Lithium iron phosphate composite cathode active material, preparation method therefor and application thereof

Abstract

The present application belongs to the technical field of secondary batteries, and specifically relates to a lithium iron phosphate composite cathode active material, and further discloses a preparation method thereof, as well as an application thereof for use in the preparation of battery plates and secondary batteries. The lithium iron phosphate composite cathode active material of the present application comprises a core containing a lithium iron phosphate material and iron phosphide particles, as well as a carbon coating layer and / or a LiBO2-containing coating layer at least partially coated on the surface of the core.

Description

[0001] LITHIUM IRON PHOSPHATE COMPOSITE CATHODE ACTIVE MATERIAL, PREPARATION METHOD THEREFOR AND APPLICATION THEREOF FIELD OF THE INVENTION The present application belongs to the technical field of secondary batteries, and specifically relates to a lithium iron phosphate composite cathode active material, and further discloses a preparation method thereof, as well as an application thereof for use in the preparation of battery plates and secondary batteries. BACKGROUND OF THE INVENTION Lithium-ion battery is a kind of energy storage device which is widely used in energy storage field, power battery and portable electronic equipment, and has the advantages of high working voltage, low self-discharge, good safety and the like. Lithium-ion battery is mainly composed of cathode material, anode material, electrolyte, separator and shell, and the cathode material system mainly includes lithium cobaltate, lithium manganate, lithium nickel-manganese oxide, lithium nickel-cobalt-manganese oxide, nickel-cobalt-aluminum oxide, lithium iron phosphate (LFP), lithium iron manganese phosphate, etc. Among them, lithium iron phosphate LiFePO4 material has the advantages of low cost, high safety, good cycling life and the like. Currently, the preparation of lithium iron phosphate particles with small particle diameter (usually 50-500 nm) and high carbon content (usually >2%) has become the main process route for developing low-temperature type lithium iron phosphate materials. This is mainly due to the fact that the materials with small particle diameter can shorten the diffusion distance of lithium ions in the material, which improves the low-temperature performance. However, the materials with small particle diameter not only severely limit the function of the electrochemical performance, but also lead to very low volumetric energy density, which is not favorable for practical applications. Therefore, how to prepare low-temperature type lithium iron phosphate cathode materials with small particle diameter, low resistivity, and high compaction performance, so that they can be applied to high-energy density batteries, is an important way to expand the practical application of lithium iron phosphate in low-temperature environments, and has positive significance for the development of high-energy density batteries. SUMMARY OF THE INVENTION The present application provides a low-temperature type lithium iron phosphate composite cathode active material, a positive electrode plate and a secondary battery, so as to solve the problem that the small particle lithium iron phosphate cathode active material in the existing technology is unable to satisfy the performance requirements of small particle diameter, low resistivity, and high compaction performance and thus affect the application performance. In a first aspect, the present application provides a lithium iron phosphate composite cathode active material, wherein the lithium iron phosphate composite cathode active material comprises a core containing a lithium iron phosphate material and iron phosphide particles, as well as a carbon coating layer and / or a LiBO2-containing coating layer at least partially coated on the surface of the core. In an optional embodiment, the lithium iron phosphate composite cathode active material further comprises a doping element M; and the doping element M includes at least one of Ti, Zr, V, Nb and Mg. In an optional embodiment, the lithium iron phosphate composite cathode active material is represented as LiFe1-xMxPO4-Fe2P / C / LiBO2; wherein, 0≤x≤0.1. Specifically, in the lithium iron phosphate composite cathode active material, “-” represents a mixed form of the materials, i.e., LiFe1-xMxPO4-Fe2P represents a mixed form of LiFe1-xMxPO4 and Fe2P, and “ / ” represents the coating layer form among the materials, i.e., LiFe1-xMxPO4-Fe2P / C / LiBO2 represents that there is a carbon coating layer and a LiBO2-containing coating layer on the outer layer of the LiFe1-xMxPO4-Fe2P core. In an optional embodiment, in the lithium iron phosphate composite cathode active material, the content of the iron phosphide is in a range from 1 ppm to 100 ppm based on the weight of the lithium iron phosphate composite cathode active material; and / or the coating amount of the carbon coating layer is in a range from 0.5 wt% to 1.0 wt% based on the weight of the core; and / or the coating amount of the LiBO2-containing coating layer is in a range from 0.1 wt% to 1.0 wt% based on the weight of the core. In an optional embodiment, the lithium iron phosphate composite cathode active material has a particle size of 50 nm to 200 nm. It should be noted that the particle size described herein characterizes the particle diameter size shown in the SEM. In a second aspect, the present application provides a method for preparing the lithium iron phosphate composite cathode active material, comprising the following steps: S1: mixing a lithium source material, an iron source material and a phosphorus source material to obtain a lithium iron phosphate precursor; S2: carrying out a reductive sintering to the lithium iron phosphate precursor under a protective atmosphere containing a reducing gas to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFePO4-Fe2P; S3: mixing LiFePO4-Fe2P obtained in S2 with a carbon source material, and subjecting to a first sintering treatment under a protective atmosphere to form the carbon coating layer on the surface of LiFePO4-Fe2P, represented as LiFePO4-Fe2P / C; and S4: mixing LiFePO4-Fe2P / C obtained in S3 with the lithium source material and a boron source material, and subjecting to a second sintering treatment to form the LiBO2-containing coating layer on the surface of LiFePO4-Fe2P / C to obtain the lithium iron phosphate composite cathode active material, represented as LiFePO4-Fe2P / C / LiBO2. In an optional embodiment, in S1, a molar ratio of lithium element in the lithium source material, iron element in the iron source material and phosphorus element in the phosphorus source material is (1.0-1.05):1:1. In an optional embodiment, S1 further comprises a step of adding a dopant containing the doping element M; and a molar ratio of lithium element in the lithium source material, iron element in the iron source material, phosphorus element in the phosphorus source material and doping element M in the dopant is (1.0-1.05): 1: 1: (0.01-0.1). In an optional embodiment, S1 further comprises grinding and drying the lithium iron phosphate precursor. In an optional embodiment, the grinding includes wet sanding or ball milling. In an optional embodiment, the drying includes spray drying. In an optional embodiment, in S2, the volume content of the reducing gas in the protective atmosphere is greater than or equal to 8%. In an optional embodiment, in S2: the reducing gas includes hydrogen gas and / or carbon monoxide; and / or the protective atmosphere includes nitrogen gas. In an optional embodiment, in S2, the reductive sintering is carried out at a temperature of 700°C to 800°C for a time period of 5h to 12h. In an optional embodiment, S3 further comprises grinding a mixture of LiFePO4-Fe2P and the carbon source material to a particle size of Dv50<0.3 μm before the first sintering treatment. In an optional embodiment, the grinding is high-energy ultrafine grinding, wherein the particle diameter of the finished particles is controlled by regulating the sanding granularity. In an optional embodiment, in S3, the amount of the carbon source material accounts for 6 wt% to 10 wt% of the amount of LiFePO4-Fe2P. In an optional embodiment, in S3, the first sintering treatment is carried out at a temperature of 700°C to 800°C for a time period of 5h to 12h. In an optional embodiment, in S4: a molar ratio of lithium element in the lithium source material to boron element in the boron source material is (1-1.05):1; and / or the amount of the boron source material accounts for 0.1 wt% to 1.0 wt% of the amount of LiFePO4-Fe2P. In an optional embodiment, in S4, the second sintering treatment is carried out at a temperature of 300°C to 500°C for a time period of 5h to 12h. In an optional embodiment, in the method for preparing the lithium iron phosphate composite cathode active material, the lithium source material includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium acetate and lithium nitrate; and / or, the iron source material includes at least one of ferric phosphate, diiron trioxide, ferrous oxalate, ferric sulfate, ferrous sulfate, ferric nitrate, ferric acetate, ferric chloride, ferrous hydroxide and ferric hydroxide; and / or, the phosphorus source material includes at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, lithium dihydrogen phosphate and ferric phosphate; and / or, the dopant includes at least one of oxides, hydroxides, oxalates, acetates, chlorides and nitrates of the doping element M; and / or, the carbon source material includes at least one of glucose, sucrose, polyethylene glycol, phenolic resin, polyvinyl alcohol and citric acid; and / or, the boron source material includes at least one of boric acid, metaboric acid and tetraboric acid. 