Iron phosphate material and preparation method therefor, positive electrode material, positive electrode sheet, and secondary battery

Abstract

Provided are an iron phosphate material and a preparation method therefor, a positive electrode material, a positive electrode sheet, and a secondary battery. The iron phosphate material is doped with titanium. At least part of the surface of primary particles of the iron phosphate material has a laminated structure. In a plurality of lamellas of the laminated structure, any two adjacent lamellas are respectively located on one side of each other. The preparation method comprises: providing a mixed solution comprising a ferrous salt, a titanium salt and a phosphate; mixing the mixed solution with a first alkali source to obtain a first slurry; adding an oxidant and a second alkali source to the first slurry to obtain a second slurry; performing a first solid-liquid separation treatment on the second slurry to obtain a filter cake; mixing the filter cake with a phosphoric acid solution, and aging the resulting mixture to obtain a third slurry; and performing a second solid-liquid separation treatment on the third slurry, then drying same, and performing a calcination treatment to obtain an iron phosphate material. Further provided are a positive electrode material comprising the iron phosphate material, a positive electrode sheet comprising a positive electrode active layer of the positive electrode material, and a secondary battery comprising the positive electrode sheet.

Description

Iron phosphate material, preparation method thereof, positive electrode material, positive electrode sheet and secondary battery

[0001] The present application claims priority to the Chinese patent application No. 202411256393.2, filed on September 6, 2024, entitled "Iron phosphate material, preparation method thereof, positive electrode material, positive electrode sheet and secondary battery", the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium ion batteries, in particular to an iron phosphate material, a preparation method thereof, a positive electrode material, a positive electrode sheet and a secondary battery. BACKGROUND

[0003] Lithium iron phosphate (LiFePO4) is a kind of positive electrode material. Compared with other kinds of positive electrode materials, lithium iron phosphate has high safety and suitable specific discharge capacity, and is widely used in lithium ion batteries.

[0004] Iron phosphate material is a common precursor material for producing lithium iron phosphate. A common process route is to use iron phosphate, lithium carbonate and glucose to form lithium iron phosphate through solid phase reaction. The properties and performance of the prepared lithium iron phosphate are largely determined by the iron phosphate material.

[0005] The micro-morphology of iron phosphate has a significant influence on the macro-performance of iron phosphate. For example, the particle morphology of the iron phosphate material directly affects the bulk density and contact area of the iron phosphate material as a whole. The primary particles of the iron phosphate material in the traditional technology are mostly dispersed irregular particles or spherical particles in microstructure. Spherical shape is the smallest specific surface area geometry, which leads to a small overall specific surface area and contact area with the outside of the iron phosphate material. The charge and discharge efficiency of the positive electrode material prepared therefrom is low. Some traditional technologies also propose iron phosphate materials with flaky primary particles, but the flaky particles are randomly and disorderly distributed, and there is obvious contact stacking and horizontal and vertical interlacing between the flakes, which has limited improvement on the specific surface area and contact area. The improvement on the charge and discharge efficiency of the prepared positive electrode material is also limited.

[0006] Based on the above, further research is still needed to improve the specific surface area and contact area of the iron phosphate material.

[0007] SUMMARY

[0008] In view of the technical problems in the background art, the present application provides an iron phosphate material, a preparation method thereof, a positive electrode material, a positive electrode sheet and a secondary battery. The iron phosphate material of the present application can effectively improve the specific surface area and contact area of the iron phosphate material through structural design.

[0009] In a first aspect, the embodiments of the present application provide a ferric phosphate material, wherein titanium elements are doped in the ferric phosphate material, at least part of the surface of primary particles of the ferric phosphate material is in a laminated structure, and the laminated structure has a plurality of layers, and any two adjacent layers are respectively located on one side of each other in a stacking direction.

[0010] In the embodiments, the stacking direction intersects with the extension direction of the layers.

[0011] In the embodiments, titanium elements are doped in the ferric phosphate material, the titanium elements cause defects in the crystallization process of the ferric phosphate material, hinder the growth of the ferric phosphate material in a specific direction, and are beneficial to the formation of the laminated structure on the surface of the ferric phosphate particles. The surface of the primary particles of the ferric phosphate material is in the laminated structure. Any two adjacent layers in the plurality of layers of the laminated structure are respectively located on one side of each other in a stacking direction. Unlike the traditional disordered distribution of the lamellar ferric phosphate material, the laminated structure forms a short-range ordered morphology, avoids the space waste caused by the horizontal and vertical interlacing of the layers, and can significantly improve the specific surface area and the contact area of the primary particles. In addition, since the ionic radius of titanium ions is smaller than that of iron ions, the ionic channels of the sites occupied by titanium ions are widened. This is also beneficial to improve the ion migration efficiency of the positive electrode material prepared therefrom in the charging and discharging process.

[0012] In some embodiments of the present application, the surface of the primary particles of the ferric phosphate material is split into the laminated structure; and / or,

[0013] The surface of a single primary particle of the ferric phosphate material has a plurality of laminated structures; and / or,

[0014] In a single laminated structure, the number of layers is 2-4.

[0015] In the embodiments, the connection between the laminated structure formed by the split of the surface of the primary particles and the matrix in which it is located is more stable, which is beneficial to maintaining the structural integrity in the subsequent processing process, and thus the structure of the ferric phosphate material can be maintained more stably in the subsequent processing and charging and discharging process.

[0016] In the embodiments, the plurality of laminated structures can further increase the specific surface area and the contact area of a single primary particle, which is beneficial to further improve the charging and discharging performance of the positive electrode material prepared therefrom.

[0017] In the embodiments, 2-4 layers are arranged in the laminated structure, which is beneficial to balance the large specific surface area and the stability of a single laminated structure.

[0018] In some embodiments of the present application, the particle size of a single primary particle is 300 nm to 1000 nm; and / or,

[0019] In the laminated structure, the thickness of each of the layers is 40 nm to 60 nm; and / or,

[0020] In the stacking direction, the maximum spacing between two adjacent layers is 20 nm to 50 nm.

[0021] In this embodiment, the particle size of a single primary particle is within the above range, which is conducive to forming a laminated structure with a large size and gap on the surface of a single primary particle, thereby generating a larger specific surface area and contact area.

[0022] In this embodiment, the thickness of the layers and the maximum spacing between two adjacent layers are within the above range, which is conducive to making the iron phosphate material have both high structural stability and a large specific surface area.

[0023] In some embodiments of the present application, the chemical formula of the iron phosphate material is: Fe (1-n) Ti 0.75n PO4, wherein 0.02≤n≤0.04.

[0024] In this embodiment, the iron phosphate material with the chemical formula has a more regular lattice structure and a higher degree of crystallization. This is conducive to ensuring the lattice integrity of the positive electrode material prepared therefrom, so that the positive electrode material has a good ion mobility.

[0025] In some embodiments of the present application, the specific surface area of the iron phosphate material is 9.4 m 2 / g to 14 m 2 / g; and / or,

[0026] The D50 particle size of the iron phosphate material is 30 μm to 60 μm; and / or,

[0027] The tap density of the iron phosphate material is 0.9 g / cm 3 to 1.05 g / cm 3 .

[0028] In this embodiment, the iron phosphate material with the above specific surface area is conducive to making the positive electrode material prepared therefrom have better charge-discharge performance.

[0029] In this embodiment, the iron phosphate material with the above particle size characteristics is conducive to improving the electrochemical performance of the positive electrode material prepared therefrom.

[0030] In this embodiment, the iron phosphate material with the above tap density has a large specific surface area itself, and is also conducive to improving the compaction density of the positive electrode material prepared therefrom.

[0031] In a second aspect, the embodiments of the present application provide a preparation method of the iron phosphate material, comprising the following steps:

[0032] providing a mixed solution comprising ferrous salt, titanium salt and phosphate salt;

[0033] mixing the mixed solution with a first alkali source to obtain a first slurry;

[0034] adding an oxidizing agent and a second alkali source to the first slurry to obtain a second slurry;

[0035] performing a first solid-liquid separation treatment on the second slurry to obtain a filter cake;

[0036] mixing the filter cake with a phosphoric acid solution, and performing an aging treatment to obtain a third slurry;

[0037] performing a second solid-liquid separation treatment on the third slurry, drying, and performing a calcination treatment to obtain the iron phosphate material.

[0038] In this embodiment, when the first alkali source is added, ferrous ions and titanium ions can co-precipitate to form mixed ferrous phosphate precipitate and titanium phosphate precipitate. Then, the oxidizing agent and the second alkali source are added, so that titanium elements can be doped into the iron phosphate precipitate in situ during the precipitation process. Then, during the aging process, the amorphous iron phosphate precipitate is dissolved and recrystallized to convert into iron phosphate dihydrate crystals. During the calcination process, the titanium elements doped in the iron phosphate make the crystal lattice have defects, thereby hindering the growth of the iron phosphate in a specific direction, and the surface of the iron phosphate particles splits to form multiple pieces arranged in sequence and spaced apart at the top end, forming a laminated structure. The preparation method of this embodiment can prepare the iron phosphate material in any of the above embodiments.

[0039] In some embodiments of the present application, in the mixed solution, the ratio of the amount of substance of the iron element to the amount of substance of the titanium element is 1:(0.015-0.03); and / or,

[0040] In the mixed solution, the ratio of the amount of substance of the iron element to the amount of substance of the phosphorus element is 1:(0.97-1.07); and / or,

[0041] The oxidizing agent comprises hydrogen peroxide, and during the process of adding the oxidizing agent and the second alkali source to the first slurry, the ratio of the amount of substance of hydrogen peroxide in the added hydrogen peroxide to the amount of substance of ferrous ions in the mixed solution is (0.55-0.75):1; and / or,

[0042] The oxidizing agent comprises oxygen or compressed air, and the ratio of the amount of substance of the oxygen added to the first slurry to the amount of substance of the ferrous ions in the mixed solution is (0.275-0.375):1, or the ratio of the amount of substance of the oxygen in the compressed air added to the first slurry to the amount of substance of the ferrous ions in the mixed solution is (0.275-0.375):1.

