Iron phosphate and a preparation method thereof, graded iron phosphate and lithium iron phosphate cathode material

Abstract

The application relates to a kind of ferric phosphate and its preparation method, graded ferric phosphate and ferric lithium phosphate positive electrode material, wherein the ferric phosphate is hexagonal system anhydrous ferric phosphate, crystallinity is more than 98%; The particle size Dv10 of ferric phosphate is 0.5 mu m, Dv50 is 1.6 mu m-2 mu m, Dv90 is 5 mu m, and ((Dv90-Dv10) / Dv50) is 2.22; The tap density is 1g / cm3-1.2g / cm3, the specific surface area is 1m2 / g-3.0m2 / g, the iron-phosphorus atomic ratio is 0.97 or more, and the sphericity is 0.806 or more; The ferric phosphate has high sphericity, high intergranular dispersity, is easier to disperse in the preparation process of ferric lithium phosphate, and has better grading effect; And the specific surface area of ferric phosphate is low, the flowability is good, it is easier to filter, and is not easy to agglomerate; The particle size of ferric phosphate is small, the grading structure is more compact, and is suitable for preparing high-pressure high-energy-density ferric lithium phosphate positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of battery material technology, and relates to a lithium-ion battery material, particularly to an iron phosphate and its preparation method, graded iron phosphate and lithium iron phosphate cathode materials. Background Technology

[0002] In recent years, new energy materials technology has continued to develop. Lithium iron phosphate (LiFePO4, LFP) has shown outstanding application advantages in large-scale energy storage and new energy vehicles due to its superior cycle performance, thermal stability and safety performance compared with traditional cathode materials such as lithium cobalt oxide and ternary cathode materials.

[0003] As a core precursor for the preparation of lithium iron phosphate, the morphology, structure, particle size, and purity of iron phosphate directly determine the performance of the final lithium iron phosphate product. Therefore, the preparation process of iron phosphate has attracted much attention. Currently, there are various methods for preparing iron phosphate, including solid-phase synthesis, hydrothermal synthesis, homogeneous precipitation, and oxidation. Among them, the mainstream synthesis routes in industrial production are mainly divided into three categories: First, using ferric nitrate solution as the iron source, reacting with phosphoric acid, and then neutralizing the excess acid in the system with ammonia water to obtain iron phosphate; Second, using ferrous sulfate as the iron source, first reacting with ammonium dihydrogen phosphate, oxidizing with hydrogen peroxide, and then adjusting the pH of the reaction system with sodium hydroxide or ammonia water to generate iron phosphate; Third, directly reacting phosphoric acid with iron scrap or iron block to prepare ferrous hydrogen phosphate intermediate, and then oxidizing with hydrogen peroxide to obtain iron phosphate.

[0004] However, existing industrial preparation methods all have significant drawbacks: when using ferric nitrate as the iron source, not only are the raw material costs high, but the washing waste liquid also contains a large amount of nitrate ions, making subsequent treatment difficult and costly; in the process of using ferrous sulfate as raw material, a large amount of ammonia nitrogen wastewater is generated, and the treatment process for such wastewater is complex and costly; while when using metallic iron as raw material, a large amount of hydrogen gas is released during the reaction, posing a great safety hazard, and high-cost hydrogen peroxide is required as an oxidant, further increasing the total production cost of ferric phosphate. All of the above problems limit the large-scale, low-cost production of ferric phosphate. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide an iron phosphate and its preparation method, graded iron phosphate, and lithium iron phosphate cathode materials. The present invention can use inexpensive iron oxide red as an iron source, and by controlling the addition rate of phosphorus source, obtains spherical dispersed small particles of iron phosphate with small particle size and narrow particle size distribution, which can better obtain graded iron phosphate and optimize the packing density, thereby obtaining lithium iron phosphate cathode materials with high compaction density and high capacity.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides an iron phosphate, wherein the iron phosphate is an anhydrous iron phosphate with a hexagonal crystal system and a crystallinity of 98% or more.

[0008] The iron phosphate has a particle size of Dv10 ≥ 0.5 μm, Dv50 of 1.6 μm ~ 2 μm, Dv90 ≤ 5 μm, and ((Dv90-Dv10) / Dv50) ≤ 2.22;

[0009] The tap density of the ferric phosphate is 1 g / cm³. 3 ~1.2g / cm 3 Specific surface area is 1m 2 / g~3.0m 2 / g, iron-phosphorus atomic ratio of 0.97 or higher, sphericity of 0.806 or higher.

