A composite lithium iron phosphate material, a preparation method thereof, a positive electrode sheet, and a secondary battery

Abstract

The application provides a composite lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery, and belongs to the technical field of secondary batteries. y Co x Fe (1‑x) O u (PO4) z wherein x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, and u is 0.2 to 0.5; and the content of carbon in the composite lithium iron phosphate material is 1% to 1.9% by weight. The composite lithium iron phosphate material based on the application has a high-voltage platform, high capacity and high coulomb efficiency, and when the composite lithium iron phosphate material is applied to a secondary battery, the secondary battery has high energy density.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a composite lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium secondary batteries have become an indispensable energy source in portable electronic devices, electric vehicles and large-scale energy storage systems due to their high energy density, long cycle life and environmental friendliness. In lithium secondary batteries, the selection of the positive electrode material has an important influence on the overall performance of the battery. At present, lithium iron phosphate (LiFePO4, LFP for short) with an olivine structure has become the main choice for positive electrode materials of vehicle power lithium batteries due to its stable crystal structure, good safety and low production cost, and occupies an important position in the field of electric vehicles. The theoretical capacity of LFP material is 170 mAh / g, the voltage platform is 3.4 V, and the true density is about 3.5 g / mL. The upper limit of the theoretical energy density limits the improvement of the overall energy density of the battery.

[0003] In addition, in order to improve the electrical conductivity and cycle performance of lithium iron phosphate, a carbon coating is usually performed in the conventional preparation process. Although the carbon coating can effectively improve the electrical conductivity, the presence of the carbon coating layer often leads to a decrease in the compaction density of the material, so the energy density is further reduced. Under the market demand of pursuing higher energy density, for example, the requirement of electric vehicles for longer cruising range, the performance improvement of LFP positive electrode materials becomes crucial. Therefore, finding a new type of lithium iron phosphate material to improve the energy density of the material has become a key problem that needs to be solved in the current lithium battery technology field. SUMMARY

[0004] In view of the technical problems in the background art, the present application provides a composite lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery, aiming to solve the technical problem of low energy density of lithium iron phosphate in the prior art.

[0005] In a first aspect, the embodiments of the present application provide a composite lithium iron phosphate material, which comprises a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material. The composition general formula of the phosphate / oxide composite material is Li y Co x Fe (1-x) O u (PO4) z wherein x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, and u is 0.2 to 0.5, and the content of carbon in the composite lithium iron phosphate material is 1% to 1.9% in terms of weight percentage.

[0006] In the technical scheme of the embodiment of the present application, the composite lithium iron phosphate material is composed of lithium cobalt iron composite oxide and olivine lithium cobalt iron phosphate. The lithium cobalt iron composite oxide has high capacity. The composite lithium iron phosphate material with high capacity is obtained by combining the lithium cobalt iron composite oxide with the lithium cobalt iron phosphate. In addition, the composite lithium iron phosphate material is designed to be lithium-rich, which can improve the coulomb efficiency of the composite lithium iron phosphate material. Furthermore, the introduction of cobalt element in the composite lithium iron phosphate material can improve the voltage platform of the composite lithium iron phosphate material. Finally, the composite and coating of carbon can improve the conductivity and stability of the electrode material, which is conducive to improving the charge and discharge efficiency of the battery, increasing the cycle life of the battery, reducing the polarization of lithium ions in the charge and discharge process, and improving the coulomb efficiency. The composite lithium iron phosphate material based on the embodiment of the present application has high voltage platform, high capacity and high coulomb efficiency, and has excellent electrical performance, which can be used to prepare a battery with high capacity, high energy density and high coulomb efficiency.

[0007] In some embodiments, the D50 particle size of the composite lithium iron phosphate material is 1.2 to 1.5 pm.

[0008] In this embodiment, the small particle size of the composite lithium iron phosphate material is conducive to increasing the specific surface area of the material, increasing the contact area of the electrode material and the electrolyte, and improving the migration rate of lithium ions. Specifically, when the D50 particle size of the composite lithium iron phosphate material is within the above range, it is helpful to speed up the diffusion speed of lithium ions in the composite lithium iron phosphate material, shorten the ion migration path, and improve the charge and discharge rate and cycle performance of the battery, while maintaining a high compaction density.

[0009] In some embodiments, the compaction density of the composite lithium iron phosphate material is 2.57 to 2.78 g / mL.

[0010] In this embodiment, the compaction density within the above range is conducive to obtaining a composite lithium iron phosphate material with high crystallinity and few internal pores, thereby improving the tightness, compaction density and conductivity of the composite lithium iron phosphate material, which is conducive to improving the energy density and charge and discharge efficiency of the battery, reducing the volume and weight of the battery, and having significant advantages for portable electronic devices and electric vehicles and other applications.

[0011] In some embodiments, the tap density of the composite lithium iron phosphate material is 1.38 to 1.55 g / mL.

[0012] In this embodiment, the tap density reflects the tightness that the material can achieve when subjected to vibration. The tap density of the composite lithium iron phosphate material within the above range is conducive to obtaining an electrode material with high compaction density during battery assembly, thereby improving the energy density of the battery.

[0013] In some embodiments, the specific surface area of the composite lithium iron phosphate material is 11.39 to 14.28 m² / g.

[0014] In the embodiment, the higher specific surface area can increase the contact area of the composite lithium iron phosphate material with the electrolyte, promote the rapid deintercalation and intercalation of lithium ions, accelerate the electrochemical reaction rate, and thus improve the conductivity, charge and discharge rate, and capacity of the composite lithium iron phosphate material. The specific surface area of the composite lithium iron phosphate material is within the above range, so that the high compactness of the composite lithium iron phosphate material is obtained, and the charge and discharge rate and energy density of the battery are improved.

[0015] In a second aspect, the embodiments of the present application provide a preparation method of a composite lithium iron phosphate material, including the following steps:

[0016] a mixed solution containing a cobalt source, an iron source, and a lithium source is provided;

[0017] carbonates and phosphates are added to the mixed solution, and the pH value of the mixed solution is adjusted to 3.5-5.0, so as to obtain a lithium-cobalt-iron coprecipitate through reaction;

[0018] the lithium-cobalt-iron coprecipitate and an organic carbon source are mixed, and a slurry is obtained through slurry treatment, and then the slurry is dried and calcined to obtain the composite lithium iron phosphate material;

[0019] The composite lithium iron phosphate material includes a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material, and the phosphate / oxide composite material has a general formula of Li y Co x Fe (1-x) O u (PO4) z wherein x is 0.1-0.2, y is 1.1-1.3, z is 0.8-0.9, and u is 0.2-0.5, and the content of carbon in the composite lithium iron phosphate material is 1%-1.9% by weight.

[0020] In the technical scheme of the embodiments of the present application, the carbonates and phosphates are used as precipitants to coprecipitate iron, cobalt, and lithium to obtain a lithium-cobalt-iron coprecipitate, and then the lithium-cobalt-iron coprecipitate is mixed with a carbon source and subjected to slurry treatment, drying, and calcination. In the process, the phosphate groups form lithium cobalt iron phosphate, and the carbonates decompose at high temperature to form lithium cobalt iron composite oxides, so as to obtain a composite material of lithium cobalt iron phosphate, lithium cobalt iron composite oxides, and carbon. Understandably, in the preparation process of the precursor, the uniform coprecipitation of lithium, cobalt, iron, and phosphorus is realized by adding carbonates and phosphates and controlling the pH value, so that the constituent elements are fully mixed and uniform. Therefore, the migration distance between ions can be effectively reduced during high-temperature calcination, the fusion between particles is more sufficient, the particle growth is more dense at the same temperature, and thus the compactness of the material is effectively improved.

[0021] Further, the capacity of lithium cobalt iron complex oxide is high, and the capacity of the composite lithium iron phosphate material can be improved by compounding the lithium cobalt iron complex oxide with lithium cobalt iron phosphate. Meanwhile, the compounding of the two can improve the compaction density of the lithium iron phosphate material. In addition, the voltage plateau of the composite lithium iron phosphate material can also be improved due to the introduction of cobalt elements.

[0022] Further, the coprecipitate of lithium cobalt iron is mixed with an organic carbon source. The addition of the organic carbon source forms a carbon coating layer in the subsequent high-temperature calcination process, which can improve the conductivity and stability of the material, thereby improving the charge and discharge efficiency of the battery, increasing the cycle life of the battery, reducing polarization during charging and discharging, and improving the coulomb efficiency. Moreover, calcination helps to decompose the carbonate to produce carbon dioxide gas, form a porous structure in the composite lithium iron phosphate material, and form a stable phosphate-based composite material through the interaction of phosphate and metal ions at high temperatures. The inert gas protection environment and the organic carbon source can prevent the oxidation of divalent iron and divalent cobalt components at high temperatures, which helps to obtain a high-purity composite lithium iron phosphate material.

[0023] Therefore, in the present application, through coprecipitation, slurry, drying, and high-temperature calcination, a composite lithium iron phosphate material with a composition of Li y Co x Fe (1-x) O u (PO4) z / C (x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, u is 0.2 to 0.5, the content of carbon in the composite lithium iron phosphate material is 1% to 1.9% in weight percentage) with uniform particle size, ordered structure, and high compaction density can be prepared. The capacity and voltage plateau of the composite lithium iron phosphate material are significantly improved. Compared with traditional lithium iron phosphate materials, the energy density of the battery prepared from the composite lithium iron phosphate material can be increased by more than 10%.