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 phosphate composite cathode active material or a lithium iron phosphate composite cathode active material prepared by the method. In a forth aspect, the present application also provides a secondary battery, comprising the positive electrode plate. In a fifth aspect, the present application also provides a powered device, comprising the secondary battery. The technical solution of the present application has the following advantages: the lithium iron phosphate composite cathode active material described in the present application improves the low-temperature performance of lithium iron phosphate by reducing the primary particle diameter of the material, improves the compaction density of the lithium iron phosphate of small particles by reducing the carbon content, improves the electrical conductivity of the material by generating iron phosphide on the surface of the particles, compensating for the decrease in the electrical conductivity of the material due to the decrease in the carbon content, establishes a stable interface on the surface of the material by constructing a thin layer of LiBO2 coating on the surface of iron phosphide, which inhibits the dissolution of iron phosphide by corrosion of HF generated by the decomposition of LiPF6, and then prepares and obtains the lithium iron phosphate material with small particle diameter and low carbon content, which is characterized by excellent low-temperature performance, low resistivity and high compaction performance, and has the advantage of high energy density compared with the traditional small particle lithium iron phosphate material. 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 an SEM pattern of the lithium iron phosphate composite cathode active material described in Example 1 of the present application; FIG. 2 is an XRD pattern of the lithium iron phosphate composite cathode active material described in Example 1 and a material of Comparative Example 2 of the present application. DETAILED DESCRIPTION OF THE INVENTION Reference will be made clearly and completely 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 “includes” 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 included 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. In the description of the present application, the term “a plurality of” refers to two or more (including two), and similarly, “a plurality of groups” refers to two or more (including two) groups, and “a plurality of pieces” refers to two or more (including two) pieces. In order to develop low-temperature type lithium iron phosphate materials, it is often necessary to prepare lithium iron phosphate particles with a small particle diameter (typically 50-500 nm) and a high carbon content (typically >2%). This is mainly due to the fact that small particle diameter materials can shorten the diffusion distance of lithium ions in the material, thereby improving the low-temperature performance. However, without wishing to be limited to any theory, small particle diameter materials simultaneously lead to an increase in the number of material interfaces, which affects the material to have a very high electrical resistivity, severely limiting the functioning of electrochemical properties. Moreover, the relatively low compaction density of dispersed small nanoscale particles leads to a relatively low volumetric energy density, which is also not favorable for practical applications. Therefore, in order to obtain a higher discharge capacity, it is usually necessary to increase the carbon content of the coating of the lithium iron phosphate material to greater than 2%, i.e., an attempt is made to induce the small particle diameter lithium iron phosphate to exert excellent electrochemical performance by increasing the electrical conductivity. However, the higher carbon content further reduces the compaction density of the lithium iron phosphate material, also limiting the application performance of the small particle diameter lithium iron phosphate in high-energy density batteries. In order to solve the above problems existed in the prior art, according to a first aspect of the present application, a lithium iron phosphate composite cathode active material is provided, wherein the lithium iron phosphate composite cathode active material comprises a core containing a lithium iron phosphate material and iron phosphide particles, as well as a carbon coating layer and / or a LiBO2-containing coating layer at least partially coated on the surface of the core. For the lithium iron phosphate composite cathode active material of the present application, in the system of lithium iron phosphate active material, by using the reducing gas for the reductive sintering, small particle iron phosphide material particles form on the surface of the lithium iron phosphate material particles, or iron phosphide material particles coexist with the lithium iron phosphate material particles, which can effectively improve the electrical conductivity of the material, i.e., the electrical conductivity of the material can be guaranteed in the case of reducing the amount of carbon coating, and at the same time, by utilizing synergistic effects of iron phosphide and carbon, the small particle diameter material still has good electrical conductivity under high compaction. Moreover, the synergistic effect of iron phosphide and LiBO2 coating reduces the contact between iron phosphide and electrolyte, inhibits its corrosion decomposition, and guarantees the stable electrical conductivity of the material. At the same time, the synergistic effect of double-layer coating including carbon coating and LiBO2 coating can reduce the contact area between LFP particles and electrolyte, reduce the occurrence of side reactions, and inhibit to a certain extent the structural damage caused by excessive deintercalation of lithium in small particles due to the lower lithium intercalation barriers and higher reactivity, resulting in a faster capacity attenuation. While fully utilizing the advantages of small particles, their disadvantages are effectively suppressed. The lithium iron phosphate composite cathode active material provided in the present application is a lithium iron phosphate material with small particle diameter and low carbon content, which has excellent low-temperature performance, low resistivity and high compaction performance, and has the advantage of high energy density compared with the small particle lithium iron phosphate material of prior art. For the lithium iron phosphate composite cathode active material of the present application, in the system of lithium iron phosphate active material, by using reducing gas for reductive sintering, small particle iron phosphide material particles form on the surface of the lithium iron phosphate material particles, or iron phosphide material particles coexist with the lithium iron phosphate material particles, which can effectively improve the electrical conductivity of the material, i.e., the electrical conductivity of the material can be guaranteed in the case of reducing the amount of carbon coating. This is mainly due to the excellent electronic conductivity of the resulting Fe2P, whose electronic conductivity is close to that of metals and much higher than that of the conventional coating carbon, while most of the conventional coating carbon on the surface of lithium iron phosphate is amorphous carbon, with an electronic conductivity less than 10-3S·cm-1, which is much lower than that of metals. Therefore, the lithium iron phosphate composite cathode active material described in the present application can improve the electrical conductivity of lithium iron phosphate by reducing and generating an appropriate amount of Fe2P, and at the same time, by using the synergistic effect of iron phosphide and carbon, the small particle diameter material can still have a good electrical conductivity under high compaction. In order to solve the problem that direct contact between Fe2P and electrolyte will lead to decomposition in the charging and discharging process and loss of its function, through the further formation of structure of LiBO2 coating on the surface, LiBO2-containing coating layer can form a similar “physical isolation” form, reduce the contact between iron phosphide and electrolyte, inhibit its corrosion and decomposition, and ensure the stable electrical conductivity of the material. This is also the key for using Fe2P to improve the electrical conductivity of lithium iron phosphate. Further, the lithium iron phosphate composite cathode active material of the present application utilizes the synergistic effect of double-layer coating including carbon coating and LiBO2 coating, which can reduce the contact area between LFP particles and electrolyte, reduce the occurrence of side reactions, and inhibit to a certain extent the structural damage caused by excessive deintercalation of lithium in small particles due to the lower lithium intercalation barriers and higher reactivity, resulting in a faster capacity attenuation. While fully utilizing the advantages of small particles, their disadvantages are effectively suppressed. In some embodiments, in the lithium iron phosphate composite cathode active material, the lithium iron phosphate material further comprises a doping element M. In some embodiments, the doping element M includes at least one of Ti, Zr, V, Nb and Mg. The lithium iron phosphate composite cathode active material provided in the present application can improve the application performance of the material by doping metal elements in the lithium iron phosphate material. In some embodiments, the lithium iron phosphate composite cathode active material is represented as LiFe1-xMxPO4-Fe2P / C / LiBO2; wherein, 0≤x≤0.1. Specifically, in the lithium iron phosphate composite cathode active material, represents a mixed form of the material, i.e., LiFe1-xMxPO4-Fe2P represents a mixed form of LiFe1-xMxPO4 and Fe2P, and “ / ” represents the coating layer form among the materials, i.e., LiFe1-xMxPO4-Fe2P / C / LiBO2 represents that there is a carbon coating layer and a LiBO2-containing coating layer on the outer layer of the LiFe1-xMxPO4-Fe2P core. In the lithium iron phosphate composite cathode active material provided by the present application, the content of the dopant element is less than or equal to 10 wt%, for example, the content of the dopant element is controlled to be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, or is in a range composed of the above any values. In an optional embodiment, the content of the iron phosphide is in a range from 1 ppm to 100 ppm based on the weight of the lithium iron phosphate composite cathode active material. For example, by adjusting the content of the reducing gas as well as the sintering