[0010] The iron phosphate provided by this invention has a spherical dispersed small particle structure, and the particle size distribution indicates that the particles are uniformly distributed. The iron phosphate includes secondary particles formed by stacking primary particles, and multiple primary particles form cracks on the surface of the secondary particles, with no obvious agglomeration between the secondary particles. The iron phosphate provided by this invention has a moderate particle size and a narrow particle size distribution, which can better grade with iron phosphate of other particle sizes, fill the gaps between particles, optimize the particle packing density, thereby achieving a more ideal gradation effect, maintaining the stability of the gradation structure, and thus facilitating the construction of lithium iron phosphate cathode materials with high compaction density and high capacity.

[0011] Secondly, the present invention provides a method for preparing iron phosphate, the method comprising the following steps:

[0012] Activate iron oxide red, then mix the activated iron oxide red with sulfuric acid solution to obtain a solution; add a phosphorus source to the solution while stirring; after the phosphorus source is added, keep warm to allow precipitation and crystallization; filter, wash, dry and calcine the precipitated crystal product to obtain the iron phosphate described in the first aspect;

[0013] The phosphorus source is added to the solution over a period of 5 to 7 hours.

[0014] The manufacturing method provided by this invention uses inexpensive iron oxide red from steel mills as the iron source. A small amount of sulfuric acid is added during dissolution to promote the dissolution of iron oxide red. There is no need to adjust the pH of the reaction system. Spherical iron phosphate is synthesized by slowly adding a phosphorus source. The resulting spherical iron phosphate has a moderate particle size and a narrow particle size distribution, which can better grade with iron phosphate of other particle sizes, fill the gaps between particles, optimize the particle packing density, and thus achieve a more ideal gradation effect and maintain the stability of the gradation structure. This is beneficial for constructing lithium iron phosphate cathode materials with high compaction density and high capacity.

[0015] In some embodiments, the stirring speed is 1300 r / min to 1400 r / min.

[0016] In some embodiments, the insulation temperature is 90°C to 95°C.

[0017] In some embodiments, the heat preservation time is 10h to 15h.

[0018] In some embodiments, the molar ratio of iron in the activated iron oxide red to phosphorus in the phosphorus source is 1:1 to 1:1.5.

[0019] In some embodiments, the phosphorus source includes phosphoric acid.

[0020] In some embodiments, the calcination temperature is 600°C to 650°C.

[0021] In some embodiments, the calcination time is 2h to 4h.

[0022] In some embodiments, the activated iron oxide red contains more than 99 wt% Fe2O3 and less than 100 ppm of other metal elements.

[0023] In some embodiments, activation refers to heat-treating the iron oxide red at 350°C to 450°C.

[0024] In some embodiments, the activation time is 1.5h to 2.5h.

[0025] In some embodiments, the molar ratio of the activated iron oxide red to sulfuric acid in the sulfuric acid solution is 1:2.9 to 1:3.1.

[0026] In some embodiments, the concentration of the sulfuric acid solution is 0.2 mol / L to 0.4 mol / L.

[0027] In some embodiments, the activated iron oxide red is mixed with sulfuric acid solution at a temperature of 60°C to 80°C and a stirring speed of 300 r / min to 400 r / min.

[0028] Thirdly, the present invention provides a graded ferric phosphate, wherein the graded ferric phosphate includes compounded amorphous small-particle ferric phosphate, compounded large-particle ferric phosphate, and the ferric phosphate described in the first aspect;

[0029] The particle size Dv50 of the compounded amorphous ferric phosphate is 0.3 μm to 0.5 μm;

[0030] The particle size Dv50 of the compounded large-particle iron phosphate is 3μm~5μm;

[0031] The mass ratio of the compounded amorphous small-particle ferric phosphate, the ferric phosphate mentioned in the first aspect, and the compounded large-particle ferric phosphate is (4-6):(2-3):(1-3).

[0032] Among them, compounded large-particle ferric phosphate serves as the skeleton to support the compaction structure, compounded amorphous small-particle ferric phosphate serves as the small-particle filler, and the ferric phosphate mentioned in the first aspect serves as the medium-particle filler.

[0033] Fourthly, the present invention provides a lithium iron phosphate cathode material, which is prepared from the graded iron phosphate described in the third aspect.

[0034] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The manufacturing method provided by the present invention uses inexpensive iron oxide red from steel plants as iron source. A small amount of sulfuric acid is added during dissolution to promote the dissolution of iron oxide red. There is no need to adjust the pH of the reaction system. By slowly adding phosphorus source, spherical dispersed iron phosphate particles are synthesized.