[0043] In this embodiment, the above-mentioned ratio of iron elements to titanium elements is selected, which is beneficial to the full doping of titanium elements into the iron phosphate in the subsequent deposition process while ensuring that the formed iron phosphate has a good crystal lattice structure. This is further beneficial to the formation of a larger laminated structure on the surface of the iron phosphate particles.

[0044] In this embodiment, the above-mentioned ratio of iron elements to phosphorus elements is selected, which is beneficial to the full deposition of iron elements and titanium elements in the subsequent deposition process and improves the utilization rate of iron elements and titanium elements.

[0045] In this embodiment, the oxidation product of hydrogen peroxide is only water, so that the selection of hydrogen peroxide as an oxidizing agent basically does not mix impurity elements into the iron phosphate precipitate, which is beneficial to obtaining a relatively pure iron phosphate precipitate. Further, by controlling the ratio of the amount of substance of hydrogen peroxide to the amount of substance of ferrous ions in the above-mentioned range, the full oxidation of ferrous ions can be ensured while avoiding the introduction of too many impurities. Oxygen and compressed air are gases, and when the oxidizing agent is selected as oxygen or compressed air, impurity elements are also avoided from being mixed into the iron phosphate precipitate, which is beneficial to obtaining a relatively pure iron phosphate precipitate. Correspondingly, the amount of oxygen in the oxygen or compressed air is controlled to have a ratio of the amount of substance of the oxygen to the amount of substance of ferrous ions within the corresponding range, which is beneficial to the full oxidation of ferrous ions.

[0046] In some embodiments of the present application, the pH value of the mixed solution is 0.5-1.5; and / or,

[0047] After the first alkali source is added to the mixed solution, the pH value of the first slurry formed is controlled to be 2.0-2.5; and / or,

[0048] In the process of adding the oxidizing agent and the second alkali source to the first slurry, the pH value is controlled to be 2.0-2.5.

[0049] In this embodiment, the mixed solution with the pH value in the above-mentioned range has strong acidity, which can inhibit the hydrolysis and precipitation of ferrous ions and titanium ions, and further ensure that ferrous ions and titanium ions can form a uniform co-precipitate in the subsequent preparation process.

[0050] In this embodiment, the pH value of the first slurry is controlled to be 2.0-2.5, which is beneficial to control the low precipitation rate of ferrous ions and titanium ions, so that the particle size of ferrous phosphate precipitate and titanium phosphate precipitate is smaller and more uniform, and the two can be fully mixed during the precipitation process. Further, the titanium element can be uniformly doped into the iron phosphate precipitate during the subsequent deposition process.

[0051] In this embodiment, the pH value of the first slurry is controlled to be 2.0-2.5, which can control the moderate transformation rate of ferrous ions to iron ions and the precipitation rate of iron ions, and is beneficial to the in-situ doping of titanium element into the formed iron phosphate precipitate during the precipitation process of iron element. Moreover, in the actual preparation process, when the pH value is too high, magnesium ions and potassium ions and other impurities will be mixed into the iron phosphate material, thereby negatively affecting the powder resistance and discharge efficiency of the finally prepared positive electrode material.

[0052] In some embodiments of the present application, the aging treatment step comprises: heating the mixture of the filter cake and the phosphoric acid solution to 85-98°C; and / or,

[0053] In the step of mixing the filter cake with the phosphoric acid solution, the ratio of the amount of substance of phosphoric acid in the phosphoric acid solution to the amount of substance of phosphate in the mixture is (0.1-0.2):1.

[0054] In this embodiment, after dispersing the iron phosphate precipitate in the phosphoric acid solution and then heating to 85-98°C, the iron phosphate is allowed to nucleate and grow again after dissolution, which can fully convert the yellow amorphous iron phosphate precipitate into white iron phosphate dihydrate crystals.

[0055] In this embodiment, the ratio of the amount of substance of phosphoric acid to the amount of substance of phosphate in the mixture is controlled within the above range, which can make the iron phosphate precipitate fully dissolve while reducing or even avoiding the introduction of anionic impurities due to phosphoric acid. Moreover, when the ratio of the amount of substance of phosphoric acid in the phosphoric acid solution to the amount of substance of phosphate in the mixture is within the above range, it is more beneficial to the occurrence and full performance of the crystal type conversion.

[0056] In some embodiments of the present application, the calcination treatment step comprises: calcining the dried material at a temperature of 600-800°C for 2-3h to form the iron phosphate material.

[0057] In this embodiment, the above temperature range and heating time are used for calcination treatment, which can fully remove the crystal water in the crystal, and promote the iron phosphate to nucleate and grow again, and split into laminated structures on the surface of the particles during the growth process.

[0058] In a third aspect, the application further provides a positive electrode material, wherein a raw material for preparing the positive electrode material comprises the iron phosphate material according to any one of the above embodiments, or a raw material for preparing the positive electrode material comprises the iron phosphate material prepared by the preparation method according to any one of the above embodiments.

[0059] In this embodiment, the iron phosphate material has a laminated structure and a large specific surface area. Accordingly, the positive electrode material prepared by using the iron phosphate material as a raw material also has a large contact area and a low internal resistance, and the doped titanium ions are also beneficial to widening the ion channel in the positive electrode material.

[0060] In some embodiments of the application, the positive electrode material comprises one or more of lithium iron phosphate and lithium manganese iron phosphate.

[0061] In a fourth aspect, the application further provides a positive electrode tab, wherein the positive electrode tab comprises a current collector and a positive electrode active layer arranged on the current collector, and the positive electrode active layer comprises the positive electrode material according to any one of the above embodiments.

[0062] In this embodiment, the positive electrode tab comprises the positive electrode material described above, and the positive electrode material has a large contact area and a low internal resistance, so that the positive electrode tab also has good charge-discharge performance.

[0063] In a fifth aspect, the application further provides a secondary battery, wherein the secondary battery comprises a positive electrode tab and a negative electrode tab, and the positive electrode tab of the secondary battery is the positive electrode tab according to the above embodiments.

[0064] In this embodiment, the secondary battery comprising the positive electrode tab described above can exhibit high charge-discharge efficiency and cycle stability during the charge-discharge process.

[0065] The above description is only a summary of the technical solutions of the application. In order to enable the technical means of the application to be more clearly understood, the following detailed description of the embodiments of the application can be implemented according to the content of the description, and in order to make the above and other purposes, characteristics and advantages of the application more obvious and easy to understand, the following detailed description of the embodiments of the application. BRIEF DESCRIPTION OF DRAWINGS

[0066] In order to more clearly illustrate the technical solutions of the application, the following will briefly introduce the drawings used in the application. Obviously, the drawings described below are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creating any creative labor on the basis of these drawings.

[0067] FIG. 1 is a schematic diagram of the steps of the preparation method of the iron phosphate material of the application;

[0068] FIG. 2 is a schematic diagram of the preparation process of the iron phosphate material of the application;

[0069] Figure 3 is a surface morphology diagram of the iron phosphate material prepared in Example 1;

[0070] Figure 4 is a surface morphology diagram of the iron phosphate material prepared in Comparative Example 1;

[0071] Figure 5 is a graph of the X-ray diffraction test results of the iron phosphate materials prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0072] The embodiments of the technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0073] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application; the terms "include" and "have" and any variations thereof used in the specification and the claims and the above description of drawings shall not be construed as excluding any additional elements or steps.

[0074] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is two or more, unless otherwise explicitly and specifically limited.

[0075] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily mutually exclusive or alternative to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0076] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0077] In the description of the embodiments of the present application, the term "multiple" refers to two or more (including two), and similarly, "multiple groups" refers to two or more groups (including two groups), and "multiple pieces" refers to two or more pieces (including two pieces).

[0078] In the description of the embodiments of the present application, the orientations or positional relationships indicated by the technical terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the embodiments of the present application.

[0079] In the description of the embodiments of the present application, unless otherwise explicitly specified and limited, the technical terms "mounting", "connecting", "connecting", "fixing" and the like should be understood in a broad sense, for example, can be fixedly connected, or can be detachably connected, or can be integrated; can be mechanically connected, or can be electrically connected; can be directly connected, or can be indirectly connected through an intermediate medium; can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0080] As analyzed in the background art of the present application, the primary particles of the iron phosphate material in the conventional technology are mostly spherical or flaky, and the actual specific surface area is relatively low, which also limits the improvement of the specific surface area and contact area of the positive electrode material prepared therefrom.

[0081] In order to improve the above problems, one embodiment of the present application provides an iron phosphate material, the iron phosphate material is doped with titanium elements, at least part of the surface of the primary particles of the iron phosphate material is in a laminated structure, the laminated structure has a plurality of lamellas, and any two adjacent lamellas are located on one side of each other in a stacking direction; wherein the stacking direction intersects with the extension direction of the lamella.

[0082] In this embodiment, the titanium element is doped in the iron phosphate material, which makes the iron phosphate material produce defects in the crystallization process, hinders the growth of the iron phosphate in a specific direction, and is conducive to the formation of a laminated structure on the surface of the iron phosphate particle. The surface of the primary particle of the iron phosphate material has a laminated structure. Any two adjacent layers in the plurality of layers of the laminated structure are located on one side of each other in the stacking direction. Unlike the traditional randomly distributed flaky iron phosphate material, the laminated structure forms a short-range ordered morphology, avoids the situation of space waste caused by the horizontal and vertical interlacing of the layers, and can significantly improve the specific surface area and contact area of the primary particles. In addition, since the ionic radius of titanium ions is smaller than that of iron ions, the ionic channels of the sites occupied by titanium ions are widened. This is also conducive to improving the ion migration efficiency of the positive electrode material prepared therefrom during the charging and discharging process.

