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

Abstract

The invention provides a lithium iron phosphate material and a preparation method thereof, a positive pole piece and a secondary battery, and belongs to the field of secondary batteries, the average particle size of primary particles of the lithium iron phosphate material is 0.23-0.26 [mu] m, in the primary particles of the lithium iron phosphate material, the number of the primary particles with the particle size larger than 0 nm and smaller than 200 nm accounts for 47%-51%, and the number of the secondary particles with the particle size larger than 0 nm and smaller than 200 nm accounts for 45%-55%. The quantity of the lithium iron phosphate with the particle size of more than or equal to 200nm and less than or equal to 2mu m accounts for 49%-53%, and the compaction density of the lithium iron phosphate material is 2.61 g / mL-2. 67g / mL. The lithium iron phosphate material provided by the invention is relatively small in average particle size, the stability of the electrochemical performance of the lithium iron phosphate material is favorably maintained, and the compaction density of the lithium iron phosphate material is improved by utilizing relatively large particles in a proper proportion, so that the volume energy density of the lithium iron phosphate material is improved; the electrochemical performance of the lithium iron phosphate material in the aspects of discharge performance, rate capability and the like is improved by utilizing relatively small particles in a proper proportion; in addition, the maximum size of primary particles of the lithium iron phosphate material is smaller than or equal to 2 microns, and the polarization problem of the lithium iron phosphate material is solved.

Description

Technical Field

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

[0002] Lithium iron phosphate is widely used in new energy applications, such as new energy vehicles. Existing lithium iron phosphate materials are typically prepared using a high-temperature carbothermal reduction method. Due to process limitations, the ratio of large to small particles in lithium iron phosphate materials prepared using this method is difficult to control. A higher number of large particles increases the compaction density, but this can lead to electrochemical performance issues such as increased polarization, reduced discharge efficiency, and poor rate performance. A higher number of small particles makes it difficult to increase the compaction density, resulting in a difficulty in improving the volumetric energy density of the lithium iron phosphate material.

[0003] Therefore, there is an urgent need to provide a lithium iron phosphate material and its preparation method, a positive electrode plate and a secondary battery to solve the problem that it is difficult to optimize the ratio of large particles and small particles of lithium iron phosphate materials, resulting in difficulty in balancing the improvement of electrochemical properties such as discharge efficiency and rate performance of lithium iron phosphate materials and the increase of volume energy density. Summary of the Invention

[0004] In view of the technical problems existing in the background technology, the present application provides a lithium iron phosphate material and its preparation method, a positive electrode plate and a secondary battery, aiming to solve the technical problem that it is difficult to optimize the ratio of large particles and small particles of lithium iron phosphate materials, resulting in difficulty in balancing the improvement of the electrochemical properties such as discharge efficiency and rate performance of lithium iron phosphate materials and the improvement of volume energy density.

[0005] In a first aspect, an embodiment of the present application provides a lithium iron phosphate material, wherein the average particle size of the primary particles of the lithium iron phosphate material is 0.23 μm to 0.26 μm, and among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, and the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm accounts for 44.8% to 58%; The compaction density of the lithium iron phosphate material is 2.61 g / mL to 2.67 g / mL.

[0006] In the technical solution of the embodiment of the present application, by controlling the average particle size of the lithium iron phosphate material to be smaller, it is beneficial to maintain the stability of the electrochemical properties of the lithium iron phosphate material. By controlling the ratio of smaller particles with a particle size greater than 0 nm and less than 200 nm to larger particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm in the primary particles of the lithium iron phosphate material, an appropriate proportion of larger particles is used to increase the compaction density of the lithium iron phosphate material, thereby increasing the volume energy density of the lithium iron phosphate material, while an appropriate proportion of smaller particles is used to improve the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.; in addition, the maximum size of the primary particles of the lithium iron phosphate material is less than or equal to 2 μm, which improves the polarization problem of the lithium iron phosphate material.

[0007] In some embodiments, the lithium iron phosphate material includes a lithium iron phosphate core and a carbon coating layer coated on the surface of the lithium iron phosphate core. The general formula of the lithium iron phosphate core is: LiFe x Ti y (PO4), 0.964≤x≤1.0, 0.0≤y≤0.036; The mass fraction of the carbon coating layer in the lithium iron phosphate material is 1.16% to 1.34%.

[0008] In the above embodiment, the lithium iron phosphate core having the above general formula has a suitable molar ratio of lithium, iron and phosphorus elements, and provides an appropriate amount of lithium ions for embedding and de-embedding while maintaining structural stability, thereby improving the charging and discharging performance of the lithium iron phosphate material. By controlling the mass fraction of the lithium iron phosphate material in the carbon coating layer, it is beneficial to form a uniform carbon coating layer with appropriate thickness on the surface of the lithium iron phosphate core, reducing the electrons of the lithium iron phosphate material, providing a smoother path for the embedding and de-embedding of lithium ions, improving the electron transmission efficiency and lithium ion diffusion efficiency, and protecting the lithium iron phosphate core, thereby improving the charging performance, discharge performance, rate performance and cycle life of the lithium iron phosphate material.

[0009] In some embodiments, the primary particles of the lithium iron phosphate material include first particles and second particles, the particle size of the first particles is greater than 0.0 nm and less than 200 nm, the average particle size of the first particles is 23 nm~26 nm, the particle size of the second particles is 200 nm~2000 nm, and the average particle size of the second particles is 340 nm~410 nm.

[0010] In the above embodiment, by combining the first particles with smaller particle size with the first particles with larger particle size, the particle size distribution of the second particles with larger particle size is more concentrated, which is beneficial to improving the compaction density of the lithium iron phosphate material while maintaining the discharge performance and rate performance. The particle size distribution of the first particles with smaller particle size is more concentrated, which is beneficial to improving the electrochemical properties of the lithium iron phosphate material such as the discharge performance and rate performance.

[0011] In some embodiments, among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42%~55.2%, the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 600 nm accounts for 34%~50.5%, the number of primary particles with a particle size greater than 600 nm and less than or equal to 1 μm accounts for 2%~11%, and the number of primary particles with a particle size greater than 1 μm and less than or equal to 2 μm accounts for 0.3%~8.5%.

[0012] In the above embodiment, by controlling the particle size distribution of the primary particles of the lithium iron phosphate material and matching lithium iron phosphate materials of different particle sizes, it is beneficial to further improve the compaction density of the lithium iron phosphate material and thereby increase the volume energy density while improving the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0013] In a second aspect, an embodiment of the present application provides a method for preparing a lithium iron phosphate material, comprising: A first iron source is mixed with a first lithium source, a first carbon source, and a first solvent to obtain a first slurry; The first slurry is subjected to a first grinding process to obtain a second slurry; The second slurry is mixed with a water retaining agent to obtain a third slurry; The third slurry is subjected to a first solid-liquid separation process to obtain a first dry material; The first dried material is subjected to a first calcination treatment to obtain a first calcined material; The first calcined material is mixed with a second iron source, a second lithium source, a second carbon source and a second solvent to obtain a fourth slurry; The fourth slurry is subjected to a second solid-liquid separation process to obtain a second dry material; The second dried material is subjected to a second calcination process to obtain a lithium iron phosphate material; Wherein, the first iron source and the second iron source are independently selected from ferric phosphate dihydrate materials, and the mass fraction of carbon element in the calcined material is 0.12% to 0.38%; The average particle size of the primary particles of the lithium iron phosphate material is 0.23 μm to 0.26 μm. Among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, and the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm accounts for 44.8% to 58%; The compaction density of the lithium iron phosphate material is 2.61 g / mL to 2.67 g / mL.

[0014] In the technical solution of the embodiment of the present application, by using a water-retaining agent in the synthesis of the burnt material, sufficient contact between the raw materials is promoted, the occurrence of side reactions is reduced, and the carbon content in the burnt material is controlled. In the first calcination treatment, the uniform formation of lithium iron phosphate particles is promoted, and the generation of lithium iron phosphate particles with excessively large particle sizes is reduced, thereby facilitating the reduction of the average particle size of the finally obtained lithium iron phosphate material and controlling the particle size of the maximum particles of the lithium iron phosphate material; through the second calcination treatment after the burnt material is mixed with the second iron source, the second lithium source, and the second carbon source and solid-liquid separation, while increasing the particle size of the lithium iron phosphate particles formed by the burnt material, more lithium iron phosphate particles with small particle sizes are formed, thereby obtaining a lithium iron phosphate material with an appropriate ratio of smaller particles with a primary particle size greater than 0 nm and less than 200 nm to larger particles with a primary particle size greater than or equal to 200 nm and less than or equal to 2 μm. The appropriate proportion of larger particles is used to increase the compaction density of the lithium iron phosphate material, thereby increasing the volume energy density of the lithium iron phosphate material, and the appropriate proportion of smaller particles is used to improve the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0015] In some embodiments, the steps of preparing the ferric phosphate dihydrate material include: The ferrous source, the phosphorus source and the first oxidant are subjected to a first mixing process to obtain a first mixture; The first mixture and the first pH adjuster are subjected to a second mixing process to obtain a second mixture; The second mixture is subjected to a first reaction to obtain a third mixture; The third mixture is subjected to a third solid-liquid separation process to obtain the ferric phosphate dihydrate material; The iron-phosphorus ratio of the ferric phosphate dihydrate material is 0.968-0.992.

[0016] In the above embodiment, a dihydrate iron phosphate material with an iron-phosphorus ratio of 0.968~0.992 is obtained through the above preparation steps. The higher iron-phosphorus ratio is beneficial to reducing the impurities in the first-fired material, and the above iron-phosphorus ratio is used to control the coarsening degree of the grains of the lithium iron phosphate particles formed in the first calcination treatment and the second calcination treatment by the surplus phosphate, thereby controlling the particle size of the lithium iron phosphate particles formed in the first calcination treatment and the second calcination treatment, reducing the average particle size of the lithium iron phosphate material finally obtained and controlling the particle size of the largest particles of the lithium iron phosphate material, thereby improving the stability of the electrochemical properties of the lithium iron phosphate material finally obtained.

[0017] In some embodiments, the molar ratio of the lithium element in the first lithium source to the first iron source is (1.00-1.08):1, the mass ratio of the first carbon source to the first iron source is (7.1-10.5):100, and the mass ratio of the water retaining agent to the first iron source is (0.2-1):100.

[0018] In the above embodiment, controlling the molar ratio of lithium and iron elements in the first lithium source and the first iron source is beneficial to the full reaction of the two in the first calcination treatment, thereby improving the purity of the lithium iron phosphate obtained from the first calcination material; controlling the mass ratio of the first carbon source to the first iron source is beneficial to controlling the mass fraction of the carbon element in the first calcination material, reducing the inhibition of the carbon element on the growth of the lithium iron phosphate particles formed in the first calcination treatment, and increasing the average particle size of the lithium iron phosphate particles in the first calcination material, thereby facilitating an appropriate ratio of larger and smaller primary particles in the lithium iron phosphate material finally obtained; controlling the mass ratio of the water retaining agent to the first iron source is beneficial to fully exerting its role in promoting full contact between the raw materials, reducing the occurrence of side reactions, and promoting the uniform formation of lithium iron phosphate particles in the first calcination treatment.

[0019] In some embodiments, the molar ratio of the lithium element in the second lithium source to the second iron source is (1.02-1.08):1, the mass ratio of the second carbon source to the second iron source is (7.3-12.5):100, and the mass ratio of the second iron source to the first-fired material is (20-37):100.

[0020] In the above embodiment, controlling the mass ratio of the lithium element in the second lithium source to the iron element in the second iron source is beneficial to improving the structural integrity, charging performance, and discharge performance of the lithium iron phosphate particles in the second calcination treatment; controlling the mass ratio of the second carbon source to the second iron source is beneficial to controlling the mass fraction of the carbon element in the lithium iron phosphate material finally obtained, improving the electron transmission efficiency and lithium ion diffusion efficiency of the lithium iron phosphate material, and protecting the lithium iron phosphate core, thereby improving the charging performance, discharge performance, rate performance and cycle life of the lithium iron phosphate material; controlling the mass ratio of the second iron source to the first calcined material is beneficial to regulating the number of lithium iron phosphate particles with smaller primary particle size in the lithium iron phosphate material finally obtained, thereby obtaining a lithium iron phosphate material with an appropriate ratio of smaller particles with a primary particle size greater than 0 nm and less than 200 nm to larger particles with a primary particle size greater than or equal to 200 nm and less than or equal to 2 μm.