[0037] (2) The iron phosphate provided by the present invention is a spherical dispersed small particle structure with uniform particle size distribution on the surface; and it is a secondary particle formed by stacking primary particles, with multiple primary particles forming cracks on the surface of the secondary particles, and no obvious agglomeration phenomenon between the secondary particles; the iron phosphate provided by the present invention has a moderate particle size and a narrow particle size distribution, which can better grade with iron phosphate of other particle sizes, fill the gaps between particles, optimize the particle packing density, thereby achieving a more ideal gradation effect, maintaining the stability of the gradation structure, and thus facilitating the construction of lithium iron phosphate cathode material with high compaction density and high capacity. Attached Figure Description

[0038] Figure 1 This is a SEM image of the iron phosphate obtained in Example 1 of the present invention;

[0039] Figure 2 The XRD pattern of ferric phosphate obtained in Example 1 of this invention;

[0040] Figure 3 This is a SEM image of the lithium iron phosphate cathode material prepared from iron phosphate obtained in Example 1 of the present invention. Detailed Implementation

[0041] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0042] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0043] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0044] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0045] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0046] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0047] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0048] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0049] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0050] In this invention, "optional" means that something is optional, that is, it refers to either "with" or "without". If there are multiple "optional" options in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, then each "optional" option is independent.

[0051] In a first aspect, an embodiment of the present invention provides an iron phosphate, wherein the iron phosphate is anhydrous iron phosphate with a hexagonal crystal system and a crystallinity of 98% or more;

[0052] The iron phosphate has a particle size of Dv10 ≥ 0.5 μm, Dv50 of 1.6 μm ~ 2 μm, Dv90 ≤ 5 μm, and ((Dv90-Dv10) / Dv50) ≤ 2.22;

[0053] The tap density of the ferric phosphate is 1 g / cm³. 3 ~1.2g / cm 3 Specific surface area is 1m 2 / g~3.0m 2 / g, iron-phosphorus atomic ratio of 0.97 or higher, sphericity of 0.806 or higher.

[0054] The iron phosphate provided by this invention has a spherical, dispersed small particle structure with a uniform particle size distribution on the surface. The iron phosphate includes secondary particles formed by stacking primary particles, and multiple primary particles form cracks on the surface of the secondary particles, with no obvious agglomeration between the secondary particles. The iron phosphate provided by this invention has a moderate particle size and a narrow particle size distribution, which can better grade it with iron phosphate of other particle sizes, fill the gaps between particles, optimize the particle packing density, thereby achieving a more ideal gradation effect and maintaining the stability of the gradation structure. This is beneficial for constructing lithium iron phosphate cathode materials with both high compaction density and high capacity.

[0055] Secondly, an embodiment of the present invention provides a method for preparing ferric phosphate, the method comprising the following steps:

[0056] Activate iron oxide red, then mix the activated iron oxide red with sulfuric acid solution to obtain a solution; add a phosphorus source to the solution while stirring; after the phosphorus source is added, keep warm to allow precipitation and crystallization; filter, wash, dry and calcine the precipitated crystal product to obtain the iron phosphate described in the first aspect;

[0057] The phosphorus source is added to the solution over a period of 5 to 7 hours.

[0058] The manufacturing method provided by this invention uses inexpensive iron oxide red from steel mills as the iron source. A small amount of sulfuric acid is added during dissolution to promote the dissolution of iron oxide red. There is no need to adjust the pH of the reaction system. Spherical iron phosphate is synthesized by slowly adding a phosphorus source. The resulting spherical iron phosphate has a moderate particle size and a narrow particle size distribution, which can better grade with iron phosphate of other particle sizes, fill the gaps between particles, optimize the particle packing density, and thus achieve a more ideal gradation effect and maintain the stability of the gradation structure. This is beneficial for constructing lithium iron phosphate cathode materials with high compaction density and high capacity.

[0059] The time required for the phosphorus source to be added to the solution can effectively control the supersaturation of the reaction system, which is beneficial for the formation of spherical iron phosphate. Specifically, in one embodiment of the present invention, the time for adding the phosphorus source to the solution is 5h to 7h, for example, 5h, 5.5h, 6h, 6.5h or 7h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0060] If the phosphorus source is added to the solution for less than 5 hours in this invention, it may cause the phosphorus source to enter the reaction system rapidly, leading to a sharp increase in the supersaturation of the system. This makes it difficult to effectively control the crystal nucleation and growth rate, often resulting in the formation of spherical particles and the formation of irregular large particles or agglomerates. Furthermore, the particle size distribution tends to increase, posing a risk of difficulty in achieving ideal gradation with other iron phosphate particles of different sizes. It is also difficult to effectively fill particle voids and optimize packing density, which is detrimental to the construction of lithium iron phosphate cathode materials with both high compaction density and high capacity. If the phosphorus source is added for more than 7 hours, the supersaturation of the reaction system may remain at a low level, resulting in a slower crystal nucleation rate and a longer growth cycle. This can easily lead to uneven particle growth, with some particles potentially growing excessively large. This may also disrupt the narrow range characteristics of the particle size distribution, adversely affecting the gradation effect. At the same time, excessively long feeding time will reduce production efficiency, increase process costs, and the product particle morphology may deviate from spherical shape, making it difficult to stably guarantee the compaction density and capacity performance of the subsequent lithium iron phosphate cathode material.