[0083] It can be understood that the primary particle refers to a single minimum particle directly formed in the chemical synthesis process. A plurality of primary particles form secondary particles after agglomeration. The laminated structure in the embodiment of the present application is directly formed on the surface of the particle during synthesis, and thus part of the structure in the primary particle is the laminated structure.

[0084] In this embodiment, "any two adjacent layers are located on one side of each other in the stacking direction" means that in any two adjacent layers, the first layer is located on one side of the second layer in the stacking direction, and the second layer is also located on one side of the first layer in the stacking direction. The stacking direction can be perpendicular or obliquely intersected with the extension direction of the layer.

[0085] As an example of this embodiment, the surface of the primary particle of the iron phosphate material is split into a laminated structure. The doped titanium element makes the iron phosphate crystal structure produce defects, and the growth of the iron phosphate crystal structure in a specific direction is hindered, so that the surface can be split to form a laminated structure.

[0086] In this example, the connection between the laminated structure formed by the split of the surface of the primary particle and the matrix in which it is located is more stable, which is conducive to maintaining the integrity of the structure during subsequent processing, and thus the structure of the iron phosphate material can be maintained more stably during subsequent processing and charging-discharging.

[0087] As an example of this embodiment, the surface of a single primary particle of the iron phosphate material has a plurality of laminated structures.

[0088] In this example, the plurality of laminated structures can further increase the specific surface area and contact area of a single primary particle, which is conducive to further improving the charging-discharging performance of the positive electrode material prepared therefrom.

[0089] It can be understood that in other examples, the surface of a primary particle of a single iron phosphate material can also only have one laminar structure. Further, although not explicitly defined or described, in other examples, the surface of a primary particle of an iron phosphate material can have one or more randomly distributed sheet structures in addition to the ordered laminar structure, or can also have a plurality of sheet structures that are otherwise ordered.

[0090] As an example of this embodiment, in a single laminar structure, the number of sheet layers is 2-4. For example, in a laminar structure, the number of sheet layers can be 2, 3, 4, or the number of sheet layers can also be between any two of the above.

[0091] In this example, the laminar structure is provided with 2-4 sheet layers, which is beneficial to balance the larger specific surface area and the stability of a single laminar structure.

[0092] As an example of this embodiment, the particle size of a single primary particle is 300-1000 nm. For example, the particle size of a single primary particle is 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, or the particle size of a single primary particle can also be between any two of the above.

[0093] In this example, the particle size of a single primary particle is within the above range, which is beneficial to form a laminar structure with a large size and gap on the surface of a single primary particle, thereby generating a larger specific surface area and contact area.

[0094] As an example of this embodiment, in a laminar structure, the thickness of each sheet layer is 40-60 nm. For example, the thickness of each sheet layer can be 40 nm, 42 nm, 45 nm, 47 nm, 50 nm, 52 nm, 55 nm, 57 nm, 60 nm, or the thickness of each sheet layer can also be between any two of the above.

[0095] It can be understood that in this embodiment, the two adjacent sheet layers can be completely spaced apart or have a gap in at least a partial region. "Completely spaced apart" means that there is no region of contact between the two adjacent sheet layers. "There is a gap in at least a partial region" means that a partial region of the two adjacent sheet layers is in contact, and another partial region is spaced apart to form a gap.

[0096] As an example of this embodiment, the distance between the two adjacent sheet layers on the side close to the substrate is less than the distance between the two adjacent sheet layers on the side away from the substrate.

[0097] As an example of this embodiment, the maximum spacing between two adjacent layers in the stacking direction is 20 nm to 50 nm. For example, the maximum spacing between two adjacent layers is 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, or 50 nm, or the maximum spacing between two adjacent layers can be between any two of the above spacings. It will be understood that the spacing between two adjacent layers in the stacking direction can be non-uniform, i.e., the spacing is larger in some regions and smaller in some regions. The maximum spacing refers to the maximum distance between two adjacent layers in the stacking direction.

[0098] In this example, the thickness of the layers and the maximum spacing between two adjacent layers are within the above ranges, which is beneficial to achieving a higher structural stability and a larger specific surface area of the iron phosphate material.

[0099] As an example of this embodiment, the chemical formula of the iron phosphate material is: (1-n) Ti 0.75n PO4, where 0.02≤n≤0.04. For example, in the chemical formula of the iron phosphate material, n can be 0.02, 0.022, 0.024, 0.026, 0.028, 0.03, 0.032, 0.034, 0.036, 0.038, 0.04, or n can be between any two of the above values.

[0100] In this example, the iron phosphate material with this chemical formula has a more regular lattice structure and a higher degree of crystallization. This is beneficial to ensuring the lattice integrity of the positive electrode material prepared therefrom, so that the positive electrode material has a better ion mobility.

[0101] As a further example of this embodiment, in the iron phosphate material, the mass content of titanium elements is 4000 ppm to 10000 ppm. For example, the mass content of titanium elements can be 4000 ppm, 5000 ppm, 6000 ppm, 7000 ppm, 8000 ppm, 9000 ppm, or 10000 ppm, or the mass content of titanium elements can be between any two of the above mass contents.

[0102] In this example, when the mass content of titanium elements is within the above range, it is beneficial to significantly improve the specific surface area of the iron phosphate material while minimizing the negative impact on the charge and discharge capacity caused by the introduction of titanium elements.

[0103] As a further example of this embodiment, the iron phosphate material has an iron-to-phosphorus ratio of 0.94 to 0.98. The mass content of iron elements in the iron phosphate material is 35.8% to 36.4%. The mass content of phosphorus elements in the iron phosphate material is 20.58% to 20.64%.

[0104] The higher the titanium element content in the iron phosphate material, the lower the value of the iron-phosphorus ratio. When the iron-phosphorus ratio of the iron phosphate material is controlled in the range of 0.94-0.98 and the iron content and the phosphorus content are in the above range, the iron phosphate material is more beneficial as a precursor material for preparing the positive electrode material, and ensures that the prepared positive electrode material has good charge-discharge performance. The iron-phosphorus ratio of the iron phosphate material is the ratio of the amount of substance of iron element to phosphorus element in the iron phosphate material.

[0105] As an example of this embodiment, the specific surface area of the iron phosphate material is 9.4 m 2 / g-14 m 2 / g. For example, the specific surface area of the iron phosphate material can be 9.4 m 2 / g, 9.5 m 2 / g, 9.6 m 2 / g, 9.8 m 2 / g, 10 m 2 / g, 10.5 m 2 / g, 11 m 2 / g, 11.5 m 2 / g, 12 m 2 / g, 12.5 m 2 / g, 13 m 2 / g, 13.5 m 2 / g, 14 m 2 / g, or the specific surface area of the iron phosphate material can also be between any two of the above specific surface areas.

[0106] In this embodiment, the iron phosphate material with the above specific surface area is beneficial to enable the positive electrode material prepared to have better charge-discharge performance.

[0107] As an example of this embodiment, the D50 particle size of the iron phosphate material is 30 μm-60 μm. For example, the D50 particle size of the iron phosphate material can be 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or the D50 particle size of the iron phosphate material can also be between any two of the above particle sizes.

[0108] The D50 particle size refers to the particle size corresponding to the cumulative particle size distribution of the iron phosphate material reaching 50%, and its physical meaning is that the particles with a particle size less than this value account for 50% of the total particles. The D50 particle size can be measured by a laser particle size instrument.

[0109] In this example, the iron phosphate material with the above particle size characteristics is beneficial to improve the electrochemical performance of the positive electrode material prepared therefrom.

[0110] As an example of this embodiment, the tap density of the iron phosphate material is 0.9 g / cm3 ~1.05g / cm 3 For example, the tap density of the iron phosphate material can be 0.9g / cm 3 , 0.92g / cm 3 , 0.94g / cm 3 , 0.96g / cm 3 , 0.98g / cm 3 , 1g / cm 3 , 1.02g / cm 3 , 1.04g / cm 3 , 1.05g / cm 3 Alternatively, the tap density of the iron phosphate material can also be between any two tap densities mentioned above.

[0111] In this example, the iron phosphate material with the tap density itself has a larger specific surface area, which is also conducive to improving the tap density of the positive electrode material prepared therefrom.

[0112] In a second aspect, the application also provides a preparation method of the iron phosphate material. FIG. 1 is a schematic diagram of the steps of the preparation method of the iron phosphate material. Referring to FIG. 1, the preparation method includes the following steps S1-S6.

[0113] Step S1, providing a mixed solution in which ferrous salt, titanium salt and phosphate are dissolved.

[0114] As an example of this embodiment, the ferrous salt is selected from a salt capable of being dissolved in water and ionizing ferrous ions. For example, the ferrous salt can be selected from, but not limited to, any one or more of ferrous sulfate, ferrous chloride, ferrous oxalate. The ferrous salt has good solubility, low material cost, and the anion in the ferrous salt is easy to remove from the surface of the subsequently formed iron phosphate, with less residual amount, which is conducive to ensuring the purity of the iron phosphate material.

[0115] As an example of this embodiment, the titanium salt is selected from a salt capable of being dissolved in water and ionizing titanium ions. For example, the titanium salt can be selected from, but not limited to, any one or more of titanyl sulfate and titanium tetrachloride. The titanium salt has good solubility, and the anion therein is easy to remove from the surface of the subsequently formed iron phosphate, with less residual amount, which is conducive to ensuring the purity of the iron phosphate material.