temperature and time during the reduction process, the content of the iron phosphide particles is controlled to be between 1 ppm and 100 ppm, for example, 1 ppm, 5 ppm, 10 ppm, 15 ppm, 20 ppm, 25 ppm, 30 ppm, 35 ppm, 40 ppm, 45 ppm, 50 ppm, 55 ppm, 60 ppm, 65 ppm, 70 ppm, 75 ppm, 80 ppm, 85ppm, 90ppm, 95ppm, 100ppm, or in a range composed of the above any values. In an optional embodiment, in the lithium iron phosphate composite cathode active material, the coating amount of the carbon coating layer is in a range from 0.5 wt% to 1.0 wt% based on the weight of the core. For example, by adjusting the amount of the carbon source material added during the carbon coating step, the coating amount of the carbon coating layer is controlled to be in a range from 0.5 wt% to 1.0 wt%, e.g., 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, or in a range composed of the above any values. The lithium iron phosphate composite cathode active material provided by the present application has a low coating amount of carbon, which can ensure the electrical conductivity properties of the material while reducing the coating amount of carbon, and at the same time it can ensure the compaction properties of the material with a small particle diameter. In an optional embodiment, in the lithium iron phosphate composite cathode active material, the coating amount of the LiBO2-containing coating layer is in a range from 0.1 wt% to 1.0 wt% based on the weight of the core. For example, by adjusting the amounts of the boron source material and the lithium source material added during the process of sintering of the coating layer, the coating amount of the LiBO2-containing coating layer is adjusted to be in a range from 0.1 wt% to 1.0 wt%, e.g., 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, or in a range composed of the above any values. In an optional embodiment, the lithium iron phosphate composite cathode active material has a particle size of 50 nm to 200 nm. It should be noted that the particle size described herein characterizes the particle diameter size shown in the SEM. In the lithium iron phosphate composite cathode active material provided in the present application, the particle size of the lithium iron phosphate composite cathode active material is adjusted by means of ultrafine grinding treatment prior to the carbon coating and sintering, e.g., the particle size of the lithium iron phosphate composite cathode active material is controlled to be 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, or in a range composed of the above any values. In a second aspect, the present application also provides a method for preparing the lithium iron phosphate composite cathode active material, comprising the following steps: S1: mixing a lithium source material, an iron source material and a phosphorus source material to obtain a lithium iron phosphate precursor; S2: carrying out a reductive sintering to the lithium iron phosphate precursor under a protective atmosphere containing a reducing gas to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFePO4-Fe2P; in this step, the reductive sintering process can over-reduce the lithium iron phosphate to produce Fe2P; the lithium iron phosphate material with Fe2P particles attached to the surface can be obtained by this step; S3: mixing LiFePO4-Fe2P obtained in S2 with a carbon source material, and subjecting to a first sintering treatment under a protective atmosphere to form the carbon coating layer on the surface of LiFePO4-Fe2P, represented as LiFePO4-Fe2P / C; and S4: mixing LiFePO4-Fe2P / C obtained in S3 with the lithium source material and a boron source material, and subjecting to a second sintering treatment to form the LiBO2-containing coating layer on the surface of LiFePO4-Fe2P / C to obtain the lithium iron phosphate composite cathode active material, represented as LiFePO4-Fe2P / C / LiBO2. The method for preparing the lithium iron phosphate composite cathode active material provided in the present application improves the low-temperature performance of lithium iron phosphate by reducing the primary particle diameter of the material, improves the compaction density of the small particle lithium iron phosphate by reducing the carbon content, improves the electrical conductivity of the material by generating iron phosphide on the surface of the particles, compensating for the decrease in the electrical conductivity of the material due to the decrease in the carbon content, establishes a stable interface on the surface of the material by constructing a thin layer of LiBO2 coating on the surface of iron phosphide, which inhibits the dissolution of iron phosphide by corrosion of HF generated by the decomposition of LiPF6, and then prepare and obtain the lithium iron phosphate material with small particle diameter and low carbon content, which is characterized by excellent low-temperature performance, low resistivity and high compaction performance, and has the advantage of high energy density compared with the traditional small particle lithium iron phosphate material. In some embodiments, in S1, a molar ratio of lithium element in the lithium source material, iron element in the iron source material and phosphorus element in the phosphorus source material is (1.0-1.05):1:1, i.e., the desired lithium iron phosphate material is obtained by regulating the ratio of the three, e.g., controlling the ratio of the three to be 1:1:1 or 1.05:1:1 or to be in a range composed of the above any values. In some embodiments, S1 further comprises a step of adding a dopant containing the doping element M. In some embodiments, the doping element M includes at least one of Ti, Zr, V, Nb and Mg. In some embodiments, a molar ratio of lithium element in the lithium source material, iron element in the iron source material, phosphorus element in the phosphorus source material and doping element M in the dopant is controlled to be (1.0-1.05): 1: 1: (0.01-0.1), or (1.0-1.05): 1: 1: (0.02-0.1), or (1.0-1.05): 1: 1: (0.03-0.1), or (1.0-1.05): 1: 1: (0.04-0.1), or (1.0-1.05): 1: 1: (0.05-0.1), or (1.0-1.05): 1: 1: (0.06-0.1), or (1.0-1.05): 1: 1: (0.07-0.1), or (1.0-1.05): 1: 1: (0.08-0.1), or (1.0-1.05): 1: 1: (0.09-0.1), or (1.0-1.05): 1: 1: 0.1, i.e., the ratio of elements in the lithium iron phosphate material is controlled by regulating the amount of the dopant added. In an optional embodiment, S1 further comprises a step of grinding and drying the lithium iron phosphate precursor. In some embodiments, the grinding includes wet sanding or ball milling. In some embodiments, the drying includes spray drying. The method for preparing the lithium iron phosphate composite cathode active material provided in the present application can help to control the particle size of the entire material by regulating parameters such as the grinding particle diameter of the lithium iron phosphate precursor. In some embodiments, in S2, the volume content of the reducing gas in the protective atmosphere is sufficient to be able to reduce to produce Fe2P, preferably, its volume content is greater than or equal to 8%. For example, the content of the reducing gas can be controlled to be, such as, 1%, 3%, 5%, and 7%, preferably, the content of the reducing gas can be regulated to be, such as, 8%, 10%, 12%, 15%, 18%, 20%, 25%, 30%, 35%, 40%, etc., and the content of iron phosphide can be controlled by regulating the content of the reducing gas. In some embodiments, in S2, the reducing gas comprises a feasible reducing gas in the art, such as, hydrogen gas and / or carbon monoxide, etc.; wherein hydrogen gas has a stronger reducing activity, and thus, the introduction of hydrogen gas is a preferred way to have a better reducing effect. In some embodiments, in S2, the protective atmosphere comprises nitrogen gas, or a gas that can be used as a protective atmosphere in conventional processes in the present art. In some embodiments, in S2, the reductive sintering is carried out at a temperature of 700°C to 800°C for a time period of 5h to 12h. In this step, the desired Fe2P can be formed on the surface of the material by high temperature sintering. In some embodiments, S3 further comprises a step of grinding a mixture of LiFePO4-Fe2P and the carbon source material to particles with a particle size Dv50 less than 0.3 μm before the first sintering treatment. In this step, small particles of the material can be obtained by regulating the size of the particles to satisfy the requirements of low-temperature type materials, e.g., by means of grinding to regulate the particle size Dv50 to be 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, or in a range composed the above any values. In some embodiments, the grinding is high-energy ultrafine grinding, wherein the particle diameter of the finished particles is controlled by regulating the sanding granularity. In some embodiments, in S3, the amount of the carbon source material accounts for 6 wt% to 10 wt% of the amount of LiFePO4-Fe2P, e.g., controlling the amount of the carbon source material to be 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%, can regulate the coating amount of the carbon coating layer. In some embodiments, in S3, the first sintering treatment is carried out at a temperature of 700°C to 800°C for a time period of 5h to 12h, e.g., the temperature is controlled to be 700°C, 720°C, 750°C, 780°C, 800°C, etc., for a time period of, such as, 5h, 6h, 7h, 8h, 9h, 10h, 11h, or 12h. In some embodiments, in S4, a molar ratio of lithium element in the lithium source material to boron element in the boron source material is (1-1.05):1, and the two can chemically react after the second sintering treatment to form the desired LiBO2-containing coating layer. In some embodiments, in S4, the amount of the boron source material accounts for 0.1 wt% to 1.0 wt% of the amount of LiFePO4-Fe2P, i.e., the coating amount of the LiBO2-containing coating layer is regulated by the amount of the boron source material, e.g., the amount of the boron source material can be selected to be 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8wt%, 0.9wt%, 1.0wt%, or in a range composed of any of these values. In some embodiments, in S4, the second sintering treatment is carried out at a temperature of 300°C to 500°C for a time period of 5h to 12h. In this step, the desired LiBO2-containing coating layer can be formed under sintering at this temperature. In some embodiments, in the method for preparing lithium iron phosphate composite cathode active material, optional lithium source material, iron source material, phosphorus source material, dopant, carbon source material, and boron source material, are conventional materials in the art, and are schematically described herein, for example: the lithium source material includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium acetate and lithium nitrate; and / or, the iron source material includes at least one of ferric phosphate, diiron trioxide, ferrous oxalate, ferric sulfate, ferrous sulfate, ferric nitrate, ferric acetate, ferric chloride, ferrous hydroxide and ferric hydroxide; and / or, the phosphorus source material includes at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, lithium dihydrogen phosphate and ferric phosphate; and / or, the dopant includes at least one of oxides, hydroxides, oxalate salts, acetate salts, chlorides and nitrate salts of the doping element M; and / or, the carbon source material includes at least one of glucose, sucrose, polyethylene glycol, phenolic resin, polyvinyl alcohol and citric acid; and / or, the boron source material includes at least one of boric acid, metaboric acid and tetraboric acid. 