Composite lithium iron phosphate material and preparation method thereof, positive pole piece and secondary battery

Abstract

The invention provides a composite lithium iron phosphate material and a preparation method thereof, a positive pole piece and a secondary battery, and belongs to the technical field of secondary batteries, the composite lithium iron phosphate material comprises a phosphate / oxide composite material and carbon at least partially coating the surface of the phosphate / oxide composite material, the general formula of the phosphate / oxide composite material is LiyCoxFe (1-x) Ou (PO4) z, x ranges from 0.1 to 0.2, y ranges from 1.1 to 1.3, z ranges from 0.8 to 0.9, u ranges from 0.2 to 0.5, and the content of carbon in the composite lithium iron phosphate material ranges from 1% to 1.9% by weight. Based on the composite lithium iron phosphate material, the composite lithium iron phosphate material has a high voltage platform, high capacity and high coulombic efficiency, and has high energy density when being applied to a secondary battery.

Description

Technical Field

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

[0002] Lithium secondary batteries, with their high energy density, long cycle life, and environmental friendliness, have become an indispensable energy source for portable electronic devices, electric vehicles, and large-scale energy storage systems. In lithium secondary batteries, the choice of cathode material has a significant impact on the overall performance of the battery. Currently, olivine-structured lithium iron phosphate (LiFePO4, LFP for short) has become the primary choice for cathode materials in automotive power lithium batteries due to its stable crystal structure, good safety, and low production cost, especially in the field of electric vehicles. The theoretical capacity of LFP material is 170mAh / g, the voltage platform is 3.4V, and the true density is approximately 3.5g / mL. Its theoretical energy density upper limit limits the improvement of the overall energy density of the battery.

[0003] In addition, in order to improve the conductivity and cycle performance of lithium iron phosphate, carbon coating is usually performed during the conventional preparation process. Although carbon coating can effectively improve the 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 for higher energy density, for example, the requirement of electric vehicles for longer driving range, the performance improvement of LFP positive electrode materials has become crucial. Therefore, finding new lithium iron phosphate materials to improve the energy density of the materials has become a key issue that needs to be urgently addressed in the current field of lithium battery technology. Summary of the Invention

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

[0005] In the first aspect, the embodiment of the present application provides a composite lithium iron phosphate material, which includes a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material. The 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. In terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1% to 1.9%.

[0006] In the technical solution of the embodiment of the present application, the composite lithium iron phosphate material is a composite of lithium cobalt iron composite oxide and olivine phosphate iron cobalt lithium phosphate. The lithium cobalt iron composite oxide has a high capacity. By combining it with iron cobalt lithium phosphate, a high-capacity composite lithium iron phosphate material is obtained. In addition, the composite lithium iron phosphate material adopts a lithium-rich design to improve the coulombic efficiency of the composite lithium iron phosphate material. Furthermore, the introduction of cobalt elements into the composite lithium iron phosphate material can improve the voltage platform of the composite lithium iron phosphate material. Finally, the carbon composite and coating 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, and reduce the polarization of lithium ions during the charge and discharge process, thereby improving the coulombic efficiency. The composite lithium iron phosphate material based on the embodiment of the present application has a high voltage platform, high capacity, high coulombic efficiency, and excellent electrical properties. It can be used to prepare batteries with high capacity, high energy density and high coulombic efficiency.

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

[0008] In this embodiment, the small particle size of the composite lithium iron phosphate material is beneficial for increasing the material's specific surface area, increasing the contact area between 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-mentioned range, it helps to accelerate the diffusion rate of lithium ions in the composite lithium iron phosphate material, shorten the ion migration path, and improve the battery's charge and discharge rate and cycle performance, while maintaining a high compaction density.

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

[0010] In this embodiment, the compaction density is within the above range, which is conducive to obtaining a composite lithium iron phosphate material with high crystallinity and few internal pores, thereby improving the compactness, 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 has significant advantages for applications such as portable electronic devices and electric vehicles.

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

[0012] In this embodiment, the tap density reflects the degree of compactness that a material can achieve when subjected to vibration. The tap density of the composite lithium iron phosphate material is within the above range, which helps to obtain an electrode material with a high tap density during the battery assembly process, 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 m² / g to 14.28 m² / g.

[0014] In this embodiment, the higher specific surface area increases the contact area between the composite lithium iron phosphate material and the electrolyte, promoting rapid intercalation and intercalation of lithium ions, accelerating the electrochemical reaction rate, and thereby improving 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 falling within the aforementioned range achieves a higher compaction density of the composite lithium iron phosphate material, which in turn helps improve the charge and discharge rate and energy density of the battery.

[0015] In a second aspect, an embodiment of the present application provides a method for preparing a composite lithium iron phosphate material, comprising the following steps:

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

[0017] adding carbonate and phosphate to the mixed solution, and adjusting the pH value of the mixed solution to 3.5-5.0, to obtain a lithium-cobalt-iron coprecipitate through reaction;

[0018] The lithium cobalt iron coprecipitate and the organic carbon source are mixed and slurried to obtain a slurry, and then the slurry is dried and calcined to obtain a 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. The 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. In terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1% to 1.9%.

[0020] In the technical solution of the embodiment of the present application, carbonate and phosphate are first used as precipitants to co-precipitate iron, cobalt and lithium to obtain a co-precipitate of lithium cobalt iron, and then the co-precipitate of lithium cobalt iron is mixed with a carbon source, and slurried, dried and calcined, wherein phosphate forms lithium iron cobalt phosphate with lithium, cobalt and iron, and the carbonate precipitate decomposes at high temperature to form a lithium cobalt iron composite oxide, thereby obtaining a composite material of lithium iron cobalt phosphate, lithium cobalt iron composite oxide and carbon. It can be understood that in the precursor preparation process, by adding carbonate and phosphate and controlling the pH, uniform coprecipitation of lithium, cobalt, iron and phosphorus can be achieved, so that the various constituent elements can be fully mixed and evenly mixed. Therefore, during high-temperature calcination, the migration distance between ions can be effectively reduced, so that the fusion between particles is more sufficient. At the same temperature, the particles grow more densely, thereby effectively improving the compaction density of the material.

[0021] Furthermore, lithium cobalt iron composite oxide has a high capacity. Combining it with lithium iron cobalt phosphate can increase the capacity of the composite lithium iron phosphate material. At the same time, the combination of the two can increase the compaction density of the lithium iron phosphate material. In addition, due to the introduction of cobalt elements, the voltage platform of the composite lithium iron phosphate material can also be improved.