[0061] In some embodiments, the rate at which the phosphorus source is added to the solution is constant.

[0062] In some embodiments, the stirring speed is 1300 r / min to 1400 r / min, for example, it can be 1300 r / min, 1320 r / min, 1350 r / min, 1360 r / min, 1380 r / min or 1400 r / min, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0063] This invention controls the stirring speed to 1300 r / min~1400 r / min, which synergizes with the slow addition of phosphorus source over a period of 5h~7h. This promotes rapid and uniform dispersion of the phosphorus source within the reaction system, avoiding excessively high or low phosphorus source concentrations in localized areas. This maintains the system's supersaturation within a suitable range, providing a stable environment for the nucleation and growth of spherical ferric phosphate. It also helps ensure a narrower particle size distribution of the product, facilitating subsequent ideal gradation and optimized bulk density with other ferric phosphates of different particle sizes. If the stirring speed is lower than 1300 r / min... The uniformity of material mixing within the system may be insufficient, and phosphorus source is prone to local enrichment, causing a sudden increase in local supersaturation, making it difficult to form regular spherical particles. This will have an adverse effect on the subsequent gradation effect and the compaction density and capacity performance of lithium iron phosphate cathode material. If the stirring rate is higher than 1400 r / min, the strong shear force may impact the iron phosphate crystals in the growth stage, causing the crystal morphology to deviate from spherical shape and breakage. This may also destroy the narrow range characteristics of particle size distribution. At the same time, excessively high stirring rates may also increase equipment energy consumption and process costs.

[0064] In some embodiments, the insulation temperature is 90°C to 95°C, for example, it can be 90°C, 91°C, 92°C, 93°C, 94°C or 95°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0065] In some embodiments, the heat preservation time is 10h to 15h, for example, it can be 10h, 11h, 12h, 13h, 14h or 15h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0066] This invention controls the temperature and time of heat preservation, which helps to ensure that the nucleation and growth rate of iron phosphate are in a suitable state and provides a sufficient and stable growth environment for crystal growth, thus facilitating the control of the particle size distribution of iron phosphate. If the heat preservation time is too short, the crystal may not grow sufficiently, easily forming irregularly shaped particles or incompletely crystallized intermediates, and the particle size distribution may become wider, making it difficult to meet the gradation requirements. If the heat preservation time is too long, the crystal may overgrow, and some particles may become too large, which may also destroy the narrow range characteristics of the particle size distribution, while also increasing process energy consumption and production costs.

[0067] In some embodiments, the molar ratio of iron in the activated iron oxide red to phosphorus in the phosphorus source is 1:1 to 1:1.5, for example, it can be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0068] In some embodiments, the phosphorus source includes phosphoric acid.

[0069] In some embodiments, the calcination temperature is 600°C to 650°C, for example, 600°C, 610°C, 620°C, 630°C, 640°C or 650°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0070] In some embodiments, the calcination time is 2h to 4h, for example, it can be 2h, 2.5h, 3h, 3.5h or 4h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0071] In some embodiments, the activated iron oxide red contains more than 99 wt% Fe2O3 and less than 100 ppm of other metal elements.

[0072] In this invention, other metallic elements refer to metallic elements other than iron.

[0073] This invention removes organic impurities through activation.

[0074] In some embodiments, activation refers to heat-treating the iron oxide red at 350°C to 450°C, for example, 350°C, 360°C, 380°C, 400°C, 420°C, 440°C or 450°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0075] In some embodiments, the activation time is 1.5h to 2.5h, for example, it can be 1.5h, 1.6h, 1.8h, 2h, 2.1h, 2.4h or 2.5h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0076] In some embodiments, the molar ratio of the activated iron oxide red to sulfuric acid in the sulfur solution is 1:2.9 to 1:3.1, for example, it can be 1:2.9, 1:3 or 1:3.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0077] In some embodiments, the concentration of the sulfuric acid solution is 0.2 mol / L to 0.4 mol / L, for example, it can be 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L or 0.4 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0078] In some embodiments, the activated iron oxide red is mixed with sulfuric acid solution at a temperature of 60°C to 80°C and a stirring speed of 300 r / min to 400 r / min.

[0079] The mixing temperature is 60℃~80℃, for example, it can be 60℃, 65℃, 70℃, 75℃ or 80℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0080] The mixing speed is 300r / min to 400r / min, for example, it can be 300r / min, 320r / min, 350r / min, 360r / min, 380r / min or 400r / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0081] As a preferred embodiment of the preparation method provided in the second aspect of the present invention, the preparation method includes the following steps:

[0082] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0083] The activated iron oxide red contains more than 99 wt% Fe2O3 and less than 100 ppm of other metal elements.

