following method: dispersing the above-mentioned components for preparing the positive electrode plate, such as the cathode active material, the conductive agent, the binder and any other components, in a solvent (such as N-methyl pyrrolidone) to form a positive electrode slurry; and coating the positive electrode slurry on the positive electrode current collector, and obtaining the positive electrode plate after drying, cold pressing and other processes. [Negative electrode plate] As an example, the negative electrode current collector has two surfaces facing in opposite directions along the thickness direction itself, and the anode active material layer is provided on either or both of the two surfaces facing in opposite directions. In an optional embodiment, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may comprise a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector may be formed by forming a metallic material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a substrate of a high molecular material such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc. Specific type of the anode active material is not limited, and those skilled in the art can make a selection according to actual requirements. As an example, the anode active material may comprise at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate and the like. The silicon-based material may be at least one selected from elemental silicon, silicon-oxygen compounds, silicon-carbon complexes, silicon-nitrogen complexes, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. In an optional embodiment, the anode active material layer optionally comprises a binder. The binder may be at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). In an optional embodiment, the anode active material layer optionally comprises a conductive agent. The conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In an optional embodiment, the anode active material layer optionally comprises other adjuvants, such as thickeners (e.g. sodium carboxymethylcellulose (CMC-Na)). In an optional embodiment, the negative electrode plate may be prepared by: dispersing the above-mentioned components for preparing the negative electrode plate, such as the anode active material, the conductive agent, the binder and any other components in a solvent (such as deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and obtaining the negative electrode plate after drying, cold pressing and other processes. [Electrolyte] The electrolyte serves to conduct ions between the positive electrode plate and the negative electrode plate. The kind of the electrolyte is not particularly limited in the present application, and may be selected according to requirements. For example, the electrolyte may be liquid, gel, or solid. In an optional embodiment, the electrolyte is an electrolyte solution. The electrolyte solution comprises an electrolyte salt and a solvent. In an optional embodiment, the electrolyte salt may comprise at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bis-trifluoromethane sulfonimide, lithium triflate, lithium difluorophosphate, lithium difluorooxalato borate, lithium dioxalato borate, lithium difluorooxalato phosphate, and lithium tetrafluorooxalato phosphate. In an optional embodiment, the solvent may comprise at least one selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone. In an optional embodiment, the electrolyte solution optionally comprises an additive. For example, the additive may comprise a negative electrode film-forming additive, a positive electrode film-forming additive, and may further comprise an additive capable of improving properties of the battery, such as an additive for improving overcharge properties of the battery, and an additive for improving high-temperature or low-temperature properties of the battery. In an optional embodiment, the electrolyte is a solid electrolyte, which may be a variety of lithium ion solid electrolytes commonly used in the art. Examples of the lithium ion solid electrolyte comprise, but are not limited to: LISICON type materials, for example, γ-Li3PO4, etc.; NASICON type materials, for example, Li(1+x1)QxM(2-x1)(PO4)3, where 0 ≤ x1 <1, and Q comprises at least one selected from Al, Cr, Ba, Fe, Sc, In, Lu, Y and La; Garnet type materials, for example, Li(7-x2)La3Zr(2-x2)Mx2O12, where 0 ≤ x2 <1, and M comprises at least one selected from Sb, Nb, Ta, Te and W; LIPON type