[0024] In some embodiments, the molar ratio of cobalt elements in the cobalt source, iron elements in the iron source, and lithium elements in the lithium source is (0.1-0.2):(0.8-0.9):(1.2-1.4).

[0025] In this embodiment, a higher lithium content helps to improve the coulomb efficiency of the material, a higher iron content (0.8-0.9) is beneficial to improve the stability and electrochemical activity of the material structure, and the introduction of cobalt can improve the voltage plateau of the material. By controlling the amount of raw materials within the above range, it is beneficial to obtain a composite lithium iron phosphate material with high voltage plateau, high capacity, high coulomb efficiency, and high energy density through subsequent coprecipitation and subsequent high-temperature calcination.

[0026] In some embodiments, the molar ratio of carbonate in the carbonate salt and phosphate in the phosphate salt and iron element in the iron source is (0.15-0.25):(0.8-0.9):(0.8-0.9).

[0027] In this embodiment, by controlling the molar ratio of carbonate, phosphate and iron element to be (0.15-0.25):(0.8-0.9):(0.8-0.9), it is beneficial for subsequent uniform co-precipitation of lithium, cobalt, iron and phosphorus, so as to obtain a high-compaction-density, high-voltage-plateau and high-capacity composite lithium iron phosphate material after subsequent high-temperature calcination.

[0028] In some embodiments, the amount of the organic carbon source added accounts for 7% to 10.5% based on the sum of the weight of the co-precipitate of lithium, cobalt and iron and the organic carbon source being 100%.

[0029] In this embodiment, the carbon content is within the above range, which is beneficial to improve the conductivity and structural stability of the composite lithium iron phosphate material, improve the charge-discharge efficiency, cycle life and coulombic efficiency of the battery, and at the same time maintain a high compaction density.

[0030] In some embodiments, the reaction temperature is controlled to be 35°C to 55°C during the process of obtaining the co-precipitate of lithium, cobalt and iron.

[0031] In this embodiment, the reaction temperature affects the co-precipitation reaction rate and the crystal structure of the product. Within the above reaction temperature range, it can promote the uniform co-precipitation of cobalt ions, iron ions and lithium ions, which is helpful to form uniform precursor particles and reduce the generation of by-products, so as to obtain a high-purity, structure-stable and good-conductivity composite lithium iron phosphate material after subsequent preparation, which is beneficial to improve the energy density of the composite lithium iron phosphate material.

[0032] In some embodiments, the carbonate salt is selected from one or a combination of any of sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate and ammonium bicarbonate.

[0033] In this embodiment, alkali metal carbonates such as sodium carbonate and sodium bicarbonate, and ammonium salts such as ammonium carbonate and ammonium bicarbonate have high solubility, which is beneficial to provide carbonate ions and control pH value, promote uniform co-precipitation, so as to obtain a composite lithium iron phosphate material with uniform particle size, high purity and stable structure after subsequent preparation.

[0034] In some embodiments, the phosphate salt is selected from one or a combination of any of sodium phosphate, potassium phosphate, ammonium phosphate, sodium hydrogen phosphate, potassium hydrogen phosphate, ammonium hydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate and ammonium dihydrogen phosphate.

[0035] In this embodiment, the above-mentioned phosphate salt has high solubility, which is beneficial to provide phosphate ions and control pH value, promote uniform co-precipitation, and is helpful to form a composite lithium iron phosphate material with uniform particle size.

[0036] In some embodiments, the cobalt source is selected from one or a combination of any of cobalt sulfate, cobalt nitrate, and cobalt chloride.

[0037] In this embodiment, cobalt sulfate, cobalt nitrate, and cobalt chloride can provide necessary divalent cobalt ions for the co-precipitation reaction to generate the co-precipitate of lithium cobalt iron and for the calcination to generate lithium cobalt iron composite oxide and cobalt lithium iron phosphate.

[0038] In some embodiments, the iron source is selected from one or a combination of any of ferrous sulfate, ferrous nitrate, ferrous chloride, ferric sulfate, ferric nitrate, and ferric chloride.

[0039] In this embodiment, ferrous sulfate, ferric nitrate, and ferric chloride, etc. can provide divalent iron ions or trivalent iron ions for the co-precipitation reaction to generate the co-precipitate of lithium cobalt iron and for the subsequent calcination in the presence of an organic carbon source and an inert gas atmosphere to generate lithium cobalt iron composite oxide and cobalt lithium iron phosphate.

[0040] In some embodiments, the lithium source is selected from one or a combination of any of lithium sulfate, lithium nitrate, lithium acetate, and lithium chloride.

[0041] In this embodiment, lithium sulfate, etc. can provide necessary lithium ions for the reaction to generate the composite lithium iron phosphate material.

[0042] In some embodiments, the organic carbon source is selected from one or a combination of any of sucrose, glucose, and water-soluble starch.

[0043] In this embodiment, the organic carbon source can be converted into a carbon coating layer during high-temperature calcination, which can significantly enhance the electrical conductivity of the material, reduce the internal resistance of the electrode, and improve the charge and discharge efficiency of the battery. The organic carbon sources such as sucrose, glucose, and water-soluble starch are beneficial to the formation of a uniform and continuous carbon layer due to their pyrolysis characteristics at high temperatures, and the distribution of these organic carbon sources on the surface of the co-precipitate is relatively uniform, which is conducive to the formation of a carbon coating layer with uniform thickness and distribution. The use of the above-mentioned organic carbon sources during high-temperature calcination can improve the electrical conductivity of the composite lithium iron phosphate material, reduce the internal resistance of the battery, and improve the charge and discharge rate and coulombic efficiency. In addition, the uniform carbon coating layer formed by the decomposition of the above-mentioned organic carbon sources is beneficial to improving the cycle stability and thermal stability of the material and prolonging the service life of the battery.

[0044] In some embodiments, the cobalt source, the iron source, and the lithium source are added to pure water, and stirred and dissolved to obtain a mixed solution containing the cobalt source, the iron source, and the lithium source.

[0045] In this embodiment, water is used as the solvent, which is beneficial to controlling the production cost and green production. In addition, uniformly dispersing each metal ion source in water is beneficial to avoiding uneven co-precipitation caused by local over-concentration, improving the purity and uniformity of the co-precipitate, and reducing the agglomeration of the composite lithium iron phosphate material.

[0046] In some embodiments, the concentration of lithium element in the mixed solution containing the cobalt source, the iron source and the lithium source is 1 mol / L to 3 mol / L.

[0047] In this embodiment, the concentration of lithium source is within the above range, which helps to promote the uniform embedding of lithium element in the composite lithium iron phosphate material, form a stable lithium-cobalt-iron composite structure, and improve the capacity and stability of the composite lithium iron phosphate material.

[0048] In some embodiments, the carbonates and phosphates are first mixed to obtain a mixture, and then the mixture is added to the mixed solution containing the cobalt source, the iron source and the lithium source.

[0049] In this embodiment, the carbonates and phosphates are added in the form of a mixture, which is conducive to the uniform co-precipitation of cobalt, iron and lithium ions, and the uniform introduction of carbonate and phosphate into the co-precipitate, forming a co-precipitate with uniform particle size, stable structure and regular morphology, improving the purity and uniformity of the distribution of each component of the composite lithium iron phosphate material, and improving the capacity, energy density and other electrochemical properties of the composite lithium iron phosphate material.

[0050] In some embodiments, the time for adding the carbonates and phosphates is 30 min to 60 min.

[0051] In this embodiment, by gradually adding the carbonates and phosphates and controlling the addition time, the morphology and structure of the co-precipitate can be optimized, the purity and uniformity of the distribution of each component of the prepared composite lithium iron phosphate material can be improved, and the capacity, energy density and other electrochemical properties of the composite lithium iron phosphate material can be improved.

[0052] In some embodiments, the carbonates and phosphates are added to the mixed solution of the cobalt source, the iron source and the lithium source during stirring, and then the aqueous solution of the alkaline substance is added during stirring until the pH of the system is 3.5 to 5.0, and then the stirring speed is controlled to be 100 r / min to 300 r / min, and the stirring aging time is controlled to be 30 min to 60 min, to obtain the lithium-cobalt-iron co-precipitate.

[0053] In this embodiment, the pH value is within the above range (3.5 to 5.0), which is conducive to the co-precipitation of cobalt, iron and lithium ions, forming a co-precipitate with uniform particle size and composition and regular morphology; controlling the stirring speed within the above range and controlling the stirring aging time helps to uniformly distribute each reactant and accelerate the collision and reaction between ions, reducing the decomposition or structural damage of the material caused by local overheating. By precisely controlling the pH value and the stirring conditions, the microstructure of the material can be optimized, forming a co-precipitate with uniform composition and consistent structure, and improving the capacity, energy density, voltage platform and cycle stability and other electrochemical properties of the composite lithium iron phosphate material.

[0054] In some embodiments, the aqueous solution of the basic substance is selected from one or any of the following: an aqueous solution of ammonium bicarbonate, sodium hydroxide, potassium hydroxide, NH3·H2O.

[0055] In this embodiment, the addition of the basic substance in the process of preparing the composite lithium iron phosphate material is to adjust the pH value of the reaction system, so that the cobalt ions, iron ions and lithium ions can be co-precipitated. The above-mentioned ammonium bicarbonate (NH4HCO3) and sodium hydroxide (NaOH) and the like are all common basic substances, and can all increase the pH value of the solution, promote the precipitation of metal ions, and form lithium-cobalt-iron co-precipitates with uniform particle size and morphology.