[0084] The activation method includes heat treatment at a temperature of 350℃~450℃ for a time of 1.5h~2.5h;

[0085] The molar ratio of the activated iron oxide red to sulfuric acid in the sulfuric acid solution is 1:2.9 to 1:3.1, and the concentration of the sulfuric acid solution is 0.2 mol / L to 0.4 mol / L.

[0086] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 60℃~80℃ and a stirring speed of 300r / min~400r / min.

[0087] (2) Add phosphoric acid to the solution while stirring;

[0088] The stirring speed is 1300 r / min to 1400 r / min;

[0089] The phosphoric acid is added to the solution over a period of 5 to 7 hours.

[0090] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1 to 1:1.5.

[0091] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0092] The insulation temperature is 90℃~95℃, and the time is 10h~15h;

[0093] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0094] The calcination temperature is 600℃~650℃, and the time is 2h~4h.

[0095] Thirdly, an embodiment of the present invention provides a graded ferric phosphate, the graded ferric phosphate comprising compounded amorphous small-particle ferric phosphate, compounded large-particle ferric phosphate, and ferric phosphate as described in any embodiment;

[0096] The particle size Dv50 of the compounded amorphous ferric phosphate is 0.3μm to 0.5μm, for example, it can be 0.3μm, 0.35μm, 0.4μm, 0.45μm or 0.5μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0097] The particle size Dv50 of the compounded large-particle iron phosphate is 3μm to 5μm, for example, it can be 3μm, 3.5μm, 4μm, 4.5μm or 5μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0098] The particle size Dv50 of the iron phosphate is 1.6μm to 2μm, for example, it can be 1.6μm, 1.7μm, 1.8μm, 1.9μm or 2μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0099] The mass ratio of the compounded amorphous small-particle ferric phosphate, the ferric phosphate in any embodiment, and the compounded large-particle ferric phosphate is (4-6):(2-3):(1-3), for example, it can be 4:2:1, 6:3:3, 4:3:1, 4:3:3 or 5:2:2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0100] Among them, compounded large-particle ferric phosphate serves as the skeleton to support the compaction structure, compounded amorphous small-particle ferric phosphate serves as the small-particle filler, and the ferric phosphate mentioned in the first aspect serves as the medium-particle filler.

[0101] Fourthly, an embodiment of the present invention provides a lithium iron phosphate cathode material, which is prepared from the graded iron phosphate described in the third aspect.

[0102] Optionally, the preparation method of lithium iron phosphate cathode material includes the following steps: mixing lithium source, graded iron phosphate and carbon source, and calcining under a protective atmosphere to obtain the lithium iron phosphate cathode material.

[0103] In one embodiment, the lithium source may be lithium carbonate.

[0104] In one embodiment, the molar ratio of lithium in the lithium source to iron in the graded iron phosphate can be 0.51:1 or higher.

[0105] In one embodiment, the carbon source may be glucose, and the amount of carbon source used is 7wt% to 10wt% of the total mass of lithium source, graded iron phosphate and carbon source.

[0106] In one embodiment, the protective atmosphere used includes nitrogen and / or an inert gas.

[0107] In one embodiment, the calcination temperature can be 720℃~750℃, and the time can be 10h~14h.

[0108] The lithium iron phosphate cathode material prepared by the present invention is coated with a uniform carbon layer with a particle size Dv50 of 0.8 μm to 1.4 μm and a compaction density of 2.55 g / cm³. 3 ~2.65g / cm 3 Under conditions of 3.65V vs. Li+ / Li and 0.1C rate, the discharge specific capacity can reach 155mAh / g~161mAh / g, and the cycle capacity retention rate after 1000 cycles at 1C rate is over 90%.

[0109] To clearly illustrate the technical solution of the present invention, in the following examples and comparative examples, the Fe2O3 content in the iron oxide red after activation is 99.2%. The above description of iron oxide red is only for the purpose of clearly illustrating the technical solution of the present invention and is not considered as a further limitation of the present invention.

[0110] Example 1

[0111] This embodiment provides a method for preparing ferric phosphate, including the following steps:

[0112] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0113] The activation method includes heat treatment at 400°C for 2 hours;

[0114] The molar ratio of the activated iron oxide red to the sulfuric acid in the sulfuric acid solution is 1:3, and the concentration of the sulfuric acid solution is 0.25 mol / L.

[0115] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 70°C and a stirring speed of 350 r / min.

[0116] (2) Add phosphoric acid to the solution while stirring;

[0117] The stirring speed is 1300 r / min;

[0118] The phosphoric acid was added to the solution over a period of 5 hours, and the flow rate remained stable during the addition process.