materials, for example, Lix3POy1Nz1; where 0<x3≤1, 0<y1≤1, and 0<z1≤1; Perovskite type materials, for example, Lix4Q(2 / 3-x4)MO3, where 0.04 <x4 <0.17, Q comprises at least one selected from La, Sr, Ba and Nd, M comprises at least one selected from Al, Ti and Ge; Anti-Perovskite type materials, for example, Li3OCl; Thio-LiSICON type materials, for example, Li(3+x5)My2A(1-y2)Q(4-z2)Tz2, where -1 <x5 <2, 0 ≤ y2 ≤ 1, and 0 ≤ z2 ≤ 2, M comprises at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A comprises at least one selected from P, As, Sb and Bi, Q comprises at least one selected from S or Se, and T comprises at least one selected from F, Cl, Br and I; sulfide solid electrolytes, comprising: Thiophosphate type materials, for example, Li3PS4, Argyrodite type materials, for example, Li6PS5Cl, Halide type materials, for example, Li3InCl6, Hydride type materials, for example at least one selected from 0.7Li(CB9H10) - 0.3Li(CB11H12); for example, a material of Li(10+x6)M(1+y3)A(2-y3)Q(12-z3)Hz3, where -2 < x6 <2, 0 ≤ y3 ≤ 2, and 0 ≤ z3 ≤ 2, M comprises at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A comprises at least one selected from P, As, Sb and Bi, Q comprises at least one selected from S and Se, H comprises at least one selected from F, Cl, Br and I, for another example, a material of (100-x7)Li2S•x7M•y4Q, where 20 ≤ x7 ≤ 30 and 0 ≤ y4 ≤ 50, M comprises at least one selected from B2S3, Al2S3, In2S3, SiS2, GeS2, SnS2, P2S5, As2S3, Sb2S5, Bi2S3, WS2 and MoS2, Q comprises at least one selected from B2O3, Al2O3, In2O3, SiO2, GeO2, SnO2, P2O5, Sb2O5, Bi2O3, WO2, WO3, MoO2, MoO3, Fe2O3, ZnO, MgO, CuO, CaO, LiN, Li2O, LiF, LiCl, LiBr and LiI; Argyrodite type materials, for example, Li(6+x8)My5A(1-y5)Q(5-z5)T(1+z5), where -1 ≤ x8 ≤ 1, 0 ≤ y5 ≤ 1 and -1 <z5 ≤ 1, M comprises at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A comprises at least one selected from P, As, Sb and Bi, Q comprises at least one selected from S and Se, T comprises at least one selected from F, Cl, Br and I; Halide type materials, for example, Li3MJ or Li2Sc2 / 3J, where M comprises at least one selected from Y, Er, In, Sc and Ga, and J comprises at least one selected from F, Cl, Br and I. The sulfide solid electrolyte comprises, but is not limited to, sulfur silver germanium mineral electrolytes; binary sulfide solid materials such as Li2S-P2S5, Li2S-SiS2, Li2S-GeS and Li2S-B2S3, and ternary materials such as Li2S-Me-P2S5, where Me is selected from Si, Ge, Sn and Al. Specifically, the sulfide electrolyte is selected from at least one of Li2S-P2S5, Li2S-SiS2, Li2S-GeS, Li2S-B2S3 and Li2S-Me-P2S5. [Separator] The type of the separator is not particularly limited in the present application, and any known separator having a porous structure and good chemical and mechanical stability may be used. In an optional embodiment, the material of the separator may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, which is not limited in the present application. In a case where the separator is a multilayer composite film, the materials of individual layers may be the same or different. In an optional embodiment, the positive electrode plate, the negative electrode plate, and the separator of a secondary battery may be prepared into an electrode assembly by a winding process or a lamination process. In an optional embodiment, a secondary battery may comprise an outer package. The outer package may be used to encapsulate the electrode assembly and the electrolyte. In an optional embodiment, the outer package of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell and the like. Alternatively, the outer package of the secondary battery may be a soft package, such as a soft bag. The soft bag may be made of a polymer material such as plastics, polypropylene, polybutylene terephthalate and polybutylene succinate. The shape of the secondary battery may be cylindrical, square or any other shape, which is not limited in the present application. In a fifth aspect, the present application also provides a powered device, comprising the secondary battery described in the fourth aspect. The present application provides a powered device that utilizes a secondary battery provided in the present application and thus has the advantage of good electrochemical performance. In an optional embodiment, the above powered device may also comprise a battery module or a battery pack obtained by assembling the secondary battery. The