[0022] Furthermore, the lithium-cobalt-iron coprecipitate is mixed with an organic carbon source. The addition of the organic carbon source forms a carbon coating during 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, and reducing polarization during the charge and discharge process, thereby improving the coulombic efficiency. Furthermore, calcination helps the carbonate decompose to produce carbon dioxide gas, forming a porous structure in the composite lithium iron phosphate material, and the phosphate radicals and metal ions interact at high temperatures to form a stable phosphate-based composite material. The protective environment of the inert gas and the organic carbon source can prevent the oxidation of components such as divalent iron and divalent cobalt at high temperatures, which helps to obtain a high-purity composite lithium iron phosphate material.

[0023] Therefore, in this application, a 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, and the carbon content in the composite lithium iron phosphate material is 1% to 1.9% by weight), 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. Compared with traditional lithium iron phosphate materials, the energy density of batteries prepared with the composite lithium iron phosphate material can be increased by more than 10%.

[0024] In some embodiments, the molar ratio of the cobalt element in the cobalt source, the iron element in the iron source, and the lithium element 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 improve the material's Coulombic efficiency, a higher iron content (0.8-0.9) enhances the material's structural stability and electrochemical activity, and the introduction of cobalt can increase the material's voltage platform. By controlling the raw material dosage within the aforementioned range, subsequent coprecipitation and high-temperature calcination facilitate the production of a composite lithium iron phosphate material with a high voltage platform, high capacity, high Coulombic efficiency, and high energy density.

[0026] In some embodiments, 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).

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

[0028] 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 lithium-cobalt-iron co-precipitate and the organic carbon source being 100%.

[0029] In this embodiment, the carbon content is within the above range, which is beneficial to improving the conductivity and structural stability of the composite lithium iron phosphate material, improving the charge and discharge efficiency, cycle life and coulombic efficiency of the battery, while maintaining a relatively high compaction density.

[0030] In some embodiments, during the reaction to obtain the lithium-cobalt-iron coprecipitate, the reaction temperature is controlled to be 35°C to 55°C.

[0031] In this embodiment, the reaction temperature affects the coprecipitation reaction rate and the product crystal structure. Within the above reaction temperature range, uniform coprecipitation of cobalt, iron, and lithium ions is promoted, helping to form uniform precursor particles and reducing the production of byproducts. This allows for the subsequent preparation of a high-purity, structurally stable, and conductive composite lithium iron phosphate material, which is beneficial for improving the energy density of the composite lithium iron phosphate material.

[0032] In some embodiments, the carbonate is selected from one or a combination 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 for providing carbonate ions and controlling the pH value, promoting uniform co-precipitation, so as to subsequently prepare a composite lithium iron phosphate material with uniform particle size, high purity and stable structure.

[0034] In some embodiments, the phosphate is selected from one 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, or any combination thereof.

[0035] In this embodiment, the phosphate has a relatively high solubility, which is beneficial for providing phosphate ions and controlling the pH value, promoting uniform co-precipitation, and facilitating the formation of 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 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 coprecipitation reaction to generate lithium cobalt iron coprecipitate and the calcination to generate lithium cobalt iron composite oxide and lithium iron cobalt phosphate.

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

[0039] In this embodiment, ferrous sulfate, ferric nitrate, ferric chloride, etc. can provide divalent iron ions or trivalent iron ions for the coprecipitation reaction to generate a lithium cobalt iron coprecipitate, and subsequently calcined in an organic carbon source and inert gas atmosphere to generate lithium cobalt iron composite oxide and lithium iron cobalt phosphate.

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

[0041] In this embodiment, lithium sulfate and the like can provide the 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 sucrose, glucose, and water-soluble starch.

[0043] In this embodiment, the organic carbon source can be converted into a carbon coating when subjected to high-temperature calcination, which can significantly enhance the electrical conductivity of the material, reduce the electron transfer resistance inside the electrode, and improve the charge and discharge efficiency of the battery. Organic carbon sources such as sucrose, glucose, and water-soluble starch are conducive to forming a uniform, continuous carbon layer due to their pyrolysis characteristics at high temperatures, and these organic carbon sources are more evenly distributed on the surface of the coprecipitate, which helps to form a carbon coating with uniform thickness and distribution. Using the above-mentioned organic carbon source during high-temperature calcination can improve the electrical conductivity of the composite lithium iron phosphate material, reduce the internal resistance of the battery, and increase the charge and discharge rate and coulomb efficiency. In addition, the uniform carbon coating formed by the decomposition of the above-mentioned organic carbon source helps to improve the cycle stability and thermal stability of the material and extend the battery life.

[0044] In some embodiments, a cobalt source, an iron source, and a lithium source are added to pure water, and stirred to dissolve 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 for controlling production costs and promoting green production. In addition, uniformly dispersing the metal ion sources in water first helps avoid uneven co-precipitation caused by local overconcentration, improves the purity and uniformity of the co-precipitate, and reduces agglomeration of the composite lithium iron phosphate material.

[0046] In some embodiments, the concentration of lithium in the mixed solution comprising 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 the lithium source is within the above range, which helps to promote the uniform embedding of lithium elements 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, carbonate and phosphate are first mixed to obtain a mixture, and then the mixture is added to a mixed solution containing a cobalt source, an iron source, and a lithium source.

[0049] In this embodiment, carbonate and phosphate 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, thereby improving the purity of the composite lithium iron phosphate material and the uniformity of the distribution of each component, and improving the electrochemical properties of the composite lithium iron phosphate material, such as capacity and energy density.

[0050] In some embodiments, the time taken to add the carbonate and phosphate salts is 30 min to 60 min.

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

[0052] In some embodiments, carbonate and phosphate are added to the mixed solution of the cobalt source, the iron source and the lithium source during stirring. Subsequently, an aqueous solution of an alkaline substance is added during stirring until the pH of the system is 3.5 to 5.0, and then the mixture is stirred and aged for 30 to 60 minutes at a stirring speed of 100 to 300 r / min to obtain a co-precipitate of lithium, cobalt and iron.