[0119] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1.2;

[0120] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0121] The insulation temperature is 95℃ and the time is 10 hours;

[0122] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0123] Washing includes primary pulping with pure water, followed by filtration and washing for 25 minutes; the washed material is then pulped a second time with pure water, filtered, and washed for 15 minutes.

[0124] The calcination temperature was 650℃ and the time was 2 hours.

[0125] The SEM image of the ferric phosphate obtained in this embodiment is as follows: Figure 1 As shown, by Figure 1 As can be seen, the iron phosphate obtained in this embodiment has a near-spherical structure. The XRD pattern of the iron phosphate obtained in this embodiment is shown below. Figure 2 As shown, by Figure 2 It can be seen that the iron phosphate obtained in this embodiment is hexagonal anhydrous iron phosphate.

[0126] Example 2

[0127] This embodiment provides a method for preparing ferric phosphate, including the following steps:

[0128] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0129] The activation method includes heat treatment at 400°C for 2 hours;

[0130] The molar ratio of the activated iron oxide red to the sulfuric acid in the sulfuric acid solution is 1:3, and the concentration of the sulfuric acid solution is 0.2 mol / L.

[0131] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 75°C and a stirring speed of 350 r / min.

[0132] (2) Add phosphoric acid to the solution while stirring;

[0133] The stirring speed is 1320 r / min;

[0134] The phosphoric acid was added to the solution over a period of 6 hours, and the flow rate remained stable during the addition process.

[0135] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1.2;

[0136] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0137] The insulation temperature is 90℃ and the time is 15 hours;

[0138] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0139] Washing includes primary pulping with pure water, followed by filtration and washing for 20 minutes; the washed material is then pulped a second time with pure water, followed by filtration and washing for 15 minutes.

[0140] The calcination temperature was 625℃ and the time was 2 hours.

[0141] Example 3

[0142] This embodiment provides a method for preparing ferric phosphate, including the following steps:

[0143] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0144] The activation method includes heat treatment at 400°C for 2 hours;

[0145] The molar ratio of the activated iron oxide red to the sulfuric acid in the sulfuric acid solution is 1:3, and the concentration of the sulfuric acid solution is 0.22 mol / L.

[0146] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 75°C and a stirring speed of 350 r / min.

[0147] (2) Add phosphoric acid to the solution while stirring;

[0148] The stirring speed is 1350 r / min;

[0149] The phosphoric acid was added to the solution over a period of 5.5 hours, with a stable flow rate during the addition process.

[0150] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1.2;

[0151] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0152] The insulation temperature is 90℃, and the time is 15 hours;

[0153] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0154] Washing includes primary pulping with pure water, followed by filtration and washing for 20 minutes; the washed material is then pulped a second time with pure water, followed by filtration and washing for 15 minutes.

[0155] The calcination temperature was 635℃ and the time was 2 hours.

[0156] Example 4

[0157] This embodiment provides a method for preparing ferric phosphate, including the following steps:

[0158] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0159] The activation method includes heat treatment at 400°C for 2 hours;

[0160] The molar ratio of the activated iron oxide red to the sulfuric acid in the sulfuric acid solution is 1:3, and the concentration of the sulfuric acid solution is 0.2 mol / L.

[0161] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 80°C and a stirring speed of 350 r / min.

[0162] (2) Add phosphoric acid to the solution while stirring;

[0163] The stirring speed is 1350 r / min;

[0164] The phosphoric acid was added to the solution over a period of 6.5 hours, with a stable flow rate during the addition process.

[0165] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1.2;

[0166] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0167] The insulation temperature is 90℃, and the time is 15 hours;

[0168] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0169] Washing includes primary pulping with pure water, followed by filtration and washing for 20 minutes; the washed material is then pulped a second time with pure water, followed by filtration and washing for 15 minutes.

[0170] The calcination temperature was 650℃ and the time was 2 hours.

[0171] Example 5

[0172] This embodiment provides a method for preparing ferric phosphate, including the following steps:

[0173] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0174] The activation method includes heat treatment at 350°C for 2.5 hours.

[0175] The molar ratio of the activated iron oxide red to sulfuric acid in the sulfuric acid solution is 1:2.9, and the concentration of the sulfuric acid solution is 0.4 mol / L.

[0176] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 60°C and a stirring speed of 400 r / min.

[0177] (2) Add phosphoric acid to the solution while stirring;

[0178] The stirring speed is 1300 r / min;

[0179] The phosphoric acid was added to the solution over a period of 7 hours, and the flow rate remained stable during the addition process.

[0180] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1;

[0181] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0182] The insulation temperature is 95℃ and the time is 10 hours;

[0183] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0184] Washing includes primary pulping with pure water, followed by filtration and washing for 25 minutes; the washed material is then pulped a second time with pure water, filtered, and washed for 15 minutes.