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 phosphate composite cathode active material or a lithium iron phosphate composite cathode active material prepared by the method. In a fourth aspect, the present application provides a secondary battery, comprising the positive electrode plate. Typically, a secondary battery includes a positive electrode plate, a negative electrode plate, an electrolyte, and 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 includes a positive electrode current collector and a cathode active material layer disposed on at least one surface of the positive electrode current collector. The cathode active material layer includes the cathode active material. 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 includes 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) system. As an example, in addition thereto, the cathode active material suitable for the battery system may also include 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 secondary battery provided in the present application is a secondary battery in which lithium iron phosphate (LiFePO4) cathode active material is selected. 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 some embodiments, the cathode active material layer can also include a binder. As an example, the binder may include 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 some embodiments, the cathode active material layer can also include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, 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] The negative electrode plate includes a negative electrode current collector and an anode active material layer disposed on at least one surface of the negative electrode current collector. The anode active material layer includes an anode active material. 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 some embodiments, the anode active material may be an anode active material known in the art. As an example, the anode active material may include 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. The present application is not limited to these materials, and other traditional materials that may be used as an anode active material for a battery may be used. These anode active materials may be used separately or in combination (for example two or more kinds of materials are used). In some embodiments, the anode active material layer optionally includes 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 some embodiments, the anode active material layer optionally includes 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 some embodiments, the anode active material layer optionally includes other adjuvants, such as thickeners (e.g. sodium carboxymethylcellulose (CMC-Na)). In some embodiments, 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 some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent. In some embodiments, the electrolyte salt may include 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 some embodiments, the solvent may include 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 some embodiments, the electrolyte solution optionally includes an additive. For example, the additive may include a negative electrode film-forming additive, a positive electrode film-forming additive, and may further include 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 some embodiments, 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 include, 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 includes 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 includes 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 includes at least one selected from La, Sr, Ba and Nd, M includes 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 includes at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A includes at least one selected from P, As, Sb and Bi, Q includes at least one selected from S or Se, and T includes at least one selected from F, Cl, Br and I; sulfide solid electrolytes, including: 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 includes at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A includes at least one selected from P, As, Sb and Bi, Q includes at least one selected from S and Se, H includes 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 includes at least one selected from B2S3, Al2S3, In2S3, SiS2, GeS2, SnS2, P2S5, As2S3, Sb2S5, Bi2S3, WS2 and MoS2, Q includes 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 includes at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A includes at least one selected from P, As, Sb and Bi, Q includes at least one selected from S and Se, T includes at least one selected from F, Cl, Br and I; Halide type materials, for example, Li3MJ or Li2Sc2 / 3J, where M includes at least one selected from Y, Er, In, Sc and Ga, and J includes at least one selected from F, Cl, Br and I. The sulfide solid electrolyte includes, 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] In some embodiments, the separator is further included in the secondary battery. 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 some embodiments, 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 some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be prepared into an electrode assembly by a winding process or a lamination process. In some embodiments, a secondary battery may include an outer package. The outer package may be used to encapsulate the electrode assembly and the electrolyte. In some embodiments, 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 provides a powered device, comprising the secondary battery described in the fourth aspect. In some embodiments, 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 include, 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 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. Example 1 The method for preparing the lithium iron phosphate composite cathode active material described in this example comprised the following steps: S1: weighing Li2CO3, FePO4 and TiO2 respectively in accordance with a molar ratio of 0.505:1:0.01, mixing, adding deionized water and mixing to obtain a slurry, controlling the solid content in the slurry to be 40%, grinding the slurry to achieve a particle size Dv50 of 0.4-0.6 μm, and spray drying to obtain a precursor in the form of powder; S2: placing the precursor obtained in S1 in a graphite sagger and sintering under an H2 and N2 atmosphere, controlling the flow ratio of H2 to N2 to be 10% : 90%, raising the temperature to 750°C at a heating rate of 3°C / min and holding at this temperature for reductive sintering, controlling the time for holding at this temperature to be 10h, allowing the lithium iron phosphate to be excessively reduced, then producing Fe2P which was attached to the surface of the lithium iron phosphate material particles to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFe0.99Ti0.01PO4-Fe2P; S3: mixing LiFe0.99Ti0.01PO4-Fe2P obtained in S2 with a glucose carbon source, controlling the weight ratio of the glucose to LiFe0.99Ti0.01PO4-Fe2P to be 8:92; adding deionized water to obtain a slurry, controlling the solid content in the slurry to be 50%; subjecting the slurry to high-energy ultrafine grinding until the particle size Dv50 is 0.2-0.3μm, spray drying to obtain a precursor in the form of powder; continuing to place the precursor in the form of powder in a graphite sagger, carrying out a first sintering treatment under N2 atmosphere, raising the temperature at a heating rate of 3 ℃ / min to 700 ℃ and holding at this temperature for the first sintering treatment, and controlling the time for holding at this temperature to be 10h, and forming a carbon coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P, represented as LiFe0.99Ti0.01PO4-Fe2P / C; and S4: weighing and mixing LiFe0.99Ti0.01PO4-Fe2P / C obtained in S3, Li2CO3 and H3BO3, controlling a weight ratio of LiFe0.99Ti0.01PO4-Fe2P / C: Li2CO3: H3BO3 to be 100:0.3:0.5 and a molar ratio of Li in Li2CO3 and B in H3BO3 to be 1:1; mixing the above materials using a high mixer for 30 min, and raising the temperature to 350°C at a heating rate of 3°C / min and holding at this temperature for a second sintering treatment, controlling the time for holding at this temperature to be 10 h, and forming a LiBO2-containing coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P / C, represented as LiFe0.99Ti0.01PO4-Fe2P / C / LiBO2. Example 2 The method for preparing the lithium iron phosphate composite cathode active material described in this example comprised the following steps: S1: weighing LiH2PO4, Fe2O3 and MgO respectively in accordance with a molar ratio of 1.01:0.5:0.01, mixing, adding deionized water and mixing to obtain a slurry, controlling the solid content in the slurry to be 40%, grinding the slurry to achieve a particle size Dv50 of 0.4-0.6 μm, and spray drying to obtain a precursor in the form of powder; S2: placing the precursor obtained in S1 in a graphite sagger and sintering under an H2 and N2 atmosphere, controlling the flow ratio of H2 to N2 to be 