[0021] In some embodiments, the step of subjecting the first dried material to a first calcination treatment to obtain a first calcined material comprises: Under a first protective atmosphere, the first dried material is calcined at a first temperature and for a first calcination time to obtain a first discharged material; The first discharged material is subjected to a first pulverization process to obtain the first burned material; The step of obtaining lithium iron phosphate material by subjecting the second dried material to a second calcination treatment comprises: Under a second protective atmosphere, the second dried material is calcined at a second temperature and for a second time to obtain a second discharged material; The second discharged material is subjected to a second pulverization process to obtain the lithium iron phosphate material; Wherein, the first calcination temperature is 500°C to 700°C, the first calcination time is 5h to 11h, the second calcination temperature is 720°C to 820°C, and the second calcination time is 7h to 16h; The first protective atmosphere and the second protective atmosphere are independently selected from at least one of a nitrogen atmosphere and a helium atmosphere; The average particle size of the primary particles of the calcined material is 490 nm to 620 nm.

[0022] In the above embodiment, controlling the first calcination temperature and the first calcination time is beneficial to the full reaction of the raw materials in the first calcination treatment, and obtaining a calcined material with an appropriate particle size, thereby obtaining lithium iron phosphate particles with improved compaction density; controlling the second calcination temperature and the second calcination time is beneficial to the full reaction of the raw materials in the second calcination treatment, and obtaining more lithium iron phosphate particles with a smaller particle size, thereby obtaining a lithium iron phosphate material with an appropriate ratio of smaller particles with a primary particle size greater than 0nm and less than 200nm and larger particles with a primary particle size greater than or equal to 200nm and less than or equal to 2μm. While utilizing an appropriate proportion of larger particles to improve the compaction density of the lithium iron phosphate material and thereby improve the volume energy density of the lithium iron phosphate material, utilizing an appropriate proportion of smaller particles to improve the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0023] In some embodiments, the first lithium source and the second lithium source are independently selected from at least one of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; The first carbon source is selected from organic matter and / or oligomers having a molecular weight less than or equal to 600; The water-retaining agent is selected from the group consisting of polymeric ferric sulfate water-retaining agents; The second carbon source is selected from organic matter and / or polymers having a molecular weight greater than or equal to 1000 and less than or equal to 100,000; When the second slurry further includes a dopant, the dopant is selected from titanium dioxide, and the ratio of the total mass of the calcined material and the second iron source to the mass of the dopant is 100:(0.35-0.0.92).

[0024] In the above embodiment, the first lithium source, the second lithium source, the first carbon source, the water retaining agent, the second carbon source and the dopant are respectively selected from the above-mentioned materials with a wide range of sources, which is conducive to reducing the process cost; wherein, the first carbon source is selected from an organic or oligomeric carbon source with a smaller molecular weight, which improves the adsorption and dispersibility of the first carbon source to the first iron source and the first lithium source, and is conducive to the full reaction of the first iron source and the first lithium source in the first calcination treatment. At the same time, the selection of the above-mentioned first carbon source is conducive to reducing the carbon content of the first carbon source after the first calcination treatment, reducing the inhibition of carbon element on the growth of lithium iron phosphate particles formed in the first calcination treatment, and increasing the average particle size of lithium iron phosphate particles in the first calcined material, thereby facilitating the final obtained lithium iron phosphate material to have a suitable ratio of larger and smaller primary particles; the selection of the above-mentioned water retaining agent is conducive to promoting full contact between the raw materials, reducing the occurrence of side reactions and promoting the reaction of phosphate in the first calcination treatment. While the particles of lithium iron phosphate particles are uniformly formed, iron elements are supplemented in the first calcination treatment, which is beneficial to increase the iron content in the first calcined material and ultimately improve the charging performance of the lithium iron phosphate material; the second carbon source is selected from organic matter or polymers with a larger molecular weight, which is beneficial to form a uniform and appropriately thick carbon coating layer on the surface of the lithium iron phosphate core of the lithium iron phosphate material finally obtained, thereby improving the electron transmission efficiency and lithium ion diffusion efficiency of the lithium iron phosphate material, and protecting the lithium iron phosphate core, thereby improving the charging performance, discharge performance, rate performance and cycle life of the lithium iron phosphate material; when the dopant is selected from titanium dioxide, titanium element is used for metal element doping, which is beneficial to further control the growth of lithium iron phosphate in the second calcination treatment, and the titanium element is doped into the lithium iron phosphate crystal structure, thereby improving the structural stability of the lithium iron phosphate particles and improving the discharge performance of the lithium iron phosphate material finally obtained.

[0025] In a third aspect, an embodiment of the present application provides a positive electrode plate, a current collector, and a positive electrode material arranged on at least one side of the current collector along its thickness direction, wherein the positive electrode material includes the lithium iron phosphate material described above, or the positive electrode material includes the lithium iron phosphate material prepared by the preparation method of the lithium iron phosphate material described above.

[0026] In this embodiment, the positive electrode plate contains the above-mentioned lithium iron phosphate material, and thus has the advantages of improved volume energy density and excellent electrochemical performance in terms of discharge performance, rate performance, etc.

[0027] In a fourth aspect, the embodiments of the present application provide a secondary battery positive electrode sheet, a negative electrode sheet and a separator; Wherein, the positive electrode plate is the positive electrode plate described above.

[0028] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has the advantages of improved volume energy density and excellent electrochemical performance in terms of discharge performance, rate performance, etc.

[0029] In a fifth aspect, an embodiment of the present application provides an electrical device comprising the secondary battery as described above.

[0030] In this embodiment, the electrical device includes the aforementioned secondary battery, thus having the advantage of overall improved electrochemical performance.

[0031] 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

[0032] 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.

[0033] Figure 1 A schematic diagram of a process flow of a method for preparing lithium iron phosphate material provided in an embodiment of the present application; Figure 2 This is the SEM result of the lithium iron phosphate material in Example 2 provided in this application; Figure 3 This is a schematic diagram of the particle size statistics of the lithium iron phosphate material in Example 2 provided in this application; Figure 4 This is the SEM result of the lithium iron phosphate material in Comparative Example 2 provided in this application; Figure 5 A schematic diagram of the particle size statistics of the lithium iron phosphate material in Comparative Example 2 provided in this application; Figure 6 This is a graph showing the charging and discharging performance results of the lithium iron phosphate material in Example 4 and Comparative Example 3 provided in this application. DETAILED DESCRIPTION

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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).

[0040] In the description of the embodiments of the present application, the first solvent, the second solvent, water, etc. are all selected from at least one of deionized water, distilled water, pure water, and ultrapure water.

[0041] In the description of the embodiments of the present application, unless otherwise specified, ppm means the mass of a test result (such as an atom, ion, element, etc.) accounts for parts per million of the mass of the sample.

[0042] In the description of the examples of the present application, the pH value test was performed at 25° C. using a clear liquid.

[0043] At present, due to the limitations of the existing preparation process of lithium iron phosphate materials, the quantitative ratio of large-size particles to small-size particles is difficult to control, resulting in the problem that it is difficult to balance the improvement of electrochemical properties such as discharge efficiency and rate performance of lithium iron phosphate materials with the increase of volume energy density.

[0044] In order to solve the technical problem that it is difficult to optimize the ratio of large particles to small particles in lithium iron phosphate materials, resulting in the difficulty in balancing the improvement of electrochemical properties such as discharge efficiency and rate performance of lithium iron phosphate materials with the improvement of volume energy density, the present application provides a lithium iron phosphate material and a preparation method thereof, a positive electrode sheet, a secondary battery and an electrical device, wherein, by providing a lithium iron phosphate material, the average particle size of the primary particles of the lithium iron phosphate material is 0.23μm~0.26μm, and among the primary particles of the lithium iron phosphate material, the particle size is greater than 0nm and less than 200nm The number of primary particles accounts for 42%~55.2%, and the number of particles with a diameter greater than or equal to 200nm and less than or equal to 2μm accounts for 44.8%~58%; the compaction density of the lithium iron phosphate material is 2.61g / mL~2.67g / mL, thereby improving the volume energy density of the lithium iron phosphate material, and while maintaining the stability of the electrochemical properties of the lithium iron phosphate material and reducing polarization problems, improving the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc., thereby improving the electrochemical performance of the positive electrode sheet, secondary battery and electrical device.

[0045] The electrical devices provided in the embodiments of the present application may be, but are not limited to, mobile phones, tablets, laptop computers, electric toys, electric tools, battery-powered vehicles, electric cars, ships, spacecraft, etc. The electric toys may include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and the spacecraft may include airplanes, rockets, space shuttles, and spacecraft, etc.

[0046] In a first aspect, an embodiment of the present application provides a lithium iron phosphate material, wherein the average particle size of the primary particles of the lithium iron phosphate material is 0.23 μm to 0.26 μm, and among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, and the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm accounts for 44.8% to 58%; The compaction density of the lithium iron phosphate material is 2.61 g / mL to 2.67 g / mL.

[0047] By controlling the average particle size of the primary particles of the lithium iron phosphate material to be small, the stability of the electrochemical performance of the lithium iron phosphate material is maintained. By controlling the number ratio of the smaller particles with a particle size greater than 0 nm and less than 200 nm and the larger particles with a particle size greater than or equal to 200 nm and less than or equal to 2 pm in the primary particles of the lithium iron phosphate material, the appropriate proportion of the larger particles is used to improve the compaction density of the lithium iron phosphate material, thereby improving the volumetric energy density of the lithium iron phosphate material, and the appropriate proportion of the smaller particles is used to improve the electrochemical performance of the lithium iron phosphate material in terms of discharge performance, rate performance, etc. In addition, the maximum size of the primary particles of the lithium iron phosphate material is less than or equal to 2 pm, which improves the polarization problem of the lithium iron phosphate material.

[0048] In some embodiments, the primary particles of the lithium iron phosphate material are single minimum particles directly formed during the preparation process of the lithium iron phosphate material. The plurality of primary particles forms secondary particles after agglomeration. The particle size of the primary particles is the distance between the two most distant points on a single minimum particle in the SEM image.

[0049] In some embodiments, the average particle size of the primary particles of the lithium iron phosphate material is preferably 0.23 pm to 0.25 pm. By controlling the average particle size of the lithium iron phosphate material to be small, the stability of the electrochemical performance of the lithium iron phosphate material is maintained, and more lithium iron phosphate material particles with appropriate sizes are beneficial to improve the electrochemical performance of the lithium iron phosphate material in terms of charging performance, discharging performance, rate performance, etc.

[0050] In some embodiments, the average particle size of the lithium iron phosphate material can be 0.23 pm, 0.24 pm, 0.25 pm, 0.26 pm, etc.

[0051] In some embodiments, the number ratio of the primary particles with a particle size greater than 0 nm and less than 200 nm in the primary particles of the lithium iron phosphate material can be 47.0% to 51.0%, and the number ratio of the primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 pm is preferably 49% to 53%. Further optimization of the number ratio of the larger primary particles and the smaller primary particles is beneficial to further improve the compaction density of the lithium iron phosphate material while improving the electrochemical performance of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0052] In some embodiments, the compaction density of the lithium iron phosphate material can be 2.61, 2.62, 2.63 g / mL, 2.64 g / mL, 2.65 g / mL, 2.66 g / mL, 2.67 g / mL, etc. This is beneficial to further improve the volumetric energy density of the lithium iron phosphate material, improve the total energy of the secondary battery using the lithium iron phosphate material, and facilitate the more extensive application of the lithium iron phosphate material in power batteries and energy storage batteries.

[0053] In some embodiments, the D50 particle size of the lithium iron phosphate material is 1.4 μm to 2.6 μm, for example, 1.4 μm, 1.5 μm, 1.6 μm, 1.9 μm, 2.0 μm, 2.2 μm, 2.5 μm, 2.6 μm, etc. Furthermore, the D50 particle size of the lithium iron phosphate material can be 1.6 μm to 2.6 μm.

[0054] In some embodiments, the lithium iron phosphate material includes a lithium iron phosphate core, and the general formula of the lithium iron phosphate core is: LiFe x Ti y (PO4), 0.964≤x≤1.0, 0.0≤y≤0.036.

[0055] The lithium iron phosphate core having the above general formula has a suitable molar ratio of lithium, iron and phosphorus elements. While maintaining structural stability, it provides an appropriate amount of lithium ions for insertion and extraction, thereby improving the charging and discharging performance of the lithium iron phosphate material.

[0056] In some embodiments, 0.964≤x≤0.980; further, 0.975≤x≤0.977. Specifically, x can be 0.966, 0.968, 0.975, 0.976, 0.978, etc.

[0057] In some embodiments, 0.020≤y≤0.036; further, 0.023≤y≤0.025. Specifically, y can be 0.022, 0.024, 0.025, 0.034, etc.

[0058] In some embodiments, the chemical formula of the lithium iron phosphate core can be LiFe 0.975 T i0.025 PO4@C, LiFe 0.977 T i0.023 PO4@C, LiFe 0.976 T i0.024 PO4@C, LiFe 0.978 T i0.022 PO4@C, LiFe 0.980 T i0.020 PO4@C, LiFe 0.966 T i0.034 PO4@C, LiFe 0.964 T i0.036 PO4@C, LiFePO4@C, etc.