[0116] As an example of this embodiment, the titanium salt can also use the filtrate obtained after mixing ilmenite with concentrated sulfuric acid, the mass fraction of titanium element in the filtrate is 7% to 10%, and the mass fraction of sulfate is 40% to 50%. For example, the mass fraction of titanium element can be 7.5%, 8%, 8.5%, 9%, 9.5%, or any value between 7% and 10%, and the mass fraction of sulfate is 42%, 45%, 46%, 48%, or any value between 40% and 50%. The mass fraction of H2SO4 in the concentrated sulfuric acid used can be 50% to 60%, for example, 52%, 54%, 55%, 56%, 58%, or any value between 50% and 60%.

[0117] As a further example of this embodiment, the titanium salt can also use the filtrate obtained after mixing ilmenite with concentrated sulfuric acid, which also includes iron elements and other impurity elements, wherein the mass fraction of iron element is 3% to 4% (such as 3.1%, 3.25%, 3.35%, 3.5%, 3.65%, 3.95%, etc.), and the impurity elements include Al, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Na, Ni, Pb and Zn, and the content of impurity elements is not more than 5000ppm.

[0118] As an example of this embodiment, the phosphate salt can be selected from, but not limited to, one or more of monobasic ammonium phosphate, dibasic ammonium phosphate and sodium hydrogen phosphate. The phosphate salt has good solubility, and the cations therein are easily removed from the surface of the subsequently formed iron phosphate, with little residual amount, which is conducive to ensuring the purity of the iron phosphate material.

[0119] As an example of this embodiment, in the mixed solution, the ratio of the amount of substance of iron element to the amount of substance of titanium element is 1:(0.015-0.03). For example, the ratio of the amount of substance of iron element to the amount of substance of titanium element can be 1:0.015, 1:0.016, 1:0.018, 1:0.02, 1:0.022, 1:0.024, 1:0.026, 1:0.028, 1:0.03, or the ratio of the amount of substance of iron element to the amount of substance of titanium element can also be between any two of the above ratios.

[0120] In this example, the above ratio of iron element to titanium element is selected, which is conducive to ensuring that the titanium element is fully doped into the iron phosphate while ensuring that the formed iron phosphate has a good lattice structure during the subsequent deposition process. This is further conducive to the formation of a good laminated structure on the surface of the iron phosphate particles.

[0121] As an example of this embodiment, the ratio of the amount of substance of the iron element to the amount of substance of the phosphorus element in the mixed solution is 1:(0.97-1.07). For example, the ratio of the amount of substance of the iron element to the amount of substance of the phosphorus element is 1:0.97, 1:0.98, 1:0.99, 1:1, 1:1.01, 1:1.02, 1:1.03, 1:1.05, 1:1.07, or the ratio of the amount of substance of the iron element to the amount of substance of the phosphorus element can also be between any two of the above ratios.

[0122] In this example, the above ratio of the iron element to the phosphorus element is selected to facilitate the sufficient precipitation of the iron element and the titanium element in the subsequent deposition process, thereby improving the utilization rate of the iron element and the titanium element.

[0123] As an example of this embodiment, the amount of substance of the phosphorus element can be selected according to the amount of substance of the titanium element. For example, when the amount of substance of the titanium element is high, the amount of substance of the phosphorus element can also be set to be high, and when the amount of substance of the titanium element is low, the amount of substance of the phosphorus element can also be set to be low.

[0124] As an example of this embodiment, the pH value of the mixed solution can be 0.5-1.5 to ensure that the iron element and the titanium element can stably exist in the aqueous solution. For example, the pH value of the mixed solution can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or the pH value of the mixed solution can also be between any two of the above pH values.

[0125] It can be understood that different salt solutions can have different pH values, and therefore a pH adjusting agent can also be added to the mixed solution to control the pH value when the mixed solution is formed. The pH adjusting agent can be selected from, but is not limited to, one or more of sulfuric acid, hydrochloric acid, sodium hydroxide, and ammonia.

[0126] In step S2, the mixed solution is mixed with a first alkali source to obtain a first slurry.

[0127] In this embodiment, the addition of the first alkali source causes the ferrous ions and the titanium ions to precipitate, thereby forming ferrous phosphate precipitates and titanium phosphate precipitates in the first slurry.

[0128] It can be understood that the first alkali source is used to regulate the pH value of the first slurry formed by the mixed solution, and the amount of the first alkali source can be selected according to the required pH value. As an example of this embodiment, after the first alkali source is added to the first mixed solution, the pH value of the first slurry formed is controlled to be 2.0-2.5 to form ferrous phosphate precipitate and titanium phosphate precipitate. For example, the pH value of the first slurry can be controlled to be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or the pH value of the first slurry can be controlled to be between any two of the above pH values.

[0129] In this example, the pH value of the first slurry is controlled to be 2.0-2.5, which is beneficial to control the low precipitation rate of ferrous ions and titanium ions, so that the particle size of the ferrous phosphate precipitate and the titanium phosphate precipitate is smaller and more uniform, so that they can be fully mixed during the precipitation process. Further, in the subsequent deposition process, titanium elements can be uniformly doped into the iron phosphate precipitate. Moreover, in the actual preparation process, if the pH value is too high, magnesium ions and potassium ions and other impurities will be mixed into the iron phosphate material, thereby negatively affecting the powder resistance and discharge efficiency of the final prepared positive electrode material.

[0130] It can be understood that the pH value of the first slurry formed can be controlled by controlling the amount of the first alkali source added.

[0131] As an example of this embodiment, the basic substance in the first alkali source can be selected from, but not limited to, one or more of ammonia, sodium hydroxide, sodium carbonate and sodium bicarbonate. The cation of the basic substance is easy to remove from the surface of the subsequently formed iron phosphate, and the residual amount is small, which is beneficial to ensure the purity of the iron phosphate material.

[0132] As an example of this embodiment, the ferrous phosphate precipitate can be ferrous dihydrogen phosphate.

[0133] In this embodiment, the ferrous ions and titanium ions are first formed into a coprecipitate before oxidation, which can make the process of forming titanium-doped iron phosphate precipitate more gentle, prevent the subsequent addition of too much second alkali source due to the pH value of the mixed solution being too low, and prevent the local iron phosphate precipitate from forming too fast and the titanium element from being difficult to uniformly doped.

[0134] Step S3, adding an oxidizing agent and a second alkali source to the first slurry to obtain a second slurry.

[0135] In this embodiment, the oxidizing agent is used to oxidize the ferrous phosphate precipitate in the first slurry, thereby forming an iron phosphate precipitate doped with titanium elements.

[0136] It can be understood that the second alkali source is used to regulate the pH value of the first slurry in the process of oxidation precipitation, and the amount of the second alkali source added into the first slurry can be selected according to the required pH value. As an example of this embodiment, the pH value is controlled to be 2.0-2.5 in the process of adding the oxidizing agent and the second alkali source into the first slurry. For example, the pH value in the process of adding the oxidizing agent and the second alkali source into the first slurry can be controlled to be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or the pH value in the process of adding the oxidizing agent and the second alkali source into the first slurry can be controlled to be between any two of the above pH values.

[0137] In the reaction between the oxidizing agent and the ferrous phosphate, the product gradually reduces the pH value of the first slurry. The second alkali source added can be used to maintain the pH value in the process of adding the oxidizing agent and the second alkali source into the first slurry, so as to ensure that the formed iron element is completely precipitated.

[0138] In this example, the pH value in the process of adding the oxidizing agent and the second alkali source into the first slurry is controlled to be 2.0-2.5, which can control the conversion rate of ferrous ions to iron ions and the precipitation rate of iron ions to be moderate, which is beneficial to the in-situ doping of titanium elements into the formed iron phosphate precipitate in the process of precipitation of iron elements. In addition, in the actual preparation process, if the pH value is too high, magnesium ions and potassium ions and other impurities will be mixed into the iron phosphate material, which will have a negative impact on the powder resistance and discharge efficiency of the finally prepared positive electrode material.

[0139] As an example of this embodiment, the oxidizing agent is selected from hydrogen peroxide. The oxidation product of hydrogen peroxide is only water, so the selection of hydrogen peroxide as the oxidizing agent basically does not mix impurity elements into the iron phosphate precipitate, which is beneficial to obtain a relatively pure iron phosphate precipitate.

[0140] As a further example of this embodiment, the mass fraction of hydrogen peroxide in the hydrogen peroxide is 15%-30%.

[0141] As a further example of this embodiment, in the process of adding the oxidizing agent and the second alkali source into the first slurry, the ratio of the amount of substance of hydrogen peroxide in the hydrogen peroxide added to the amount of substance of ferrous ions in the mixed solution is (0.55-0.75):1. For example, the ratio of the amount of substance of hydrogen peroxide to the amount of substance of ferrous ions is 0.55:1, 0.57:1, 0.6:1, 0.62:1, 0.65:1, 0.67:1, 0.7:1, 0.72:1, 0.75:1, or the ratio of the amount of substance of hydrogen peroxide to the amount of substance of ferrous ions can also be between any two of the above ratios.

[0142] In this example, although the excessive hydrogen peroxide does not affect the composition and morphology of the generated iron phosphate material, it causes partial waste of raw materials. When the hydrogen peroxide is insufficient, the oxidation reaction is not sufficient, and the purity of the obtained iron phosphate material is low, thereby further affecting the quality of the subsequent prepared positive electrode material.

[0143] As an example of this embodiment, the oxidizing agent is selected from oxygen or compressed air.

[0144] As a further example of this embodiment, during the process of adding the oxidizing agent and the second alkali source to the first slurry, the ratio of the amount of substance of the oxygen added to the amount of substance of the ferrous ions in the mixed solution is (0.275-0.375): 1. For example, the ratio of the amount of substance of the oxygen added to the amount of substance of the ferrous ions in the mixed solution can be 0.285: 1, 0.295: 1, 0.305: 1, 0.315: 1, 0.325: 1, 0.335: 1, 0.345: 1, 0.355: 1, 0.365: 1, or can be any ratio between (0.275-0.375): 1.