8% : 92%, raising the temperature to 720°C at a heating rate of 3°C / min and holding at this temperature for reductive sintering, controlling the time for holding at this temperature to be 10h, allowing the lithium iron phosphate to be excessively reduced, then producing Fe2P which was attached to the surface of the lithium iron phosphate material particles to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFe0.99Mg0.01PO4-Fe2P; S3: mixing LiFe0.99Mg0.01PO4-Fe2P obtained in S2 with a glucose carbon source, controlling the weight ratio of the glucose to LiFe0.99Mg0.01PO4-Fe2P to be 5:95; adding deionized water to obtain a slurry, controlling the solid content in the slurry to be 50%; subjecting the slurry to high-energy ultrafine grinding until the particle size Dv50 is 0.2-0.3μm, spray drying to obtain a precursor in the form of powder; continuing to place the precursor in the form of powder in a graphite sagger, carrying out a first sintering treatment under N2 atmosphere, raising the temperature at a heating rate of 3 ℃ / min to 720 ℃ and holding at this temperature for the first sintering treatment, and controlling the time for holding at this temperature to be 10h, and forming a carbon coating layer on the surface of LiFe0.99Mg0.01PO4-Fe2P, represented as LiFe0.99Mg0.01PO4-Fe2P / C; and S4: weighing and mixing LiFe0.99Mg0.01PO4-Fe2P / C obtained in S3, Li2CO3 and H3BO3, controlling a weight ratio of LiFe0.99Mg0.01PO4-Fe2P / C: Li2CO3: H3BO3 to be 100:0.3:0.5 and a molar ratio of Li in Li2CO3 and B in H3BO3 to be 1:1; mixing the above materials using a high mixer for 30 min, and raising the temperature to 350°C at a heating rate of 3°C / min and holding at this temperature for a second sintering treatment, controlling the time for holding at this temperature to be 10 h, and forming a LiBO2-containing coating layer on the surface of LiFe0.99Mg0.01PO4-Fe2P / C, represented as LiFe0.99Mg0.01PO4-Fe2P / C / LiBO2. Example 3 The method for preparing the lithium iron phosphate composite cathode active material described in this example comprised the following steps: S1: weighing Li2CO3, FePO4 and TiO2 respectively in accordance with a molar ratio of 0.505:1:0.01, mixing, adding deionized water and mixing to obtain a slurry, controlling the solid content in the slurry to be 40%, grinding the slurry to achieve a particle size Dv50 of 0.4-0.6 μm, and spray drying to obtain a precursor in the form of powder; S2: placing the precursor obtained in S1 in a graphite sagger and sintering under an H2 and N2 atmosphere, controlling the flow ratio of H2 to N2 to be 10% : 90%, raising the temperature to 750°C at a heating rate of 3°C / min and holding at this temperature for reductive sintering, controlling the time for holding at this temperature to be 10h, allowing the lithium iron phosphate to be excessively reduced, then producing Fe2P which was attached to the surface of the lithium iron phosphate material particles to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFe0.99Ti0.01PO4-Fe2P; S3: mixing LiFe0.99Ti0.01PO4-Fe2P obtained in S2 with a glucose carbon source, controlling the weight ratio of the glucose to LiFe0.99Ti0.01PO4-Fe2P to be 8:92; adding deionized water to obtain a slurry, controlling the solid content in the slurry to be 50%; subjecting the slurry to high-energy ultrafine grinding until the particle size Dv50 is 0.5-0.6μm, spray drying to obtain a precursor in the form of powder; continuing to place the precursor in the form of powder in a graphite sagger, carrying out a first sintering treatment under N2 atmosphere, raising the temperature at a heating rate of 3 ℃ / min to 700 ℃ and holding at this temperature for the first sintering treatment, and controlling the time for holding at this temperature to be 10h, and forming a carbon coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P, represented as LiFe0.99Ti0.01PO4-Fe2P / C; and S4: weighing and mixing LiFe0.99Ti0.01PO4-Fe2P / C obtained in S3, Li2CO3 and H3BO3, controlling a weight ratio of LiFe0.99Ti0.01PO4-Fe2P / C: Li2CO3: H3BO3 to be 100:0.3:0.5 and a molar ratio of Li in Li2CO3 and B in H3BO3 to be 1:1; mixing the above materials using a high mixer for 30 min, and raising the temperature to 350°C at a heating rate of 3°C / min and holding at this temperature for a second sintering treatment, controlling the time for holding at this temperature to be 10 h, and forming a LiBO2-containing coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P / C, represented as LiFe0.99Ti0.01PO4-Fe2P / C / LiBO2. Example 4 The method for preparing the lithium iron phosphate composite cathode active material described in this example comprised the following steps: S1: weighing Li2CO3, FePO4 and TiO2 respectively in accordance with a molar ratio of 0.505:1:0.01, mixing, adding deionized water and mixing to obtain a slurry, controlling the solid content in the slurry to be 40%, grinding the slurry to achieve a particle size Dv50 of 0.4-0.6 μm, and spray drying to obtain a precursor in the form of powder; S2: placing the precursor obtained in S1 in a graphite sagger and sintering under an H2 and N2 atmosphere, controlling the flow ratio of H2 to N2 to be 5% : 95%, raising the temperature to 700°C at a heating rate of 3°C / min and holding at this temperature for reductive sintering, controlling the time for holding at this temperature to be 10h, allowing the lithium iron phosphate to be excessively reduced, then producing Fe2P which was attached to the surface of the lithium iron phosphate material particles to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFe0.99Ti0.01PO4-Fe2P; S3: mixing LiFe0.99Ti0.01PO4-Fe2P obtained in S2 with a glucose carbon source, controlling the weight ratio of the glucose to LiFe0.99Ti0.01PO4-Fe2P to be 2:98; adding deionized water to obtain a slurry, controlling the solid content in the slurry to be 50%; subjecting the slurry to high-energy ultrafine grinding until the particle size Dv50 is 0.2-0.3μm, spray drying to obtain a precursor in the form of powder; continuing to place the precursor in the form of powder in a graphite sagger, carrying out a first sintering treatment under N2 atmosphere, raising the temperature at a heating rate of 3 ℃ / min to 720 ℃ and holding at this temperature for the first sintering treatment, and controlling the time for holding at this temperature to be 10h, and forming a carbon coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P, represented as LiFe0.99Ti0.01PO4-Fe2P / C; and S4: weighing and mixing LiFe0.99Ti0.01PO4-Fe2P / C obtained in S3, Li2CO3 and H3BO3, controlling a weight ratio of LiFe0.99Ti0.01PO4-Fe2P / C: Li2CO3: H3BO3 to be 100:0.06:0.1 and a molar ratio of Li in Li2CO3 and B in H3BO3 to be 1:1; mixing the above materials using a high mixer for 30 min, and raising the temperature to 350°C at a heating rate of 3°C / min and holding at this temperature for a second sintering treatment, controlling the time for holding at this temperature to be 10 h, and forming a LiBO2-containing coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P / C, represented as LiFe0.99Ti0.01PO4-Fe2P / C / LiBO2. Example 5 The method for preparing the lithium iron phosphate composite cathode active material described in this example comprised the following steps: S1: weighing Li2CO3, FePO4 and TiO2 respectively in accordance with a molar ratio of 0.505:1:0.01, mixing, adding deionized water and mixing to obtain a slurry, controlling the solid content in the slurry to be 40%, grinding the slurry to achieve a particle size Dv50 of 0.4-0.6 μm, and spray drying to obtain a precursor in the form of powder; S2: placing the precursor obtained in S1 in a graphite sagger and sintering under an H2 and N2 atmosphere, controlling the flow ratio of H2 to N2 to be 20% : 80%, raising the temperature to 760°C at a heating rate of 3°C / min and holding at this temperature for reductive sintering, controlling the time for holding at this temperature to be 10h, allowing the lithium iron phosphate to be excessively reduced, then producing Fe2P which was attached to the surface of the lithium iron phosphate material particles to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFe0.99Ti0.01PO4-Fe2P; S3: mixing LiFe0.99Ti0.01PO4-Fe2P obtained in S2 with a glucose carbon source, controlling the weight ratio of the glucose to LiFe0.99Ti0.01PO4-Fe2P to be 10:90; adding deionized water to obtain a slurry, controlling the solid content in the slurry to be 50%; subjecting the slurry to high-energy ultrafine grinding until the particle size Dv50 is 0.2-0.3μm, spray drying to obtain a precursor in the form of powder; continuing to place the precursor in the form of powder in a graphite sagger, carrying out a first sintering treatment under N2 atmosphere, raising the temperature at a heating rate of 3 ℃ / min to 700 ℃ and holding at this temperature for the first sintering treatment, and controlling the time for holding at this temperature to be 10h, and forming a carbon coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P, represented as LiFe0.99Ti0.01PO4-Fe2P / C; and S4: weighing and mixing LiFe0.99Ti0.01PO4-Fe2P / C obtained in S3, Li2CO3 and H3BO3, controlling a weight ratio of LiFe0.99Ti0.01PO4-Fe2P / C: Li2CO3: H3BO3 to be 100:0.54:0.9 and a molar ratio of Li in Li2CO3 and B in H3BO3 to be 1:1; mixing the above materials using a high mixer for 30 min, and raising the temperature to 350°C at a heating rate of 3°C / min and holding at this temperature for a second sintering treatment, controlling the time for holding at this temperature to be 10 h, and forming a LiBO2-containing coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P / C, represented as LiFe0.99Ti0.01PO4-Fe2P / C / LiBO2. Example 6 The method for preparing the lithium iron phosphate composite cathode active material described in this example comprised the following steps: S1: weighing Li2CO3, FePO4 and TiO2 respectively in accordance with a molar ratio of 0.505:1:0.01, mixing, adding deionized water and mixing to obtain a slurry, controlling the solid content in the slurry to be 40%, grinding the slurry to achieve a particle size Dv50 of 0.4-0.6 μm, and spray drying to obtain a precursor in the form of powder; S2: placing the precursor obtained in S1 in a graphite sagger and sintering under an H2 and N2 atmosphere, controlling the flow ratio of H2 to N2 to be 30% : 70%, raising