secondary battery, battery module, or battery pack may be used as a power source for the powered device or as an energy storage unit for the powered device. The powered device may comprise, but is not limited to, mobile devices (e.g., cell phones, laptop computers, etc.), 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.), electric trains, ships and satellites, energy storage systems, and the like. As the powered device, a secondary battery, a battery module or a battery pack can be selected according to the needs of its use. As an example, for a powered device that is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc., a battery pack or a battery module may be used to meet the demand for high power and high energy density of the secondary battery of the powered device. As another example, the powered device may be a cell phone, a tablet computer, a laptop computer, and the like. This device usually requires thinness and lightness, and a secondary battery may be used as the power source. The present application is described in further detail below in connection with specific examples, which are not to be construed as limiting the scope of the protection claimed herein. In all examples and comparative examples of the present application, the unit % represents weight percent content. Example 1 The present example provided a method for preparing a lithium iron manganese phosphate composite material, comprising the following steps: S1, weighing manganese carbonate, ferrous oxalate and lithium dihydrogen phosphate in accordance with a molar ratio of Li:Mn:Fe:P=1:0.6:0.4:1, and then weighing an appropriate amount of glucose (glucose accounts for 4% of the total weight of manganese carbonate, ferrous oxalate and lithium dihydrogen phosphate), placing the above raw materials into a stirring ball mill, adding ethanol as solvent to make the solid content to be 50%, grinding for 5h to make the particle size D50 less than 1 μm, placing into an oven for drying at 100 °C for 8 h to obtain a precursor; placing the precursor in a graphite crucible, placing the crucible in a box type furnace, introducing nitrogen gas, raising the temperature from room temperature to 500 °C at a heating rate of 3 °C / min for sintering for 10h, and cooling to obtain a first sintered material LiMn0.6Fe0.4PO4; S2, placing 100g of the first sintered material prepared in S1 on a flat-bottomed glass rotating (20 rpm) within the deposition chamber of an atomic layer deposition (ALD) system, and after the temperature of the deposition chamber reached 400 °C, introducing an inert carrier gas (argon gas) carrying with lithium tert-butoxide, trimethylaluminum, bis(methylcyclopentadienyl)magnesium, and deionized water in a molar ratio of 1:0.5:0.3:10 into the deposition chamber in a pulsed mode, setting the pulse time to be 0.2 s and the number of deposition cycles to be 50, so as to form a LAMO coating layer on the surface of the first sintered material, which was used as an intermediate; and S3, mixing the intermediate obtained from S2 with glucose at a weight ratio of 95:7, placing in a stirring ball mill, adding ethanol to make the solid content to be 50%, grinding for 5h, putting into an oven to dry at 100°C for 8h, then transferring into a graphite crucible, and putting the crucible into a box type furnace, heating up to 700°C at a heating rate of 3°C / min under the nitrogen gas atmosphere, and holding at the temperature for 10h to obtain a LMFP composite material coated with two layers, i.e., LAMO layer and carbon layer, which is denoted as LiMn0.6Fe0.4PO4 / LiAl0.5Mg0.3O1.55 / C. FIGS.1 and 2 are SEM and STEM-EDX pictures, respectively, of the LMFP composite material made in this example. As can be seen from FIG. 1, the particle diameters of the primary particles of the LMFP composite material are mainly centered in a range from 100 nm to 500 nm. As can be seen from FIG.2, the LMFP composite material contains O, Fe, Al, and Mg, and these elements are uniformly distributed in this composite material. FIG. 3 is a comparison graph of XRD measured and standardized graphs of the core material in Example 1 of the present application. As can be seen from FIG. 3, the XRD diffraction peaks of the LMFP composite material prepared in Example 1 correspond to the standard diffraction diagrams of lithium iron manganese phosphate (PDF#83-2092), all of which are consistent with the olivine type structure of orthorhombic system, and