[0059] In some embodiments, the lithium iron phosphate material further includes a carbon coating layer coated on the surface of the lithium iron phosphate core, and the mass fraction of the carbon coating layer in the lithium iron phosphate material is 1.16% to 1.34%. The mass fraction of the carbon coating layer in the lithium iron phosphate material can be 1.16%, 1.19%, 1.24%, 1.26%, 1.28%, 1.3%, 1.34%, etc.

[0060] By controlling the mass fraction of the lithium iron phosphate material in the carbon coating, it is beneficial to form a uniform and appropriately thick carbon coating on the surface of the lithium iron phosphate core, reduce the electrons of the lithium iron phosphate material, provide a smoother path for the insertion and extraction of lithium ions, improve the electron transmission efficiency and lithium ion diffusion efficiency, and protect the lithium iron phosphate core, thereby improving the charging performance, discharge performance, rate performance and cycle life of the lithium iron phosphate material.

[0061] In some embodiments, the mass fraction of the carbon coating layer in the lithium iron phosphate material is further 1.16% to 1.28%.

[0062] In some embodiments, the primary particles of the lithium iron phosphate material include first particles and second particles, the particle size of the first particles is greater than 0.0 nm and less than 200 nm, the average particle size of the first particles is 23 nm~26 nm, the particle size of the second particles is 200 nm~2000 nm, and the average particle size of the second particles is 340 nm~410 nm.

[0063] By combining the first particles with smaller particle size with the first particles with larger particle size, the particle size distribution of the second particles with larger particle size is more concentrated, which is beneficial to improving the compaction density of the lithium iron phosphate material while maintaining the discharge performance and rate performance. The particle size distribution of the first particles with smaller particle size is more concentrated, which is beneficial to improving the electrochemical properties of the lithium iron phosphate material such as the discharge performance and rate performance.

[0064] In some embodiments, the average particle size of the first particles is 23 nm to 26 nm. Smaller first particles are more concentrated, which helps maintain the consistency and stability of the first particles' discharge performance while maintaining good discharge performance and rate performance, and effectively increases the compaction density of the lithium iron phosphate material.

[0065] In some embodiments, the average particle size of the first particles may be 23 nm, 24 nm, 25 nm, 26 nm, etc.

[0066] In some embodiments, the average particle size of the second particles is preferably 340 nm to 410 nm. The larger the second particles, the more concentrated the particle size, which is beneficial to effectively improve the electrochemical performance of the lithium iron phosphate material in terms of discharge performance and rate performance.

[0067] In some embodiments, the average particle size of the second particles may be 340 nm, 350 nm, 360 nm, 370 nm, 380 nm, 390 nm, 400 nm, 410 nm, etc.

[0068] In some embodiments, among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 600 nm accounts for 34% to 50.5%, the number of primary particles with a particle size greater than 600 nm and less than or equal to 1 μm accounts for 2% to 11%, and the number of primary particles with a particle size greater than 1 μm and less than or equal to 2 μm accounts for 0.3% to 8.5%. By controlling the particle size distribution of the primary particles of the lithium iron phosphate material and combining lithium iron phosphate materials of different particle sizes, it is beneficial to further improve the compaction density of the lithium iron phosphate material, thereby increasing the volume energy density, while improving the electrochemical performance of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0069] Furthermore, among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 47% to 51%, the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 600 nm accounts for 34% to 37%, the number of primary particles with a particle size greater than 600 nm and less than or equal to 1 μm accounts for 7.5% to 11%, and the number of primary particles with a particle size greater than 1 μm and less than or equal to 2 μm accounts for 4.5% to 8.5%.

[0070] Furthermore, among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 47.1% to 51%, the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 600 nm accounts for 34.9% to 36.7%, the number of primary particles with a particle size greater than 600 nm and less than or equal to 1 μm accounts for 7.6% to 10.7%, and the number of primary particles with a particle size greater than 1 μm and less than or equal to 2 μm accounts for 4.7% to 8.5%.

[0071] By further optimizing the particle size distribution of the primary particles of the lithium iron phosphate material and combining lithium iron phosphate materials with different particle sizes, it is beneficial to further improve the compaction density of the lithium iron phosphate material, thereby increasing the volume energy density, while improving the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0072] In some embodiments, the lithium iron phosphate material has a magnetic foreign matter content of 0 ppm to 0.083 ppm, for example, 0 ppm, 0.002 ppm, 0.007 ppm, 0.013 ppm, 0.026 ppm, 0.046 ppm, 0.072 ppm, 0.083 ppm, etc. Furthermore, the lithium iron phosphate material has a magnetic foreign matter content of 0 ppm to 0.08 ppm. A low magnetic foreign matter content helps reduce damage to the lithium iron phosphate material during the charge-discharge process when used in a secondary battery due to the presence of magnetic foreign matter, thereby extending the service life of the secondary battery using the lithium iron phosphate material.

[0073] In some embodiments, the specific surface area of ​​the lithium iron phosphate material is 11.5 m 2 / g~14.5m 2 / g, for example, it can be 11.5m 2 / g, 11.8m 2 / g、12m 2 / g, 12.2m 2 / g, 12.4m 2 / g, 12.8m 2 / g、13m 2 / g, 13.2m 2 / g, 13.5m 2 / g, 13.8m 2 / g、14m 2 / g, 14.5m 2 / g, etc.; Further, the specific surface area of ​​the lithium iron phosphate material is 11.5m 2 / g~13.2m 2 / g. The appropriate specific surface area is conducive to ensuring a suitable contact area between the lithium iron phosphate material and the electrolyte while providing sufficient active sites, which is beneficial to improving the electrochemical performance of the lithium iron phosphate material when used in secondary batteries.

[0074] In some embodiments, the static iron dissolution amount of the lithium iron phosphate material is 0 ppm to 14.3 ppm, for example, 0 ppm, 0.3 ppm, 1.1 ppm, 1.2 ppm, 1.4 ppm, 1.5 ppm, 1.8 ppm, 2 ppm, 2.2 ppm, 2.5 ppm, 2.6 ppm, 2.8 ppm, 3 ppm, 4 ppm, 5 ppm, 8 ppm, 10 ppm, 12 ppm, 14.3 ppm, etc.; further, the static iron dissolution amount of the lithium iron phosphate material is 0 ppm to 3 ppm; further, the static iron dissolution amount of the lithium iron phosphate material is 0 ppm to 1.85 ppm. Low static iron dissolution amount is beneficial to improving the stability of the lithium iron phosphate material and extending the service life of secondary batteries using the lithium iron phosphate material.

[0075] In some embodiments, the powder resistivity of the lithium iron phosphate material is 8Ω·cm to 13Ω·cm, for example, 8Ω·cm, 8.2Ω·cm, 8.5Ω·cm, 8.6Ω·cm, 8.8Ω·cm, 9Ω·cm, 9.2Ω·cm, 9.5Ω·cm, 9.8Ω·cm, 10Ω·cm, 10.4Ω·cm, 10.8Ω·cm, 11Ω·cm, 11.2Ω·cm, 11.5Ω·cm, 11.8Ω·cm, 12Ω·cm, 12.2Ω·cm, 12.5Ω·cm, 12.8Ω·cm, 13Ω·cm, etc.; further, the powder resistivity of the lithium iron phosphate material is 8Ω·cm to 12.5Ω·cm. Low powder resistivity is beneficial to reducing heat generation of the lithium iron phosphate material during charge-discharge, improving the performance stability of the lithium iron phosphate material, and extending the cycle life of the lithium iron phosphate material.

[0076] In some embodiments, the free lithium element content of the lithium iron phosphate material is 90ppm~300ppm, for example, it can be 90ppm, 95ppm, 100ppm, 105ppm, 110ppm, 120ppm, 150ppm, 180ppm, 200ppm, 220ppm, 260ppm, 300ppm, etc.; further, the free lithium element content of the lithium iron phosphate material is 90ppm~120ppm. A low free lithium element content is beneficial to improving the stability of the lithium iron phosphate material and extending the service life of the secondary battery using the lithium iron phosphate material.

[0077] In some embodiments, the coulombic efficiency of the lithium iron phosphate material at 25°C and 0.1C is 98%~99.9%, further 98.2%~99.9%, and can be 98%, 98.2%, 98.5%, 98.6%, 99%, 99.2%, 99.3%, 99.5%, 99.6%, 99.8%, 99.9%, etc.

[0078] In some embodiments, the constant voltage charging specific capacity of the lithium iron phosphate material at 25°C and 0.1C is 1.2%~3.4%, further 1.2%~1.62%, and can be 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.5%, 1.6%, 2%, 2.5%, 2.7%, 3%, 3.2%, 3.4%, etc.

[0079] In some embodiments, the lithium iron phosphate material has a coulomb efficiency of 86.5% to 92.5%, further 90.5% to 92.5%, and can be 87%, 88%, 90%, 90.5%, 90.8%, 91%, 91.2%, 91.5%, 91.6%, 91.8%, 92%, 92.2%, 92.5%, etc. at 25°C under 1C condition.

[0080] In some embodiments, the lithium iron phosphate material has a discharge specific capacity of 134mAh / g to 138mAh / g, further 134.4mAh / g to 138mAh / g, and can be 134mAh / g, 134.4mAh / g, 134.5mAh / g, 134.8mAh / g, 135mAh / g, 135.2mAh / g, 135.4mAh / g, 135.6mAh / g, 135.8mAh / g, 136mAh / g, 136.5mAh / g, 137mAh / g, 137.4mAh / g, 137.8mAh / g, 138mAh / g, etc. at 25°C under 1C condition and 3.2V.

[0081] In some embodiments, the lithium iron phosphate material has a discharge specific capacity retention rate of 95.2% to 96.8%, further 95.8% to 96.8%, and can be 95.2%, 95.5%, 95.8%, 96%, 96.2%, 96.4%, 96.5%, 96.6%, 96.8%, etc. after 100 cycles of charge-discharge at 25°C under 1C condition.

[0082] Please refer to Figure 1 , in a second aspect, the embodiments of the present application provide a preparation method of a lithium iron phosphate material, comprising: S100, a first iron source is mixed with a first lithium source, a first carbon source and a first solvent to obtain a first slurry.

[0083] S200, the first slurry is subjected to a first grinding treatment to obtain a second slurry.

[0084] S300, the second slurry is mixed with a water-retaining agent to obtain a third slurry.

[0085] S400, the third slurry is subjected to a first solid-liquid separation treatment to obtain a first dry material.

[0086] S500, the first dry material is subjected to a first calcination treatment to obtain a calcined material; S600, the calcined material is mixed with a second iron source, a second lithium source, a second carbon source and a second solvent to obtain a fourth slurry.

[0087] S700, the fourth slurry is subjected to a second solid-liquid separation treatment to obtain a second dry material.

[0088] S800: The second dried material is subjected to a second calcination process to obtain a lithium iron phosphate material.

[0089] Wherein, the first iron source and the second iron source are independently selected from ferric phosphate dihydrate materials, and the mass fraction of carbon element in the calcined material is 0.12% to 0.38%; The average particle size of the primary particles of the lithium iron phosphate material is 0.23 μm to 0.26 μm. Among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, and the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm accounts for 44.8% to 58%; The compaction density of the lithium iron phosphate material is 2.61 g / mL to 2.67 g / mL.

[0090] The preparation method of the lithium iron phosphate material provided in the present application promotes full contact between the raw materials, reduces the occurrence of side reactions, and combines the control of the carbon content in the first-burned material with the use of a water-retaining agent in the synthesis of the first-burned material to promote the uniform formation of lithium iron phosphate particles and reduce the generation of lithium iron phosphate particles with excessively large particle sizes, thereby facilitating the reduction of the average particle size of the finally obtained lithium iron phosphate material and controlling the particle size of the largest particles of the lithium iron phosphate material; through the second calcination treatment after the first-burned material is mixed with the second iron source, the second lithium source, and the second carbon source and solid-liquid separation, the first-burned material is mixed with the second iron source, the second lithium source, and the second carbon source, and solid-liquid separation is carried out, thereby promoting the uniform formation of lithium iron phosphate particles and reducing the generation of lithium iron phosphate particles with excessively large particle sizes; While the particle size of the lithium iron phosphate particles formed by the sintering material is reduced, more lithium iron phosphate particles with small particle sizes are formed, thereby obtaining a lithium iron phosphate material with an appropriate ratio of smaller particles with a primary particle size greater than 0nm and less than 200nm and larger particles with a primary particle size greater than or equal to 200nm and less than or equal to 2μm. While utilizing an appropriate proportion of larger particles to improve the compaction density of the lithium iron phosphate material and thus the volume energy density of the lithium iron phosphate material, utilizing an appropriate proportion of smaller particles to improve the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0091] In some embodiments, the lithium iron phosphate material prepared by the method for preparing the lithium iron phosphate material provided in the present application is the lithium iron phosphate material as described above.