[0145] As a further example of this embodiment, during the process of adding the oxidizing agent and the second alkali source to the first slurry, the ratio of the amount of substance of the oxygen in the compressed air added to the amount of substance of the ferrous ions in the mixed solution is (0.275-0.375): 1. For example, the ratio of the amount of substance of the oxygen in the compressed air added to the amount of substance of the ferrous ions in the mixed solution can be 0.285: 1, 0.295: 1, 0.305: 1, 0.315: 1, 0.325: 1, 0.335: 1, 0.345: 1, 0.355: 1, 0.365: 1, or can be any ratio between (0.275-0.375): 1.

[0146] In this example, oxygen and compressed air are gases, and when the oxidizing agent is selected as oxygen or compressed air, impurity elements are also avoided from being mixed into the iron phosphate precipitation, which is conducive to obtaining a relatively pure iron phosphate precipitation. Accordingly, the amount of oxygen in the oxygen or compressed air is controlled to be within a corresponding range relative to the amount of substance of the ferrous ions, which is conducive to the sufficient oxidation of the ferrous ions.

[0147] As an example of this embodiment, the basic substance in the second alkali source can be selected from, but not limited to, one or more of ammonia, sodium hydroxide, sodium carbonate, and sodium bicarbonate. The cation of the basic substance is easily removed from the surface of the subsequently formed iron phosphate, and the residual amount is small, which is conducive to ensuring the purity of the iron phosphate material.

[0148] As a further example of this embodiment, the second alkali source is the same as the first alkali source. Using the same first alkali source and second alkali source can avoid introducing as many different cations as possible, which is conducive to ensuring the purity of the iron phosphate material.

[0149] As an example of this embodiment, the total time of the process of adding the oxidizing agent and the second alkali source to the first slurry is 20 min to 60 min. For example, the total time of the process of adding the oxidizing agent and the second alkali source can be 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 65 min, or the total time can also be between any two of the above times.

[0150] In this example, controlling the total time of adding the oxidizing agent and the second alkali source within the above range is conducive to controlling the generation process of the iron phosphate precipitate to be more gentle, which is conducive to improving the uniformity of the distribution of titanium elements in the iron phosphate precipitate.

[0151] As a further example of this embodiment, after the addition of the oxidizing agent and the second alkali source is completed, the time for continuing to stir the first slurry can be 30 min to 240 min. Continuing to stir is used to make the reaction for forming the iron phosphate precipitate proceed sufficiently.

[0152] Step S4, performing a first solid-liquid separation treatment on the second slurry to obtain a filter cake.

[0153] As an example of this embodiment, the way of performing the first solid-liquid separation treatment on the second slurry can be filtration.

[0154] As an example of this embodiment, after performing the first solid-liquid separation treatment on the second slurry, a step of performing a first rinsing treatment on the iron phosphate precipitate is further included.

[0155] In this example, the first rinsing treatment is used to remove anionic and cationic impurities attached to the surface of the iron phosphate precipitate, so as to obtain a more pure iron phosphate precipitate, and reduce the impurities in the interior of the iron phosphate dihydrate crystals formed subsequently, and improve the quality of the crystals.

[0156] As a further example of this embodiment, the step of performing the first rinsing treatment on the iron phosphate precipitate includes: rinsing the iron phosphate precipitate with flowing pure water, and monitoring the conductivity of the rinsing water flowing out, and stopping the first rinsing treatment when the conductivity of the rinsing water flowing out is lower than a first preset conductivity value. The first preset conductivity value can be set according to actual needs.

[0157] As a further example of this embodiment, the first preset conductivity value can be 2 mS / cm ~ 10 mS / cm. For example, the first preset conductivity value can be 2 mS / cm, 3 mS / cm, 4 mS / cm, 5 mS / cm, 6 mS / cm, 7 mS / cm, 8 mS / cm, 9 mS / cm, 10 mS / cm, or the first preset conductivity value can also be between any two conductivities described above.

[0158] In step S5, the filter cake is mixed with the phosphoric acid solution, and is subjected to aging treatment to obtain a third slurry.

[0159] In this embodiment, after the filter cake is mixed with the phosphoric acid solution, the ferric phosphate precipitate in the filter cake is dissolved in the phosphoric acid solution, and the subsequent aging treatment is used to recrystallize the ferric phosphate in the solution to form the ferric phosphate dihydrate crystals.

[0160] As an example of this embodiment, the step of aging treatment includes heating the mixture of the filter cake and the phosphoric acid solution to 85℃ ~ 98℃ to form the ferric phosphate dihydrate crystals. In some examples, the mixture of the filter cake and the phosphoric acid solution can be heated to 85℃, 86℃, 87℃, 88℃, 90℃, 92℃, 94℃, 95℃, 96℃, 97℃, 98℃, or the mixture of the filter cake and the phosphoric acid solution can also be heated to between any two temperatures described above.

[0161] In this example, after the ferric phosphate precipitate is dispersed in the phosphoric acid solution and then heated, the ferric phosphate is allowed to nucleate and grow again after being dissolved, which can fully convert the yellow amorphous ferric phosphate precipitate into white ferric phosphate dihydrate crystals. In addition, the impurity ions attached to the surface of the amorphous ferric phosphate can also be dissolved in the phosphoric acid solution to be reduced.

[0162] As a further example of this embodiment, after the mixture of the filter cake and the phosphoric acid solution is heated to 85℃ ~ 98℃, it is kept at this temperature for 3h ~ 5h.

[0163] As an example of this embodiment, in the phosphoric acid solution, the ratio of the amount of substance of phosphoric acid to the amount of substance of phosphate in the mixture is (0.1 ~ 0.2):1. For example, the ratio of the amount of substance of phosphoric acid to the amount of substance of phosphate in the mixture can be 0.1:1, 0.11:1, 0.12:1, 0.13:1, 0.14:1, 0.15:1, 0.16:1, 0.18:1, 0.2:1, or the ratio of the amount of substance of phosphoric acid to the amount of substance of phosphate in the mixture can also be between any two ratios described above. It can be understood that the phosphate ions in the phosphate in the mixture are basically completely transferred to the ferric phosphate.

[0164] In this example, the ratio of the amount of substance of phosphoric acid to the amount of substance of phosphate in the mixed solution is within the above range, which can make the iron phosphate precipitate fully dissolved while reducing or even avoiding the introduction of anionic impurities due to phosphoric acid. Moreover, when the ratio of the amount of substance of phosphoric acid in the phosphoric acid solution to the amount of substance of phosphate in the mixed solution is within the above range, the occurrence and sufficient progress of the crystal transformation are more favorable.

[0165] As a further example of this embodiment, the pH value of the third slurry can be 1.0-2.0 to ensure that the iron phosphate can be completely dissolved in a normal temperature state. For example, the pH value of the third slurry can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.7, 1.9, 2.0, or the pH value of the third slurry can also be between any two of the above pH values.

[0166] In step S6, the third slurry is subjected to a second solid-liquid separation treatment and then dried, and is subjected to a calcination treatment to obtain an iron phosphate material.

[0167] In this embodiment, the third slurry is subjected to a second solid-liquid separation treatment to obtain iron phosphate dihydrate crystals. The iron phosphate dihydrate crystals can form the desired iron phosphate material with a laminated structure after a calcination treatment.

[0168] As an example of this embodiment, the third slurry is subjected to a second solid-liquid separation treatment by filtration.

[0169] As an example of this embodiment, after the third slurry is subjected to a second solid-liquid separation treatment, a step of subjecting the iron phosphate dihydrate crystals to a second rinsing treatment is further included.

[0170] In this example, the second rinsing treatment is used to remove anionic and cationic impurities attached to the surface of the iron phosphate dihydrate crystals, so as to avoid as much as possible the influence of the impurity ions on the morphology of the iron phosphate material in the subsequent calcination process.

[0171] As a further example of this embodiment, the step of subjecting the iron phosphate dihydrate crystals to a second rinsing treatment includes: rinsing the iron phosphate dihydrate crystals with flowing pure water, and monitoring the conductivity of the flowing rinsing water, and stopping the second rinsing treatment when the conductivity of the flowing rinsing water is lower than a second preset conductivity value. The second preset conductivity value can be set according to actual needs.

[0172] As a further example of this embodiment, the second preset conductivity value can be 100 μS / cm ~ 500 μS / cm. For example, the first preset conductivity value can be 100 μS / cm, 150 μS / cm, 200 μS / cm, 250 μS / cm, 300 μS / cm, 350 μS / cm, 400 μS / cm, 450 μS / cm, 500 μS / cm, or the second preset conductivity value can also be between any two conductivities described above.

[0173] In this embodiment, the drying after the second solid-liquid separation treatment of the third slurry is performed by drying, and the drying temperature can be 80°C to 140°C, and the drying time can be 8h (hours) to 16h. For example, the drying temperature can be 85, 90, 92°C, 95°C, 96°C, 98°C, 100°C, 110°C, 120°C, 130°C, or any temperature between 80°C and 140°C, and the drying time can be 9h, 10h, 12h, 15h, or any length of time between 8h and 16h. The drying can be performed in an air atmosphere. Further, the content of free water (in mass fraction) in the obtained iron phosphate dihydrate crystal after drying is below 1%. Among them, the free water is relative to the crystal water, and the crystal water refers to the water molecules combined with iron phosphate and jointly constitute the crystal, and the free water is the water molecules not combined with iron phosphate and remaining in the interior or surface of the iron phosphate dihydrate crystal.