no heterogeneous peaks appeared, which indicates that the product prepared in Example 1 is pure phase of lithium iron manganese phosphate. Example 2 The present example provided a method for preparing a lithium iron manganese phosphate composite material, comprising the following steps: S1, weighing manganese carbonate, ferrous phosphate and lithium carbonate in accordance with a molar ratio of Li:Mn:Fe:P=1:0.7:0.3:1, and then weighing an appropriate amount of glucose (glucose accounts for 4% of the total weight of manganese carbonate and ferrous phosphate), placing the above raw materials into a stirring ball mill, adding ethanol as solvent to make the solid content to be 50%, grinding for 5h to make the particle size D50 less than 1 μm, placing into an oven for drying at 100 °C for 8 h to obtain a precursor; placing the precursor in a graphite crucible, placing the crucible in a box type furnace, introducing nitrogen gas, raising the temperature from room temperature to 500 °C at a heating rate of 3 °C / min for sintering for 10h, and cooling to obtain a first sintered material LiMn0.7Fe0.3PO4; S2, placing 100g of the first sintered material prepared in S1 on a flat-bottomed glass rotating (20 rpm) within the deposition chamber of an atomic layer deposition (ALD) system, and after the temperature of the deposition chamber reached 400 °C, introducing an inert carrier gas (argon gas) carrying with lithium tert-butoxide, trimethylaluminum, zirconium(IV) acetylacetonate, and deionized water in a molar ratio of 1:0.1:0.9:10 into the deposition chamber in a pulsed mode, setting the pulse time to be 0.2 s and the number of deposition cycles to be 50, so as to form a LAMO coating layer on the surface of the first sintered material, which was used as an intermediate; and S3, mixing the intermediate obtained from S2 with glucose at a weight ratio of 1:0.06, placing in a stirring ball mill, adding ethanol to make the solid content to be 50%, grinding for 5h, putting into an oven to dry at 100°C for 8h, then transferring into a graphite crucible, and putting the crucible into a box type furnace, heating up to 600°C at a heating rate of 4°C / min under the nitrogen gas atmosphere, and holding at the temperature for 5h to obtain a LMFP composite material coated with two layers, i.e., LAMO layer and carbon layer, which is denoted as LiMn0.7Fe0.3PO4 / LiAl0.1Zr0.9O2.45 / C. Example 3 The present example provided a method for preparing a lithium iron manganese phosphate composite material, comprising the following steps: S1, weighing manganese dioxide, ferric oxide, phosphoric acid and lithium hydroxide in accordance with a molar ratio of Li:Mn:Fe:P=1:0.8:0.2:1, and then weighing an appropriate amount of glucose (glucose accounts for 6% of the total weight of manganese dioxide, ferric oxide, and phosphoric acid), placing the above raw materials into a stirring ball mill, adding ethanol as solvent to make the solid content to be 50%, grinding for 5h to make the particle size D50 less than 1 μm, placing into an oven for drying at 100 °C for 8 h to obtain a precursor; placing the precursor in a graphite crucible, placing the crucible in a box type furnace, introducing nitrogen gas, raising the temperature from room temperature to 500 °C at a heating rate of 3 °C / min for sintering for 10h, and cooling to obtain a first sintered material LiMn0.8Fe0.2PO4; S2, placing 100g of the first sintered material prepared in S1 on a flat-bottomed glass rotating (20 rpm) within the deposition chamber of an atomic layer deposition (ALD) system, and after the temperature of the deposition chamber reached 400 °C, introducing an inert carrier gas (nitrogen gas) carrying with lithium tert-butoxide, trimethylaluminum, yttrium tris(2-methoxyethanolate), and deionized water in a molar ratio of 1:0.9:0.1:10 into the deposition chamber in a pulsed mode, setting the pulse time to be 0.2 s and the number of deposition cycles to be 50, so as to form a LAMO coating layer on the surface of the first sintered material, which was used as an intermediate; and S3, mixing the intermediate obtained from S2 with glucose at a weight ratio of 1:0.12, placing in a stirring ball mill, adding ethanol to make the solid content to be 50%, grinding for 5h, putting into an oven to dry at 100°C for 8h, then transferring into a graphite crucible, and putting the crucible into a box type furnace, heating up to 800°C at a heating rate of 5°C / min under the nitrogen gas atmosphere, and holding at the temperature for 7.5h to obtain a LMFP composite material coated with two layers, i.e., LAMO layer and carbon layer, which is denoted as LiMn0.8Fe0.2PO4 / LiAl0.9Y0.1O2 / C. Example 4 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, with the difference that in the present example, the number of deposition cycles in S2 was adjusted to 100. Example 5 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, with the difference that in the present example, the number of deposition cycles in S2 was adjusted to 150. Example 6 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, and the difference lies in the deposition process parameters in S2, i.e., in the present example, the deposition temperature was 200 °C, the pulse time was 2 s, and the number of deposition cycles was 25. Example 7 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, and the difference lies in the deposition process parameters in S2, i.e., in the present example, the deposition temperature was 600 °C, the pulse time was 1 s, and the number of deposition cycles was 10. Example 8 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, with the difference that a carbon coating layer was prepared by vapor deposition in S3, i.e., in this example, the intermediate prepared in S2 was placed in a vapor deposition apparatus, heated to 700°C, a mixture of acetylene and nitrogen gas with a volume ratio of 1:1 was introduced therein, the pressure of the apparatus was adjusted to 0.2 MPa, and maintained for 8h. Example 9 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, and the difference lies in the amounts of raw materials in S1, i.e., in this example, manganese carbonate, ferrous oxalate, vanadium pentoxide, and lithium dihydrogen phosphate were weighed according to a molar ratio of Li:Mn:Fe:V:P=1:0.6:0.2:0.2:1, and then glucose was weighed in an amount of 4% of the total weight of manganese carbonate, ferrous oxalate, vanadium pentoxide, and lithium dihydrogen phosphate. Example 10 The present example provided a method for preparing a lithium iron manganese phosphate composite material, which was essentially the same as Example 1, and the difference lies in the amounts of raw materials in S1, i.e., in this example, manganese carbonate, ferrous oxalate, titanium dioxide, niobium pentoxide, and lithium dihydrogen phosphate were weighed according to a molar ratio of Li:Mn:Fe:Ti:Nb:P of 1:0.6:0.2:0.1:0.1:1, and then glucose was weighed in an amount of 4% of the total weight of the manganese carbonate, ferrous oxalate, titanium dioxide, niobium pentoxide, and lithium dihydrogen phosphate. Comparative Example 1 This comparative example was essentially the same as Example 1, with the difference that, in this comparative example, S2 was omitted, and the first sintered material prepared by S1 was directly coated with carbon. Comparative Example 2 This comparative example was essentially the same as Example 1, with the difference that, in this comparative example, the step of carbon coating of S3 was omitted. Comparative Example 3 This comparative example was essentially the same as Example 1, with the difference that, in S2 of this comparative example, an inert carrier gas carrying with lithium tert-butoxide, trimethylaluminum, and deionized water was introduced into the deposition chamber of ALD, i.e., a LiAlO2 (abbreviated as LAO) coating layer was formed. Comparative Example 4 The comparative example was essentially the same as Example 2, with the difference that, in S2 of this comparative example, an inert carrier gas carrying with lithium tert-butoxide, trimethylaluminum, and deionized water was introduced into the deposition chamber of ALD, i.e., a LiAlO2 (abbreviated as LAO) coating layer was formed. Comparative Example 5 The comparative example was essentially the same as Example 3, with the difference that, in S2 of this comparative example, an inert carrier gas carrying with lithium tert-butoxide, trimethylaluminum, and deionized water was introduced into the deposition chamber of ALD, i.e., a LiAlO2 (abbreviated as LAO) coating layer was formed. Test Example 1. Test of carbon content HF-2000B high frequency infrared carbon and sulfur analyzer was used to test the carbon content. 2. Test of thickness of coating layer Scanning Transmission Electron Microscope (STEM) was used for analysis by Z-contrast imaging and Energy Dispersive X-ray Spectroscopy (EDX) to obtain the thickness of the coating layer. 