[0053] In this embodiment, the pH value is within the aforementioned range (3.5 to 5.0), which promotes the coprecipitation of cobalt, iron, and lithium ions, forming a coprecipitate with uniform particle size, composition, and regular morphology. Controlling the stirring speed within the aforementioned range and the stirring aging time helps evenly distribute the reactants, accelerates collisions and reactions between ions, and reduces material decomposition or structural damage caused by local overheating. Precisely controlling the pH value and stirring conditions helps optimize the material's microstructure, forming a coprecipitate with uniform composition and consistent structure, and improving the electrochemical properties of the composite lithium iron phosphate material, including capacity, energy density, voltage platform, and cycling stability.

[0054] In some embodiments, the aqueous solution of the alkaline substance is selected from one or more aqueous solutions of ammonium bicarbonate, sodium hydroxide, potassium hydroxide, and NH 3 ·H 2 O.

[0055] In this embodiment, during the preparation of the composite lithium iron phosphate material, an alkaline substance is added to adjust the pH of the reaction system to enable coprecipitation of cobalt, iron, and lithium ions. Commonly used alkaline substances, such as ammonium bicarbonate (NH₄HCO₃) and sodium hydroxide (NaOH), can increase the pH of the solution, promote the precipitation of metal ions, and form a lithium-cobalt-iron coprecipitate with uniform particle size and morphology.

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

[0057] In this embodiment, the above-mentioned filtration and washing can effectively remove impurities, improve the purity and consistency of the composite lithium iron phosphate material, help improve coulombic efficiency and cycle life, reduce internal resistance, and improve the electrochemical activity of the composite lithium iron phosphate material.

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

[0059] In this embodiment, during the high-temperature calcination process, the co-precipitate of lithium, cobalt and iron is converted into a composite lithium iron phosphate material composed of lithium cobalt iron composite oxide and lithium iron cobalt phosphate; carbonates are pyrolyzed to release CO2, forming a uniform porous structure in the composite lithium iron phosphate material; the organic carbon source is pyrolyzed to form a carbon conductive material, which is at least partially coated on the composite material of lithium cobalt iron composite oxide and lithium iron cobalt phosphate; at the same time, the high-temperature calcination promotes further reaction between metal ions and phosphate radicals to form a stable iron phosphate-based composite oxide structure. Under the above-mentioned calcination conditions, it is conducive to the formation of a high-purity, structurally stable composite lithium iron phosphate material, which helps to improve the electrochemical properties 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 process is 1° C. / min to 3° C. / min.

[0061] In this embodiment, heating at the above rate helps to uniformly heat the components inside the composite lithium iron phosphate material, thereby helping to improve the structural integrity of the components and reduce 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 coprecipitate 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 lithium-cobalt-iron coprecipitate is dispersed in the aqueous solution to form a slurry. This is conducive to full contact and uniform mixing of the coprecipitate and the organic carbon source in the solution, and helps the organic carbon source to evenly cover the surface of the coprecipitate to form a carbon layer of 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 organic carbon source solution contains 10% to 20% organic carbon source by weight.

[0065] In this embodiment, the concentration of the organic carbon source affects the thickness of the carbon coating layer. A concentration within the aforementioned range helps form a carbon coating layer with uniform thickness, excellent conductivity, and stability. This also allows the composite lithium iron phosphate material to maintain a high compaction density and energy density, improving the electrochemical performance of the battery.

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

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

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

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

[0070] In some embodiments, the drying process further includes 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 into the above-mentioned 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, and improves the electrochemical activity of the material. At the same time, a composite lithium iron phosphate material with a higher compaction density is obtained, which helps to improve the electrochemical properties of the composite lithium iron phosphate material, such as capacity, energy density, cycle stability, and charge and discharge rate.

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

[0073] In this embodiment, the low humidity and low oxygen content atmosphere is beneficial to reducing the oxidation and moisture 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.

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

[0075] In this embodiment, the temperature control and pulverization steps of the calcined material facilitate the subsequent processing and use of the composite lithium iron phosphate material. Iron removal and packaging in a constant temperature and humidity room help prevent moisture absorption and oxidation of the composite lithium iron phosphate material, 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 room is 24° C. to 26° C., and the humidity is 8% to 10%.

[0077] In this embodiment, the treatment is carried out in the above-mentioned constant temperature and humidity room, which is beneficial 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.

[0078] In a third aspect, an embodiment of the present application provides a positive electrode plate, comprising the composite lithium iron phosphate material as described above, or the composite lithium iron phosphate material prepared by the method for preparing the composite lithium iron phosphate material as described above.

[0079] In this embodiment, the positive electrode plate includes the above-mentioned composite lithium iron phosphate material, and thus has the advantages of high compaction density, high capacity, high energy density and high coulombic efficiency.

[0080] In a fourth aspect, an embodiment of the present application provides a secondary battery, including a positive electrode, and the electrode used for the positive electrode is the positive electrode electrode as described above.

[0081] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has the advantages of high compaction density, high capacity, high energy density and high coulombic efficiency.

[0082] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0084] Figure 1 This is an SEM image of the composite lithium iron phosphate material prepared in Example 1 of the present application;

[0085] Figure 2 This is a 0.1C first charge-discharge curve of the composite lithium iron phosphate material prepared in Example 1 of the present application;

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

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

[0088] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0089] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0090] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

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

[0092] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0093] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0094] In the existing technology, the theoretical energy density upper limit of lithium iron phosphate limits the improvement of the overall energy density of the battery. In addition, 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.

[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 and its preparation method, a positive electrode plate and a secondary battery, wherein, by using lithium cobalt iron composite oxide and olivine phosphate iron cobalt lithium phosphate to composite, a composite lithium iron phosphate material with high capacity, high voltage platform, high coulombic efficiency and high energy density is obtained, thereby improving the electrochemical properties of the positive electrode plate and the secondary battery, such as capacity, coulombic efficiency, energy density, etc.

[0096] In a first aspect, the present invention provides a composite lithium iron phosphate material comprising a phosphate / oxide composite material and carbon at least partially coated on the surface of the phosphate / oxide composite material. The 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. In terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1% to 1.9%.

[0097] Wherein, x can be 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 1.1, 1.12, 1.15, 1.18, 1.2, 1.22, 1.25, 1.28, 1.3, z can be 0.8, 0.82, 0.85, 0.88, 0.9, and u can be 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. The carbon content 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%, and 1.9%. Of course, it 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 iron cobalt lithium phosphate, wherein the lithium cobalt iron composite oxide has a high capacity, and it is combined with iron cobalt lithium phosphate to obtain a high-capacity composite lithium iron phosphate material; in addition, the composite lithium iron phosphate material adopts a lithium-rich design to improve the coulombic efficiency of the composite lithium iron phosphate material; furthermore, the introduction of cobalt elements into 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, and reduce the polarization of lithium ions during the charge and discharge process, thereby improving the coulombic efficiency. The composite lithium iron phosphate material based on the embodiment of the present application has a high voltage platform, high capacity and high coulombic efficiency, has excellent electrical properties, and can be used to prepare batteries with high capacity, high energy density and high coulombic efficiency.