[0174] As an example of this embodiment, the step of calcining the iron phosphate dihydrate crystal includes heating the iron phosphate dihydrate crystal at a temperature of 600°C to 800°C for 2h or more to form the iron phosphate material.

[0175] As a further example of this embodiment, the calcining temperature during the calcining treatment can be 600°C, 620°C, 640°C, 650°C, 680°C, 700°C, 720°C, 740°C, 750°C, 780°C, 800°C, or the calcining temperature during the calcining treatment can also be between any two temperatures described above.

[0176] As a further example of this embodiment, the calcining time during the calcining treatment is 2h to 3h. For example, the calcining time can be 2h, 2.1h, 2.2h, 2.3h, 2.5h, 2.7h, 2.9h, 3h, or the calcining time can also be between any two times described above.

[0177] As a further example of this embodiment, the heating rate for heating to the calcining temperature during the calcining treatment can be 3°C / min to 8°C / min, for example, can be 4°C / min, 5°C / min, 6°C / min, 7°C / min, or any heating rate between 3°C / min and 8°C / min.

[0178] As a further example of this embodiment, the calcination treatment can be performed under an air atmosphere.

[0179] In this example, the calcination treatment is performed on the iron phosphate dihydrate crystals using the above-mentioned temperature range and heating time, which can sufficiently remove the crystal water in the crystals and promote the re-nucleation and growth of the iron phosphate, and the growth of the crystals splits the surface of the particles to form the laminated structure.

[0180] After the above-mentioned steps S1-S6, the iron phosphate material in the present application can be prepared.

[0181] In this embodiment, the ferrous salt solution, the titanium salt solution and the phosphate solution are pre-mixed. When the first alkali source is added, the ferrous ions and the titanium ions can co-precipitate to form mixed ferrous phosphate precipitate and titanium phosphate precipitate. Then, the oxidizing agent and the second alkali source are added, so that the titanium element can be doped into the iron phosphate precipitate in situ during the precipitation process. Then, during the aging process, the amorphous iron phosphate precipitate dissolves and recrystallizes to convert into iron phosphate dihydrate crystals. During the calcination process, the titanium element doped in the iron phosphate causes defects in the crystal lattice, which hinders the growth of the iron phosphate in a specific direction, and the surface of the iron phosphate particles splits to form multiple laminated plates arranged in sequence and spaced apart at the top end, forming a laminated structure. The preparation method of this embodiment can prepare the iron phosphate material in any of the above-mentioned embodiments.

[0182] In a third aspect, another embodiment of the present application further provides a positive electrode material, the preparation raw material of the positive electrode material comprises the iron phosphate material according to any of the above-mentioned embodiments, or the preparation raw material of the positive electrode material comprises the iron phosphate material prepared by the preparation method according to any of the above-mentioned embodiments.

[0183] In this embodiment, the iron phosphate material has a laminated structure and has a large specific surface area. Correspondingly, the positive electrode material prepared by using the iron phosphate material as the raw material also has a large contact area and a low internal resistance, and the doped titanium ions are also beneficial to widening the ion channel in the positive electrode material.

[0184] As an example of this embodiment, the positive electrode material comprises one or more of lithium iron phosphate and lithium manganese iron phosphate.

[0185] In a fourth aspect, another embodiment of the present application further provides a positive electrode tab, the positive electrode tab comprises a current collector and a positive electrode active layer arranged on the current collector, and the positive electrode active layer comprises the positive electrode material according to any of the above-mentioned embodiments.

[0186] In this embodiment, the positive electrode tab comprises the above-mentioned positive electrode material, and the positive electrode material has a large contact area and a low internal resistance, so that the positive electrode tab also has good charge and discharge performance.

[0187] As an example of this embodiment, the positive electrode active layer can further include one or more of a conductive agent and a binder.

[0188] As a further example of this embodiment, in the positive electrode active layer, the conductive agent can be selected from one or more of conductive carbon black, carbon nanotube, graphene, and carbon nanofiber.

[0189] As a further example of this embodiment, in the positive electrode active layer, the binder can be selected from one or more of polyvinylpyrrolidone, polyvinylidene fluoride, polyethylene oxide, polytetrafluoroethylene, carboxymethyl cellulose, and a copolymer of styrene and butadiene.

[0190] In a fifth aspect, another embodiment of the present application also provides a secondary battery, the secondary battery including a positive electrode tab and a negative electrode tab, the positive electrode tab of the secondary battery being the positive electrode tab of the above-described embodiment.

[0191] In this embodiment, the secondary battery including the above-described positive electrode tab can exhibit high charge-discharge efficiency and cycle stability during charge-discharge.

[0192] As an example of this embodiment, the negative electrode tab includes a current collector and a negative electrode active layer disposed on the current collector, the negative electrode active layer including a negative electrode active material.

[0193] As a further example of this embodiment, the negative electrode active material includes one or more of graphite, silicon, silicon-carbon, metallic lithium, and lithium titanate.

[0194] As a further example of this embodiment, the negative electrode active layer can further include one or more of a conductive agent and a binder.

[0195] As a further example of this embodiment, in the negative electrode active layer, the conductive agent can be selected from one or more of conductive carbon black, carbon nanotube, graphene, and carbon nanofiber.

[0196] As a further example of this embodiment, in the negative electrode active layer, the binder can be selected from one or more of polyvinylpyrrolidone, polyvinylidene fluoride, polyethylene oxide, polytetrafluoroethylene, carboxymethyl cellulose, and a copolymer of styrene and butadiene.

[0197] The present application will be further described in conjunction with specific embodiments, which are not to be understood as limiting the scope of the present application.

[0198] Unless otherwise defined, all terms used in connection with the present application, the meanings commonly understood by those skilled in the art are the same. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present application.

[0199] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are used only to explain the present application and cannot be understood as a limitation of the present application. If a specific technology or condition is not specified in the examples, the technology or condition described in the literature in the art or according to the product manual is used. If the manufacturer of the reagent or instrument is not specified, it is a conventional product that can be obtained on the market.

[0200] I. Preparation method

[0201] Example 1

[0202] Anhydrous iron phosphate material was prepared using the process flow shown in Figure 2 as follows:

[0203] (1) A ferrous sulfate (FeSO4) solution (mass concentration of 191.77 g / kg, pH value of 2.8) was taken, and titanium sulfate (TiOSO4) powder and ammonium dihydrogen phosphate ((NH4)2HPO4) powder were added thereto, and the titanium sulfate powder and ammonium dihydrogen phosphate powder were fully dissolved by stirring, to serve as a mixed solution. In the mixed solution, the amount-of-substance ratio of iron element, phosphorus element and titanium element was 1:1.02:0.0225, and the pH value of the mixed solution was 1.4.

[0204] (2) Ammonia water was added to the mixed solution to raise the pH value of the first mixed solution to 2.20, so that the ferrous ions and titanium ions were converted into ferrous phosphate dihydrate precipitate and titanium phosphate precipitate, respectively, to obtain a first slurry.

[0205] (3) Hydrogen peroxide and ammonia water were gradually added to the first slurry, and the overall dropwise addition time was controlled to be 30 min, and after the dropwise addition was completed, stirring was continued for 1 h to fully react, to form iron phosphate precipitate doped with titanium element, to obtain a second slurry. The pH value of the first slurry during the reaction and the second slurry after the reaction was controlled to be 2.15. The amount-of-substance ratio of hydrogen peroxide in the added hydrogen peroxide to the amount of iron element in step (1) was 0.65:1.

[0206] (4) The iron phosphate precipitate was filtered and rinsed with pure water until the conductivity of the effluent water after rinsing reached 5 mS / cm or less.

[0207] (5) The rinsed iron phosphate was transferred to a dilute phosphoric acid solution (prepared by mixing pure water and concentrated phosphoric acid according to a mass ratio of 93.4:1.85, and the mass concentration of phosphoric acid in the concentrated phosphoric acid was 75%), and fully stirred, and the amount-of-substance ratio of phosphoric acid to the amount of ammonium dihydrogen phosphate in step (1) was 0.115:1, and the pH value of the formed slurry was 1.55, and then heated to 95°C and kept for 3 h, so that the yellow amorphous iron phosphate was completely converted into white iron phosphate dihydrate crystals, to obtain a third slurry.

[0208] (6) The third slurry is filtered and rinsed with pure water until the conductivity of the effluent water is less than 200 μS / cm, and then the rinsed iron phosphate dihydrate crystals are dried in an oven at 95 °C for 12 h in air atmosphere. The free water content in the dried iron phosphate dihydrate crystals is less than 1 wt%.

[0209] (7) The iron phosphate dihydrate crystals are crushed and heated in a muffle furnace at 600 °C for 2 h in air atmosphere, and then cooled to form white iron phosphate material with a slightly yellowish color.

[0210] Example 2

[0211] (1) A solution of ferrous chloride (FeCl2) (mass concentration of 160.01 / kg, pH value of 2.8) is prepared. Titanium ore mixed with concentrated sulfuric acid is added to the solution, and the filtrate obtained after filtration (titanium content of 9.98%) and ammonium dihydrogen phosphate (NH4H2PO4) powder are added to the solution. The solution is stirred to dissolve the titanium oxide sulfate powder and ammonium dihydrogen phosphate powder to form a mixed solution. In the mixed solution, the molar ratio of iron, phosphorus and titanium is 1:1.03:0.03, and the pH value of the mixed solution is 0.9.

[0212] (2) Sodium hydroxide solution is added to the mixed solution to increase the pH value of the mixed solution to 2.20, so that the ferrous ions and titanium ions are converted into ferrous phosphate dihydrogen and titanium phosphate precipitates, respectively, to obtain a first slurry.