[0056] In some embodiments, before the step of mixing the lithium-cobalt-iron co-precipitate and the organic carbon source, the lithium-cobalt-iron co-precipitate is further subjected to filtration and washing until the conductivity of the washing water after washing is ≤200 μS / cm.

[0057] In this embodiment, through the above-mentioned filtration and washing, impurities can be effectively removed, the purity and consistency of the composite lithium iron phosphate material can be improved, which is helpful to improve the coulombic efficiency and cycle life, reduce the internal resistance, and improve the electrochemical activity of the composite lithium iron phosphate material.

[0058] In some embodiments, the calcination temperature of the calcination treatment is 750-850°C, and the calcination time is 6-12 h.

[0059] In this embodiment, in the high-temperature calcination process, the lithium-cobalt-iron co-precipitate is converted into lithium-cobalt-iron composite oxides and the composite lithium iron phosphate material of phosphoric acid cobalt lithium iron; the carbonate pyrolysis releases CO2, forming a uniform porous structure in the composite lithium iron phosphate material; the organic carbon source pyrolysis forms a carbon conductive material, and at least partially coats the lithium-cobalt-iron composite oxides and the composite material of phosphoric acid cobalt lithium iron; at the same time, the high-temperature calcination promotes the further reaction of metal ions and phosphate to form a stable phosphoric acid iron-based composite oxide structure. Under the above-mentioned calcination conditions, it is conducive to forming a composite lithium iron phosphate material with high purity and stable structure, which is helpful to improve the electrochemical performance of the composite lithium iron phosphate material, such as capacity, cycle stability and voltage platform.

[0060] In some embodiments, the heating rate of the calcination treatment is 1-3°C / min.

[0061] In this embodiment, the above-mentioned heating rate is conducive to uniform heating of each component inside the composite lithium iron phosphate material, and is conducive to improving the structural integrity of each component and reducing the number of defects.

[0062] In some embodiments, the organic carbon source is first dispersed in water to prepare an organic carbon source solution, and then the lithium-cobalt-iron co-precipitate is dispersed in the organic carbon source solution to obtain a slurry.

[0063] In this embodiment, the organic carbon source is first dispersed in water to prepare an organic carbon source solution, and then the coprecipitate of lithium-cobalt-iron is dispersed in the aqueous solution to form a slurry, which is conducive to the full contact and uniform mixing of the coprecipitate and the organic carbon source in the solution, and helps the organic carbon source uniformly cover the surface of the coprecipitate to form a carbon layer with uniform thickness, thereby improving the conductivity, lithium ion diffusion capacity and overall structural stability of the composite lithium iron phosphate material.

[0064] In some embodiments, the content of the organic carbon source in the organic carbon source solution is 10% to 20% by weight.

[0065] In this embodiment, the concentration of the organic carbon source affects the thickness of the carbon coating layer. The concentration of the organic carbon source within the above range helps to form a carbon coating layer with uniform thickness, good conductivity and stability. At the same time, the composite lithium iron phosphate material maintains a high compaction density and energy density, thereby improving the electrochemical performance of the battery.

[0066] In some embodiments, the slurry is dried by spray drying.

[0067] In this embodiment, spray drying can achieve rapid drying of the material, improve the uniformity and flowability of the formed particles, reduce agglomeration, and improve the dispersibility and compactibility of the composite lithium iron phosphate material, thereby helping to improve the charge and discharge efficiency, energy density and cycle stability of the composite lithium iron phosphate material.

[0068] In some embodiments, the water content (mass fraction) of the dried material obtained by drying the slurry is less than 2%.

[0069] In this embodiment, the purpose of drying the slurry is to remove water from the slurry to reduce the impact of water evaporation on the structure of the composite lithium iron phosphate material during subsequent high-temperature calcination. Controlling the water content of the dried material to be below 2% helps to reduce material shrinkage and pore structure damage caused by water evaporation, improve the microstructure of the composite lithium iron phosphate material and the integrity of the carbon coating layer, thereby improving the electrochemical performance and cycle stability of the composite lithium iron phosphate material.

[0070] In some embodiments, the drying process is followed by a crushing process, and the D50 particle size of the dried material after the crushing process is 1 μm to 3 μm.

[0071] Crushing the dried material to the above particle size range helps to obtain a composite lithium iron phosphate material with a high specific surface area, promotes the rapid diffusion of lithium ions, improves the electrochemical activity of the material, and at the same time obtains a composite lithium iron phosphate material with a high compaction density, which helps to improve the capacity, energy density, cycle stability and charge-discharge rate of the composite lithium iron phosphate material.

[0072] In some embodiments, the calcination is performed in an inert gas atmosphere with humidity ≤0.5% and oxygen content lower than 1 ppm.

[0073] In this embodiment, the low-humidity and low-oxygen content atmosphere described above is conducive to reducing the oxidation and moisture absorption of the composite lithium iron phosphate material under high-temperature calcination conditions, improving the purity and structural stability of the composite lithium iron phosphate material, and improving the electrochemical performance and cycle stability of the composite lithium iron phosphate material.

[0074] In some embodiments, after the calcination is completed, the material obtained by calcination is discharged after being cooled to 100°C, the material obtained by discharging is crushed, and then iron is removed and packaged in a constant temperature and humidity chamber to obtain the composite lithium iron phosphate material.

[0075] In this embodiment, the temperature control and crushing steps of the material after calcination facilitate the subsequent processing and use of the composite lithium iron phosphate material. Removing iron and packaging in a constant temperature and humidity chamber is conducive to avoiding the moisture absorption and oxidation of the composite lithium iron phosphate material, and maintaining the chemical purity and performance stability of the composite lithium iron phosphate material.

[0076] In some embodiments, the temperature of the constant temperature and humidity chamber is 24°C to 26°C, and the humidity is 8% to 10%.

[0077] In this embodiment, processing in the constant temperature and humidity chamber described above is conducive to maintaining the chemical purity and performance stability of the composite lithium iron phosphate material, thereby maintaining the consistency and reliability of the performance of the battery.

[0078] In a third aspect, the embodiments of the present application provide a positive electrode tab, which comprises the composite lithium iron phosphate material described above or the composite lithium iron phosphate material prepared by the preparation method of the composite lithium iron phosphate material described above.

[0079] In this embodiment, the positive electrode tab contains the composite lithium iron phosphate material described above, and thus has the advantages of high compaction density, high capacity, high energy density, and high coulomb efficiency.

[0080] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises a positive electrode, and the tab of the positive electrode is the positive electrode tab described above.

[0081] In this embodiment, the secondary battery contains the positive electrode tab described above, and thus has the advantages of high compaction density, high capacity, high energy density, and high coulomb efficiency.

[0082] The above description is only a summary of the technical solutions of the present application. In order to more clearly understand the technical means of the present application, the specific embodiments of the present application can be implemented in accordance with the content of the description, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0083] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0084] Figure 1 SEM image of the composite lithium iron phosphate material prepared for Example 1 of the present application;

[0085] Figure 2 0.1C initial charge-discharge curve diagram of the composite lithium iron phosphate material prepared for Example 1 of the present application;

[0086] Figure 3 SEM image of the composite lithium iron phosphate material prepared for Example 2 of the present application;

[0087] Figure 4 SEM image of the composite lithium iron phosphate material prepared for Example 3 of the present application. DETAILED DESCRIPTION

[0088] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0089] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above description of drawings are intended to cover non-exclusive inclusion.

[0090] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0091] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily independent or alternative embodiments to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0092] In the description of the embodiments of the present application, the term "and / or" is only to describe the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0093] In the description of the embodiments 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 groups (including two groups), and "a plurality of pieces" refers to two or more pieces (including two pieces).

[0094] In the prior art, the upper limit of the theoretical energy density of lithium iron phosphate limits the improvement of the overall energy density of the battery. In addition, the existence of the carbon coating layer often leads to a decrease in the compaction density of the material, so the energy density is further reduced.

[0095] In order to solve the technical problem of low energy density of lithium iron phosphate, the present application provides a composite lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery. By using lithium cobalt iron composite oxide and olivine phosphate lithium cobalt iron phosphate, a composite lithium iron phosphate material with high capacity, high voltage platform, high coulomb efficiency and high energy density is obtained, and the capacity, coulomb efficiency and energy density of the positive electrode sheet and the secondary battery are also improved.

[0096] In the first aspect, the embodiments of the present application provide a composite lithium iron phosphate material, which comprises a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material. The composition general formula of the phosphate / oxide composite material is Li y Co x Fe (1-x) O u (PO4) z , wherein x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, and u is 0.2 to 0.5. The content of carbon in the composite lithium iron phosphate material is 1% to 1.9% by weight.

[0097] Wherein, x can be specifically 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, y can be specifically 1.1, 1.12, 1.15, 1.18, 1.2, 1.22, 1.25, 1.28, 1.3, z can be specifically 0.8, 0.82, 0.85, 0.88, 0.9, u can be specifically 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.38, 0.4, 0.42, 0.44, 0.46, 0.48, 0.5, and the content of carbon in the composite lithium iron phosphate material can be specifically 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and of course can also be other values within the above range.