[0099] Furthermore, 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 solutions of the embodiments of the present application, small-particle-size composite lithium iron phosphate materials are beneficial for increasing the specific surface area of ​​the material, increasing the contact area between 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-mentioned range, it helps to accelerate the diffusion rate 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 of the composite lithium iron phosphate material.

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

[0102] In the technical solution of the embodiment of the present application, the compaction density range is within the above-mentioned range, which is conducive to obtaining a composite lithium iron phosphate material with high crystallinity and few internal pores, thereby improving the compactness, 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 has significant advantages for applications such as portable electronic devices and electric vehicles.

[0103] Furthermore, 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 solutions of the embodiments of this application, tap density reflects the degree of compactness a material can achieve when subjected to vibration. The tap density of the composite lithium iron phosphate material is within the above range, which helps to obtain an electrode material with a high tap density during battery assembly, thereby improving the battery's energy density.

[0105] Furthermore, 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 solutions of the embodiments of this application, a higher specific surface area can increase the contact area between the composite lithium iron phosphate material and the electrolyte, promote the rapid deintercalation and intercalation of lithium ions, accelerate the electrochemical reaction rate, and thereby 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, achieving a higher compaction density of the composite lithium iron phosphate material while also helping to improve the charge and discharge rate and energy density of the battery.

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

[0108] In the technical solution of the embodiment of the present application, lower powder internal resistance means that the structural integrity of the composite lithium iron phosphate material is higher, which helps the insertion and extraction of lithium ions, and thus helps to improve the charge and discharge capacity of the composite lithium iron phosphate material.

[0109] Furthermore, in some embodiments, the content of magnetic substance in the composite lithium iron phosphate material is less than 0.15 ppm.

[0110] In the technical solution of the embodiment of the present application, a lower content of magnetic material means that the purity of the composite lithium iron phosphate material is higher, which is beneficial to improving 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, an embodiment of the present application provides a method for preparing a composite lithium iron phosphate material, comprising the following steps:

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

[0113] adding carbonate and phosphate to the mixed solution, and adjusting the pH value of the mixed solution to 3.5-5.0, to obtain a lithium-cobalt-iron coprecipitate through reaction;

[0114] The lithium cobalt iron coprecipitate and the organic carbon source are mixed and slurried to obtain a slurry, and then the slurry is dried and calcined to obtain a composite lithium iron phosphate material;

[0115] 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. The 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. In terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1% to 1.9%.

[0116] In the technical solution of the embodiment of the present application, carbonate and phosphate are first used as precipitants to co-precipitate iron, cobalt and lithium to obtain a co-precipitate of lithium cobalt iron, and then the co-precipitate of lithium cobalt iron is mixed with a carbon source, and slurried, dried and calcined, wherein phosphate forms lithium iron cobalt phosphate with lithium, cobalt and iron, and the carbonate precipitate decomposes at high temperature to form a lithium cobalt iron composite oxide, thereby obtaining a composite material of lithium iron cobalt phosphate, lithium cobalt iron composite oxide and carbon. It can be understood that in the precursor preparation process, by adding carbonate and phosphate and controlling the pH, uniform coprecipitation of lithium, cobalt, iron and phosphorus can be achieved, so that the various constituent elements can be fully mixed and uniform. Therefore, during high-temperature calcination, the migration distance between ions can be effectively reduced, so that the fusion between particles is more sufficient. At the same temperature, the particles grow more densely, thereby effectively improving the compaction density of the material.

[0117] Furthermore, lithium cobalt iron composite oxide has a high capacity. Combining it with lithium iron cobalt phosphate can increase the capacity of the composite lithium iron phosphate material. At the same time, the combination of the two can increase the compaction density of the lithium iron phosphate material. In addition, due to the introduction of cobalt elements, the voltage platform of the composite lithium iron phosphate material can also be improved.

[0118] Furthermore, the lithium-cobalt-iron coprecipitate is mixed with an organic carbon source. The addition of the organic carbon source forms a carbon coating during the subsequent high-temperature calcination process, which can improve the conductivity and stability of the material, improve the charge and discharge efficiency of the battery, increase the cycle life of the battery, and reduce polarization during the charge and discharge process, thereby improving the coulombic efficiency. Furthermore, calcination helps the carbonate decompose to produce carbon dioxide gas, forming a porous structure in the composite lithium iron phosphate material, and the phosphate radicals and metal ions interact at high temperatures to form a stable phosphate-based composite material. The protective environment of the inert gas and the organic carbon source can prevent the oxidation of components such as divalent iron and divalent cobalt at high temperatures, which helps to obtain a high-purity composite lithium iron phosphate material.

[0119] Therefore, in this application, a 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, and the carbon content in the composite lithium iron phosphate material is 1% to 1.9% by weight), 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. Compared with traditional lithium iron phosphate materials, the energy density of batteries prepared with the composite lithium iron phosphate material can be increased by more than 10%.

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

[0121] In this embodiment, a higher lithium content helps improve the material's Coulombic efficiency, a higher iron content helps enhance the material's structural stability and electrochemical activity, and the introduction of cobalt can increase the material's voltage platform. By controlling the raw material dosage within the aforementioned range, subsequent coprecipitation and subsequent high-temperature calcination facilitate the production of a composite lithium iron phosphate material with a high voltage platform, high capacity, high Coulombic efficiency, and high energy density.

[0122] Specifically, the molar ratio of the cobalt element in the cobalt source, the iron element in the iron source, and the lithium element 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] Furthermore, in some embodiments, the molar ratio of carbonate in carbonate, phosphate in phosphate, and iron 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 elements within the range of (0.15-0.25): (0.8-0.9): (0.8-0.9), it is beneficial to the subsequent uniform co-precipitation of lithium, cobalt, iron and phosphorus, so that a composite lithium iron phosphate material with high compaction density, high voltage platform, high capacity and high energy density can be obtained by subsequent high-temperature calcination.