[0185] The calcination temperature was 600℃ and the time was 4 hours.

[0186] Example 6

[0187] This embodiment provides a method for preparing ferric phosphate, including the following steps:

[0188] (1) Activate iron oxide red, and then mix the activated iron oxide red with sulfuric acid solution to obtain a solution;

[0189] The activation method includes heat treatment at 450°C for 1.5 hours.

[0190] The molar ratio of the activated iron oxide red to sulfuric acid in the sulfuric acid solution is 1:3.1, and the concentration of the sulfuric acid solution is 0.2 mol / L.

[0191] The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 80°C and a stirring speed of 300 r / min.

[0192] (2) Add phosphoric acid to the solution while stirring;

[0193] The stirring speed is 1400 r / min;

[0194] The phosphoric acid was added to the solution over a period of 5 hours, and the flow rate remained stable during the addition process.

[0195] The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphoric acid is 1:1.5;

[0196] (3) After the phosphoric acid is added, keep it at a warm temperature to allow precipitation and crystallization;

[0197] The insulation temperature is 95℃ and the time is 10 hours;

[0198] (4) The precipitated crystallized product is filtered, washed, dried and calcined to obtain the iron phosphate;

[0199] Washing includes primary pulping with pure water, followed by filtration and washing for 25 minutes; the washed material is then pulped a second time with pure water, filtered, and washed for 15 minutes.

[0200] The calcination temperature was 650℃ and the time was 2 hours.

[0201] Example 7

[0202] This embodiment provides a method for preparing ferric phosphate, which is the same as in Example 1 except that the stirring speed is 1200 r / min.

[0203] Example 8

[0204] This embodiment provides a method for preparing ferric phosphate, which is the same as in Example 1 except that the stirring speed is 1500 r / min.

[0205] Example 9

[0206] This embodiment provides a method for preparing ferric phosphate, which is the same as in Example 1 except that the heat preservation time is 8 hours.

[0207] Example 10

[0208] This embodiment provides a method for preparing ferric phosphate, which is the same as in Example 1 except that the heat preservation time is 18 hours.

[0209] Comparative Example 1

[0210] This comparative example provides a method for preparing iron phosphate, in which phosphoric acid is added in one step, and all other steps are the same as in Example 1.

[0211] Comparative Example 2

[0212] This comparative example provides a method for preparing ferric phosphate, which is the same as in Example 1 except that phosphoric acid is added all at once and the stirring is done at 400 r / min.

[0213] Performance Characterization

[0214] The iron phosphate provided in Example 1 was mixed with compound iron phosphate to obtain graded iron phosphate. The graded iron phosphate included compound amorphous small-particle iron phosphate and compound large-particle iron phosphate. The compound amorphous small-particle iron phosphate had a particle size Dv50 of 0.4 μm as small particle filler, the iron phosphate prepared in Example 1 as medium particle filler, and the compound large-particle iron phosphate had a Dv50 of 4.6 μm as a framework support and compaction structure. The mass ratio of the compound amorphous small-particle iron phosphate, the iron phosphate provided in Example 1, and the compound large-particle iron phosphate in the graded iron phosphate was 4:3:3. Lithium carbonate, graded iron phosphate, and glucose (the molar ratio of lithium in lithium carbonate to iron in graded iron phosphate was 0.51:1, and the amount of glucose was 9 wt% of the total mass of the lithium source, graded iron phosphate, and glucose) were mixed and calcined at 750°C for 10 h under a nitrogen atmosphere to obtain lithium iron phosphate cathode material. The SEM image of the lithium iron phosphate cathode material is shown below. Figure 3 As shown, by Figure 3 It is known that lithium iron phosphate consists of spherical particles, including large, medium, and small particles. The small and medium particles fill the gaps between the large particles to form a densely packed structure.

[0215] The mass percentage of Fe, mass percentage of P, iron-to-phosphorus atomic ratio, sulfur content, particle size, specific surface area, and tap density of the iron phosphate provided in the above examples and comparative examples were measured, and the results are shown in Table 1.