[0092] In some embodiments, the first iron source and the second iron source are independently selected from ferric phosphate dihydrate materials, and the steps of preparing the ferric phosphate dihydrate material include: The ferrous source, the phosphorus source and the first oxidant are subjected to a first mixing process to obtain a first mixture; The first mixture and the first pH adjuster are subjected to a second mixing process to obtain a second mixture; The second mixture is subjected to a first reaction to obtain a third mixture; The third mixture is subjected to a third solid-liquid separation process to obtain the ferric phosphate dihydrate material; The iron-phosphorus ratio (the ratio of the amount of iron to the amount of phosphorus) of the ferric phosphate dihydrate material is 0.968-0.992.

[0093] In some embodiments, the ferrous iron source is derived from a byproduct of titanium dioxide.

[0094] In some embodiments, when the ferrous source is derived from a by-product of titanium dioxide, the ferrous source is obtained by the following preparation steps: The titanium dioxide byproduct is dissolved in water by heating, and then 3% to 7% by mass of liquid caustic soda is added and the solid-liquid separation is carried out to obtain a ferrous solution, which is the ferrous source. The mass fraction of the liquid caustic soda can be 3%, 4%, 5%, 6%, 7%, etc.

[0095] The concentration of ferrous ions in the ferrous solution is 1.5 mol / L to 2 mol / L, for example, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2 mol / L, etc.

[0096] In some embodiments, the solid-liquid separation after adding liquid alkali can be carried out by at least one of normal pressure filtration, pressure filtration, and suction filtration.

[0097] In some embodiments, the pH value of the ferrous solution is 2.4-3.2, for example, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, etc.

[0098] In some embodiments, the phosphorus source is from at least one of triammonium phosphate, monoammonium phosphate, and diammonium phosphate.

[0099] In some embodiments, taking the phosphorus source as monoammonium phosphate as an example, the phosphorus source can be obtained by the following preparation steps: Industrial-grade monoammonium phosphate is dissolved in water and separated into solid and liquid to obtain a monoammonium phosphate solution; Ammonia gas is introduced into the monoammonium phosphate solution to adjust the pH value of the monoammonium phosphate solution to 6.8-7.2 to obtain the phosphorus source.

[0100] In some embodiments, the concentration of phosphate ions in the phosphorus source is 1.5 mol / L to 3 mol / L, for example, 1.5 mol / L, 1.6 mol / L, 1.8 mol / L, 2 mol / L, 2.4 mol / L, 2.6 mol / L, 2.8 mol / L, 3 mol / L, etc.

[0101] In some embodiments, the solid-liquid separation after industrial-grade monoammonium phosphate is dissolved in water can be carried out by at least one of normal pressure filtration, pressure filtration, and suction filtration.

[0102] In some embodiments, the first oxidant is selected from hydrogen peroxide, and the mass fraction of hydrogen peroxide in hydrogen peroxide can be 25% to 45%.

[0103] In some embodiments, the molar ratio of the iron element, the phosphorus element and the first oxidant in the ferrous source, the phosphorus source and the first oxidant is 1:(1.4~1.6):(2.9~3.1), for example, it can be 1:1.4:2.9, 1:1.5:3, 1:1.6:3.1, etc.

[0104] In some embodiments, the ferrous source, the phosphorus source, and the first oxidant are subjected to a first mixing process to obtain a first mixture, comprising: A phosphorus source and a first oxidant are added to the stirred ferrous solution respectively at a first stirring temperature to obtain a first mixture.

[0105] In some embodiments, the phosphorus source and the first oxidant can be added to the ferrous solution separately via peristaltic pumps.

[0106] In some embodiments, the phosphorus source can be added to the ferrous solution by dropwise addition, and the first oxidant can be added to the ferrous solution by dropwise addition.

[0107] In some embodiments, the phosphorus source may be added for 40 to 65 minutes, for example, 40 minutes, 42 minutes, 45 minutes, 48 ​​minutes, 50 minutes, 52 minutes, 55 minutes, 60 minutes, 65 minutes, etc. The first oxidant may be added for 50 to 65 minutes, for example, 50 minutes, 52 minutes, 55 minutes, 56 minutes, 58 minutes, 60 minutes, 62 minutes, 65 minutes, etc.

[0108] In some embodiments, the first stirring temperature may be 50° C. to 65° C., for example, 50° C., 52° C., 55° C., 56° C., 58° C., 60° C., 62° C., 65° C., etc. The temperature of the ferrous solution may be 50° C. to 65° C., for example, 50° C., 52° C., 55° C., 56° C., 58° C., 60° C., 62° C., 65° C., etc. Maintaining a temperature close to or the same as the first stirring temperature of the ferrous solution is conducive to a sufficient reaction of the ferrous ions, phosphate ions, and the first oxidant.

[0109] In some embodiments, the first mixture and the first pH adjuster are subjected to a second mixing process to obtain a second mixture, comprising: The first mixture and the first pH adjuster are stirred at a second temperature to obtain a second mixture, and the pH value of the second mixture is 0.8-1.3.

[0110] In some embodiments, the second stirring temperature may be 50°C to 65°C, for example, 50°C, 52°C, 55°C, 56°C, 58°C, 60°C, 62°C, 65°C, etc.

[0111] In some embodiments, the first pH adjuster may be selected from phosphoric acid to avoid introducing impurities while adjusting the pH.

[0112] In some embodiments, the pH value of the second mixture may be 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, etc.

[0113] In some embodiments, the step of subjecting the second mixture to a first reaction to obtain a third mixture comprises: After the second mixture is reacted at the first reaction temperature and for the first reaction time, a third mixture is obtained.

[0114] In some embodiments, the first reaction temperature may be 92°C to 98°C, for example, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, 98°C, etc.

[0115] In some embodiments, the first reaction time is 1.5 h to 2.5 h, for example, 1.5 h, 1.6 h, 1.8 h, 2 h, 2.2 h, 2.4 h, 2.5 h, etc.

[0116] In some embodiments, the pH value of the third mixture is 1.6-2.0, for example, 1.6, 1.7, 1.8, 1.9, or 2.0.

[0117] In some embodiments, the step of subjecting the third mixture to a third solid-liquid separation process to obtain the ferric phosphate dihydrate material comprises: The third mixture is subjected to solid-liquid separation, rinsing, and drying to obtain ferric phosphate dihydrate material.

[0118] In some embodiments, the solid-liquid separation of the third mixture can be performed by atmospheric pressure filtration, pressure filtration, suction filtration, centrifugation, etc.

[0119] In some embodiments, the solid material obtained by solid-liquid separation of the third mixture can be rinsed with rinsing water until the conductivity of the rinsing water is less than or equal to 500 μS / cm to obtain the material to be dried.

[0120] In some embodiments, the material to be dried is flash evaporated to obtain ferric phosphate dihydrate material.

[0121] By adjusting the molar ratio of the iron element, phosphorus element and the first oxidant in the ferrous source, phosphorus source and the first oxidant, and combining the above preparation steps, it is advantageous to obtain a dihydrate ferric phosphate material with a higher iron-phosphorus ratio. In some embodiments, the iron-phosphorus ratio of the dihydrate iron phosphate material is 0.968~0.992, for example, it can be 0.968, 0.976, 0.98, 0.985, 0.988, 0.99, 0.991, 0.992, etc. Further, the iron-phosphorus ratio of the dihydrate iron phosphate material can be 0.976~0.992. The dihydrate iron phosphate material with a higher iron-phosphorus ratio is beneficial to reducing the impurities in the first-fired material, and using the above-mentioned iron-phosphorus ratio to control the coarsening degree of the grains of the lithium iron phosphate particles formed in the first calcination treatment and the second calcination treatment by the surplus phosphate, thereby controlling the particle size of the lithium iron phosphate particles formed in the first calcination treatment and the second calcination treatment, reducing the average particle size of the lithium iron phosphate material finally obtained and controlling the particle size of the largest particles of the lithium iron phosphate material, thereby improving the stability of the electrochemical properties of the lithium iron phosphate material finally obtained.

[0122] In some embodiments, the molar ratio of the lithium element in the first lithium source to the first iron source is (1.00-1.08):1, for example, it can be 1.00:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.06:1, 1.08:1, etc.; further, the molar ratio of the lithium element in the first lithium source to the first iron source is (1.01-1.04):1, which is conducive to providing sufficient lithium element to fully react with the first iron source, thereby improving the purity and electrochemical properties of the final lithium iron phosphate material.

[0123] In some embodiments, the first lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium oxalate, and lithium acetate. These materials are widely available and are beneficial for reducing process costs.

[0124] In some embodiments, the mass ratio of the first carbon source to the first iron source is (7.1~10.5):100, which can be 7.1:100, 7.5:100, 8:100, 8.5:100, 9:100, 9.5:100, 10:100, 10.5:100, etc.; further, the mass ratio of the first carbon source to the first iron source is (7.1~9.7):100, which is beneficial to control the mass fraction of carbon element in the first-fired material, reduce the inhibition of carbon element on the growth of lithium iron phosphate particles formed in the first calcination treatment, and increase the average particle size of lithium iron phosphate particles in the first-fired material, thereby facilitating that the ratio of larger particles and smaller particles of primary particles in the finally obtained lithium iron phosphate material is appropriate.

[0125] In some embodiments, the first carbon source is selected from organic matter and / or oligomers with a molecular weight less than or equal to 600. The wide range of material sources is conducive to reducing process costs while improving the adsorption and dispersibility of the first carbon source on the first iron source and the first lithium source, which is conducive to the full reaction of the first iron source and the first lithium source in the first calcination treatment. At the same time, the selection of the above-mentioned first carbon source is conducive to reducing the carbon content of the first carbon source after the first calcination treatment, controlling the mass fraction of carbon elements in the first-calcined material, reducing the inhibition of carbon elements on the growth of lithium iron phosphate particles formed in the first calcination treatment, and increasing the average particle size of lithium iron phosphate particles in the first-calcined material, thereby facilitating the final obtained lithium iron phosphate material to have a suitable ratio of larger and smaller primary particles.

[0126] In some embodiments, the first carbon source is selected from at least one of triethyl glycol dibutyl ether, propylene glycol butyl ether, diglycerol, monoglycerol fatty acid ester, diglycerol fatty acid ester, and the like.

[0127] In some embodiments, the solid content of the first slurry is 60.5% to 67.5%, that is, the mass fraction of solid particles in the first slurry is 60.5% to 67.5%, for example, 60.5%, 62%, 63%, 64%, 65%, 66%, 67%, 67.5%, etc. Maintaining the solid content of the first slurry at 60.5% to 67.5% is conducive to controlling the content of solid particles in the first slurry to be moderate, facilitating subsequent processes.

[0128] In some embodiments, in the step of obtaining the second slurry by subjecting the first slurry to a first grinding process, the first slurry may be ground by sanding.

[0129] In some embodiments, the first grinding process may be performed by a grinding machine, for example, a planetary ball mill, or the like.

[0130] In some embodiments, the D50 particle size of the solid particles in the second slurry is 0.42 μm to 0.48 μm, for example, 0.42 μm, 0.43 μm, 0.45 μm, 0.46 μm, 0.47 μm, 0.48 μm, etc. By controlling the D50 particle size of the solid particles in the second slurry, it is facilitated to fully mix the solid particles in the second slurry with the water-retaining agent, and to control the particle size of the subsequently obtained first dried material, thereby facilitating control of the average particle size of the lithium iron phosphate particles in the first-fired material.

[0131] In some embodiments, the water retaining agent is selected from a polyferric sulfate water retaining agent, which promotes sufficient contact between the raw materials, reduces the occurrence of side reactions, and promotes the uniform formation of lithium iron phosphate particles in the first calcination treatment. At the same time, it supplements iron elements in the first calcination treatment, which is beneficial to increase the iron content in the first calcined material and ultimately improve the charging performance of the lithium iron phosphate material.

[0132] In some embodiments, the polyferric sulfate water-retaining agent can be selected from at least one of polyferric sulfate, basic ferric sulfate, and the like.

[0133] In some embodiments, the mass ratio of the water-retaining agent to the first iron source is (0.2-1):100, for example, 0.2:100, 0.3:100, 0.4:100, 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.1:100, etc.; further, the mass ratio of the water-retaining agent to the first iron source is (0.3-0.8):100. Controlling the mass ratio of the water-retaining agent to the first iron source helps the water-retaining agent fully exert its role in promoting sufficient contact between the raw materials, reducing the occurrence of side reactions, and promoting uniform formation of lithium iron phosphate particles during the first calcination process.

[0134] In some embodiments, the step of subjecting the third slurry to a first solid-liquid separation process to obtain a first dry material comprises: The third slurry is subjected to solid-liquid separation to obtain a first wet material; The first wet material is flash evaporated to obtain the first dry material.