[0125] Specifically, the molar ratio of carbonate in carbonate, phosphate in phosphate, and iron 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] Furthermore, 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 lithium-cobalt-iron co-precipitate and the organic carbon source being 100%.

[0127] In this embodiment, the carbon content is within the above range, which is beneficial to improving the conductivity and structural stability of the composite lithium iron phosphate material, improving the charge and discharge efficiency, cycle life and coulombic efficiency of the battery, while maintaining a relatively high compaction 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] Furthermore, in some embodiments, during the reaction to obtain the lithium-cobalt-iron coprecipitate, the reaction temperature is controlled to be 35° C. to 55° C.

[0130] In the technical solutions of the embodiments of the present application, the reaction temperature affects the coprecipitation reaction rate and the product crystal structure. Within the above reaction temperature range, uniform coprecipitation of cobalt, iron, and lithium ions is promoted, which helps form uniform precursor particles and reduces the production of byproducts. This allows for the subsequent preparation of a high-purity, structurally stable, and highly conductive composite lithium iron phosphate material, which is beneficial for improving the energy density of the composite lithium iron phosphate material.

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

[0132] In the technical solution of the embodiment of the present application, 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 providing carbonate ions and controlling the pH value, promoting uniform co-precipitation, so as to subsequently prepare a composite lithium iron phosphate material with uniform particle size, high purity and stable structure.

[0133] Furthermore, in some embodiments, the phosphate 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.

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

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

[0136] In the technical solution of the embodiment of the present application, cobalt sulfate, cobalt nitrate and cobalt chloride can provide necessary divalent cobalt ions for the coprecipitation reaction to generate lithium cobalt iron coprecipitate and calcination to generate lithium cobalt iron composite oxide and lithium iron cobalt phosphate.

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

[0138] In the technical solution of the embodiment of the present application, ferrous sulfate, ferric nitrate, ferric chloride, etc. can provide divalent iron ions or trivalent iron ions for the coprecipitation reaction to generate a lithium cobalt iron coprecipitate, and subsequently calcined in an organic carbon source and inert gas atmosphere to generate lithium cobalt iron composite oxide and lithium iron cobalt phosphate.

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

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

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

[0142] In the technical solution of the embodiment of the present application, the organic carbon source can be converted into a carbon coating when subjected to high-temperature calcination, which can significantly enhance the electrical conductivity of the material, reduce the electron transfer resistance inside the electrode, and improve the charge and discharge efficiency of the battery. Organic carbon sources such as sucrose, glucose, and water-soluble starch are conducive to forming a uniform, continuous carbon layer due to their pyrolysis characteristics at high temperatures, and these organic carbon sources are more evenly distributed on the surface of the coprecipitate, which helps to form a carbon coating with uniform thickness and distribution. When the above-mentioned organic carbon source is used during high-temperature calcination, the electrical conductivity of the composite lithium iron phosphate material can be improved, the internal resistance of the battery can be reduced, and the charge and discharge rate and coulomb efficiency can be increased. In addition, the uniform carbon coating formed by the decomposition of the above-mentioned organic carbon source helps to improve the cycle stability and thermal stability of the material and extend the battery life.

[0143] Furthermore, in some embodiments, the cobalt source, the iron source, and the lithium source are added into 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 solution of the embodiments of this application, water is used as the solvent, which helps control production costs and promotes green production. In addition, uniformly dispersing the metal ion sources in water helps avoid uneven co-precipitation caused by local overconcentration, improves the purity and uniformity of the co-precipitate, and reduces agglomeration of the composite lithium iron phosphate material.

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

[0146] In the technical solution of the embodiment of the present application, the concentration of the lithium source is within the above-mentioned range, which helps to promote the uniform embedding of lithium elements in the composite lithium iron phosphate material, forming a stable lithium-cobalt-iron composite structure with good conductivity, improving the capacity and stability of the composite lithium iron phosphate material, and improving the energy density of the composite lithium iron phosphate material in the embodiment of the present application when applied to secondary batteries.

[0147] Furthermore, in some embodiments, carbonate and phosphate are first mixed to obtain a mixture, and then the mixture is added to a mixed solution containing a cobalt source, an iron source, and a lithium source.

[0148] In the technical solution of the embodiment of the present application, carbonate and phosphate 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, thereby improving the purity of the composite lithium iron phosphate material and the uniformity of the distribution of each component, and improving the electrochemical properties such as the capacity of the composite lithium iron phosphate material.

[0149] Furthermore, in some embodiments, the time taken to add the carbonate and phosphate is 30 min to 60 min.

[0150] In the technical solution of the embodiment of the present application, by gradually adding carbonates and phosphates and controlling the addition time, the morphology and structure of the co-precipitate can be optimized, the purity of the prepared composite lithium iron phosphate material and the uniformity of the distribution of each component can be improved, and the electrochemical properties such as the capacity of the composite lithium iron phosphate material can be improved.

[0151] Furthermore, in some embodiments, carbonate and phosphate are added to the mixed solution of the cobalt source, iron source and lithium source during stirring, and then an aqueous solution of an alkaline substance is added during stirring until the pH of the system is 3.5 to 5.0, and then the mixture is stirred and aged for 30 to 60 minutes at a stirring speed of 100 to 300 r / min to obtain a co-precipitate of lithium, cobalt and iron.

[0152] In the technical solutions of the embodiments of this application, the pH value within the aforementioned range (3.5 to 5.0) promotes the coprecipitation of cobalt, iron, and lithium ions, forming a coprecipitate with uniform particle size and composition and a regular morphology. Controlling the stirring speed within the aforementioned range and the stirring aging time helps evenly distribute the reactants, accelerates collisions and reactions between ions, and reduces material decomposition or structural damage caused by local overheating. Precisely controlling the pH value and stirring conditions helps optimize the material's microstructure, forming a coprecipitate with uniform composition and consistent structure, and improving the electrochemical properties of the composite lithium iron phosphate material, including capacity, energy density, voltage platform, and cycling stability.

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

[0154] In the technical solution of the embodiments of this application, during the preparation of the composite lithium iron phosphate material, an alkaline substance is added to adjust the pH of the reaction system to enable the coprecipitation of cobalt ions, iron ions, and lithium ions. The aforementioned ammonium bicarbonate and sodium hydroxide are both commonly used alkaline substances that can increase the pH of the solution, promote the precipitation of metal ions, and form a lithium-cobalt-iron coprecipitate with uniform particle size and morphology.