[0098] In the present application, the composite lithium iron phosphate material is a composite of lithium cobalt iron composite oxide and olivine phosphate lithium cobalt iron phosphate, wherein the lithium cobalt iron composite oxide has high capacity, and the composite of the lithium cobalt iron composite oxide and the lithium cobalt iron phosphate obtains a composite lithium iron phosphate material with high capacity; in addition, the composite lithium iron phosphate material adopts a lithium-rich design, which can improve the coulomb efficiency of the composite lithium iron phosphate material; furthermore, the introduction of cobalt element in the composite lithium iron phosphate material can improve the voltage platform of the composite lithium iron phosphate material; finally, the composite and coating of carbon can improve the conductivity and stability of the electrode material, which is beneficial to improve the charge and discharge efficiency of the battery, increase the cycle life of the battery, reduce the polarization of lithium ions in the charge and discharge process, and improve the coulomb efficiency. The composite lithium iron phosphate material based on the embodiments of the present application has high voltage platform, high capacity and high coulomb efficiency, and has excellent electrical performance, which can be used to prepare a battery with high capacity, high energy density and high coulomb efficiency.

[0099] Further, in some embodiments, the D50 particle size of the composite lithium iron phosphate material is 1.2 μm to 1.5 μm.

[0100] In the technical solution of the embodiments of the present application, the small particle size composite lithium iron phosphate material is beneficial to increase the specific surface area of the material, increase the contact area of the electrode material and the electrolyte, and improve the migration rate of lithium ions. Specifically, when the D50 particle size of the composite lithium iron phosphate material is within the above range, it is helpful to speed up the diffusion speed of lithium ions in the composite lithium iron phosphate material, shorten the ion migration path, and improve the charge and discharge rate and cycle performance of the battery, while maintaining a high compactness of the composite lithium iron phosphate material.

[0101] Further, in some embodiments, the compactness of the composite lithium iron phosphate material is 2.57 g / mL to 2.78 g / mL.

[0102] In the technical scheme of the embodiment of the present application, the compaction density range is within the above range, which is beneficial to obtain the composite lithium iron phosphate material with high crystallinity and few internal pores, thereby improving the tightness, compaction density and conductivity of the composite lithium iron phosphate material, and being beneficial to improve the energy density and charge-discharge efficiency of the battery, and reduce the volume and weight of the battery, which has significant advantages for portable electronic devices and electric vehicles and other applications.

[0103] Further, in some embodiments, the tap density of the composite lithium iron phosphate material is 1.38 g / mL to 1.55 g / mL.

[0104] In the technical scheme of the embodiment of the present application, the tap density reflects the tightness that the material can reach when subjected to vibration. The tap density of the composite lithium iron phosphate material is within the above range, which is helpful to obtain the electrode material with high compaction density in the battery assembly process, and further improve the energy density of the battery.

[0105] Further, in some embodiments, the specific surface area of the composite lithium iron phosphate material is 11.39 m² / g to 14.28 m² / g.

[0106] In the technical scheme of the embodiment of the present application, the higher specific surface area can increase the contact area of the composite lithium iron phosphate material and the electrolyte, promote the rapid deintercalation and intercalation of lithium ions, accelerate the electrochemical reaction rate, and further improve the conductivity, charge-discharge rate and capacity of the composite lithium iron phosphate material. The specific surface area of the composite lithium iron phosphate material is within the above range, which is beneficial to improve the charge-discharge rate and energy density of the battery while obtaining higher compaction density of the composite lithium iron phosphate material.

[0107] Further, in some embodiments, the powder resistance of the composite lithium iron phosphate material is ≤22 Ω·cm.

[0108] In the technical scheme of the embodiment of the present application, the lower powder resistance means that the structural integrity of the composite lithium iron phosphate material is higher, which is helpful for the intercalation and deintercalation of lithium ions, and further improves the charge-discharge capacity of the composite lithium iron phosphate material.

[0109] Further, in some embodiments, the content of magnetic substances in the composite lithium iron phosphate material is <0.15 ppm.

[0110] In the technical scheme of the embodiment of the present application, the lower content of magnetic substances means that the purity of the composite lithium iron phosphate material is higher, which is beneficial to improve the consistency and energy density of the battery, and also helps to reduce the self-discharge rate of the battery, thereby improving the cycle performance of the composite lithium iron phosphate material.

[0111] In a second aspect, the embodiment of the present application provides a preparation method of a composite lithium iron phosphate material, comprising the following steps:

[0112] a mixed solution containing a cobalt source, an iron source and a lithium source is provided;

[0113] carbonate and phosphate are added to the mixed solution, and the pH value of the mixed solution is adjusted to 3.5-5.0, to obtain a lithium cobalt iron co-precipitate through reaction;

[0114] The lithium cobalt iron co-precipitate and an organic carbon source are mixed, and a slurry is obtained through slurry treatment, after which the slurry is dried and calcined to obtain a composite lithium iron phosphate material;

[0115] The composite lithium iron phosphate material comprises a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material, and the phosphate / oxide composite material has a general formula of Li y Co x Fe (1-x) O u (PO4) z wherein x is 0.1-0.2, y is 1.1-1.3, z is 0.8-0.9, and u is 0.2-0.5, and the content of carbon in the composite lithium iron phosphate material is 1%-1.9% by weight.

[0116] In the technical scheme of the embodiments of the present application, the iron, cobalt and lithium are co-precipitated by using carbonate and phosphate as precipitants to obtain a lithium cobalt iron co-precipitate, and then the lithium cobalt iron co-precipitate is mixed with a carbon source, and the slurry is obtained through slurry treatment, drying and calcination. In the composite material of lithium cobalt iron phosphate, phosphate, lithium, cobalt and iron form lithium cobalt iron phosphate, and the carbonate decomposes at high temperature to form lithium cobalt iron composite oxide. Understandably, in the preparation of the precursor, the uniform co-precipitation of lithium, cobalt, iron and phosphorus can be achieved by adding carbonate and phosphate and controlling the pH value, so that the constituent elements are fully mixed and uniform, and thus the migration distance between ions can be effectively reduced at high temperature calcination, and the fusion between particles is more sufficient, and the particle growth is more compact at the same temperature, thereby effectively improving the compaction density of the material.

[0117] Further, the lithium cobalt iron composite oxide has high capacity, and the combination of the lithium cobalt iron composite oxide and lithium cobalt iron phosphate can improve the capacity of the composite lithium iron phosphate material, and the combination of the two can improve the compaction density of the lithium iron phosphate material, and in addition, the introduction of cobalt can also improve the voltage platform of the composite lithium iron phosphate material.

[0118] Further, the coprecipitate of lithium cobalt iron is mixed with an organic carbon source, the addition of the organic carbon source forms a carbon coating layer in the subsequent high-temperature calcination process, which can improve the conductivity and stability of the material, improve the charge-discharge efficiency of the battery, increase the cycle life of the battery, and reduce the polarization in the charge-discharge process, and improve the coulomb efficiency. Moreover, calcination helps to decompose the carbonate to produce carbon dioxide gas, form a porous structure in the composite lithium iron phosphate material, and the interaction between phosphate and metal ions at high temperature forms a stable phosphoric acid-based composite material, and the protection of the inert gas environment and the organic carbon source can prevent the oxidation of divalent iron, divalent cobalt and other components at high temperature, which helps to obtain a high-purity composite lithium iron phosphate material.

[0119] Therefore, in the present application, through coprecipitation, slurry, drying and high-temperature calcination, a composite lithium iron phosphate material with a composition of Li y Co x Fe (1-x) O u (PO4) z / C (x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, u is 0.2 to 0.5, the content of carbon in the composite lithium iron phosphate material is 1% to 1.9% in weight percentage), and the composite lithium iron phosphate material has uniform particle size, ordered structure and high compaction density. The capacity and voltage platform of the composite lithium iron phosphate material are significantly improved, and the energy density of the battery prepared by the composite lithium iron phosphate material can be increased by more than 10% compared with the traditional lithium iron phosphate material.

[0120] Further, in some embodiments, the molar ratio of cobalt elements in the cobalt source, iron elements in the iron source, and lithium elements in the lithium source is (0.1-0.2):(0.8-0.9):(1.2-1.4).

[0121] In this embodiment, higher lithium content helps to improve the coulomb efficiency of the material, higher iron content is beneficial to improve the stability and electrochemical activity of the material structure, and the introduction of cobalt can improve the voltage platform of the material. By controlling the amount of raw materials within the above range, it is beneficial to obtain a composite lithium iron phosphate material with high voltage platform, high capacity, high coulomb efficiency and high energy density through subsequent coprecipitation and subsequent high-temperature calcination.

[0122] Specifically, the molar ratio of cobalt elements in the cobalt source, iron elements in the iron source, and lithium elements in the lithium source can be 0.1:0.8:1.2, 0.1:0.9:1.4, 0.2:0.8:1.2, 0.2:0.9:1.4 or any value within the range of (0.1-0.2):(0.8-0.9):(1.2-1.4).

[0123] Further, in some embodiments, the molar ratio of carbonate in the carbonate salt and phosphate in the phosphate salt and iron element in the iron source is (0.15-0.25):(0.8-0.9):(0.8-0.9).

[0124] In this embodiment, by controlling the molar ratio of carbonate, phosphate and iron element to be (0.15-0.25):(0.8-0.9):(0.8-0.9), it is beneficial for the subsequent uniform co-precipitation of lithium, cobalt, iron and phosphorus, so as to obtain a composite lithium iron phosphate material with high tap density, high voltage platform, high capacity and high energy density after subsequent high-temperature calcination.