[0135] In some embodiments, the solid-liquid separation of the third slurry can be selected from at least one of normal pressure filtration, pressure filtration, suction filtration, centrifugation, and the like.

[0136] In some embodiments, the first dried material is subjected to a first calcination treatment to obtain a first calcined material, comprising: Under a first protective atmosphere, the first dried material is calcined at a first temperature and for a first calcination time to obtain a first discharged material; The first discharged material is subjected to a first pulverization process to obtain the first burned material.

[0137] In some embodiments, the first calcination temperature is 500°C~700°C, for example, it can be 500°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 660°C, 700°C, etc.; further, the first calcination temperature is 550°C~660°C.

[0138] In some embodiments, the first calcination time is 5 h to 11 h, for example, 5 h, 6 h, 6.5 h, 7 h, 7.5 h, 8 h, 8.5 h, 9 h, 10 h, 11 h, etc.; further, the first calcination time is 6 h to 9 h.

[0139] In some embodiments, the first protective atmosphere is selected from at least one of a nitrogen atmosphere and a helium atmosphere.

[0140] By controlling the first calcination temperature and the first calcination time of the first calcination treatment under the first protective atmosphere, it is beneficial to fully react the raw materials in the first calcination treatment, obtain a calcined material with a suitable particle size, and thus obtain lithium iron phosphate particles with increased compaction density.

[0141] In some embodiments, during the first calcination process, the heating rate to reach the first calcination temperature may be 4°C / min to 7°C / min, for example, 4°C / min, 5°C / min, 6°C / min, 7°C / min, etc.

[0142] In some embodiments, the average particle size of the primary particles of the calcined material is 490 nm to 620 nm, for example, 520 nm, 530 nm, 540 nm, 550 nm, 560 nm, 570 nm, 580 nm, 590 nm, 600 nm, 610 nm, 620 nm, 630 nm, 640 nm, 650 nm, etc. Furthermore, the average particle size of the primary particles of the calcined material is 540 nm to 620 nm. This facilitates obtaining a calcined material with increased compaction density, thereby increasing the compaction density of the lithium iron phosphate material.

[0143] In some embodiments, the mass fraction of carbon in the first-burned material is 0.12% to 0.38%, for example, 0.12%, 0.16%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, 0.3%, 0.31%, 0.33%, 0.35%, etc.; further, the mass fraction of carbon in the first-burned material is 0.18% to 0.33%; further, the mass fraction of carbon in the first-burned material is 0.18% to 0.23%. By controlling the carbon content in the first-burned material, the effect of carbon on the growth of lithium iron phosphate particles during the first calcination process is reduced, promoting the uniform formation of lithium iron phosphate particles during the first calcination process. At the same time, the low carbon mass fraction makes the first-burned material easier to crush and disperse, facilitating subsequent processes.

[0144] In some embodiments, the first comminution process may be performed by a vibrating screen.

[0145] In some embodiments, in the step of mixing the first calcined material with a second iron source, a second lithium source, a second carbon source, and a second solvent to obtain a fourth slurry, the second lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium oxalate, and lithium acetate, the second iron source is selected from a dihydrated iron phosphate material, and the second carbon source is selected from an organic matter and / or polymer having a molecular weight greater than or equal to 1000 and less than or equal to 100,000. The above materials have a wide range of sources, which is beneficial to reducing process costs. Among them, the second iron source is selected from a dihydrated iron phosphate material, which increases the moisture content in the second calcination treatment, improves the reaction efficiency, and utilizes the second calcination treatment to perform dehydration shrinkage, so that the lithium iron phosphate particles formed by the second calcination treatment are more tightly combined with the lithium iron phosphate particles formed by the first calcined material, which is beneficial to improving the compaction density of the lithium iron phosphate material. The second carbon source is selected from organic matter or polymers with a larger molecular weight, which is conducive to forming a uniform and appropriately thick carbon coating layer on the surface of the lithium iron phosphate core of the lithium iron phosphate material finally obtained, thereby improving the electron transmission efficiency and lithium ion diffusion efficiency of the lithium iron phosphate material, and protecting the lithium iron phosphate core, thereby improving the charging performance, discharge performance, rate performance and cycle life of the lithium iron phosphate material.

[0146] In some embodiments, the second carbon source is selected from the group consisting of pasty starch with a molecular weight of 2,000-40,000, β-cyclodextrin with a molecular weight of 1,000-1,500, and polyether polyamide type ultra-dispersed hyperbranched polyester polyamino acid ester with a molecular weight of 20,000-100,000.

[0147] In some embodiments, the molar ratio of the lithium element in the second lithium source to the second iron source is (1.02-1.08):1, for example, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, etc.; further, the molar ratio of the lithium element in the second lithium source to the second iron source is (1.04-1.06):1. Controlling the molar ratio of the lithium element in the second lithium source to the iron element in the second iron source is beneficial for improving the structural integrity, charging performance, and discharging performance of the lithium iron phosphate particles during the second calcination process.

[0148] In some embodiments, the mass ratio of the second carbon source to the second iron source is (7.3-12.5):100, for example, 7.3:100, 8.3:100, 9.3:100, 9.5:100, 10.3:100, 10.5:100, 10.8:100, 11:100, 11.2:100, 11.5:100, 12.5:100, etc.; further, the mass ratio of the second carbon source to the second iron source is (9.3-11.2):100. Controlling the mass ratio of the second carbon source to the second iron source is beneficial for controlling the carbon mass fraction of the resulting lithium iron phosphate material, improving the electron transport efficiency and lithium ion diffusion efficiency of the lithium iron phosphate material, protecting the lithium iron phosphate core, and improving the charging performance, discharge performance, rate performance, and cycle life of the lithium iron phosphate material.

[0149] In some embodiments, the mass ratio of the second iron source to the calcined material is (20-37):100, for example, 20:100, 22:100, 24:100, 25:100, 28:100, 29:100, 30:100, 32:100, 34:100, 35:100, 37:100, etc. Furthermore, the mass ratio of the second iron source to the calcined material is (24-37):100. Controlling the mass ratio of the second iron source to the calcined material facilitates regulating the number of lithium iron phosphate particles with smaller primary particle sizes in the resulting lithium iron phosphate material, thereby obtaining a lithium iron phosphate material having an appropriate ratio of smaller particles with a primary particle size greater than 0 nm and less than 200 nm to larger particles with a primary particle size greater than or equal to 200 nm and less than or equal to 2 μm.

[0150] In some embodiments, the second slurry further includes a dopant selected from titanium dioxide. The mass ratio of the total mass of the calcined material and the second iron source to the dopant is 100:(0.35-0.92), for example, 100:0.35, 100:0.38, 100:0.4, 100:0.41, 100:0.43, 100:0.45, 100:0.5, 100:0.55, 100:0.6, 100:0.65, 100:0.7, 100:0.75, 100:0.8, 100:0.9, 100:0.92, etc. Furthermore, the mass ratio of the total mass of the calcined material and the second iron source to the dopant is 100:(0.41-0.87). When the dopant is selected from titanium dioxide, the titanium element is used for metal doping, which is beneficial to further control the growth of lithium iron phosphate in the second calcination treatment, and the titanium element is doped into the lithium iron phosphate crystal structure, thereby improving the structural stability of the lithium iron phosphate particles and improving the discharge performance of the lithium iron phosphate material finally obtained.

[0151] In some embodiments, the solid content of the second slurry is 50%~59.6%, that is, the mass fraction of solid particles in the second slurry is 50%~59.6%, for example, it can be 50%, 51%, 53%, 55%, 56%, 57%, 59.6%, etc.

[0152] In some embodiments, the step of subjecting the fourth slurry to a second solid-liquid separation process to obtain a second dry material comprises: The fourth slurry is subjected to solid-liquid separation to obtain a second wet material; The second wet material is flash evaporated to obtain the second dry material.

[0153] In some embodiments, the solid-liquid separation of the fourth slurry can be at least one selected from the group consisting of atmospheric filtration, filter press, suction filtration, and centrifugation. Filter press is preferred, as it facilitates the full removal of moisture from the second wet material and facilitates full contact and fusion between the lithium iron phosphate particles formed in the subsequent second calcination process and the lithium iron phosphate particles in the first calcined material, thereby forming more lithium iron phosphate particles with a more suitable particle size.

[0154] The fourth slurry is not ground but directly subjected to the second solid-liquid separation treatment, thereby avoiding the destruction of the structure of the lithium iron phosphate particles in the fired material by the grinding treatment and avoiding the change of the particle size distribution of the primary particles of the lithium iron phosphate particles in the fired material, which is conducive to easier control of the proportion of larger particles and smaller particles in the primary particles of the lithium iron phosphate material finally obtained.

[0155] In some embodiments, the second dried material is subjected to a second calcination process to obtain a lithium iron phosphate material, comprising: Under a second protective atmosphere, the second dried material is calcined at a second temperature and for a second time to obtain a second discharged material; The second discharged material is subjected to a second pulverization process to obtain the lithium iron phosphate material.

[0156] In some embodiments, the second calcination temperature is 720°C to 820°C, for example, 720°C, 750°C, 780°C, 785°C, 790°C, 795°C, 800°C, 820°C, etc.; further, the second calcination temperature is 780°C to 800°C.

[0157] In some embodiments, the second calcination time is 7h~16h, for example, it can be 7h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, 12h, 12.5h, 13h, 13.5h, 14h, 16h, etc.; further, the second calcination time is 8h~14h.

[0158] In some embodiments, the second protective atmosphere is selected from at least one of a nitrogen atmosphere and a helium atmosphere.

[0159] By controlling the second calcination temperature and the second calcination time under the second protective atmosphere, it is beneficial to the full reaction of the raw materials in the second calcination treatment, and more lithium iron phosphate particles with smaller particle sizes are obtained, thereby obtaining a lithium iron phosphate material with an appropriate ratio of smaller particles with a primary particle size greater than 0nm and less than 200nm and larger particles with a primary particle size greater than or equal to 200nm and less than or equal to 2μm. While utilizing an appropriate proportion of larger particles to improve the compaction density of the lithium iron phosphate material and thus the volume energy density of the lithium iron phosphate material, utilizing an appropriate proportion of smaller particles to improve the electrochemical properties of the lithium iron phosphate material in terms of discharge performance, rate performance, etc.

[0160] The second calcination treatment is directly carried out on the second dried material, which saves the steps of drying and spray drying, reduces the process cost, and at the same time increases the potting volume during the second calcination treatment by more than 20%, thereby improving the process capacity.

[0161] The second pulverization treatment of the second discharged material is beneficial for sufficient dispersion of particles in the obtained lithium iron phosphate material, thereby avoiding the subsequent peeling that affects the compaction density and causes internal resistance polarization problems.

[0162] In some embodiments, during the second calcination process, the heating rate to reach the second calcination temperature may be 2° C. / min to 4° C. / min, for example, 2° C. / min, 3° C. / min, 4° C. / min, etc.

[0163] In a third aspect, an embodiment of the present application provides a positive electrode plate, a current collector, and a positive electrode material arranged on at least one side of the current collector along its thickness direction, wherein the positive electrode material includes the lithium iron phosphate material described above, or the positive electrode material includes the lithium iron phosphate material prepared by the preparation method of the lithium iron phosphate material described above.

[0164] In this embodiment, the positive electrode plate contains the above-mentioned lithium iron phosphate material, and thus has the advantages of improved volume energy density and excellent electrochemical performance in terms of discharge performance, rate performance, etc.

[0165] In a fourth aspect, the embodiments of the present application provide a secondary battery positive electrode sheet, a negative electrode sheet and a separator; Wherein, the positive electrode plate is the positive electrode plate described above.

[0166] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has the advantages of improved volume energy density and excellent electrochemical performance in terms of discharge performance, rate performance, etc.

[0167] In a fifth aspect, an embodiment of the present application provides an electrical device comprising the secondary battery as described above.

[0168] In this embodiment, the electrical device includes the secondary battery, thus having the advantage of overall improved electrochemical performance.

[0169] 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.

[0170] 1. Preparation method Example 1 Step 1: Preparation of ferric phosphate dihydrate 1) heating and dissolving the titanium dioxide byproduct containing ferrous sulfate, adding 5% by mass of liquid caustic soda for sedimentation, and filtering to remove impurities to obtain a ferrous solution with a ferrous ion concentration of 1.7 mol / L and a pH of 2.7; 2) Preparation of phosphorus source: dissolve industrial monoammonium phosphate, filter it, add ammonia water and adjust the pH value to 7.0 to obtain phosphorus source solution, in which PO4 3- The content is 2.0mol / L; 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 45 minutes, the hydrogen peroxide solution was added dropwise for 50 minutes, and the first stirring temperature during the addition was 50° C. After the addition was completed, phosphoric acid was added at a second stirring temperature of 50° C. to adjust the first pH value to 0.9. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.980.