[0155] Furthermore, in some embodiments, the calcination temperature is 750° C. to 850° C., and the calcination time is 6 h to 12 h.

[0156] In the technical solution of the embodiment of the present application, during the high-temperature calcination process, the co-precipitate of lithium, cobalt and iron is converted into a composite material of lithium, cobalt and iron composite oxide and lithium iron cobalt phosphate; carbonates are pyrolyzed to release CO2, forming a uniform porous structure in the composite lithium iron phosphate material; the organic carbon source is pyrolyzed to form a carbon conductive material, which is at least partially coated on the composite material of lithium, cobalt and iron composite oxide and lithium iron cobalt phosphate; at the same time, the high-temperature calcination promotes further reaction between metal ions and phosphate radicals to form a stable iron phosphate-based composite oxide structure. Under the above calcination conditions, it is conducive to the formation of a high-purity, structurally stable composite lithium iron phosphate material, which helps to improve the electrochemical properties of the composite lithium iron phosphate material, such as capacity, cycle stability and voltage platform.

[0157] Furthermore, in some embodiments, the heating rate of the calcination process is 1° C. / min to 3° C. / min.

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

[0159] Furthermore, 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 coprecipitate is dispersed in the organic carbon source solution to obtain a slurry.

[0160] In the technical solution 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 lithium cobalt iron coprecipitate is dispersed in the aqueous solution to form a slurry, which is conducive to full contact and uniform mixing of the coprecipitate and the organic carbon source in the solution, and helps to uniformly cover the organic carbon source on the surface of the coprecipitate to form a carbon layer of uniform thickness, thereby improving the conductivity, lithium ion diffusion capacity and overall structural stability of the composite lithium iron phosphate material.

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

[0162] In the technical solutions of the embodiments of this application, the concentration of the organic carbon source affects the thickness of the carbon coating layer. A concentration within the above range helps form a carbon coating layer with uniform thickness, excellent conductivity, and stability. This also maintains a high compaction density of the composite lithium iron phosphate material, improving battery performance.

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

[0164] In the technical solution of the embodiment of the present application, spray drying can achieve rapid drying of the material, improve the uniformity and fluidity of the formed particles and reduce agglomeration, improve the dispersibility and compactness of the composite lithium iron phosphate material, and help improve the charge and discharge efficiency, energy density and cycle stability of the composite lithium iron phosphate material.

[0165] Furthermore, in some embodiments, the moisture content of the dried material obtained by drying the slurry is less than 2%.

[0166] In the technical solution of the embodiments of this application, the purpose of slurry drying is to remove moisture from the slurry and reduce the impact of water evaporation during the subsequent high-temperature calcination process on the structure of the composite lithium iron phosphate material. Controlling the moisture content of the dried material to below 2% helps reduce material shrinkage and pore structure damage caused by water evaporation, improves 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.

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

[0168] Crushing the dried material into the above-mentioned 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 simultaneously obtains a higher compaction density of the composite lithium iron phosphate material, which helps to improve the electrochemical properties of the composite lithium iron phosphate material, such as capacity, cycle stability, and charge and discharge rate.

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

[0170] In the technical solution of the embodiment of the present application, the above-mentioned low humidity and low oxygen content atmosphere is beneficial to reducing the oxidation and moisture 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 properties and cycle stability of the composite lithium iron phosphate material.

[0171] Furthermore, in some embodiments, after the calcination is completed, the calcined material is cooled to 100° C. and then discharged. The discharged material is crushed, and then iron is removed and packaged in a constant temperature and humidity room to obtain a composite lithium iron phosphate material.

[0172] In the technical solutions of the embodiments of this application, the temperature control and pulverization steps of the calcined material facilitate the subsequent processing and use of the composite lithium iron phosphate material. Iron removal and packaging in a constant temperature and humidity chamber help prevent moisture absorption and oxidation of the composite lithium iron phosphate material, maintaining its chemical purity and performance stability.

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

[0174] In the technical solution of the embodiment of the present application, processing in the above-mentioned constant temperature and humidity room is beneficial 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] Furthermore, in some embodiments, before the step of mixing the lithium-cobalt-iron coprecipitate and the organic carbon source, the step further includes filtering and washing the lithium-cobalt-iron coprecipitate until the conductivity of the washing water after washing is ≤200 μS / cm.

[0176] In the technical solution of the embodiment of the present application, the above-mentioned filtration and washing can effectively remove impurities, improve the purity and consistency of the composite lithium iron phosphate material, help improve the coulombic efficiency and cycle life, reduce internal resistance, and improve the electrochemical activity of the composite lithium iron phosphate material.

[0177] In a third aspect, an embodiment of the present application provides a positive electrode plate, comprising the composite lithium iron phosphate material as described above, or the composite lithium iron phosphate material prepared by the method for preparing the composite lithium iron phosphate material as described above.

[0178] In a fourth aspect, an embodiment of the present application provides a secondary battery, including a positive electrode, wherein the electrode used for the positive electrode is the positive electrode electrode as described above.

[0179] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0180] 1. Preparation method

[0181] Example 1

[0182] A method for preparing a composite lithium iron phosphate material comprises the following steps:

[0183] Step 1: Add cobalt sulfate, ferrous sulfate, and lithium sulfate into pure water, stir and dissolve to obtain a mixed solution containing a cobalt source, an iron source, and a lithium source; add a mixture of sodium carbonate and sodium phosphate to the mixed solution containing the cobalt source, the iron source, and the lithium source during stirring; then add ammonia water during stirring until the pH of the system is 4.5; then stir and age at 40°C for 40 minutes; filter the system after the reaction to obtain a filtrate; wash the filtrate until the conductivity of the washing water is ≤200μS / cm, and obtain a co-precipitate of lithium cobalt iron.

[0184] The concentration of lithium in the mixed solution containing a cobalt source, an iron source and a lithium source is 2 mol / L; the molar ratio of cobalt in the cobalt source, iron in ferrous sulfate and lithium in lithium sulfate is 0.15:0.85:1.31; sodium carbonate and sodium phosphate are added in a molar ratio of carbonate, phosphate and iron in the iron source of 0.2:0.85:0.85, and the time used to add the mixture of sodium carbonate and sodium phosphate is 50 minutes; the stirring speed during the stirring and aging process is 200 r / min.