[0125] Specifically, the molar ratio of carbonate in the carbonate salt and phosphate in the phosphate salt and iron element in the iron source can be 0.15:0.8:0.8, 0.15:0.9:0.9, 0.25:0.8:0.8, 0.25:0.9:0.9, or any value within the range of (0.15-0.25):(0.8-0.9):(0.8-0.9).

[0126] Further, in some embodiments, the amount of the organic carbon source added is 7% to 10.5%, based on the sum of the weight of the co-precipitate of lithium, cobalt and iron and the organic carbon source being 100%.

[0127] In this embodiment, the carbon content within the above range is beneficial to improve the conductivity and structural stability of the composite lithium iron phosphate material, improve the charge and discharge efficiency, cycle life and coulombic efficiency of the battery, and at the same time maintain a high tap density.

[0128] Specifically, the amount of the organic carbon source added can be 7%, 8%, 9%, 10%, 10.5%, or any value within the range of 7% to 10.5%.

[0129] Further, in some embodiments, the reaction temperature is controlled to be 35°C to 55°C during the process of obtaining the co-precipitate of lithium, cobalt and iron.

[0130] In the technical scheme of the embodiments of the present application, the reaction temperature affects the co-precipitation reaction rate and the crystal structure of the product. Within the above reaction temperature range, it can promote the uniform co-precipitation of cobalt ions, iron ions and lithium ions, help to form uniform precursor particles and reduce the generation of by-products, so as to obtain a composite lithium iron phosphate material with high purity, structural stability and good conductivity during subsequent preparation, which is beneficial to improve the energy density of the composite lithium iron phosphate material.

[0131] Further, in some embodiments, the carbonate salt is selected from one or a combination of any of sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate.

[0132] In the technical scheme of the embodiment of the present application, alkali metal carbonates such as sodium carbonate and sodium bicarbonate, ammonium salts such as ammonium carbonate and ammonium bicarbonate have high solubility, which is conducive to providing carbonate ions and controlling pH value, promoting uniform co-precipitation, so as to subsequently prepare the composite lithium iron phosphate material with uniform particle size, high purity and stable structure.

[0133] Further, in some embodiments, the phosphate salt is selected from one or a combination of any number of sodium phosphate, potassium phosphate, ammonium phosphate, sodium hydrogen phosphate, potassium hydrogen phosphate, ammonium hydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, and ammonium dihydrogen phosphate.

[0134] In the technical scheme of the embodiment of the present application, the above-mentioned phosphate salt has high solubility, which is conducive to providing phosphate ions and controlling pH value, promoting uniform co-precipitation, and helping to form the composite lithium iron phosphate material with uniform particle size.

[0135] Further, in some embodiments, the cobalt source is selected from one or a combination of any number of cobalt sulfate, cobalt nitrate, and cobalt chloride.

[0136] In the technical scheme of the embodiment of the present application, cobalt sulfate, cobalt nitrate, and cobalt chloride can provide necessary divalent cobalt ions for the co-precipitate of lithium cobalt iron generated by the co-precipitation reaction and the lithium cobalt iron composite oxide and the phosphorocobalt lithium iron generated by calcination.

[0137] Further, in some embodiments, the iron source is selected from one or a combination of any number of ferrous sulfate, ferrous nitrate, ferrous chloride, ferric sulfate, ferric nitrate, and ferric chloride.

[0138] In the technical scheme of the embodiment of the present application, ferrous sulfate, ferric nitrate, and ferric chloride can provide divalent iron ions or trivalent iron ions for the co-precipitate of lithium cobalt iron generated by the co-precipitation reaction and the lithium cobalt iron composite oxide and the phosphorocobalt lithium iron generated by subsequent calcination in an inert gas atmosphere.

[0139] Further, in some embodiments, the lithium source is selected from one or a combination of any number of lithium sulfate, lithium nitrate, lithium acetate, and lithium chloride.

[0140] In the technical scheme of the embodiment of the present application, lithium sulfate and the like can provide necessary lithium ions for the reaction to generate the composite lithium iron phosphate material.

[0141] Further, in some embodiments, the organic carbon source is selected from one or a combination of any number of sucrose, glucose, and water-soluble starch.

[0142] In the technical scheme of the embodiment of the present application, the organic carbon source can be converted into a carbon coating layer during high-temperature calcination, which can significantly enhance the conductivity of the material, reduce the electron transmission resistance in the electrode, and improve the charge and discharge efficiency of the battery. The organic carbon sources such as sucrose, glucose and water-soluble starch are beneficial to the formation of a uniform and continuous carbon layer due to their pyrolysis characteristics at high temperatures, and the distribution of these organic carbon sources on the surface of the coprecipitate is uniform, which is helpful to the formation of a carbon coating layer with uniform thickness and distribution. The use of the above-mentioned organic carbon sources during high-temperature calcination can improve the conductivity of the composite lithium iron phosphate material, reduce the internal resistance of the battery, and improve the charge and discharge rate and coulombic efficiency. In addition, the uniform carbon coating layer formed by the decomposition of the above-mentioned organic carbon sources is helpful to improve the cycle stability and thermal stability of the material, and prolong the service life of the battery.

[0143] Further, in some embodiments, the cobalt source, the iron source and the lithium source are added to pure water, and stirred and dissolved to obtain a mixed solution containing the cobalt source, the iron source and the lithium source.

[0144] In the technical scheme of the embodiment of the present application, water is used as the solvent, which is beneficial to control the production cost and green production. In addition, the metal ion sources are uniformly dispersed in water in advance, which is beneficial to avoid uneven coprecipitation caused by local over-concentration, improve the purity and uniformity of the coprecipitate, and reduce the agglomeration of the composite lithium iron phosphate material.

[0145] Further, in some embodiments, the concentration of lithium element in the mixed solution containing the cobalt source, the iron source and the lithium source is 1 mol / L to 3 mol / L.

[0146] In the technical scheme of the embodiment of the present application, the concentration of the lithium source is within the above range, which is helpful to promote the uniform embedding of lithium element in the composite lithium iron phosphate material, form a stable and conductive lithium-cobalt-iron composite structure, improve the capacity and stability of the composite lithium iron phosphate material, and improve the energy density when the composite lithium iron phosphate material in the embodiment of the present application is applied to a secondary battery.

[0147] Further, in some embodiments, the carbonates and the phosphates are mixed to obtain a mixture, and then the mixture is added to the mixed solution containing the cobalt source, the iron source and the lithium source.

[0148] In the technical scheme of the embodiment of the present application, the carbonates and the phosphates are added in the form of a mixture, which is beneficial to the uniform coprecipitation of cobalt ions, iron ions and lithium ions, and the uniform introduction of carbonate ions and phosphate ions into the coprecipitate, so as to form a coprecipitate with uniform particle size, stable structure and regular morphology, improve the purity of the composite lithium iron phosphate material and the uniformity of the distribution of each component, and improve the capacity and other electrochemical properties of the composite lithium iron phosphate material.

[0149] Further, in some embodiments, the time for adding the carbonates and the phosphates is 30 min to 60 min.

[0150] In the technical scheme of the embodiments of the present application, by gradually adding carbonates and phosphates and controlling the adding time, the morphology and structure of the coprecipitate can be optimized, the purity and the uniformity of the distribution of each component of the prepared composite lithium iron phosphate material can be improved, and the capacity and other electrochemical properties of the composite lithium iron phosphate material can be improved.

[0151] Further, in some embodiments, the carbonates and phosphates are added to the mixed solution of the cobalt source, the iron source and the lithium source during stirring, and then the aqueous solution of the alkaline substance is added during stirring until the pH of the system is 3.5 to 5.0, and then the stirring and aging is performed at a stirring speed of 100 r / min to 300 r / min for 30 min to 60 min to obtain the lithium-cobalt-iron coprecipitate.

[0152] In the technical scheme of the embodiments of the present application, the pH value is in the above range (3.5 to 5.0), which is beneficial to promote the coprecipitation of cobalt ions, iron ions and lithium ions, and form a coprecipitate with uniform particle size and regular morphology; controlling the stirring speed in the above range and the stirring and aging time helps the uniform distribution of each reactant and accelerates the collision and reaction between ions, and reduces the decomposition or structural damage of the material caused by local overheating. By precisely controlling the pH value and the stirring conditions, the microstructure of the material can be optimized, the coprecipitate with uniform composition and consistent structure is formed, and the capacity, energy density, voltage platform and cycle stability and other electrochemical properties of the composite lithium iron phosphate material are improved.

[0153] Further, in some embodiments, the aqueous solution of the alkaline substance is selected from one or any combination of aqueous solutions of ammonium bicarbonate, sodium hydroxide, potassium hydroxide and NH3·H2O.

[0154] In the technical scheme of the embodiments of the present application, during the preparation of the composite lithium iron phosphate material, the alkaline substance is added to adjust the pH value of the reaction system, so that cobalt ions, iron ions and lithium ions can coprecipitate. The above-mentioned ammonium bicarbonate and sodium hydroxide and the like are common alkaline substances, and can all increase the pH value of the solution, promote the precipitation of metal ions, and form a lithium-cobalt-iron coprecipitate with uniform particle size and morphology.