[0171] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 7.1% of triethyl glycol dibutyl ether (first carbon source) by mass of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 61.4%. The first slurry was sand milled to adjust the D50 particle size to 0.42 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material of polyferric sulfate to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 550° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.7 μm to obtain a burned material with a carbon content of 0.21% and an average primary particle size of 0.54 μm.

[0172] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 24% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.04, 9.3% of the mass of the second iron source paste starch (molecular weight: 2000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 50.0%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 780° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 14 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 1.6 μm to obtain a lithium iron phosphate material with a carbon content of 1.16%.

[0173] Example 2 This embodiment provides a method for preparing anhydrous iron phosphate and carbon-coated lithium iron phosphate. The specific steps of the preparation method are as follows: Step 1: Preparation of ferric phosphate dihydrate 1) heating and dissolving the titanium dioxide byproduct containing ferrous sulfate, adding 7% by mass of liquid caustic soda for sedimentation, and filtering to remove impurities to obtain a ferrous solution with a ferrous ion mass fraction of 2.0 mol / L and a pH value of 2.4; 2) Preparation of phosphorus source: dissolve industrial monoammonium phosphate, filter it, add ammonia water and adjust the pH value to 7.2 to obtain phosphorus source solution, in which PO4 3- The content is 3mol / L; 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 45%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 50 min, and the hydrogen peroxide solution was added dropwise for 50 min. The first stirring temperature during the dropwise addition was 55° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 55° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 h (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.976.

[0174] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.04, and 8.5% of the mass of propylene glycol butyl ether (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 67.5%. The first slurry was sand milled to adjust the D50 particle size to 0.48 μm to obtain a second slurry. The second slurry was added with 0.8% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.8 μm to obtain a burned material. The burned material had a carbon content of 0.19% and an average primary particle size of 0.61 μm.

[0175] The calcined material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 37% of the calcined material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.06, β-cyclodextrin (molecular weight: 1000) of 11.2% of the mass of the second iron source, and titanium dioxide of 0.75% of the total mass of the calcined material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 59.6%. The fourth slurry was filtered into a filter cake and then flash evaporated to obtain a second dry material. The second dry material was heated to 800°C at 3°C / min under a nitrogen protective atmosphere, kept warm for 8 hours, and then naturally cooled to below 120°C and taken out of the furnace to obtain the second out-of-furnace material. The second out-of-furnace material was crushed to a D50 particle size of 1.9um to obtain a lithium iron phosphate material with a carbon content of 1.28%. The SEM results and particle size statistics of the obtained lithium iron phosphate material are shown as follows: Figure 2 and Figure 3 shown.

[0176] Example 3 1) heating and dissolving the titanium dioxide byproduct containing ferrous sulfate, adding 3% by mass of liquid caustic soda for sedimentation, and filtering to remove impurities to obtain a ferrous solution with a ferrous ion concentration of 1.5 mol / L and a pH of 3.2; 2) Preparation of phosphorus source: dissolve industrial monoammonium phosphate, filter it, add ammonia water and adjust the pH value to 6.8 to obtain phosphorus source solution, in which PO4 3- The content is 1.5mol / L; 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 25%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 50 minutes, and the hydrogen peroxide solution was added dropwise for 60 minutes. The first stirring temperature during the addition was 60° C. After the addition was completed, the pH value was adjusted to 0.8 with phosphoric acid at a second stirring temperature of 60° C., and then the temperature was raised to a first reaction temperature of 95° C. The reaction was continued with stirring for 2 hours (first reaction time) until the pH value was 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.986.

[0177] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.04, and 9.7% of the mass of propylene glycol butyl ether (first carbon source) of the ferric phosphate dihydrate material were added with pure water and stirred to form a first slurry with a solid content of 67.5%. The first slurry was sand milled to adjust the D50 particle size to 0.48 μm to obtain a second slurry. The second slurry was added with 0.4% of the mass of the ferric phosphate dihydrate material of polyferric sulfate to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 660° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.8 μm to obtain a burned material with a carbon content of 0.18% and an average primary particle size of 0.58 μm.

[0178] The burnt material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 37% of the burnt material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.06, β-cyclodextrin (molecular weight: 1500) of 11.2% of the mass of the second iron source, and titanium dioxide of 0.75% of the total mass of the burnt material and the second iron source were stirred and mixed with pure water to form a second slurry with a solid content of 59.6%. The second slurry was filtered into a filter cake and flash evaporated to obtain a second dry material. The second dry material was heated to 800° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 8 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to obtain a lithium iron phosphate material with a D50 particle size of 2.6 μm and a carbon content of 1.24%.

[0179] Example 4 1) Same as Example 2.

[0180] 2) Preparation of phosphorus source: dissolve industrial monoammonium phosphate, filter it, add ammonia water and adjust the pH value to 7.0 to obtain phosphorus source solution, in which PO4 3- The content is 1.5mol / L; 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.992.

[0181] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.04, and 9.7% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added with pure water and stirred to form a first slurry with a solid content of 64.5%. The first slurry was sand milled to adjust the D50 particle size to 0.46 μm to obtain a second slurry. The second slurry was added with 0.4% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 660° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.6 μm to obtain a burned material. The burned material had a carbon content of 0.18% and an average primary particle size of 0.56 μm.

[0182] The calcined material, a dihydrate iron phosphate material (second iron source) with a mass fraction of 37% of the calcined material, a lithium source (lithium carbonate) with a molar ratio of lithium to the second iron source of 1.06, β-cyclodextrin (molecular weight: 1200) with a mass fraction of 11.2% of the second iron source, and titanium dioxide with a mass fraction of 0.75% of the total mass of the calcined material and the second iron source were added to pure water and stirred to form a fourth slurry with a solid content of 59.6%. The fourth slurry was filtered into a filter cake and then flash evaporated to obtain a second dry material. The second dry material was heated to 780°C at 3°C / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120°C and removed from the furnace to obtain a second discharged material. The second discharged material was pulverized to a D50 particle size of 1.9um to obtain a lithium iron phosphate material with a carbon content of 1.26%. The charging and discharging performance diagrams of the obtained lithium iron phosphate material are as shown in the figure. Figure 6 shown.

[0183] Example 5 1) Same as Example 2.

[0184] 2) Same as Example 4.

[0185] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.992.

[0186] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.20% and an average primary particle size of 0.54 μm.

[0187] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0188] Example 6 1) Same as Example 2.

[0189] 2) Same as Example 4.

[0190] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 65 minutes. The first stirring temperature during the dropwise addition was 65° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 65° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.985.

[0191] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.03, and 9.7% of the mass of propylene glycol butyl ether (first carbon source) of the ferric phosphate dihydrate material were added with pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.42 μm to obtain a second slurry. The second slurry was added with 0.6% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 620° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.8 μm to obtain a burned material. The burned material had a carbon content of 0.23% and an average primary particle size of 0.63 μm.

[0192] The burnt material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 32% of the burnt material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, the paste starch (molecular weight: 20000) with a mass fraction of 11.2% of the second iron source, and the titanium dioxide with a total mass of 0.75% of the burnt material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 59.0%. The fourth slurry was filter-pressed to form a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 780° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 14 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was pulverized and the pulverized D50 particle size was 2.6 μm to obtain a lithium iron phosphate material with a carbon content of 1.19%.

[0193] Example 7 1) Same as Example 5.

[0194] 2) Same as Example 5.

[0195] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.4 mol of phosphorus element and a hydrogen peroxide solution containing 3.1 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.980.

[0196] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.50 μm to obtain a burned material. The burned material had a carbon content of 0.22% and an average primary particle size of 0.51 μm.

[0197] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0198] Example 8 1) Same as Example 5.

[0199] 2) Same as Example 5.

[0200] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.6 mol of phosphorus element and a hydrogen peroxide solution containing 2.9 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.986.

[0201] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.19% and an average primary particle size of 0.61 μm.

[0202] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0203] Example 9 1) Same as Example 5.

[0204] 2) Same as Example 5.

[0205] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.2 mol of phosphorus element and a hydrogen peroxide solution containing 3.3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.988.

[0206] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.23% and an average primary particle size of 0.52 μm.

[0207] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0208] Example 10 1) Same as Example 5.

[0209] 2) Same as Example 5.

[0210] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.8 mol of phosphorus element and a hydrogen peroxide solution containing 2.6 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 95° C. and stirred for 2 hours (first reaction time) until the pH value reached 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.968.

[0211] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.24% and an average primary particle size of 0.62 μm.

[0212] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0213] Example 11 1) Same as Example 5.

[0214] 2) Same as Example 5.

[0215] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 40 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 1.3. The mixture was then heated to a first reaction temperature of 92° C. and stirred for 1.5 hours (first reaction time) until the pH value reached 2.0. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.991.

[0216] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.16% and an average primary particle size of 0.52 μm.

[0217] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0218] Example 12 1) Same as Example 5.

[0219] 2) Same as Example 5.

[0220] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 minutes, and the hydrogen peroxide solution was added dropwise for 55 minutes. The first stirring temperature during the dropwise addition was 60° C. After the dropwise addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The mixture was then heated to a first reaction temperature of 98° C. and stirred for 2.5 hours (first reaction time) until the pH value reached 1.6. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.992.

[0221] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.17% and an average primary particle size of 0.52 μm.

[0222] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0223] Example 13 1) Same as Example 5.

[0224] 2) Same as Example 5.

[0225] 3) A ferrous solution with 1 mol of ferrous ions was added into a reaction kettle with slow stirring, and then a phosphorus source solution with 1.5 mol of phosphorus elements and a hydrogen peroxide solution with 3 mol of hydrogen peroxide (mass fraction of hydrogen peroxide was 30%) were simultaneously added into the reaction kettle through a peristaltic pump with slow dripping, the dripping time of the phosphorus source solution was 35 min, the dripping time of the hydrogen peroxide solution was 45 min, the first stirring temperature during dripping was 45°C, after the dripping was completed, the first pH value was adjusted to 0.6 by adding phosphoric acid at a second stirring temperature of 45°C, and then the first reaction temperature was increased to 88°C, and the stirring reaction was continued for 1.2 h (the first reaction time) until the pH value was 1.4, pressure filtration, rinsing (the conductivity of the rinsing water after rinsing was less than or equal to 500 μS / cm), and flash evaporation were performed to obtain a ferrous phosphate dihydrate material, and the iron-phosphorus ratio was 0.978.

[0226] Step two: preparation of a lithium iron phosphate positive electrode material The ferrous phosphate dihydrate material prepared in step one, a lithium source (lithium carbonate) with a lithium element to lithium element and ferrous phosphate dihydrate material molar ratio of 1.02, 8.5% of dipropyl glycerol (the first carbon source) based on the mass of the ferrous phosphate dihydrate material, and pure water were stirred and mixed to form a first slurry with a solid content of 60.5%, the first slurry was adjusted to a D50 particle size of 0.44 um through sand milling to obtain a second slurry, the second slurry was added to 0.3% of polymeric ferric sulfate based on the mass of the ferrous phosphate dihydrate material to obtain a third slurry, the third slurry was pressure filtered through a pressure filter and then flash evaporated to obtain a first dry material, the first dry material was heated to 600°C at a temperature increase rate of 5°C / min under a nitrogen protective atmosphere, the temperature was maintained for 9 h, and then the temperature was naturally decreased to below 120°C to discharge the material to obtain a first discharged material, the first discharged material was crushed, the crushed D50 particle size was 0.5 um, and a first calcined material was obtained, the carbon element content of the first calcined material was 0.21%, and the average primary particle size was 0.59 um.

[0227] The first calcined material, 28% of the ferrous phosphate dihydrate material (the second iron source) based on the mass of the first calcined material, a lithium source (lithium carbonate) with a lithium element to second iron source molar ratio of 1.05, 11.2% of paste starch (molecular weight: 10000) based on the mass of the second iron source, and 0.75% of titanium white powder based on the total mass of the first calcined material and the second iron source were stirred and mixed with pure water to form a fourth slurry with a solid content of 56.8%, the fourth slurry was pressure filtered into a filter cake and then flash evaporated to obtain a second dry material, the second dry material was heated to 790°C at a temperature increase rate of 3°C / min under a nitrogen protective atmosphere, the temperature was maintained for 10 h, and then the temperature was naturally decreased to below 120°C to discharge the material to obtain a second discharged material, the second discharged material was crushed, the crushed D50 particle size was 2.2 um, and a lithium iron phosphate material was obtained, and the carbon element content was 1.28%.

[0228] Example 14 1) Same as Example 5.

[0229] 2) Same as Example 5.