3. Test of weight percentage of LAMO coating layer The weight percentage of the LAMO coating layer was obtained by testing the Al element content with an ICP-AES instrument. 4. Test of Button cell specific capacity 1) the cathode active material, acetylene black and polyvinylidene fluoride in a weight ratio of 80:10:10 were dissolved in N-methyl-pyrrolidone, stirred uniformly, coated on an aluminum foil, and then dried at 100°C in a blasting blowing drying oven to obtain a positive electrode plate precursor; 2) the dried positive electrode plate precursor was punched and sliced into small round pieces with a diameter of 12 mm as the positive electrode plate; and 3) lithium metal piece was used as negative electrode plate, polypropylene microporous membrane was used as a separator, and a mixture of 1 mol / L of LiPF6 in EC (ethylene carbonate) and DMC (diethyl carbonate) (volume ratio of EC to DMC is 1:1) was used as the electrolyte solution to assemble the CR2025-type button cell in a glove box filled with argon gas. The battery specific capacity was tested using a button cell tester under 2.0 to 4.3V according to standard 20202915-T-610, “Electrochemical performance test of lithium iron phosphate-Test method for specific capacity and charge-discharge efficiency of the first cycle”. 5. Test of Electrochemical performance NEWARE BTS-5V / 5mA battery testing system was used to test the charging and discharging performance of the battery with a voltage of 4.3V to 2.5 V. (1) Charge specific capacity and discharge specific capacity: in a constant temperature box at 25℃, the assembled battery was charged to 4.3V at constant current at a rate of 0.1C, then charged at a constant voltage of 4.3V until the cut-off current is 0.05C, and then discharged to 2.5V at a rate of 0.1C to get the charge specific capacity and discharge specific capacity at a rate of 0.1C. (2) Cycling performance: in a constant temperature box at 25℃, the assembled battery was charged to 4.3V at constant current at a rate of 1C, then charged at constant voltage of 4.3V until the cut-off current is 0.05C, and then discharged to 2.5V at a rate of 1C, and the obtained specific capacity was recorded as the initial specific capacity (C1). The above steps were repeated for the same battery as mentioned above, and the discharge specific capacity after the 200th cycling (C200) was recorded. Capacity retention rate after 200th cycling = C200 / C1×100%. (3) Test of rate capability: the assembled battery was charged to 4.3V at constant current at a rate of 0.5C, then charged at a constant voltage of 4.3V until the current is reduced to 0.05C, and then discharged to 2.5V at a rate of 5C to getthe discharge specific capacity at a rate of 5C. 6. Test of Manganese dissolution The battery was fully charged at 0.1C and stored in an oven at 60°C for 7 days, then the battery was removed and disassembled, the lithium negative electrode plate was dissolved in 0.1M of hydrochloric acid, the amount of Mn ions in the solution after dissolving was tested, and the ratio of the weight of the dissolved Mn element to the total weight of the active material LMFP in the positive electrode plate was taken as the degree of dissolution of Mn element. The test results were shown in Tables 1 and 2. Table 1. Structure of lithium iron manganese phosphate composite material First coating layer (LAMO)WeightCore Weight Chemical Thickness percentage of percentage Formula (nm) (%) second coating layer (carbon) (%) Example 1LiMn0.6Fe0.4PO4 LiAl0.5Mg0.3O1.55 5.0 1.52 2.0Example 2LiMn0.7Fe0.3PO4LiAl0.1Zr0.9O2.455.0 1.50 1.0Example 3LiMn0.8Fe0.2PO4 LiAl0.9Y0.1O2 5.0 1.51 3.0Example 4LiMn0.6Fe0.4PO4LiAl0.5Mg0.3O1.557.8 2.35 1.9Example 5LiMn0.6Fe0.4PO4 LiAl0.5Mg0.3O1.55 10.0 3.00 2.1Example 6LiMn0.6Fe0.4PO4LiAl0.5Mg0.3O1.553.0 0.98 2.0Example 7LiMn0.6Fe0.4PO4 LiAl0.5Mg0.3O1.55 1.0 0.29 1.9Example 8LiMn0.6Fe0.4PO4LiAl0.5Mg0.3O1.554.9 1.50 1.8Example 9LiMn0.6Fe0.2V0.2PO4 LiAl0.5Mg0.3O1.55 5.0 1.52 1.9Example 10LiMn0.6Fe0.2Ti0.1Nb0.1PO4LiAl0.5Mg0.3O1.554.9 1.53 1.9Comparative LiMn0.6Fe0.4PO4 — 0 0 2.0 Example 1 Comparative LiMn0.6Fe0.4PO4LiAl0.5Mg0.3O1.554.9 1.52 0.02 Example 2 Comparative LiMn0.6Fe0.4PO4 LiAlO2 5.0 1.51 2.0 Example 3 Comparative LiMn0.7Fe0.3PO4LiAlO25.0 1.52 1.9 Example 4 Comparative LiMn0.8Fe0.2PO4LiAlO24.9 1.49 2.0 Example 5 Table 2. Battery performance Charge specific capacity and Cycle Concentration of