[0155] Further, in some embodiments, the calcination temperature of the calcination treatment is 750℃ to 850℃, and the calcination time is 6h to 12h.

[0156] In the technical scheme of the embodiment of the present application, during high-temperature calcination, the coprecipitate of lithium-cobalt-iron is converted into lithium-cobalt-iron composite oxide and a composite material of cobalt-lithium-iron phosphate; carbonates are pyrolyzed to release CO2, forming a uniform porous structure in the composite lithium-iron-phosphate material; the pyrolysis of the organic carbon source forms carbon conductive material, which at least partially coats the lithium-cobalt-iron composite oxide and the composite material of cobalt-lithium-iron phosphate; at the same time, high-temperature calcination promotes the further reaction of metal ions and phosphate to form a stable phosphoric acid iron-based composite oxide structure. Under the above calcination conditions, it is conducive to forming a composite lithium-iron-phosphate material with high purity and stable structure, and helps to improve the capacity, cycle stability and voltage platform of the composite lithium-iron-phosphate material.

[0157] Further, in some embodiments, the heating rate of the calcination treatment is 1-3 ℃ / min.

[0158] In the technical scheme of the embodiment of the present application, heating at the above rate helps to uniformly heat each component of the composite lithium-iron-phosphate material, and helps to improve the structural integrity of each component and reduce the number of defects.

[0159] Further, in some embodiments, the organic carbon source is first dispersed in water to prepare an organic carbon source solution, and then the coprecipitate of lithium-cobalt-iron is dispersed in the organic carbon source solution to obtain a slurry.

[0160] In the technical scheme of the embodiment of the present application, the organic carbon source is first dispersed in water to prepare an organic carbon source solution, and then the coprecipitate of lithium-cobalt-iron is dispersed in the aqueous solution to form a slurry, which is conducive to the full contact and uniform mixing of the coprecipitate and the organic carbon source in the solution, and helps the organic carbon source to uniformly cover the surface of the coprecipitate to form a carbon layer with uniform thickness, thereby improving the conductivity of the composite lithium-iron-phosphate material, the lithium ion diffusion capacity, and the stability of the overall structure.

[0161] Further, in some embodiments, the content of the organic carbon source in the organic carbon source solution is 10-20% by weight.

[0162] In the technical scheme of the embodiment of the present application, the concentration of the organic carbon source affects the thickness of the carbon coating layer. When the concentration of the organic carbon source is within the above range, it is conducive to forming a carbon coating layer with uniform thickness, good conductivity and stability. At the same time, the compaction density of the composite lithium-iron-phosphate material is maintained at a high level, thereby improving the performance of the battery.

[0163] Further, in some embodiments, the slurry is dried by spray drying.

[0164] In the technical scheme of the embodiment of the present application, spray drying can realize rapid drying of the material, improve the uniformity and flowability of the formed particles, reduce agglomeration, and improve the dispersibility and compactibility of the composite lithium iron phosphate material, thereby helping to improve the charge-discharge efficiency, energy density and cycle stability of the composite lithium iron phosphate material.

[0165] Further, in some embodiments, the water content of the dry material obtained by drying the slurry is less than 2%.

[0166] In the technical scheme of the embodiment of the present application, the purpose of drying the slurry is to remove water in the slurry, so as to reduce the influence of water evaporation in the subsequent high-temperature calcination process on the structure of the composite lithium iron phosphate material. Controlling the water content of the dry material to be less than 2% is conducive to reducing the material shrinkage and pore structure damage caused by water evaporation, improving the microstructure inside the composite lithium iron phosphate material and the integrity of the carbon coating layer, thereby improving the electrochemical performance and cycle stability of the composite lithium iron phosphate material.

[0167] Further, in some embodiments, the drying process further includes a crushing process, and the D50 particle size of the dry material after the crushing process is 1-3 μm.

[0168] Crushing the dry material to the above particle size range is conducive to obtaining a composite lithium iron phosphate material with high specific surface area, promoting the rapid diffusion of lithium ions, improving the electrochemical activity of the material, and at the same time obtaining a high compactness of the composite lithium iron phosphate material, which is conducive to improving the electrochemical performance of the composite lithium iron phosphate material such as capacity, cycle stability and charge-discharge rate.

[0169] Further, in some embodiments, the calcination is carried out in an inert gas atmosphere with a humidity of ≤0.5% and an oxygen content of less than 1 ppm.

[0170] In the technical scheme of the embodiment of the present application, the above low-humidity and low-oxygen content atmosphere is conducive to reducing the oxidation and water adsorption of the composite lithium iron phosphate material under high-temperature calcination conditions, improving the purity and structural stability of the composite lithium iron phosphate material, and improving the electrochemical performance and cycle stability of the composite lithium iron phosphate material.

[0171] Further, in some embodiments, after the calcination is completed, the material obtained by calcination is discharged after being cooled to 100°C, the discharged material is crushed, and then iron removal and packaging are carried out in a constant-temperature and constant-humidity chamber to obtain the composite lithium iron phosphate material.

[0172] In the technical scheme of the embodiment of the present application, the temperature control and crushing steps of the material after calcination facilitate the subsequent processing and use of the composite lithium iron phosphate material. Carrying out iron removal and packaging in a constant-temperature and constant-humidity chamber is conducive to avoiding the hygroscopicity and oxidation of the composite lithium iron phosphate material, and maintaining the chemical purity and performance stability of the composite lithium iron phosphate material.

[0173] Further, in some embodiments, the temperature of the constant temperature and humidity room is 24-26°C, and the humidity is 8-10%.

[0174] In the technical scheme of the embodiments of the present application, the above-mentioned constant temperature and humidity room is used for processing, which is conducive to maintaining the chemical purity and performance stability of the composite lithium iron phosphate material, thereby maintaining the consistency and reliability of the battery performance.

[0175] Further, in some embodiments, before the step of mixing the coprecipitate of lithium-cobalt-iron and the organic carbon source, the method further comprises: filtering and washing the coprecipitate of lithium-cobalt-iron until the conductivity of the washing water after washing is ≤200 μS / cm.

[0176] In the technical scheme of the embodiments of the present application, through the above-mentioned filtering and washing, impurities can be effectively removed, the purity and consistency of the composite lithium iron phosphate material can be improved, which is helpful to improve the coulomb efficiency and cycle life, reduce the internal resistance, and improve the electrochemical activity of the composite lithium iron phosphate material.

[0177] In a third aspect, the embodiments of the present application provide a positive electrode tab, which comprises the composite lithium iron phosphate material as above, or the composite lithium iron phosphate material prepared by the preparation method as above.

[0178] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises a positive electrode, and the tab of the positive electrode is the positive electrode tab as above.

[0179] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not specified in the embodiments, the technology or condition described in the literature in the art or according to the product manual is used. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0180] I. Preparation method

[0181] Embodiment 1

[0182] A preparation method of a composite lithium iron phosphate material, comprising the following steps:

[0183] Step one, adding cobalt sulfate, ferrous sulfate and lithium sulfate into pure water, stirring and dissolving to obtain a mixed solution containing a cobalt source, an iron source and a lithium source, adding a mixture of sodium carbonate and sodium phosphate into the mixed solution containing the cobalt source, the iron source and the lithium source during stirring, then adding ammonia water during stirring until the pH of the system is 4.5, then stirring and aging at 40℃ for 40min, filtering the system after the reaction is completed to obtain a filtrate, washing the filtrate until the conductivity of the washing water is ≤200μS / cm to obtain a lithium cobalt iron co-precipitate.

[0184] In the mixed solution containing the cobalt source, the iron source and the lithium source, the concentration of lithium element is 2mol / L; the molar ratio of cobalt element in the cobalt source, iron element in the ferrous sulfate and lithium element in the lithium sulfate is 0.15:0.85:1.31; the mixture of sodium carbonate and sodium phosphate is added according to the molar ratio of carbonate, phosphate and iron element in the iron source is 0.2:0.85:0.85, the time for adding the mixture of sodium carbonate and sodium phosphate is 50min; the stirring speed during stirring and aging is 200r / min.

[0185] Step two, dispersing an organic carbon source in water to prepare an organic carbon source solution, then adding the organic carbon source solution into the lithium cobalt iron co-precipitate, stirring to obtain a slurry, drying the slurry to a water content of 1.8% (by weight) by spray drying, then stopping drying, obtaining a dried material, then crushing the dried material to a D50 particle size of 1.4μm, then calcining under nitrogen protection, after calcination, cooling the calcined material to 100℃ and discharging, crushing the discharged material to a D50 particle size of 1.2μm, then removing iron and packaging in a constant temperature and humidity chamber (temperature 25℃, humidity 9%) to obtain a composite lithium iron phosphate material.

[0186] In the organic carbon source solution, the content of the organic carbon source is 15% by weight; the weight of the added organic carbon source is 8.5% based on the sum of the weight of the lithium cobalt iron co-precipitate and the organic carbon source; the calcination temperature is 810℃, the holding time is 9h, the heating rate is 2.1℃ / min, and the calcination is carried out in a nitrogen atmosphere with a humidity of ≤0.5% and an oxygen content of less than 1ppm.