[0213] (3) Compressed air and sodium hydroxide solution are gradually added to the first slurry, and the total drop time is controlled to be 30 min. After the drop is completed, the solution is continuously stirred for 1 h to fully react, forming a titanium-doped iron phosphate precipitate, and obtaining a second slurry. The pH value of the first slurry during the reaction and the second slurry after the reaction is controlled to be 2.15. The molar ratio of oxygen in the added air to the molar amount of iron in step (1) is 0.325:1.

[0214] (4) The iron phosphate precipitate is filtered and rinsed with pure water until the conductivity of the effluent water is less than 5 mS / cm.

[0215] (5) The rinsed iron phosphate is transferred to a dilute phosphoric acid solution (prepared by mixing pure water and concentrated phosphoric acid in a mass ratio of 93.4:1.85, and the mass concentration of phosphoric acid in the concentrated phosphoric acid is 75%), and the solution is stirred. The molar ratio of phosphoric acid to the molar amount of ammonium dihydrogen phosphate in step (1) is 0.115:1, and the pH value of the formed slurry is 1.55. The solution is heated to 98 °C and kept for 5 h, so that the yellow amorphous iron phosphate is completely converted into white iron phosphate dihydrate crystals, and a third slurry is obtained.

[0216] (6) After the third slurry is filtered, the iron phosphate dihydrate crystals are rinsed with pure water until the conductivity of the effluent water after rinsing reaches 200 μS / cm or less, and then the rinsing is stopped. The iron phosphate dihydrate crystals are then transferred to an oven at 100°C and dried in an air atmosphere for 8 h. The content of free water in the dried iron phosphate dihydrate crystals is less than 1 wt%.

[0217] (7) The iron phosphate dihydrate crystals are crushed and placed in a muffle furnace. The furnace is heated to 800°C in an air atmosphere and held for 2 h to perform calcination. The furnace is then naturally cooled to form white iron phosphate material with a slightly yellowish color.

[0218] Example 3

[0219] The iron phosphate dihydrate crystals are crushed and placed in a muffle furnace. The furnace is heated to 800°C in an air atmosphere and held for 2 h to perform calcination. The furnace is then naturally cooled to form white iron phosphate material with a slightly yellowish color.

[0220] (1) A ferrous sulfate (FeSO4) solution (mass concentration of 191.77 g / kg, pH value of 2.8) is prepared. Titanium tetrachloride (TiCl4) and sodium phosphate monobasic (Na2HPO4) powder are added to the solution, which is stirred to fully dissolve the titanium tetrachloride and sodium phosphate monobasic powder. The mixture is used as a mixed solution. In the mixed solution, the molar ratio of iron, phosphorus, and titanium is 1:1.01:0.015, and the pH value of the mixed solution is 1.4.

[0221] (2) Sodium carbonate is added to the mixed solution to raise the pH value of the mixed solution to 2.20, so that the ferrous ions and titanium ions are converted into ferrous phosphate dihydrate precipitate and titanium phosphate precipitate, respectively, to obtain a first slurry.

[0222] (3) Hydrogen peroxide and sodium carbonate are gradually added to the first slurry, and the total dropwise addition time is controlled to be 30 min. After the dropwise addition is completed, the mixture is continuously stirred for 1 h to fully react, forming iron phosphate precipitate doped with titanium elements, to obtain a second slurry. The pH value of the first slurry during the reaction and the second slurry after the reaction is controlled to be 2.15. The molar ratio of hydrogen peroxide in the added hydrogen peroxide to the molar amount of iron in step (1) is 0.65:1.

[0223] (4) The iron phosphate precipitate is filtered and rinsed with pure water until the conductivity of the effluent water after rinsing reaches 5 mS / cm or less.

[0224] (5) The rinsed iron phosphate is transferred to a dilute phosphoric acid solution (prepared by mixing pure water and concentrated phosphoric acid with a mass ratio of 93.4:1.85, and the mass concentration of phosphoric acid in the concentrated phosphoric acid is 75%), and is fully stirred, wherein the ratio of the amount of substance of phosphoric acid to the amount of substance of ammonium monohydrogen phosphate in step (1) is 0.115:1, the pH value of the formed slurry is 1.55, and then heated to 98°C and kept for 5h, so that the yellow amorphous iron phosphate is completely converted into white iron phosphate dihydrate crystals, and a third slurry is obtained.

[0225] (6) The third slurry is filtered, and the iron phosphate dihydrate crystals are rinsed with pure water until the conductivity of the effluent water after rinsing is less than 200μS / cm, and then transferred to a 90°C oven for drying in an air atmosphere for 16h, and the content of free water in the dried iron phosphate dihydrate crystals is less than 1wt%.

[0226] (7) The iron phosphate dihydrate crystals are crushed and placed in a muffle furnace, heated to 700°C in an air atmosphere, and kept for 3h for calcination treatment, and then naturally cooled to form a white and slightly yellow iron phosphate material.

[0227] Example 4

[0228] The anhydrous iron phosphate is prepared according to the method in Example 1, and the difference between Example 4 and Example 1 is that in step (1), the amount of substance ratio of iron element, phosphorus element and titanium element is 1:1.043:0.04.

[0229] Example 5

[0230] The anhydrous iron phosphate is prepared according to the method in Example 1, and the difference between Example 5 and Example 1 is that in step (1), the amount of substance ratio of iron element, phosphorus element and titanium element is 1:0.997:0.01.

[0231] Example 6

[0232] The anhydrous iron phosphate is prepared according to the method in Example 1, and the difference between Example 6 and Example 1 is that in step (2), the amount of ammonia water is increased to increase the pH value of the mixed solution to 3.0, as the first slurry.

[0233] Example 7

[0234] The anhydrous iron phosphate is prepared according to the method in Example 1, and the difference between Example 7 and Example 1 is that in step (3), the amount of ammonia water is increased, and the pH value of the first slurry during the reaction and the second slurry after the reaction is controlled to 3.0.

[0235] Comparative Example 1

[0236] The anhydrous iron phosphate was prepared according to the method in Example 1. The difference between Comparative Example 1 and Example 1 is that in step (1), no titanium salt was added, and the molar ratio of iron element to phosphorus element was 1:1.4.

[0237] Comparative Example 2

[0238] The anhydrous iron phosphate was prepared according to the method in Example 1. The difference between Comparative Example 2 and Example 1 is that after step (4), the rinsed iron phosphate precipitate was directly placed in a muffle furnace, heated to 600℃ and kept for 2h for calcination treatment, and then naturally cooled.

[0239] II. Test Methods

[0240] 1. Titanium element content test

[0241] The inductively coupled plasma optical emission spectrometer (ICP-OES) was used to test the titanium element content in the iron phosphate materials prepared in each example and comparative example. The test results are shown in Table 1 below.

[0242] 2. Test of macroscopic performance parameters of iron phosphate materials

[0243] BET specific surface area test: The specific surface area of the iron phosphate materials prepared in each example and comparative example was tested by gas adsorption BET method. The test results are shown in Table 1 below.

[0244] Particle size: The D50 particle size of the iron phosphate materials prepared in each example and comparative example was tested by laser particle size analyzer. The test results are shown in Table 1 below.

[0245] Iron element content determination: In an acidic condition, stannous chloride was used to reduce most of Fe 3+ in the sample solution to Fe 2+ , and hydrogen peroxide was added dropwise to eliminate excess Sn 2+ if necessary. With sodium tungstate solution as an indicator, titanium trichloride was further used to reduce Fe 3+ to Fe 2+ . Excess titanium trichloride was reacted with potassium dichromate solution. Finally, the divalent iron was titrated with potassium dichromate standard titration solution, so as to obtain the iron element content. The iron element content in the iron phosphate materials prepared in each example and comparative example is shown in Table 1 below.

[0246] Phosphorus element content determination: quinoline molybdenum ketone gravimetric method. In an acidic condition, orthophosphate reacts with quinoline molybdenum ketone precipitant to form yellow quinoline phosphomolybdate precipitate. After filtration, washing and drying, the phosphorus element content can be calculated. The phosphorus element content in the iron phosphate materials prepared in each example and comparative example is shown in Table 1 below.

[0247] The ratio of the number of moles of iron to the number of moles of phosphorus in the iron phosphate material was calculated to obtain the iron-phosphorus ratio (Fe / P), and the results are shown in Table 1 below.

[0248] Tap density measurement: The tap density of each example and the comparative example was measured using a tap density instrument.

[0249] 3. Crystal form and morphology test of the iron phosphate material

[0250] The surface morphology of the iron phosphate material of Example 1 and Comparative Example 1 was observed using a scanning electron microscope, and the surface morphology of Example 1 and Comparative Example 1 is shown in FIGS. 3 and 4, respectively.

[0251] The iron phosphate material prepared in each example and Comparative Example 1 was characterized by X-ray powder diffraction (XRD), and the results are shown in FIG. 5. The peak position diagram at the bottom of FIG. 5 is a standard iron phosphate diffraction peak diagram.

[0252] 4. Application performance test of the iron phosphate material

[0253] First, the iron phosphate prepared in each example and the comparative example and the lithium source (lithium carbonate) were sintered to obtain the corresponding lithium iron phosphate positive electrode material.

[0254] The tap density of the lithium iron phosphate positive electrode material prepared in each example and the comparative example was tested using a tap density instrument, and the test pressure was 3T and the pressing time was 30S. The results are shown in Table 2.

[0255] The powder resistivity of the lithium iron phosphate positive electrode material prepared in each example and the comparative example was tested using the four-probe method under a pressure of 10 MPa. The results are shown in Table 2.