[0216] Table 1

[0217]

[0218] The iron phosphate provided in the above embodiments and comparative examples was mixed with compound iron phosphate to obtain graded iron phosphate. The graded iron phosphate included compound amorphous small-particle iron phosphate and compound large-particle iron phosphate. The particle size Dv50 of the compound amorphous small-particle iron phosphate was 0.4 μm, which was used as small particle filler. The iron phosphate provided in the above embodiments and comparative examples was used as medium particle filler. The particle size Dv50 of the compound large-particle iron phosphate was 4.6 μm, which was used as a skeleton support compaction structure. The mass ratio of compound amorphous small-particle iron phosphate, iron phosphate provided in Example 1, and compound large-particle iron phosphate in the graded iron phosphate was 4:3:3. Lithium carbonate, graded iron phosphate, and glucose (the molar ratio of lithium in lithium carbonate to iron in graded iron phosphate was 0.51:1, and the amount of glucose was 9 wt% of the total mass of lithium source, graded iron phosphate, and glucose) was mixed and calcined at 750°C for 10 h under a nitrogen atmosphere to obtain lithium iron phosphate cathode material. The compaction density of lithium iron phosphate cathode material was tested, and lithium was used as a reference electrode. The first discharge specific capacity and first charge-discharge efficiency at 0.1C were tested at 3.65V. Then, the discharge specific capacity and the cycle capacity retention rate after 1000 cycles were tested at 1C. The results are shown in Table 2.

[0219] Table 2

[0220]

[0221] In summary, the iron phosphate provided by this invention uses inexpensive iron oxide red from steel mills as the iron source. A small amount of sulfuric acid is added during dissolution to promote the dissolution of iron oxide red. There is no need to adjust the pH of the reaction system. It is synthesized by slowly adding a phosphorus source. The iron phosphate has a spherical dispersed particle structure with uniform particle size distribution. It is a secondary particle formed by the stacking of primary particles. Multiple primary particles form cracks on the surface of the secondary particles. There is no obvious agglomeration between the secondary particles. Its particle size is moderate and the particle size distribution is narrow, which can better grade it with iron phosphate of other particle sizes, fill the gaps between particles, optimize the particle packing density, and thus achieve a more ideal gradation effect, maintain the stability of the gradation structure, and is conducive to constructing lithium iron phosphate cathode materials with high compaction density and high capacity.

[0222] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A type of iron phosphate, characterized in that, The iron phosphate is hexagonal anhydrous iron phosphate with a crystallinity of over 98%. The iron phosphate has a particle size of Dv10 ≥ 0.5 μm, Dv50 of 1.6 μm ~ 2 μm, Dv90 ≤ 5 μm, and ((Dv90-Dv10) / Dv50) ≤ 2.22; The tap density of the ferric phosphate is 1 g / cm³. 3 ~1.2g / cm 3 Specific surface area is 1m 2 / g~3.0m 2 / g, iron-phosphorus atomic ratio of 0.97 or higher, sphericity of 0.806 or higher; The iron phosphate comprises secondary particles formed by stacking primary particles, with multiple primary particles forming cracks on the surface of the secondary particles, and no obvious agglomeration between the secondary particles.

2. A method for preparing ferric phosphate, characterized in that, The preparation method includes the following steps: Activate iron oxide red, then mix the activated iron oxide red with sulfuric acid solution to obtain a solution; add a phosphorus source to the solution while stirring; after the phosphorus source is added, keep warm to allow precipitation and crystallization; filter, wash, dry and calcine the precipitated crystal product to obtain the iron phosphate as described in claim 1; The phosphorus source is added to the solution over a period of 5 to 7 hours. The stirring speed is 1300 r / min to 1400 r / min; The insulation temperature is 90℃~95℃, and the time is 10h~15h; The phosphorus source includes phosphoric acid; The activation refers to heat-treating the iron oxide red at 350℃~450℃; The molar ratio of the activated iron oxide red to sulfuric acid in the sulfuric acid solution is 1:2.9 to 1:3.1; the concentration of the sulfuric acid solution is 0.2 mol / L to 0.4 mol / L.

3. The preparation method according to claim 2, characterized in that, The molar ratio of iron in the activated iron oxide red to phosphorus in the phosphorus source is 1:1 to 1:1.

5.

4. The preparation method according to claim 2, characterized in that, The calcination temperature is 600℃~650℃; And / or, the calcination time is 2h~4h.

5. The preparation method according to claim 2, characterized in that, The activated iron oxide red contains more than 99 wt% Fe2O3 and less than 100 ppm of metal elements other than iron. And / or, the activation time is 1.5h to 2.5h.

6. The preparation method according to claim 2, characterized in that, The activated iron oxide red was mixed with the sulfuric acid solution at a temperature of 60℃~80℃ and a stirring speed of 300r / min~400r / min.

7. A graded iron phosphate, characterized in that, The graded ferric phosphate includes compounded amorphous small-particle ferric phosphate, compounded large-particle ferric phosphate, and the ferric phosphate described in claim 1; The particle size Dv50 of the compounded amorphous ferric phosphate is 0.3 μm to 0.5 μm; The particle size Dv50 of the compounded large-particle iron phosphate is 3μm~5μm; The mass ratio of the compounded amorphous small-particle ferric phosphate, the ferric phosphate of claim 1, and the compounded large-particle ferric phosphate is (4-6):(2-3):(1-3).

8. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared from the graded iron phosphate as described in claim 7.

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