[0230] 3) A ferrous solution containing 1 mol of ferrous ions was added to the reactor and slowly stirred. Then, a phosphorus source solution containing 1.5 mol of phosphorus element and a hydrogen peroxide solution containing 3 mol of hydrogen peroxide (the mass fraction of hydrogen peroxide was 30%) were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 70 min, the hydrogen peroxide solution was added dropwise for 70 min, and the first stirring temperature during the addition was 70° C. After the addition was completed, phosphoric acid was added at a second stirring temperature of 70° C. to adjust the first pH value to 1.5. The mixture was then heated to a first reaction temperature of 100° C. and stirred for 3 h (first reaction time) until the pH value reached 2.2. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.990.

[0231] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.22% and an average primary particle size of 0.55 μm.

[0232] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.75% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0233] Example 15 1) Same as Example 5.

[0234] 2) Same as Example 5.

[0235] 3) Same as Example 5.

[0236] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.01, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 550° C. at 4° C. / min under a nitrogen protective atmosphere, kept warm for 6 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 1.4 μm to obtain a burned material. The burned material had a carbon content of 0.36% and an average primary particle size of 0.55 μm.

[0237] The first dry material, the second iron source of 28% of the first dry material by mass, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 11.2% of the second iron source by mass of the paste starch (molecular weight: 10000), 0.75% of the titanium white powder of the total mass of the first dry material and the second iron source, and pure water were stirred and mixed to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filtered into a filter cake and then flash evaporated to obtain a second dry material. The second dry material was heated to 790°C at a rate of 3°C / min under a nitrogen protective atmosphere, and then kept at 790°C for 10 hours. The second dry material was naturally cooled to below 120°C and discharged to obtain a second discharged material. The second discharged material was crushed to obtain a lithium iron phosphate material with a carbon element content of 1.30% and a D50 particle size of 1.9um.

[0238] Example 16 1) The same as example 5.

[0239] 2) The same as example 5.

[0240] 3) The same as example 5.

[0241] Step two: preparation of lithium iron phosphate positive electrode material The second dry material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron phosphate dihydrate material of 1.00, 6.8% of the second iron phosphate dihydrate material by mass of dipropylene glycol (first carbon source), and pure water were stirred and mixed to form a first slurry with a solid content of 59.0%. The first slurry was ground to control the D50 particle size to 0.40um to obtain a second slurry. The second slurry was added with 0.2% of the second iron phosphate dihydrate material by mass of polymeric ferric sulfate to obtain a third slurry. The third slurry was filtered by a filter press and then flash evaporated to obtain a first dry material. The first dry material was heated to 500°C at a rate of 1.5°C / min under a nitrogen protective atmosphere, and then kept at 500°C for 5 hours. The first dry material was naturally cooled to below 120°C and discharged to obtain a first discharged material. The first discharged material was crushed to obtain a first dry material with a carbon element content of 0.38% and an average primary particle size of 0.49um.

[0242] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, the paste starch (molecular weight: 10000) with a mass fraction of 11.2% of the second iron source, and the titanium dioxide with a total mass of 0.75% of the first-burned material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a D50 particle size of 1.4 μm to obtain a lithium iron phosphate material with a carbon content of 1.34%.

[0243] Example 17 1) Same as Example 5.

[0244] 2) Same as Example 5.

[0245] 3) Same as Example 5.

[0246] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.08, and 10.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 69.0%. The first slurry was sand milled to adjust the D50 particle size to 0.50 μm to obtain a second slurry. The second slurry was added with 1.0% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 700° C. at 8° C. / min under a nitrogen protective atmosphere, kept warm for 11 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 2.6 μm to obtain a burned material. The burned material had a carbon content of 0.12% and an average primary particle size of 0.62 μm.

[0247] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, the paste starch (molecular weight: 10000) with a mass fraction of 11.2% of the second iron source, and the titanium dioxide with a total mass of 0.75% of the first-burned material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to obtain a D50 particle size of 2.5 μm to obtain a lithium iron phosphate material with a carbon content of 1.24%.

[0248] Example 18 1) Same as Example 5.

[0249] 2) Same as Example 5.

[0250] 3) Same as Example 5.

[0251] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.20% and an average primary particle size of 0.54 μm.

[0252] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, 10.2% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.41% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 2° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and taken out of the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 1.8 μm to obtain a lithium iron phosphate material with a carbon content of 1.18%.

[0253] Example 19 1) Same as Example 5.

[0254] 2) Same as Example 5.

[0255] 3) Same as Example 5.

[0256] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.20% and an average primary particle size of 0.54 μm.

[0257] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, the paste starch (molecular weight: 10000) with a mass fraction of 11.2% of the second iron source, and the titanium dioxide with a total mass of 0.87% of the first-burned material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 4° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a D50 particle size of 1.5 μm to obtain a lithium iron phosphate material with a carbon content of 1.26%.

[0258] Example 20 1) Same as Example 5.

[0259] 2) Same as Example 5.

[0260] 3) Same as Example 5.

[0261] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.20% and an average primary particle size of 0.54 μm.

[0262] The first-burned material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 20% of the first-burned material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.02, 7.3% of the mass of the second iron source paste starch (molecular weight: 10000), and 0.35% of the total mass of the first-burned material and the second iron source titanium dioxide were added with pure water and stirred to form a fourth slurry with a solid content of 45%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 720° C. at 1.5° C. / min under a nitrogen protective atmosphere, kept warm for 7 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 1.4 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0263] Example 21 1) Same as Example 5.

[0264] 2) Same as Example 5.

[0265] 3) Same as Example 5.

[0266] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate material prepared in step 1, a lithium source (lithium carbonate) having a molar ratio of lithium element to ferric phosphate dihydrate material of 1.02, and 8.5% of the mass of dipropylene glycol (first carbon source) of the ferric phosphate dihydrate material were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The first slurry was sand milled to adjust the D50 particle size to 0.44 μm to obtain a second slurry. The second slurry was added with 0.3% of the mass of the ferric phosphate dihydrate material. Polyferric sulfate was added to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 h, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to a crushed D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.20% and an average primary particle size of 0.54 μm.

[0267] The burnt material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the burnt material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.08, the paste starch (molecular weight: 10000) with a mass fraction of 12.5% ​​of the second iron source, and the titanium dioxide with a total mass of 0.92% of the burnt material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 65%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 820° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 16 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to obtain a D50 particle size of 2.6 μm to obtain a lithium iron phosphate material with a carbon content of 1.16%.

[0268] Example 22 This embodiment is the same as or similar to embodiment 5, except that titanium dioxide is not added to the second slurry.

[0269] Comparative Example 1 The comparative example provides a method for preparing anhydrous iron phosphate and carbon-coated lithium iron phosphate, which are prepared by the following method: Step 1: Preparation of ferric phosphate dihydrate 1) The titanium dioxide by-product was dissolved by heating, and 5% concentration of liquid caustic soda was added for sedimentation, and the impurities were removed by filter pressing to obtain a ferrous solution with an iron ion mass fraction of 2.0 mol / L and a pH of 2.4; 2) Preparation of phosphorus source: dissolve industrial monoammonium phosphate, filter it, add ammonia water and adjust the pH value to neutral to obtain phosphorus source solution, in which PO4 3- The content is 1.5mol / L; 3) 1 mol of ferrous solution was added to the reactor and slowly stirred. Then, 1.5 mol of phosphorus source solution and 3 mol of hydrogen peroxide solution were slowly added to the reactor simultaneously through a peristaltic pump. The phosphorus source solution was added dropwise for 65 min, and the hydrogen peroxide solution was added dropwise for 55 min. The first stirring temperature during the addition was 60° C. After the addition was completed, phosphoric acid was added at a second stirring temperature of 60° C. to adjust the first pH value to 0.8. The temperature was then raised to a first reaction temperature of 95° C. The stirring reaction was continued for 2 h (first reaction time) until the pH value was 1.8. The mixture was then filtered, rinsed (the conductivity of the rinse water after rinsing was less than or equal to 500 μS / cm), and flash evaporated to obtain ferric phosphate dihydrate with an iron-phosphorus ratio of 0.992.

[0270] Step 2: Preparation of lithium iron phosphate positive electrode material The ferric phosphate dihydrate prepared in step 1, a lithium source with a molar ratio of 1.02 for ferric phosphate dihydrate, and 6.3% of glucose by mass of ferric phosphate dihydrate were added to pure water and stirred to form a first slurry with a solid content of 60.5%. The particle size of D50 was adjusted to 0.44 μm by sand milling to obtain a second slurry. 0.3% of the mass of the ferric phosphate dihydrate material of polyferric sulfate was added to the second slurry to obtain a third slurry. The third slurry was filtered through a filter press and then flashed to obtain a first dried material. The first dried material was heated to 600° C. at 5° C. / min under a nitrogen protective atmosphere, kept warm for 9 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a first discharged material. The first discharged material was crushed to obtain a D50 particle size of 0.5 μm to obtain a burned material. The burned material had a carbon content of 0.20% and an average primary particle size of 0.40 μm.

[0271] The burnt material, the dihydrate iron phosphate material (second iron source) with a mass fraction of 28% of the burnt material, the lithium source (lithium carbonate) with a molar ratio of lithium element to the second iron source of 1.05, glucose of 13.7% of the mass of the second iron source, and titanium dioxide of 0.75% of the total mass of the burnt material and the second iron source were added with pure water and stirred to form a fourth slurry with a solid content of 56.8%. The fourth slurry was filter-filtered into a filter cake and then flash-evaporated to obtain a second dry material. The second dry material was heated to 790° C. at 3° C. / min under a nitrogen protective atmosphere, kept warm for 10 hours, and then naturally cooled to below 120° C. and discharged from the furnace to obtain a second discharged material. The second discharged material was crushed to a crushed D50 particle size of 2.2 μm to obtain a lithium iron phosphate material with a carbon content of 1.28%.

[0272] Comparative Example 2 Compared with Example 5, the difference is that the polyferric sulfate is replaced with an equal mass of iron oxide, and the other steps and conditions are the same as those in Example 5. The SEM results and particle size statistics of the obtained lithium iron phosphate material are shown as follows: Figure 2 and Figure 3 shown.

[0273] Comparative Example 3 Compared with Example 5, the difference is that the second slurry is spray-dried (inlet air temperature is 220℃-260℃, outlet air temperature is 85℃-105℃) to obtain a dried material that undergoes a first calcination treatment, and the third slurry is spray-dried (inlet air temperature is 220℃-260℃, outlet air temperature is 85℃-105℃) to obtain a dried material that undergoes a second calcination treatment. The other steps and conditions are the same as those in Example 5. The charging and discharging performance diagrams of the obtained lithium iron phosphate material are as follows: Figure 6 shown.

[0274] Comparative Example 4 Compared with Example 5, the difference is that: the dihydrate iron phosphate material prepared in step one is rotary kiln dried to remove the crystal water to form an anhydrous iron phosphate material, the second iron source is replaced by the anhydrous iron phosphate prepared in step one, and other steps and conditions are the same as those in Example 5.

[0275] Comparative Example 5 Compared with Example 5, the difference is that: in step two, the calcined material is the lithium iron phosphate material.

[0276] Comparative Example 6 Compared with Example 5, the difference is that: in step two, no calcined material, the dihydrate iron phosphate material, a lithium source (lithium carbonate) with a molar ratio of lithium element to dihydrate iron phosphate material of 1.05, 11.2% of the second iron source mass of paste starch (molecular weight: ), and 0.75% of the dihydrate iron phosphate material mass of titanium white powder are mixed with pure water to form a slurry with a solid content of 56.8%. The slurry is pressure-filtered into a filter cake and then flash evaporated to obtain a dry material. The dry material is heated at a temperature increasing rate of 3℃ / min to 790℃ under a nitrogen protective atmosphere, and then kept at 790℃ for 10h. Then the temperature is naturally lowered to below 120℃ to discharge the material. The discharged material is crushed to obtain a lithium iron phosphate material with a crushed D50 particle size of 2.2um, and a carbon element content of 1.18%.

[0277] II. Test Methods 1. Property Test of Lithium Iron Phosphate Material Particle size: SEM is used to take pictures, and then Nano Measurer 1.2.5 is used for statistics.

[0278] Element composition and content: detected by ICP-OES.

[0279] Compaction density: tested by a compaction density instrument, with a test pressure of 3T and a pressing time of 30S.

[0280] Magnetic foreign matter content: 100g of the material is weighed and poured into a plastic bottle with a cover. 1000g of pure water is added, followed by the addition of a 6000GS magnet. The magnet is protected by polytetrafluoroethylene outside. Then the cover is tightly closed, the plastic bottle is placed horizontally, and the plastic bottle is rotated at a speed of 10-20r / min. After 30-45min of rotation, the rotation is stopped, the magnet is taken out, 1000g of pure water is added, and then put into the plastic bottle. The cover is tightly closed, the plastic bottle is placed horizontally, and the plastic bottle is rotated at a speed of 10-20r / min. After 5-10min of rotation, the rotation is stopped, the magnet is taken out, and aqua regia is added for dissolution. The dissolved solution is measured by ICP-OES, and the volume is adjusted. The nickel, chromium, copper, zinc and iron contents in the solution are measured, and the total mass of nickel, chromium, copper, zinc and iron is calculated. Then the total mass of nickel, chromium, copper, zinc and iron is divided by the weight of the material to obtain the magnetic foreign matter content of the material.