[0187] The SEM result of the composite lithium iron phosphate material prepared in Example 1 is shown in Figure 1 The primary particle morphology of the composite lithium iron phosphate material is regular, the particle size is uniform and the particle size is about 200nm. According to the particle size detection data, the D50 particle size of the secondary particle is 1.2μm; the 0.1C first charge-discharge curve of the composite lithium iron phosphate material prepared in Example 1 is shown in Figure 2As shown, the 0.1C first charge capacity reaches 186.5 mAh / g, the 0.1C discharge specific capacity reaches 178.6 mAh / g, and the first coulombic efficiency reaches 95.8%. The composition general formula of the composite lithium iron phosphate material is Li 1.2 Co 0.15 Fe 0.85 O 0.4 (PO4) 0.85 / C, the content of carbon in the composite lithium iron phosphate material is 1.42% in terms of weight percentage.

[0188] Example 2

[0189] A preparation method of a composite lithium iron phosphate material, comprising the following steps:

[0190] Step one, cobalt chloride, ferrous chloride and lithium chloride are added to pure water to obtain a mixed solution containing a cobalt source, an iron source and a lithium source, and the mixed solution is stirred and dissolved. During the stirring process, a mixture of ammonium carbonate and ammonium phosphate is added to the mixed solution containing the cobalt source, the iron source and the lithium source, and then an aqueous solution of sodium hydroxide is added during the stirring process until the pH of the system is 3.5. Then the system is stirred and aged at 35℃ for 60min. After the reaction is completed, the system is filtered to obtain a filterate. The filterate is washed until the conductivity of the washing water is ≤200μS / cm to obtain a lithium-cobalt-iron coprecipitate.

[0191] In the mixed solution containing the cobalt source, the iron source and the lithium source, the concentration of lithium element is 1mol / L; the molar ratio of cobalt element in cobalt chloride, iron element in ferrous chloride and lithium element in lithium chloride is 0.1:0.9:1.2; sodium carbonate and sodium phosphate are added according to the molar ratio of carbonate, phosphate and iron element in the iron source is 0.25:0.9:0.9; the time for adding the mixture of ammonium carbonate and ammonium phosphate is 30min; the stirring speed during the stirring and aging process is 100r / min.

[0192] Step two, an organic carbon source solution is prepared by dispersing an organic carbon source in water, and then the organic carbon source solution is added to the lithium-cobalt-iron coprecipitate to obtain a slurry after stirring. The slurry is dried by spray drying until the water content is 2% (in terms of weight percentage) and then the drying is stopped. A dry material is obtained, which is then crushed to a D50 particle size of 3μm. Then the material is calcined under nitrogen protection. After calcination is completed, the material obtained by calcination is cooled to 100℃ and then discharged. The discharged material is crushed to a D50 particle size of 1.4μm and then iron is removed and packaged in a constant temperature and humidity chamber (temperature is 25℃ and humidity is 9%) to obtain a composite lithium iron phosphate material.

[0193] The organic carbon source is glucose; the organic carbon source content in the organic carbon source solution is 20% by weight; the weight of the added organic carbon source is 10.1% based on the sum of the weights of the lithium cobalt iron coprecipitate and the organic carbon source being 100%; the calcination temperature is 750℃, the holding time is 12h, the heating rate is 3℃ / min, and the calcination is carried out in a nitrogen atmosphere with humidity ≤0.5% and oxygen content below 1ppm.

[0194] The SEM results of the composite lithium iron phosphate material prepared in Example 2 are as follows: Figure 3 As shown, in the composite lithium iron phosphate material, the primary particles have regular morphology, uniform particle size, and a particle size of approximately 200 nm. According to particle size analysis data, the D50 particle size of the secondary particles is 1.4 μm; the general formula of the composite lithium iron phosphate material is Li... 1.1 Co 0.1 Fe 0.9 O 0.3 (PO4) 0.9 / C, by weight percentage, the carbon content in the composite lithium iron phosphate material is 1.8%.

[0195] Example 3

[0196] A method for preparing a composite lithium iron phosphate material includes the following steps:

[0197] Step 1: Add cobalt acetate, ferrous acetate, and lithium acetate to pure water and stir to dissolve, obtaining a mixed solution containing cobalt, iron, and lithium sources. During stirring, add a mixture of potassium carbonate and potassium phosphate to the mixed solution containing cobalt, iron, and lithium sources. Then, while stirring, add potassium hydroxide aqueous solution until the pH of the system reaches 5.0. Then, stir and age at 55°C for 30 minutes. After the reaction is complete, filter the system to obtain the filtrate. Wash the filtrate until the conductivity of the wash water is ≤200 μS / cm to obtain a lithium cobalt iron coprecipitate.

[0198] The concentration of lithium in the mixed solution containing cobalt, iron, and lithium sources is 3 mol / L; the molar ratio of cobalt in cobalt acetate, iron in ferrous acetate, and lithium in lithium acetate is 0.2:0.8:1.4; sodium carbonate and sodium phosphate are added in a molar ratio of carbonate, phosphate, and iron in the iron source of 0.25:0.8:0.8; the time taken to add the mixture of potassium carbonate and potassium phosphate is 60 min; the stirring speed during the stirring and aging process is 300 r / min.

[0199] Step two, dispersing the organic carbon source in water to prepare an organic carbon source solution, then adding the organic carbon source solution to the co-precipitate of lithium-cobalt-iron, stirring to obtain a slurry, using spray drying to dry the slurry to a moisture content of 2% (by weight) and then stopping the drying, obtaining a dried material, then crushing the dried material to a D50 particle size of 1.4 μm, and then performing calcination under nitrogen protection, after calcination is completed, cooling the calcined material to 100°C and then discharging, crushing the discharged material to a D50 particle size of 1.5 μm, and then removing iron and packaging in a constant temperature and humidity chamber (temperature 25°C, humidity 9%) to obtain a composite lithium iron phosphate material.

[0200] wherein the organic carbon source is water-soluble starch; the content of the organic carbon source in the organic carbon source solution is 20% by weight; the weight of the added organic carbon source is 7.1% based on the sum of the weights of the co-precipitate of lithium-cobalt-iron and the organic carbon source being 100%; the calcination temperature is 850°C, the holding time is 12 h, the heating rate is 3°C / min, and the calcination is performed in a nitrogen atmosphere with a humidity of ≤0.5% and an oxygen content of less than 1 ppm.

[0201] The SEM results of the composite lithium iron phosphate material prepared in Example 3 are shown in Figure 4 The primary particles in the composite lithium iron phosphate material have regular morphology, uniform particle size, and a particle size of about 1 μm. According to the particle size detection data, the D50 particle size of the secondary particles is 1.5 μm; the composition general formula of the composite lithium iron phosphate material is Li 1.3 Co 0.2 Fe 0.8 O 0.5 (PO4) 0.8 / C, and the content of carbon in the composite lithium iron phosphate material is 1.0% by weight.

[0202] Example 4

[0203] The difference between it and Example 1 is only that the stirring aging is performed at 55°C.

[0204] Example 5

[0205] The difference between it and Example 1 is only that the stirring aging is performed at 65°C.

[0206] Example 6

[0207] The difference between it and Example 1 is only that the concentration of lithium element in the mixed solution containing a cobalt source, an iron source, and a lithium source is 3 mol / L.

[0208] Example 7

[0209] The difference from Example 1 is only that the concentration of lithium element in the mixed solution containing a cobalt source, an iron source and a lithium source is 5 mol / L.

[0210] Example 8

[0211] The difference from Example 1 is only that the calcination temperature is 850℃.

[0212] Example 9

[0213] The difference from Example 1 is only that the calcination temperature is 900℃.

[0214] Comparative Example 1

[0215] The difference from Example 1 is only that the amount of phosphate added is increased, and sodium carbonate and sodium phosphate are added according to the molar ratio of carbonate, phosphate and iron element in the iron source is 0.2:1:0.85.

[0216] Comparative Example 2

[0217] The difference from Example 1 is only that the amount of phosphate added is reduced, and sodium carbonate and sodium phosphate are added according to the molar ratio of carbonate, phosphate and iron element in the iron source is 0.2:0.6:0.85.

[0218] Comparative Example 3

[0219] The difference from Example 1 is only that the weight of the organic carbon source added accounts for 18.6% of the sum of the weight of the coprecipitate of lithium cobalt iron and the organic carbon source.

[0220] Comparative Example 4

[0221] The difference from Example 1 is only that the same molar amount of ferrous sulfate is used to replace cobalt sulfate in step one.

[0222] II. Test method

[0223] 1. Property test of composite lithium iron phosphate material

[0224] 1) Morphology characterization: The morphology of the material was characterized by scanning electron microscope (SEM).

[0225] 2) Elemental analysis: The elemental composition of the material was analyzed by inductively coupled plasma spectrometer, and the results are shown in Table 1.

[0226] 3) pH test: Refer to standard GB / T 5211.6-2020 for details, and the pH test results of the material are shown in Table 2.

[0227] 4) Free lithium content test: Refer to standard SJ / T 11794-2022 for details. The free lithium content test results of the materials are shown in Table 2.

[0228] 5) Powder internal resistance test: Refer to standard GB / T 45324-2025 for details. The powder internal resistance test results of the materials are shown in Table 2.

[0229] 6) Compacted density test: Refer to standard GB / T 44330-2024 for details. The compacted density test results of the materials are shown in Table 2.

[0230] 7) Specific surface area test: Refer to standard GB / T 9587-2017 for details. The specific surface area test results of the materials are shown in Table 2.

[0231] 8) Tap density test: The tap density of the materials was tested using a tap density instrument. The tap density test results of the materials are shown in Table 2.