discharge specific capacity at 0.1C retention rate Rate capability: dissolved (mAh / g) discharge specific after 200 capacity at 5C manganese Charge Discharge specific sp cycles (mAh / g) (ppm) ecific capacity capacity (%) Example 135 159.5 157.5 97.6 118.2Example 255 160.2 158.9 97.0 114.4Example 350 161.7 159.7 95.8 111.8Example 427 155.7 153.6 98.0 115.8Example 520 154.3 151.4 98.3 115.0Example 672 158.6 156.3 93.1 109.4Example 7120 158.1 155.2 90.2 100.8Example 828 160.1 157.9 97.8 119.6Example 938 159.8 157.8 97.7 118.6Example 1034 160.3 158.2 98.0 118.4Comparative 450 155.6 153.4 88.2 97.6 Example 1 Comparative 980 110 78.2 30 23.4 Example 2 Comparative 301 156.8 155.2 91.2 102.3 Example 3 Comparative 350 155.1 153.2 89.3 100.2 Example 4 Comparative 397 156.1 155.5 90.2 101.9 Example 5 As can be seen from Table 2, Examples 1 to 10 can significantly inhibit manganese dissolution, and at the same time also show higher charge specific capacity and discharge specific capacity, capacity retention rate and rate capability. Compared with Examples 1 to 3, the LAMO coating layer produced in Examples 4 and 5 with the number of deposition cycles of 100-150 is thicker, which can better inhibit manganese dissolution and improve the cycling performance, but at the same time, it also adversely affects the LMFP capacity, and since the coating layer itself is not electrochemically active, when the weight percentage of the coating layer is higher, the capacity does not increase, while the weight increase instead, leading to a decrease in specific capacity (capacity / weight) and affecting the rate capability. On the contrary, in Examples 6 and 7, the number of deposition cycles is 10 to 25, the LAMO coating layer produced is relatively thin, the inhibition effect on manganese dissolution decreases, which in turn affects the lithium ion transport rate, and cycling performance and rate capability of the battery are correspondingly not good enough. The core materials of Examples 9 and 10 are doped with element D, and then the synergistic effect among ions of different valence states through ion doping is able to increase the electrical conductivity and stabilize the lattice structure, which is conducive to the enhancement of electrochemical properties. Compared with Example 1, there is no LAMO coating layer in Comparative Example 1, the material has serious manganese dissolution and fast cycling performance degradation. There is no carbon coating layer in Comparative Example 2, which is not conducive to suppressing manganese dissolution and enhancing electrochemical performance, which is because the coated carbon layer is both a physical barrier to block the direct contact between manganese element and electrolyte solution and an excellent conductive substance. In Comparative Example 3, LAO was used as the coating layer, the capacity retention rate and rate capability of the battery are significantly inferior to that of Example 1 with a LAMO coating layer, and the same regular pattern also appears in Comparative Examples 4 and 5, which suggests that, compared with LAO coating, LAMO coating can further improve the structural stability as well as the electrochemical performance of LMFP in terms of capacity, rate capability, and cycling performance due to the synergistic effect of Al and M elements. Obviously, the above examples are merely examples for the purpose of clear illustration, and are not a limitation of the embodiments. For those skilled ordinary in the art, other variations or changes in different forms can be made on the basis of the above description. It is neither necessary nor possible to exhaust all of the embodiments herein. The obvious variations or changes derived therefrom are still within the scope of protection of the present application.

Claims

CLAIMS 1. A lithium iron manganese phosphate composite material, comprising a core, a first coating layer and a second coating layer, wherein the first coating layer is uniformly coated on the outer surface of the core, and the second coating layer is coated on the outer surface of the first coating layer; wherein, the core comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4, wherein 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; the first coating layer comprises a material having a chemical general formula LiaAlbMcOd, 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element; and the second coating layer comprises carbon.