[0281] Specific surface area: determined by gas adsorption BET method.

[0282] Powder resistivity: measured by four-probe method with a measuring pressure of 8 MPa.

[0283] Static iron dissolution: Add 1 g of the sample to be tested to 100 mL of a 0.1 mol / L hydrogen fluoride-ethanol solution, stir and dissolve at 45°C for 30 minutes, then filter. The iron content of the filtrate is measured by ICP-OES, which is the iron dissolution amount.

[0284] Free lithium content: The free lithium content is determined by potentiometric titration.

[0285] 2. Properties test of positive electrode and secondary battery The lithium iron phosphate materials prepared in the examples and comparative examples were mixed with conductive carbon black and PVDF binder in a mass ratio of 90:5:5, respectively, and coated on a 12 μm thick aluminum foil. The electrode was then placed in an oven at 110°C and dried for 10 hours. The dried electrode was punched into positive electrode discs with a diameter of 15 mm and pressed to a density of 2.5 g / cm 3 Roll-pressed, the negative electrode material is artificial graphite; the separator material is polyethylene + alumina ceramic separator; the electrolyte solvent is EC / DEC / EMC (mass ratio 2:5:3), the solute is 1 mol / L LiPF6; the electrolyte additive is 2% by mass VC, to prepare a soft-pack battery. The nominal capacity is 4700mAh and the nominal voltage is 3.2V.

[0286] Using a battery performance testing system (model: CT3002A) from Wuhan Blue Electric Electronic Technology Co., Ltd., clamped between two aluminum plates, the battery was tested at a constant temperature of 25°C ± 2°C at charge and discharge rates of 0.1C / 0.1C and 1C / 1C, with a voltage range of 2.50-3.65V. The battery was then subjected to specific capacity testing at a 0.1C constant voltage, cycle testing, and specific capacity testing at a discharge rate of 3.2V. The 1C rate test was repeated 100 times. Before cycling, the charge / discharge capacity was measured at 0.05C / 0.05C. After cycling, the capacity was calibrated.

[0287] 3. Analysis of test results of various embodiments and comparative examples Table 1 Table 2 Table 3 It can be seen from the results in Tables 1 to 3 that the present application controls the average particle size of the primary particles of the iron phosphate material to be 0.23μm~0.26μm, and further controls the primary particles of the lithium iron phosphate material to have a particle size greater than 0nm and less than 200nm, accounting for 42%~55.2%, and a particle size greater than or equal to 200nm and less than or equal to 2μm, accounting for 44.8%~58%, thereby obtaining a lithium iron phosphate material with high compaction density, small polarization, small internal resistance, and excellent electrochemical properties in terms of discharge performance, rate performance, and cycle performance.

[0288] As can be seen from the results in Tables 1 to 3, the present application promotes sufficient contact between the raw materials and reduces the occurrence of side reactions by using a water-retaining agent in the synthesis of the burnt material. In combination with controlling the mass fraction of the carbon element in the burnt material to 0.12% to 0.38%, preferably controlling the mass fraction of the carbon element in the burnt material to 0.18% to 0.33%, the uniform formation of lithium iron phosphate particles is promoted in the first calcination treatment, the generation of lithium iron phosphate particles with excessively large particle size is reduced, and the first discharged material is easily dispersed during pulverization, thereby facilitating the secondary feeding of the burnt material and eliminating the grinding step in the subsequent process. While saving costs, the average particle size of the primary particles of the lithium iron phosphate material is effectively reduced and the particle size of the maximum particles of the lithium iron phosphate material is controlled, thereby improving the compaction density of the lithium iron phosphate material and the electrochemical properties in terms of discharge performance, rate performance, and cycle performance.

[0289] It can be seen from the results in Tables 1 to 3 that the present application controls the amount ratio of the lithium element in the second lithium source to the second iron source to be (1.02~1.08):1, preferably controls the amount ratio of the lithium element in the second lithium source to the second iron source to be (1.04~1.06):1, so that the lithium source is excessive, thereby improving the structural integrity and charging and discharging performance of the lithium iron phosphate particles in the second calcination treatment, and thus improving the electrochemical properties of the lithium iron phosphate material.

[0290] It can be seen from the results in Tables 1 to 3 that the present application uses dihydrate iron phosphate material with an iron-phosphorus ratio of 0.968 to 0.992, preferably an iron-phosphorus ratio of 0.976 to 0.992, as the first iron source and the second iron source. The higher iron-phosphorus ratio is beneficial to reducing the impurities in the first-fired material, and the above-mentioned iron-phosphorus ratio is used to control the excess phosphate to coarsen the degree of grain size of the lithium iron phosphate particles formed in the first calcination treatment and the second calcination treatment. At the same time, the removal of crystallization water in the first calcination treatment and the second calcination treatment makes the lithium iron phosphate particles smaller and the lithium iron phosphate particles more tightly bonded, which is beneficial to improve the compaction density of the lithium iron phosphate material.

[0291] As can be seen from the results in Tables 1 to 3, the present application utilizes the adsorption and dispersibility of the first carbon source on the first iron source and the first lithium source by selecting the first carbon source as the first carbon source, which is conducive to the full reaction of the first iron source and the first lithium source in the first calcination treatment, reducing the carbon content of the first carbon source after the first calcination treatment, controlling the mass fraction of the carbon element in the first-fired material, and combining the second carbon source with an organic matter or polymer with a larger molecular weight to form a uniform and appropriately thick carbon coating on the surface of the lithium iron phosphate core of the lithium iron phosphate material finally obtained, thereby effectively improving the electrochemical properties of the lithium iron phosphate material in terms of charging performance, discharging performance, rate performance and cycle life.

[0292] It can be seen from the results in Tables 1 to 3 that the present application obtains the first dry material by flash evaporation treatment after filter pressing the third slurry and obtains the second dry material by flash evaporation treatment after filter pressing the fourth slurry. During the extrusion process, the particles are further brought into close contact with each other, which is beneficial to further improve the compaction density of the lithium iron phosphate material. At the same time, during the calcination process, it is beneficial to inhibit the growth of lithium iron phosphate particles with smaller particle size inside the filter cake and to facilitate the fusion and growth of lithium iron phosphate particles outside the filter cake, thereby further regulating the ratio of larger and smaller primary particles in the final obtained lithium iron phosphate material to improve the electrochemical properties of the lithium iron phosphate material.

[0293] 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 lithium iron phosphate material, characterized in that: The average particle size of the primary particles of the lithium iron phosphate material is 0.23 μm to 0.26 μm. Among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, and the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm accounts for 44.8% to 58%; The compaction density of the lithium iron phosphate material is 2.61 g / mL to 2.67 g / mL.

2. The lithium iron phosphate material according to claim 1, characterized in that The lithium iron phosphate material includes a lithium iron phosphate core and a carbon coating layer coated on the surface of the lithium iron phosphate core. The general formula of the lithium iron phosphate core is: LiFe x Ti y (PO4), 0.964≤x≤1.0, 0.0≤y≤0.036; The mass fraction of the carbon coating layer in the lithium iron phosphate material is 1.16% to 1.34%.

3. The lithium iron phosphate material according to claim 1 or 2, characterized in that: The primary particles of the lithium iron phosphate material include first particles and second particles, the particle size of the first particles is greater than 0.0 nm and less than 200 nm, the average particle size of the first particles is 23 nm to 26 nm, the particle size of the second particles is 200 nm to 2000 nm, and the average particle size of the second particles is 340 nm to 410 nm; Among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 600 nm accounts for 34% to 50.5%, the number of primary particles with a particle size greater than 600 nm and less than or equal to 1 μm accounts for 2% to 11%, and the number of primary particles with a particle size greater than 1 μm and less than or equal to 2 μm accounts for 0.3% to 8.5%.

4. A method for preparing lithium iron phosphate material, characterized in that: include: A first iron source is mixed with a first lithium source, a first carbon source, and a first solvent to obtain a first slurry; The first slurry is subjected to a first grinding process to obtain a second slurry; The second slurry is mixed with a water retaining agent to obtain a third slurry; The third slurry is subjected to a first solid-liquid separation process to obtain a first dry material; The first dried material is subjected to a first calcination treatment to obtain a first calcined material; The first calcined material is mixed with a second iron source, a second lithium source, a second carbon source and a second solvent to obtain a fourth slurry; The fourth slurry is subjected to a second solid-liquid separation process to obtain a second dry material; The second dried material is subjected to a second calcination process to obtain a lithium iron phosphate material; Wherein, the first iron source and the second iron source are independently selected from ferric phosphate dihydrate materials, and the mass fraction of carbon element in the calcined material is 0.12% to 0.38%; The average particle size of the primary particles of the lithium iron phosphate material is 0.23 μm to 0.26 μm. Among the primary particles of the lithium iron phosphate material, the number of primary particles with a particle size greater than 0 nm and less than 200 nm accounts for 42% to 55.2%, and the number of primary particles with a particle size greater than or equal to 200 nm and less than or equal to 2 μm accounts for 44.8% to 58%; The compaction density of the lithium iron phosphate material is 2.61 g / mL to 2.67 g / mL.

5. The method for preparing the lithium iron phosphate material according to claim 4, wherein: The preparation steps of the ferric phosphate dihydrate material include: The ferrous source, the phosphorus source and the first oxidant are subjected to a first mixing process to obtain a first mixture; The first mixture and the first pH adjuster are subjected to a second mixing process to obtain a second mixture; The second mixture is subjected to a first reaction to obtain a third mixture; The third mixture is subjected to a third solid-liquid separation process to obtain the ferric phosphate dihydrate material; The iron-phosphorus ratio of the ferric phosphate dihydrate material is 0.968-0.

992.

6. The method for preparing the lithium iron phosphate material according to claim 4, characterized in that: The molar ratio of the lithium element in the first lithium source to the first iron source is (1.00-1.08):1, the mass ratio of the first carbon source to the first iron source is (7.1-10.5):100, and the mass ratio of the water retaining agent to the first iron source is (0.2-1):100; The molar ratio of the lithium element in the second lithium source to the second iron source is (1.02-1.08):1, the mass ratio of the second carbon source to the second iron source is (7.3-12.5):100, and the mass ratio of the second iron source to the first-fired material is (20-37):

100.

7. The method for preparing the lithium iron phosphate material according to claim 4, characterized in that: The step of subjecting the first dried material to a first calcination treatment to obtain a first calcined material comprises: Under a first protective atmosphere, the first dried material is calcined at a first temperature and for a first calcination time to obtain a first discharged material; The first discharged material is subjected to a first pulverization process to obtain the first burned material; The step of subjecting the second dried material to a second calcination treatment to obtain a lithium iron phosphate material comprises: Under a second protective atmosphere, the second dried material is calcined at a second temperature and for a second time to obtain a second discharged material; The second discharged material is subjected to a second pulverization process to obtain the lithium iron phosphate material; Wherein, the first calcination temperature is 500°C to 700°C, the first calcination time is 5h to 11h, the second calcination temperature is 720°C to 820°C, and the second calcination time is 7h to 16h; The first protective atmosphere and the second protective atmosphere are independently selected from at least one of a nitrogen atmosphere and a helium atmosphere; The average particle size of the primary particles of the calcined material is 490nm~620nm.

8. The method for preparing the lithium iron phosphate material according to any one of claims 4 to 7, characterized in that: The first lithium source and the second lithium source are independently selected from at least one of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; The first carbon source is selected from organic matter and / or oligomers having a molecular weight less than or equal to 600; The water-retaining agent is selected from the group consisting of polymeric ferric sulfate water-retaining agents; The second carbon source is selected from organic matter and / or polymers having a molecular weight greater than or equal to 1000 and less than or equal to 100,000; When the second slurry further includes a dopant, the dopant is selected from titanium dioxide, and the ratio of the total mass of the calcined material and the second iron source to the mass of the dopant is 100:(0.35-0.92).

9. A positive electrode plate, characterized in that: It includes a current collector and a positive electrode material arranged on at least one side of the current collector along its thickness direction, the positive electrode material includes the lithium iron phosphate material as described in any one of claims 1 to 3, or the positive electrode material includes the lithium iron phosphate material prepared by the preparation method of the lithium iron phosphate material as described in any one of claims 4 to 8.

10. A secondary battery, characterized in that: Including positive electrode sheet, negative electrode sheet and separator; Wherein, the positive electrode plate is the positive electrode plate as claimed in claim 9.

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