[0232] 9) Particle size test: The D50 particle size (μm) of the materials was tested using a laser particle size instrument. The particle size test results are shown in Table 2.

[0233] 10) Magnetic substance content test: Refer to standard GB / T 41704-2022 for details. The magnetic substance content test results of the materials are shown in Table 2.

[0234] 11) Moisture content test: Refer to standard GB / T 6283 for details. The moisture content test results of the materials are shown in Table 2.

[0235] 2) Property test of secondary battery

[0236] 1) Battery assembly

[0237] The composite lithium iron phosphate material prepared in the examples and the composite lithium iron phosphate material prepared in the comparative examples were mixed with the conductive agent SP and the PVDF binder in a mass ratio of 92:4:4, respectively, homogenized, coated on aluminum foil, dried at 100°C, then rolled using a roll machine, and then punched into positive electrode sheets with a diameter of 14 mm using a sheet punching machine. The mass of the active material was obtained by weighing and deducting the mass of the aluminum foil.

[0238] After drying the positive electrode sheet, lithium sheets were used as negative electrode sheets, and CR2032 button-type half-batteries were assembled in the order of negative electrode shell, lithium sheet, electrolyte, separator, electrolyte, positive electrode sheet, gasket, spring, and positive electrode shell in a UNlab type inert gas glove box of Braun Company, Germany.

[0239] 2) Charge-discharge performance test

[0240] The electrochemical performance of CR2032 coin cells was tested using the Wuhan Landian CT2001A battery testing system. The voltage range was 2V to 4.55V, and the test temperature was 25℃. The results for the initial charge capacity at 0.1C, initial discharge capacity at 0.1C, initial coulombic efficiency at 0.1C, initial discharge capacity at 1C, capacity percentage at 3.75-4.55V, voltage plateau, energy density, and capacity retention after 500 cycles at 1C are shown in Table 3.

[0241] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0242] Table 1

[0243]

[0244] Table 2

[0245]

[0246] Table 3

[0247]

[0248] As can be seen from the data in Tables 1, 2, and 3, the composite lithium iron phosphate materials Li prepared in Examples 1 to 9 of this application... y Co x Fe (1-x) O u (PO4) z (where x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, u is 0.2 to 0.5, and the carbon content in the composite lithium iron phosphate material is 1% to 1.9% by weight), has high compaction density, high voltage plateau, high charge and discharge capacity and high energy density, and has excellent electrical performance.

[0249] A comparison of Examples 1, 4, and 5 shows that, compared to Example 5, Examples 1 and 4, which involve precipitation reactions at temperatures between 35°C and 55°C, are more conducive to the uniform co-precipitation of cobalt ions, iron ions, and lithium ions. This facilitates the subsequent preparation of high-purity, structurally stable, and highly conductive composite lithium iron phosphate materials. These composite lithium iron phosphate materials exhibit higher voltage plateaus, charge / discharge capacity, energy density, and cycle stability.

[0250] A comparison of Examples 1, 6, and 7 shows that, compared to Example 7, the lithium element concentration in the mixed solution containing cobalt, iron, and lithium sources in Examples 1 and 6 is in the range of 1 mol / L to 3 mol / L, and the prepared composite material has higher voltage plateau, charge / discharge capacity, energy density, and cycle stability.

[0251] Compared with Example 9, Example 1 and Example 8 are calcined at 750-850℃, which is beneficial to form high-purity and structure-stable composite lithium iron phosphate material, and the prepared composite lithium iron phosphate material has higher charge-discharge capacity, voltage plateau, energy density and cycle stability.

[0252] Compared with Example 1, a higher amount of phosphate is introduced in the coprecipitation stage in Comparative Example 1, and the obtained material is LiCo 0.15 Fe 0.85 PO4 / C, which does not contain lithium cobalt iron composite oxide, and its voltage plateau, charge-discharge capacity and energy density are significantly reduced.

[0253] Compared with Example 1, a lower amount of phosphate is introduced in the coprecipitation stage in Comparative Example 2, and the obtained material is Li 1.2 Co 0.15 Fe 0.85 O 0.73 (PO4) 0.6 / C, the content of lithium cobalt iron composite oxide in the material is much higher than that in Example 1, and its discharge capacity is significantly reduced.

[0254] Compared with Example 1, Comparative Example 3 uses a higher amount of organic carbon source for carbon coating, and the prepared composite lithium iron phosphate material contains a higher content of carbon, and its compaction density and energy density are reduced.

[0255] Compared with Example 1, Comparative Example 4 does not perform cobalt doping, and the prepared composite lithium iron phosphate material is Li 1.2 Fe 1.0 O 0.4 (PO4) 0.85 / C, and its charge-discharge capacity (especially the proportion of 3.75-4.55V charge capacity) and energy density are significantly reduced.

[0256] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A composite lithium iron phosphate material, characterized in that, The composite lithium iron phosphate material comprises a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material, the phosphate / oxide composite material has a general formula of Li y Co x Fe (1-x) O u (PO4) z wherein x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, and u is 0.2 to 0.5, and the content of carbon in the composite lithium iron phosphate material is 1% to 1.9% by weight.

2. The composite lithium iron phosphate material of claim 1, wherein, The D50 particle size of the composite lithium iron phosphate material is 1.2 μm to 1.5 μm.

3. The composite lithium iron phosphate material of claim 1, wherein, The compacted density of the composite lithium iron phosphate material is 2.57 g / mL to 2.78 g / mL; and / or, The tap density of the composite lithium iron phosphate material is 1.38 g / mL to 1.55 g / mL.

4. A method for preparing a composite lithium iron phosphate material, characterized in that, The method comprises the following steps: A mixed solution containing a cobalt source, an iron source and a lithium source is provided; A carbonate and a phosphate are added to the mixed solution, and the pH value of the mixed solution is adjusted to 3.5-5.0, to obtain a lithium-cobalt-iron co-precipitate through reaction; The lithium-cobalt-iron co-precipitate and an organic carbon source are mixed to obtain a slurry through slurry treatment, and then the slurry is dried and calcined to obtain the composite lithium iron phosphate material; The composite lithium iron phosphate material comprises a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material, the phosphate / oxide composite material has a general formula of Li y Co x Fe (1-x) O u (PO4) z wherein x is 0.1 to 0.2, y is 1.1 to 1.3, z is 0.8 to 0.9, and u is 0.2 to 0.5, and the content of carbon in the composite lithium iron phosphate material is 1% to 1.9% by weight.

5. The method of claim 4, wherein the lithium iron phosphate composite material is prepared by the steps of: mixing a lithium source, an iron source, and a phosphorus source; and heating the mixture at a temperature of 600 to 800°C for 1 to 10 hours in an inert atmosphere. The molar ratio of cobalt in the cobalt source, iron in the iron source and lithium in the lithium source is (0.1-0.2):(0.8-0.9):(1.2-1.4); and / or, The molar ratio of carbonate in the carbonate, phosphate in the phosphate and iron in the iron source is (0.15-0.25):(0.8-0.9):(0.8-0.9); and / or, The amount of the organic carbon source is 7% to 10.5% based on the total weight of the lithium-cobalt-iron co-precipitate and the organic carbon source.

6. The method for preparing the composite lithium iron phosphate material according to claim 4, wherein The carbonate is selected from one or a combination of any number of sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate and ammonium bicarbonate; and / or, The phosphate is selected from one or a combination of any number of sodium phosphate, potassium phosphate, ammonium phosphate, sodium hydrogen phosphate, potassium hydrogen phosphate, ammonium hydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate and ammonium dihydrogen phosphate; and / or, The cobalt source is selected from one or a combination of any number of cobalt sulfate, cobalt nitrate and cobalt chloride; and / or, The iron source is selected from one or a combination of any number of ferrous sulfate, ferrous nitrate, ferrous chloride, ferric sulfate, ferric nitrate and ferric chloride; and / or, The lithium source is selected from one or a combination of any number of lithium sulfate, lithium nitrate, lithium acetate and lithium chloride; and / or, The organic carbon source is selected from one or a combination of any number of sucrose, glucose and water-soluble starch.

7. The method of claim 4, wherein the lithium iron phosphate composite material is prepared by the steps of: mixing a lithium source, an iron source, and a phosphorus source; and heating the mixture at a temperature of 600 to 800°C for 1 to 10 hours in an inert atmosphere. The water content of the dry material obtained through the drying treatment of the slurry is less than 2 wt%; and / or, The drying treatment further comprises a crushing treatment, and the D50 particle size of the dry material after the crushing treatment is 1 μm to 3 μm; and / or, The calcination temperature of the calcination treatment is 750°C to 850°C, and the calcination time is 6 h to 12 h.

8. The method for preparing the composite lithium iron phosphate material according to claim 4, characterized in that, Before the step of mixing the lithium-cobalt-iron co-precipitate and the organic carbon source, the method further comprises: The lithium-cobalt-iron co-precipitate is filtered and washed until the conductivity of the washing water after washing is ≤200 μS / cm.

9. A positive electrode sheet characterized by comprising: The positive electrode plate comprises the composite lithium iron phosphate material according to any one of claims 1 to 3, or comprises the composite lithium iron phosphate material prepared by the preparation method of the composite lithium iron phosphate material according to any one of claims 4 to 8.

10. A secondary battery comprising a positive electrode, characterized by, The positive electrode plate used by the positive electrode is the positive electrode plate according to claim 9.

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