[0256] The lithium iron phosphate positive electrode material prepared in each example and the comparative example was mixed with conductive carbon powder and PVDF binder at a mass ratio of 90:5:5, homogenized, and then coated on an aluminum foil. After drying at 100°C, the material was rolled using a roll mill, and then a sheet was punched to obtain a 14mm diameter electrode. The mass of the active material was determined by weighing and subtracting the mass of the aluminum foil. After drying the positive electrode sheet, a CR2032 button half-cell was assembled in a UNlab type inert gas primary box of Braun Company, Germany. The button half-cell was assembled in the order of negative electrode shell, lithium sheet, electrolyte, separator, electrolyte, positive electrode sheet, gasket, spring, and positive electrode shell. The CR2032 button half-cell was tested for charge and discharge electrochemical performance using a Wuhan Landian CT2001A type battery test system, and the voltage range was 2.0-4.6V. The test results are shown in Table 2.

[0257] Table 1

[0258] The initial mixture of Comparative Example 1 does not contain titanium element, while the initial mixture of Example 1 contains titanium element. Referring to the content shown in FIG. 4, the iron phosphate material prepared in Comparative Example 1 has irregular granular shape, and each single particle is a primary particle. Referring to the content shown in FIG. 3, compared with Comparative Example 1, the iron phosphate material prepared in Example 1 has a plurality of laminated structures on the surface of the primary particles. Further, according to FIG. 3, the laminated structure generally includes 2-4 layers. The laminated structure is formed by splitting the original granular surface. The thickness of each single layer is between 40 nm and 60 nm, the particle size of each single primary particle is between 300 nm and 1000 nm, and the maximum distance between adjacent two layers is between 20 nm and 50 nm. The iron phosphate materials prepared in Example 2-Example 7 have similar morphology.

[0259] Referring to FIG. 5, the iron phosphate materials prepared in Example 1 and Comparative Example 1 have basically the same diffraction peak type, which indicates that the introduction of a small amount of titanium ions does not significantly change the phase characteristics of the iron phosphate material. Compared with the diffraction peak type of Comparative Example 1, the diffraction peak type of Example 1 has a small leftward shift as a whole, which is mainly because the radius of titanium atom is smaller than that of iron atom, and the introduction of titanium atom increases the interplanar spacing, and accordingly the incident angle corresponding to the diffraction peak is also reduced.

[0260] The main difference between Example 1-Example 5 is the relative content of titanium element and phosphorus element in the mixture. Among them, the relative content of titanium element in Example 1 is 0.0225, the relative content of titanium element in Example 2 is 0.03, the relative content of titanium element in Example 3 is 0.015, the relative content of titanium element in Example 4 is 0.04, and the relative content of titanium element in Example 5 is 0.01. The main difference between Example 6-Example 7 and Example 1 is the control of pH value in steps (2) and (3). Referring to Table 1, compared with Comparative Example 1, the specific surface area of Example 1-Example 7 increases to different degrees under the condition that the tap density is basically unchanged, which is beneficial to the subsequent preparation of the positive electrode material with low internal resistance and high specific surface area.

[0261] In addition, compared with Comparative Example 1, although Comparative Example 2 contains titanium element in the mixture, the amorphous iron phosphate precipitate is not converted into iron phosphate dihydrate crystal in the subsequent preparation process, and the iron phosphate material is directly calcined based on the amorphous iron phosphate precipitate. This makes the formed iron phosphate material have large particles and fail to form laminated structure. Accordingly, the tap density and specific surface area of Comparative Example 2 are also low.

[0262] Table 2

[0263] As shown in Tables 1 and 2, Comparative Example 1 did not contain titanium doping, and its lithium iron phosphate material did not have a laminated structure. The powder resistivity of the prepared lithium iron phosphate material was 33.3 Ω. It exhibited a discharge efficiency of only 96.3% during the charge-discharge process.

[0264] As shown in Tables 1 and 2, based on Examples 1, 2, and 4, it can be seen that with the increase of titanium in the iron phosphate material, although the specific surface area of ​​the iron phosphate material increases and the powder resistance of the prepared lithium iron phosphate material decreases, the charge-discharge efficiency shows a trend of first increasing and then decreasing. This is mainly because excessive titanium occupies the sites of iron elements, leading to a decrease in the discharge specific capacity of the prepared lithium iron phosphate material itself. Based on Examples 1, 3, and 5, it can be seen that with the decrease of titanium in the iron phosphate material, the specific surface area of ​​the lithium iron phosphate material decreases slightly, and the powder resistance of the prepared lithium iron phosphate material also increases slightly, correspondingly, its charge-discharge efficiency also decreases. This indicates that both excessive and insufficient titanium doping will have a certain negative impact on the charge-discharge efficiency of the prepared lithium iron phosphate material.

[0265] As shown in Tables 1 and 2, compared to Example 1, the lithium iron phosphate materials prepared in Examples 6 and 7 exhibit significantly higher internal resistance and significantly lower charge-discharge efficiency. This is mainly because a higher pH value was used during the preparation of the lithium iron phosphate materials, resulting in a higher content of impurities such as magnesium and potassium salts in the materials.

[0266] As shown in Tables 1 and 2, although titanium was doped in Comparative Example 2, the powder internal resistance of the prepared lithium iron phosphate material was significantly higher, even higher than that of Comparative Example 1. This is mainly because Comparative Example 2 did not recrystallize in the phosphoric acid solution to form uniform iron phosphate dihydrate before subsequent calcination, which resulted in uneven size of the final lithium iron phosphate particles, leading to a significant decrease in charge-discharge performance.

[0267] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. An iron phosphate material, characterized in that, The titanium element is doped in the iron phosphate material, at least part of the surface of the primary particles of the iron phosphate material is in a laminated structure, the laminated structure has a plurality of layers, and any two adjacent layers are located on the sides of each other in the stacking direction. The stacking direction intersects with the extension direction of the layers.

2. The iron phosphate material of claim 1, wherein, The surface of the primary particles of the iron phosphate material is split into the laminated structure; and / or, The surface of a single primary particle of the iron phosphate material has a plurality of laminated structures; and / or, In a single laminated structure, the number of layers is 2-4.

3. The iron phosphate material of claim 1, wherein, The particle size of a single primary particle is 300-1000 nm; and / or, In the laminated structure, the thickness of each layer is 40-60 nm; and / or, In the stacking direction, the maximum spacing between two adjacent layers is 20-50 nm.

4. The iron phosphate material according to any one of claims 1 to 3, characterized in that The chemical formula of the iron phosphate material is: Fe (1-n) Ti 0.75n PO4, wherein 0.02≤n≤0.

04.

5. The iron phosphate material according to any one of claims 1 to 3, characterized in that The specific surface area of the iron phosphate material is 9.4 m 2 / g to 14 m 2 / g; and / or, The D50 particle size of the iron phosphate material is 30-60 μm; and / or, The tap density of the iron phosphate material is 0.9 g / cm 3 ~ 1.05 g / cm 3 .

6. A method for producing a ferric phosphate material, characterized by, The method comprises the following steps: A mixed solution comprising ferrous salt, titanium salt and phosphate salt is provided; The mixed solution is mixed with a first alkali source to obtain a first slurry; An oxidizing agent and a second alkali source are added to the first slurry to obtain a second slurry; The second slurry is subjected to a first solid-liquid separation treatment to obtain a filter cake; The filter cake is mixed with a phosphoric acid solution, and subjected to an aging treatment to obtain a third slurry; The third slurry is subjected to a second solid-liquid separation treatment, dried, and subjected to a calcination treatment to obtain the iron phosphate material.

7. The method of producing a ferric phosphate material according to claim 6, characterized by, In the mixed solution, the ratio of the amount of substance of iron element to the amount of substance of titanium element is 1:(0.015-0.03); and / or, In the mixed solution, the ratio of the amount of substance of iron element to the amount of substance of phosphorus element is 1:(0.97-1.07); and / or, The oxidizing agent comprises hydrogen peroxide, and in the process of adding the oxidizing agent and the second alkali source to the first slurry, the ratio of the amount of substance of hydrogen peroxide in the added hydrogen peroxide to the amount of substance of ferrous ions in the mixed solution is (0.55-0.75):1; and / or, The oxidizing agent comprises oxygen or compressed air, and in the process of adding the oxidizing agent and the second alkali source to the first slurry, the ratio of the amount of substance of oxygen in the added oxygen or compressed air to the amount of substance of ferrous ions in the mixed solution is (0.275-0.375):

1.

8. The method of producing a ferric phosphate material according to any one of claims 6 to 7, characterized by, The pH value of the mixed solution is 0.5-1.5; and / or, After the first alkali source is added to the mixed solution, the pH value of the formed first slurry is controlled to be 2.0-2.5; and / or, In the process of adding the oxidizing agent and the second alkali source to the first slurry, the pH value is controlled to be 2.0-2.

5.

9. The method of producing a ferric phosphate material according to any one of claims 6 to 7, characterized by, The aging treatment step comprises heating the mixture of the filter cake and the phosphoric acid solution to 85-98°C; and / or, In the step of mixing the filter cake with the phosphoric acid solution, the amount of substance of phosphoric acid in the phosphoric acid solution is 0.5-1.5 times the amount of substance of the mixed The molar ratio of the liquid-phase phosphate to the substance is (0.1-0.2):

1.

10. The method of producing a ferric phosphate material according to any one of claims 6 to 7, characterized by, The step of calcining treatment comprises: calcining the dried material at a temperature of 600-800 DEG C for 2-3 hours to form the iron phosphate material.

11. A positive electrode material, characterized in that, The raw material for preparing the positive electrode material comprises the iron phosphate material according to any one of claims 1-5, or the raw material for preparing the positive electrode material comprises the iron phosphate material prepared by the preparation method according to any one of claims 6-10.

12. A positive electrode sheet characterized by comprising: The positive electrode plate comprises a current collector and a positive electrode active layer arranged on the current collector, and the positive electrode active layer comprises the positive electrode material according to claim 11.

13. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode plate and a negative electrode plate, and the positive electrode plate of the secondary battery is the positive electrode plate according to claim 12.

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