2. The lithium iron manganese phosphate composite material of claim 1, wherein, the content of the first coating layer is in a range from 0.1 wt% to 3.0 wt% based on the total weight of the lithium iron manganese phosphate composite material; and / or the first coating layer has a thickness ranging from 1 nm to 10 nm; and / or the content of the second coating layer is in a range from 1.0 wt % to 3.0 wt % based on the total weight of the lithium iron manganese phosphate composite material.

3. The lithium iron manganese phosphate composite material of claim 1 or 2, wherein, 0.6≤y<0.8; and / or 0≤z<0.2; and / or 0.1≤b≤0.9; and / or 0.1≤c≤0.9; and / or 1.55≤d≤2.45; and / or D comprises at least one of Ti element, Zr element, V element, Nb element and Mg element.

4. A method for preparing a lithium iron manganese phosphate composite material, comprising the following steps: 1providing a core, which comprises a material having a chemical general formula LixMnyFe1-y-zDzPO4; preparing a first coating layer on the outer surface of the core utilizing an atomic layer deposition technology, the first coating layer comprises a material having a chemical general formula LiaAlbMcOd; and preparing a second coating layer on the outer surface of the first coating layer, the second coating layer comprises carbon; wherein, 1≤x≤1.05, 0<y<1, 0≤z<1, and D is a metal element; and 0<a≤1, 0<b<1, 0<c<1, 0<d<3, and M comprises at least one of Y element, Zr element and Mg element.

5. The method for preparing a lithium iron manganese phosphate composite material of claim 4, wherein, the atomic layer deposition technology comprises: placing the material of the core in a deposition chamber and heating it to a temperature ranging from 200°C to 600°C, and then passing an inert carrier gas carrying with a precursor of lithium element, a precursor of aluminum element, a precursor of M element, and a precursor of oxygen element therein, and setting the pulse time to be 0.2s to 2s and the number of deposition cycles to be 10 to 150.

6. The method for preparing a lithium iron manganese phosphate composite material of claim 5, wherein, D comprises at least one of Ti element, Zr element, V element, Nb element and Mg element; and / or the precursor of lithium element comprises at least one of lithium tert-butoxide, tert-butyllithium, isobutyllithium and n-butyllithium; and / or the precursor of aluminum element comprises at least one of trimethylaluminum and dimethylaluminum isopropoxide; and / or the precursor of M element is an organometallic compound containing M element; and / or the precursor of oxygen element comprises at least one of deionized water, alcohol 2compounds and ketone compounds.

7. The method for preparing a lithium iron manganese phosphate composite material of any one of claims 4 to 6, wherein, the preparing a second coating layer on the outer surface of the first coating layer comprises: mixing an intermediate with a carbon source, heating it to a temperature ranging from 600°C to 800°C at a heating rate ranging from 3°C / min to 5°C / min under an inert atmosphere, and holding the temperature for a time period ranging from 5h to 10h; or placing an intermediate in a vapor deposition apparatus, heating it to a temperature ranging from 700°C to 800°C, passing a gas containing a precursor of carbon element therein, adjusting the apparatus to have a pressure ranging from 0.1 MPa to 0.5 MPa, and holding the temperature and pressure for a time period ranging from 5h to 12h; and wherein the intermediate is LixMnyFe1-y-zDzPO4 having a LiaAlbMcOd coating layer.

8. The method for preparing a lithium iron manganese phosphate composite material of claim 7, wherein, the weight ratio of the intermediate to the carbon source is in a range from 1:0.06 to 1:0.12, and the carbon source comprises at least one of glucose, sucrose, polyethylene glycol, phenolic resin, polyvinyl alcohol and citric acid; or the precursor of carbon element comprises at least one of acetylene, ethylene and methane.

9. A positive electrode plate, comprising: a positive electrode current collector, and a cathode active material layer provided on at least one side of the positive electrode current collector, wherein, the cathode active material layer comprises the lithium iron manganese phosphate composite material of any one of claims 1 to 3 or a lithium iron manganese phosphate composite material prepared by the method for preparing a lithium iron manganese phosphate composite material of any one of claims 4 to 8.

310. A secondary battery, comprising the positive electrode plate of claim 9.

11. A powered device, comprising the secondary battery of claim 10. 4

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