the temperature to 770°C at a heating rate of 3°C / min and holding at this temperature for reductive sintering, controlling the time for holding at this temperature to be 10h, allowing the lithium iron phosphate to be excessively reduced, then producing Fe2P which was attached to the surface of the lithium iron phosphate material particles to obtain a core containing lithium iron phosphate material and iron phosphide particles, represented as LiFe0.99Ti0.01PO4-Fe2P; S3: mixing LiFe0.99Ti0.01PO4-Fe2P obtained in S2 with a glucose carbon source, controlling the weight ratio of the glucose to LiFe0.99Ti0.01PO4-Fe2P to be 15:85; adding deionized water to obtain a slurry, controlling the solid content in the slurry to be 50%; subjecting the slurry to high-energy ultrafine grinding until the particle size Dv50 is 0.2-0.3μm, spray drying to obtain a precursor in the form of powder; continuing to place the precursor in the form of powder in a graphite sagger, carrying out a first sintering treatment under N2 atmosphere, raising the temperature at a heating rate of 3 ℃ / min to 700 ℃ and holding at this temperature for the first sintering treatment, and controlling the time for holding at this temperature to be 10h, and forming a carbon coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P, represented as LiFe0.99Ti0.01PO4-Fe2P / C; and S4: weighing and mixing LiFe0.99Ti0.01PO4-Fe2P / C obtained in S3, Li2CO3 and H3BO3, controlling a weight ratio of LiFe0.99Ti0.01PO4-Fe2P / C: Li2CO3: H3BO3 to be 100:0.72:1.2 and a molar ratio of Li in Li2CO3 and B in H3BO3 to be 1:1; mixing the above materials using a high mixer for 30 min, and raising the temperature to 350°C at a heating rate of 3°C / min and holding at this temperature for a second sintering treatment, controlling the time for holding at this temperature to be 10 h, and forming a LiBO2-containing coating layer on the surface of LiFe0.99Ti0.01PO4-Fe2P / C, represented as LiFe0.99Ti0.01PO4-Fe2P / C / LiBO2. Example 7 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S1, lithium acetate, ferrous oxalate, and ammonium dihydrogen phosphate as the raw materials were weighed and mixed in accordance with a molar ratio of 1: 1: 1 (measured in terms of Li, Fe, and P), respectively. Example 8 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S1, lithium hydroxide, iron nitrate and phosphoric acid as the raw materials were weighed and mixed in accordance with a molar ratio of 1.05: 1: 1 (measured in terms of Li, Fe, and P), respectively. Example 9 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S1, lithium nitrate, ferrous oxalate, ammonium dihydrogen phosphate and NbO2 as the raw materials were weighed and mixed in accordance with a molar ratio of 1.05:1:1:0.05 (measured as Li, Fe, P and Nb), respectively. Example 10 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S1, lithium carbonate, ferrous sulfate, phosphoric acid, and ZrO2 as the raw materials were weighed and mixed according to a molar ratio of 1.05: 1: 1: 1: 0.1 (measured in terms of Li, Fe, P, and Zr, respectively), respectively. Example 11 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S2, carbon monoxide gas was added for reduction in the reductive sintering. Example 12 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S2, the reductive sintering was carried out at a temperature of 800 ℃ for 5h. Example 13 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S3, the particle size Dv50 of the mixture in the grinding step was controlled to be 0.1-0.2 μm. Example 14 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S3, polyethylene glycol was used as the carbon source material, and its amount is 6 wt% of the amount of LiFePO4-Fe2P. Example 15 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S3, the first sintering treatment was carried out at a temperature of 800 ℃ for 5h. Example 16 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S3, the first sintering treatment was carried out at a temperature of 700 ℃ for 12h. Example 17 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S4, metaboric acid was used as the boron source material, the amount of the boron source material was controlled to be 1.0 wt% of the amount of LiFePO4-Fe2P. Example 18 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S4, tetraboronic acid was used as the boron source material, the amount of the boron source material was controlled to be 0.3 wt% of the amount of LiFePO4-Fe2P. Example 19 The method for preparing the lithium iron phosphate composite cathode active material described in this example was the same as that of Example 1, which differed only in that, in S4, the second sintering treatment was carried out at a temperature of 300 ℃ for 12h. Example 20 The method for preparing the lithium iron phosphate composite cathode active material described in this example as the same as that of Example 1, which differed only in that, in S4, the second sintering treatment was carried out at a temperature of 500 ℃ for 5h. Comparative Example 1 The method for preparing the lithium iron phosphate composite cathode active material described in this comparative example was the same as that of Example 1, which differed only in that, in S2, the lithium iron phosphate precursor was sintered only under an N2 atmosphere without the addition of hydrogen gas. Comparative Example 2 The method for preparing the lithium iron phosphate composite cathode active material described in this comparative example was the same as that of Example 1, which differed only in that, in S4, the second sintering treatment was performed without the addition of the boron source material and the lithium source material. Comparative Example 3 The method for preparing the lithium iron phosphate composite cathode active material described in this comparative example was the same as that of Example 1, which differed only in that the step of performing carbon coating in S3 was omitted. Comparative Example 4 The method for preparing the lithium iron phosphate composite cathode active material described in this comparative example was the same as that of Example 2, which differed only in that, in S2, the lithium iron phosphate precursor was sintered only under an N2 atmosphere without the addition of hydrogen gas. Comparative Example 5 The method for preparing the lithium iron phosphate composite cathode active material described in this comparative example was the same as that of Example 2, which differed only in that, in S4, the second sintering treatment was performed without the addition of the boron source material and the lithium source material. Comparative Example 6 The method for preparing the lithium iron phosphate composite cathode active material described in this comparative example was the same as that of Example 2, which differed only in that the step of performing carbon coating in S3 was omitted. Test Examples 1. Physical and chemical indexes and battery performance test The test parameters and methods involved in the following test examples of the present application included: (1) carbon content was tested using HF-2000B high-frequency infrared carbon and sulfur analyzer (room temperature 25±5℃); (2) resistivity was tested using a volume resistivity tester (room temperature 25±5°C), and samples were made in the manner of the instrument example; (3) powder compaction was tested using a powder compaction tester (pressure 3t, room temperature 25±5℃) / GB / T 30835-2014; (4) Fe2P content was tested using magnetic bar adsorption and Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) by Test methods of cathode materials for lithium ion battery - Determination of magnetic impurities content and residual alkali content (20194101-T-610); and (5) button cell capacity was tested using button cell tester at a voltage of 2.5-3.65V by Electrochemical performance test of lithium iron phosphate - Test method for specific capacity and charge-discharge efficiency of the first cycle (20202915-T-610). Battery assembly: 1) the cathode material / acetylene black / polyvinylidene fluoride were dissolved in N-methyl-pyrrolidone according to a weight ratio of 80:10:10, stirred uniformly, and then 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; 3) lithium metal piece were used as negative electrode plate, polypropylene microporous membrane was used as the separator, and a mixture of 1 mol / L of LiPF6, EC and DMC (volume ratio of 1: 1:1) was used as the electrolyte solution to assemble the CR2025-type button cell in a glove box filled with argon gas. Electrical performance test method: the battery was tested for charging and discharging performance with BTS-5V / 5mA battery test system at a voltage of 3.65-2.5V, which specifically included: (1) charge / discharge specific capacity: in a constant temperature box at 25℃, the assembled battery was charged to 3.65V at constant current of 0.1C rate, then charged to a cut-off current of 0.05C at a constant voltage of 3.65V, and then discharged to 2.5V at a rate of 0.1C, to obtain the charge / discharge capacity; (2) cycling performance: in a constant temperature box at 25℃, the assembled battery was charged to 3.65V at constant current of 1C rate, then charged to the cut-off current of 0.05C at constant voltage of 3.65V, and discharged to 2.5V at 1C rate, and the obtained capacity was recorded as the initial capacity (C1), the above steps were repeated for the same battery mentioned above, and the discharge capacity after the 200th cycle (C200) was recorded, and capacity retention rate after cycling = C200 / C1*100%; (3) low-temperature performance test at -20℃: in a constant temperature box at 25℃, the battery was charged to 3.65V at constant current of 0.1C rate, then charged at constant voltage of 3.65V to a cut-off current of 0.05C, activated at 0.1C at room temperature for 2 cycles, and the second discharge capacity was recorded (C25℃), and then the battery was fully charged at 0.1C constant current and constant voltage; it was placed in the constant temperature box of -20℃, and discharged to 2.5 V at 1C rate under the condition of -20℃. The discharge capacity (C-20℃) was recorded, and then the capacity retention rate of -20℃ = C-20℃ / C25℃. The relevant results of the testing of the lithium iron phosphate material in Examples 1-20 and Comparative Examples 1-6 in accordance with the foregoing method are shown in Table 1 below.