[0185] Step 2: Disperse the organic carbon source in water to prepare an organic carbon source solution, then add the organic carbon source solution to the lithium cobalt iron coprecipitate, stir to obtain a slurry, and use spray drying to dry the slurry to a moisture content of 1.8% (by weight percentage) and then stop drying to obtain a dried material, then crush the dried material to a D50 particle size of 1.4 μm, and then calcine under nitrogen protection. After the calcination is completed, the calcined material is cooled to 100°C and discharged, and the discharged material is crushed to a D50 particle size of 1.2 μm, and then removed from the material in a constant temperature and humidity room (temperature of 25°C, humidity of 9%) and packaged to obtain a composite lithium iron phosphate material.

[0186] Among them, the organic carbon source is sucrose; in terms of weight percentage, the content of the organic carbon source in the organic carbon source solution is 15%; based on the sum of the weight of the lithium cobalt iron co-precipitate and the organic carbon source as 100%, the weight of the added organic carbon source is 8.5%; the calcination temperature is 810°C, the holding time is 9h, the heating rate is 2.1°C / 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 results of the composite lithium iron phosphate material prepared in Example 1 are as follows: Figure 1 As shown, in the composite lithium iron phosphate material, the primary particles have regular morphology, uniform particle size and are about 200nm. According to the particle size detection data, the D50 particle size of the secondary particles is 1.2μm; the 0.1C first charge and discharge curve of the composite lithium iron phosphate material prepared in Example 1 is shown in FIG. Figure 2As shown, the 0.1C first charge capacity reaches 186.5mAh / g, the 0.1C discharge capacity reaches 178.6mAh / g, and the first coulombic efficiency reaches 95.8%. The 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, in terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1.42%.

[0188] Example 2

[0189] A method for preparing a composite lithium iron phosphate material comprises the following steps:

[0190] Step 1: Add cobalt chloride, ferrous chloride, and lithium chloride into pure water, stir and dissolve to obtain a mixed solution containing a cobalt source, an iron source, and a lithium source, add a mixture of ammonium carbonate and ammonium phosphate to the mixed solution containing the cobalt source, the iron source, and the lithium source during stirring, and then add an aqueous solution of sodium hydroxide during stirring until the pH of the system is 3.5, and then stir and age at 35°C for 60 minutes, filter the system after the reaction to obtain a filtrate, wash the filtrate until the conductivity of the washing water is ≤200μS / cm, and obtain a co-precipitate of lithium cobalt iron.

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

[0192] Step 2: Disperse the organic carbon source in water to prepare an organic carbon source solution, then add the organic carbon source solution to the lithium cobalt iron coprecipitate, stir to obtain a slurry, and use spray drying to dry the slurry to a moisture content of 2% (by weight percentage) and then stop drying to obtain a dried material, then crush the dried material to a D50 particle size of 3 μm, and then calcine under nitrogen protection. After the calcination is completed, the calcined material is cooled to 100°C and discharged, and the discharged material is crushed to a D50 particle size of 1.4 μm, and then removed from the material in a constant temperature and humidity room (temperature of 25°C, humidity of 9%) and packaged to obtain a composite lithium iron phosphate material.

[0193] Among them, the organic carbon source is glucose; in terms of weight percentage, the content of the organic carbon source in the organic carbon source solution is 20%; based on the sum of the weight of the lithium cobalt iron co-precipitate and the organic carbon source as 100%, the weight of the added organic carbon source is 10.1%; the calcination temperature is 750°C, the holding time is 12h, the heating rate is 3°C / 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.

[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 are about 200nm. According to the particle size detection data, the D50 particle size of the secondary particles is 1.4μm; 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, in terms of 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 comprises the following steps:

[0197] Step 1: Add cobalt acetate, ferrous acetate, and lithium acetate to pure water, stir and dissolve to obtain a mixed solution containing a cobalt source, an iron source, and a lithium source, add a mixture of potassium carbonate and potassium phosphate to the mixed solution containing the cobalt source, the iron source, and the lithium source during stirring, and then add potassium hydroxide aqueous solution to the system during stirring until the pH value is 5.0, and then stir and age at 55°C for 30 minutes, filter the system after the reaction to obtain a filtrate, wash the filtrate until the conductivity of the washing water is ≤200μS / cm, and obtain a co-precipitate of lithium cobalt iron.

[0198] The concentration of lithium in the mixed solution containing a cobalt source, an iron source and a lithium source 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, and the time used to add the mixture of potassium carbonate and potassium phosphate is 60 minutes; the stirring speed during the stirring and aging process is 300 r / min.

[0199] Step 2: Disperse the organic carbon source in water to prepare an organic carbon source solution, then add the organic carbon source solution to the lithium cobalt iron coprecipitate, stir to obtain a slurry, and use spray drying to dry the slurry to a moisture content of 2% (by weight percentage) and then stop drying to obtain a dried material, then crush the dried material to a D50 particle size of 1.4 μm, and then calcine under nitrogen protection. After the calcination is completed, the calcined material is cooled to 100°C and discharged, and the discharged material is crushed to a D50 particle size of 1.5 μm, and then removed from the material in a constant temperature and humidity room (temperature of 25°C, humidity of 9%) and packaged to obtain a composite lithium iron phosphate material.

[0200] Among them, the organic carbon source is water-soluble starch; in terms of weight percentage, the content of organic carbon source in the organic carbon source solution is 20%; based on the sum of the weight of the lithium cobalt iron co-precipitate and the organic carbon source as 100%, the weight of the added organic carbon source is 7.1%; the calcination temperature is 850°C, the holding time is 12h, the heating rate is 3°C / 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.

[0201] The SEM results of the composite lithium iron phosphate material prepared in Example 3 are as follows: Figure 4 As shown, in the composite lithium iron phosphate material, the primary particles have regular morphology, uniform particle size and are about 1 μm. According to the particle size detection data, the D50 particle size of the secondary particles is 1.5 μm; the 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, calculated as a percentage by weight, the carbon content in the composite lithium iron phosphate material is 1.0%.

[0202] Example 4

[0203] The only difference from Example 1 is that the stirring aging is carried out at 55°C.

[0204] Example 5

[0205] The only difference from Example 1 is that the stirring aging is carried out at 65°C.

[0206] Example 6

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

[0208] Example 7

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

[0210] Example 8

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

[0212] Example 9

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

[0214] Comparative Example 1

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

[0216] Comparative Example 2

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

[0218] Comparative Example 3

[0219] The only difference from Example 1 is that the weight of the added organic carbon source is 18.6%, based on the sum of the weight of the lithium-cobalt-iron coprecipitate and the organic carbon source being 100%.