[0002] Table 1. Performance Test Results Discharge Fe2P Powder 0.1C specific L Carb Resistivity ow Capacity on of B compaction discharge capacity at temperature retention c the Content ontent powder content den specific low retention rate after (ppm) C% (ppm) sity capacity temperature rate at 200 Ω·cm-1g / cc -20°C mAh / g -20°C cycles mAh / g Example 1 50.0 0.62% 30 1000 2.40 162.2 120.5 74.1% 98.1% Example 2 40.2 0.83% 48 800 2.35 161.4 121.2 75.0% 97.3% Example 3 50.1 0.60 34 1000 2.50 158.1 65.4 41.5% 99.2% Example 4 1.5 0.31% 24000 200 2.52 151.0 80.1 53.3% 88.6% Example 5 80.4 1.50% 13 1500 2.20 158.2 110.1 70.1% 98.4% Example 6 100.4 2.2% 12 2000 2.10 156.1 105.3 67.0% 99.5% Example 7 51.5 0.61% 32 1020 2.42 160.2 117.0 73.0% 97.8% Example 8 49.3 0.58% 41 995 2.37 162.7 120.5 74.1% 96.5% Example 9 49.3 0.58% 36 997 2.38 162.3 120.1 74.0% 96.2% Example 10 50.1 0.59% 34 998 2.39 162.5 120.2 74.0% 96.3% Example 11 30.5 0.57% 120 1005 2.41 160.0 112.0 70.0% 95.2% Example 12 62.1 0.61% 29 1001 2.47 159.0 110.1 69.2% 98.2% Example 13 51.3 0.61% 38 996 2.30 162.8 123.2 75.7% 95.7% Example 14 50.5 0.30% 230 1002 2.48 157.9 109.3 69.2% 97.1% Example 15 50.8 0.57% 41 1008 2.44 160.6 118.5 73.8% 98.2% Example 16 49.8 0.62% 38 1003 2.42 161.8 119.7 74.0% 97.8% Example 17 48.9 0.61% 33 1670 2.41 158.1 112.3 71.0% 98.3% Example 18 50.7 0.59% 41 654 2.39 161.5 119.2 73.8% 90.7% Example 19 50.2 0.60% 35 1002 2.41 161.7 120.2 74.3% 97.8% Example 20 49.6 0.58% 38 956 2.39 161.2 117.3 72.8% 94.2% Comparative Example 1 0 0.6% 18000 1000 2.39 150 90 60% 97% Comparative Example 2 50 0.6% 38 0 2.41 161 121 75% 92% Comparative Example 3 55.2 0 24200 1007 2.55 149.1 70.2 47.1% 95.1% Comparative Example 4 0 0.78 26400 820 2.37 150.1 85.3 56.8% 89.5% Comparative Example 5 39.5 0.81 120 805 2.36 160.8 119.5 74.3% 90.2% Comparative Example 6 40.9 0 21000 801 2.52 150.1 75.6 50.4% 89.3% As can be seen from the data in Table 1 above, the lithium iron phosphate composite cathode active material described in Examples 1 and 2 exhibits excellent comprehensive performance as a cathode active material. In Example 3, although the controlled sanding granularity is slightly larger, the primary particles of the material are relatively larger, affecting the capacity and low-temperature performance of the material, which is slightly worse than that of Examples 1 and 2, it still has good performance; in Example 4, due to the amount of Fe2P and the carbon coating amount are slightly lower, which partially affects the conductivity of the material, thereby affecting the capacity, and the material's resistivity is slightly high, and its capacity is relatively low, but the composite material still has good performance; in Examples 5-6, due to the amount of Fe2P as well as carbon coating amount and boron coating amount are slightly higher, relatively affecting the electrochemical capacity of the material, this is due to the fact that the above substances are not electrochemically active, thus affecting the overall electrochemical performance of the material, but the lithium iron phosphate composite cathode active material still has a good performance. In addition, as can be seen from the data in Table 1 above, in the material prepared in Example 3, the particle size is controlled to be larger than that of the material of Example 1 in the sanding process of S3, resulting in a particle diameter of the finally generated lithium iron phosphate material that is significantly larger than that of Example 1, but affecting the material's charge specific capacity at low temperature and retention rate that is significantly lower than that of Example 1. It proves that the smaller particle size of the material has a positive effect on the electrochemical capacity of the material at low temperature. In the preparation technical solution in Example 6, when the carbon content therein is controlled to be 2.2%, the powder compaction density of the material is only 2.10 g / cc, comparing with the material in Example 1, when the carbon content is 0.62%, the powder compaction density of the material is 2.40 g / cc. This also illustrates that the higher carbon content elaborated in the present application affects the lithium iron phosphate material with a severely reduced compaction density, which is not conducive to the enhancement of the volumetric energy density of the lithium iron phosphate battery. This is also the reason that the technical solution of the present application seeks to reduce the carbon coating amount and reduce the particle size, as well as use the help of isolation and protection function of LiBO2-containing coating layer, to maintain the stability of Fe2P, which effectively guarantees the performance of the application of lithium iron phosphate active materials. As can be seen from the data in Table 1 above, for the lithium iron phosphate system composite materials in Comparative Example 1, due to the lack of reduction process, there is no generation of Fe2P and lead to poor electrical conductivity and poor capacity of the material; and for the the lithium iron phosphate material in Comparative Example 2, because of the lack of LiBO2 coating, during the application process, the contacting of Fe2P with the electrolyte solution cannot be protected, resulting in the corrosion of Fe2P by the electrolyte solution during the cycling process and the loss of electrical conductivity, thereby resulting in serious material cycling attenuation. 2. SEM image The lithium iron phosphate composite cathode active material in Example 1 was taken for microscopic morphology observation, and its SEM image is shown in FIG.1. As can be seen from the SEM image in FIG. 1, the primary particles of the lithium iron phosphate composite cathode material prepared in Example 1 were concentrated at 50-200 nm, which was mainly obtained by the controlling of grinding granularity. 3. XRD spectra The batteries of Example 1 and Comparative Example 2 after the above battery cycling performance test, respectively, were disassembled, and the anode materials were taken and tested for their XRD, and the XRD spectra of the materials in Example 1 and Comparative Example 2 are shown in FIG. 2, in which the standardized card number of the lithium iron phosphate, JCPDS: 83-2092, is shown. As can be seen from the XRD patterns shown in FIG. 2, after charge / discharge cycling, in Example 1, the characteristic peaks of Fe2P (40.3°, 44.2°, 47.3°) are still present in the material’s XRD pattern, proving that Fe2P still exists after charge / discharge cycling and has not been dissolved by the electrolyte solution. In contrast, for the material in Comparative Example 2, after the charge / discharge cycling test, the characteristic peaks of Fe2P could not be detected, indicating that Fe2P has been decomposed by the electrolyte solution. It can be seen that the LiBO2-containing coating layer in the technical solution of the present application generated by using boron coating mode helps to establish a stable interface on the surface of the material, preventing the damage of HF to Fe2P, which can ensure the stability of the application of the material. In summary, the lithium iron phosphate composite cathode active material provided in the present application improves the low-temperature performance of lithium iron phosphate by reducing the primary particle diameter of the material, improves the compaction density of the small-particle lithium iron phosphate by decreasing the carbon content, improves the electrical conductivity of the material and compensates for the decrease in the electrical conductivity of the material due to the decrease in the carbon content by generating iron phosphate on the surface of the particles, and establishes a stable interface on the surface of the material and inhibits the dissolution of the iron phosphide by the corrosion of HF generated by the decomposition of LiPF6 by building a thin LiBO2 coating layer on the surface of the iron phosphate; and then a lithium iron phosphate material with a small particle diameter and a low content of carbon is prepared, and the material is characterized by excellent low-temperature performance, a low resistivity and a high compaction density, and has the advantage of high energy density compared with the traditional small-particle lithium iron phosphate material. 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 phosphate composite cathode active material, wherein the lithium iron phosphate composite cathode active material comprises a core containing a lithium iron phosphate material and iron phosphide particles, as well as a carbon coating layer and / or a LiBO2-containing coating layer at least partially coated on the surface of the core.