[0220] Comparative Example 4

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

[0222] 2. Test Method

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

[0224] 1) Morphology characterization: Scanning electron microscopy (SEM) was used to characterize the material morphology.

[0225] 2) Elemental analysis: The elemental composition of the materials was analyzed using inductively coupled plasma spectrometry. The results are shown in Table 1.

[0226] 3) pH test: Refer to GB / T 5211.6-2020 for specific details. The pH test results of the materials are shown in Table 2.

[0227] 4) Free lithium content test: Refer to the 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 the standard GB / T 45324-2025 for details. The powder internal resistance test results of the material are shown in Table 2.

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

[0230] 7) Specific surface area test: For specific reference, refer to the standard GB / T9587-2017. The specific surface area test results of the materials are shown in Table 2.

[0231] 8) Tap density test: The tap density of the material is tested using a tap density meter. The tap density test results of the material are shown in Table 2.

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

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

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

[0235] 2. Property test of secondary batteries

[0236] 1) Battery assembly

[0237] The composite lithium iron phosphate material prepared in the example and the composite lithium iron phosphate material prepared in the comparative example were respectively mixed with a conductive agent SP and a PVDF binder in a mass ratio of 92:4:4, and after homogenization, they were coated on an aluminum foil sheet, dried at 100°C, and rolled with a double-roller machine. Then, a positive electrode sheet with a diameter of 14 mm was obtained using a punching machine. The weight was weighed and the mass of the aluminum foil was deducted to obtain the mass of the active material.

[0238] After drying the positive electrode sheet, the lithium sheet is used as the negative electrode sheet and assembled into a CR2032 button half-cell in the UNlab inert gas glove box of Braun Company in Germany in the order of negative electrode shell, lithium sheet, electrolyte, diaphragm, electrolyte, positive electrode sheet, gasket, shrapnel, and positive electrode shell.

[0239] 2) Charge and discharge performance test

[0240] Electrochemical performance testing of CR2032 button-type half-cells was conducted using a Wuhan Blue Electric CT2001A battery testing system. The voltage range was 2V to 4.55V, and the test temperature was 25°C. Table 3 shows the results of the 0.1C initial charge capacity, 0.1C initial discharge capacity, initial coulombic efficiency at 0.1C, initial discharge capacity at 1C, 3.75-4.55V charge capacity ratio, voltage plateau, energy density, and capacity retention after 500 cycles at 1C.

[0241] 3. Analysis of test results of various embodiments and comparative examples

[0242] Table 1

[0243]

[0244] Table 2

[0245]

[0246] Table 3

[0247]

[0248] It can be seen from the data in Table 1, Table 2 and Table 3 that the composite lithium iron phosphate material Li prepared in Examples 1 to 9 of the present application 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 carbon content in the composite lithium iron phosphate material is 1% to 1.9% by weight), has a high compaction density, a high voltage platform, a high charge and discharge capacity, and a high energy density, and has excellent electrical properties.

[0249] From the comparison of Example 1, Example 4 and Example 5, it can be seen that compared with Example 5, Example 1 and Example 4 carry out the precipitation reaction at 35°C to 55°C, which is more conducive to the uniform co-precipitation of cobalt ions, iron ions and lithium ions, so as to subsequently prepare a high-purity, structurally stable and conductive composite lithium iron phosphate material. The composite lithium iron phosphate material has a higher voltage platform, charge and discharge capacity, energy density and cycle stability.

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

[0251] From the comparison of Example 1, Example 8 and Example 9, it can be seen that compared with Example 9, Example 1 and Example 8 are calcined at 750°C to 850°C, which is conducive to the formation of high-purity, structurally stable composite lithium iron phosphate materials. The prepared composite lithium iron phosphate materials have higher charge and discharge capacity, voltage platform, energy density and cycle stability.

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

[0253] Compared with Example 1, in Comparative Example 2, a lower amount of phosphate was introduced during the coprecipitation stage, and the obtained material was 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 both reduced.

[0255] Compared with Example 1, in Comparative Example 4, no cobalt doping was performed, and the prepared composite lithium iron phosphate material was Li 1.2 Fe 1.0 O 0.4 (PO4) 0.85 / C, its charge and discharge capacity (especially the proportion of 3.75-4.55V charging 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 merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in 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 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. In terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1% to 1.9%.

2. The composite lithium iron phosphate material according to claim 1, characterized in that 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 according to claim 1, characterized in that: The compaction 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 steps include: Providing a mixed solution containing a cobalt source, an iron source, and a lithium source; adding carbonate and phosphate to the mixed solution, and adjusting the pH value of the mixed solution to 3.5-5.0, to obtain a lithium-cobalt-iron coprecipitate through reaction; The lithium-cobalt-iron coprecipitate and an organic carbon source are mixed and slurried to obtain a slurry, 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 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. In terms of weight percentage, the carbon content in the composite lithium iron phosphate material is 1% to 1.9%.

5. The method for preparing the composite lithium iron phosphate material according to claim 4, characterized in that: The molar ratio of the cobalt element in the cobalt source, the iron element in the iron source, and the lithium element in the lithium source is (0.1-0.2): (0.8-0.9): (1.2-1.4); and / or, The molar ratio of the carbonate in the carbonate, the phosphate in the phosphate, and the iron in the iron source is (0.15-0.25): (0.8-0.9): (0.8-0.9); and / or, Based on the sum of the weight of the lithium-cobalt-iron coprecipitate and the organic carbon source being 100%, the amount of the organic carbon source added is 7% to 10.5%.

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

7. The method for preparing the composite lithium iron phosphate material according to claim 4, characterized in that: The moisture content of the dried material obtained by the drying process of the slurry is less than 2%; and / or, The drying process further includes a pulverization process, and the D50 particle size of the dried material after the pulverization process 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 hours to 12 hours.

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 coprecipitate with the organic carbon source, the method further comprises: The lithium-cobalt-iron coprecipitate is filtered and washed until the conductivity of the washing water is ≤200 μS / cm.

9. A positive electrode plate, characterized in that: 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 method for preparing the composite lithium iron phosphate material according to any one of claims 4 to 8.

10. A secondary battery comprising a positive electrode, characterized in that: The electrode sheet used for the positive electrode is the positive electrode sheet according to claim 9.

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