2. The lithium iron phosphate composite cathode active material of claim 1, wherein in the lithium iron phosphate composite cathode active material, the lithium iron phosphate material further comprises a doping element M; and the doping element M includes at least one of Ti, Zr, V, Nb and Mg.

3. The lithium iron phosphate composite cathode active material of claim 1 or 2, wherein the lithium iron phosphate composite cathode active material is represented as LiFe1-xMxPO4-Fe2P / C / LiBO2; wherein, 0≤x≤0.

1.

4. The lithium iron phosphate composite cathode active material of any one of claims 1 to 3, wherein in the lithium iron phosphate composite cathode active material, the content of iron phosphide is in a range from 1 ppm to 100 ppm based on the weight of the lithium iron phosphate composite cathode active material; and / or the coating amount of the carbon coating layer is in a range from 0.5 wt% to 1.0 wt% based on the weight of the core; and / or the coating amount of the LiBO2-containing coating layer is in a range from 0.1 wt% to 1.0 wt% based on the weight of the core.

5. The lithium iron phosphate composite cathode active material of any one of claims 1 to 4, wherein the lithium iron phosphate composite cathode active material has a particle size of 50 nm to 200 nm.

6. A method for preparing the lithium iron phosphate composite cathode active material of any one of claims 1 to 5, comprising the following steps: S1: mixing a lithium source material, an iron source material and a phosphorus source material to obtain a lithium iron phosphate precursor; S2: carrying out a reductive sintering to the lithium iron phosphate precursor under a protective atmosphere containing a reducing gas to obtain a core containing lithium iron 1phosphate material and iron phosphide particles, represented as LiFePO4-Fe2P; S3: mixing LiFePO4-Fe2P obtained in S2 with a carbon source material, and subjecting to a first sintering treatment under a protective atmosphere to form the carbon coating layer on the surface of LiFePO4-Fe2P, represented as LiFePO4-Fe2P / C; and S4: mixing LiFePO4-Fe2P / C obtained in S3 with the lithium source material and a boron source material, and subjecting to a second sintering treatment to form the LiBO2-containing coating layer on the surface of LiFePO4-Fe2P / C to obtain the lithium iron phosphate composite cathode active material, represented as LiFePO4-Fe2P / C / LiBO2.

7. The method for preparing the lithium iron phosphate composite cathode active material of claim 6, wherein in S1, a molar ratio of lithium element in the lithium source material, iron element in the iron source material and phosphorus element in the phosphorus source material is (1.0-1.05):1:

1.

8. The method for preparing the lithium iron phosphate composite cathode active material of claim 6 or 7, wherein S1 further comprises a step of adding a dopant containing the doping element M; and a molar ratio of lithium element in the lithium source material, iron element in the iron source material, phosphorus element in the phosphorus source material and doping element M in the dopant is (1.0-1.05): 1: 1: (0.01-0.1).

9. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 8, wherein S1 further comprises grinding and drying the lithium iron phosphate precursor.

10. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 9, wherein in S2, the volume content of the reducing gas in the protective atmosphere is greater than or equal to 8%.

11. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 10, wherein, in S2: the reducing gas includes hydrogen gas and / or carbon monoxide; and / or the protective atmosphere includes nitrogen gas.

212. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 11, wherein, in S2, the reductive sintering is carried out at a temperature of 700°C to 800°C for a time period of 5h to 12h.

13. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 12, wherein S3 further comprises grinding a mixture of LiFePO4-Fe2P and the carbon source material to achieve a particle size Dv50 less than 0.3 μm before the first sintering treatment.

14. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 13, wherein, in S3, the amount of the carbon source material accounts for 6 wt% to 10 wt% of the amount of LiFePO4-Fe2P.

15. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 14, wherein, in S3, the first sintering treatment is carried out at a temperature of 700°C to 800°C for a time period of 5h to 12h.

16. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 15, wherein, in S4: a molar ratio of lithium element in the lithium source material to boron element in the boron source material is (1-1.05):1; and / or the amount of the boron source material accounts for 0.1 wt% to 1.0 wt% of the amount of LiFePO4-Fe2P.

17. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 16, wherein, in S4, the second sintering treatment is carried out at a temperature of 300°C to 500°C for a time period of 5h to 12h.

18. The method for preparing the lithium iron phosphate composite cathode active material of any one of claims 6 to 17, wherein, the lithium source material includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium acetate and lithium nitrate; and / or, the iron source material includes at least one of ferric phosphate, diiron trioxide, ferrous oxalate, ferric sulfate, ferrous sulfate, ferric nitrate, ferric acetate, ferric chloride, ferrous 3hydroxide and ferric hydroxide; and / or, the phosphorus source material includes at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, lithium dihydrogen phosphate and ferric phosphate; and / or, the dopant includes at least one of oxides, hydroxides, oxalates, acetates, chlorides and nitrates of the doping element M; and / or, the carbon source material includes at least one of glucose, sucrose, polyethylene glycol, phenolic resin, polyvinyl alcohol and citric acid; and / or, the boron source material includes at least one of boric acid, metaboric acid and tetraboric acid.

19. 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 phosphate composite cathode active material of any one of claims 1 to 5 or a lithium iron phosphate composite cathode active material prepared by the method of any one of claims 6 to 18.

20. A secondary battery, comprising the positive electrode plate of claim 19.

21. A powered device, comprising the secondary battery of claim 20. 4

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