Iron phosphate with low sulfur content and high iron-to-phosphorus ratio, its manufacturing method, and applications.

Abstract

This invention provides iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, as well as a method for producing the same and its applications, belonging to the field of battery materials. The method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio includes the steps of: providing amorphous iron phosphate; adding water and phosphoric acid to the amorphous iron phosphate to form a slurry, then increasing the temperature to mature it and obtain a mature slurry; performing slurry washing and pressure filtration on the mature slurry to obtain a first filtration cake; adding water and a first pH adjusting agent to the first filtration cake to perform slurry washing to obtain a first slurry with a pH range of 3.0 to 5.0, and pressure filtration of the first slurry to obtain a second filtration cake; and rinsing, drying, and calcining the second filtration cake to obtain iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio. This invention is advantageous for producing iron phosphate with a low sulfur content, a high iron-to-phosphorus ratio, and a large specific surface area. The iron phosphate can be used to produce lithium iron phosphate with good electrochemical performance, and lithium iron phosphate can further be used to produce positive electrode sheets and secondary batteries with good electrochemical performance.

Description

Cross-reference of related applications

[0001] This application is based on and claims priority from the Chinese application CN No. 202410354078.7, filed on March 26, 2024, and the disclosures of the said CN application are incorporated into this application as a whole. [Technical Field]

[0002] This invention relates to the technical field of battery materials, and more specifically to iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, as well as its manufacturing method and applications. [Background technology]

[0003] As an important precursor to lithium iron phosphate, iron phosphate's sulfur content, specific surface area, morphology, and iron-to-phosphorus ratio all affect the performance of lithium iron phosphate. Currently, the method for producing iron phosphate involves first producing amorphous iron phosphate, then adding excess phosphoric acid and aging it at high temperatures to complete the crystallization transition and obtain iron phosphate. However, if there is excess phosphoric acid, acidic salt by-products such as iron hydrogen phosphate, iron dihydrogen phosphate, and ammonium iron sulfate tend to be formed, leading to problems such as a decrease in product purity and a decrease in the iron-to-phosphorus ratio. Furthermore, in the aging process of the above method, the material tends to aggregate easily, resulting in a wide particle distribution and excessively high surface energy, which causes large aggregates of the material during the subsequent drying or calcination process, resulting in a small specific surface area of ​​the produced iron phosphate.

[0004] Therefore, iron phosphate produced by existing iron phosphate production methods has problems such as a high sulfur content, a low iron-to-phosphorus ratio, and a small specific surface area.

[0005] In light of the above, I hereby submit this application. [Overview of the project]

[0006] In view of the technical problems present in the background art, this application aims to improve or solve the technical problems described in the background art by providing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, as well as a method for producing the same and its applications.

[0007] In a first aspect, the embodiments of this application provide a method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and this method is A process for providing amorphous iron phosphate, The process involves adding water and phosphoric acid to amorphous iron phosphate to form a slurry, then increasing the temperature to allow it to mature and obtain a matured slurry. A step to obtain a first filtered cake by performing slurry washing and pressure filtration on the aged slurry, The process involves adding water and a first pH adjuster to the first filter cake to perform slurry washing, obtaining a first slurry with a pH range of 3.0 to 5.0, and then pressurizing the first slurry to obtain a second filter cake. The process includes rinsing the second filtration cake, drying it, and calcining it to obtain iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio.

[0008] In the technical solution of the embodiment of this application, water is added to amorphous iron phosphate to form a slurry (first slurry) before the crystals are aged, and after the transformation of the aged crystals is complete, water is added to the first filtered cake to form a slurry again (second slurry). By adding the first pH adjuster during or after the second slurry, it is possible to avoid adding the first pH adjuster at the high temperature during aging, which would cause a phase transformation of the material and generate basic iron ammonium phosphate. Therefore, it is advantageous to lower the surface energy of the particles, and the particles are less likely to aggregate during the subsequent drying or calcination process. Furthermore, in this application, by controlling the pH value of the first slurry to 3.0 to 5.0, it is possible to avoid the hydrolysis of iron phosphate and the generation of iron hydroxide by-products due to an excessively high pH value, and it is also advantageous to avoid the problem of an excessively high iron-to-phosphorus ratio.

[0009] Next, in the technical solution of the embodiments of this application, the above two slurrying processes can exert a dispersion effect, further making the overall distribution of particles more uniform and finer. This is advantageous in increasing the reaction area between the particles and the first pH adjusting agent, reducing the formation of acidic salts, improving the purity of iron phosphate and the iron-to-phosphorus ratio, and lowering the sulfur content of iron phosphate. Therefore, the manufacturing method provided by this application can yield anhydrous iron phosphate with a large specific surface area, low sulfur content, and high iron-to-phosphorus ratio.

[0010] In some examples, the process involves adding water and phosphoric acid to amorphous iron phosphate to form a slurry, and then increasing the temperature to allow it to mature and obtain a matured slurry. The process involves adding water and phosphoric acid to amorphous iron phosphate to form a slurry and obtain an initial slurry, The process includes raising the initial slurry to 80°C to 100°C, and then maintaining the temperature for 60 to 100 minutes after the initial slurry changes color to obtain a matured slurry. However, the P / Fe molar ratio of the added phosphoric acid and amorphous iron phosphate is 0.2:1 to 0.3:1.

[0011] In this embodiment, by controlling the maturation temperature to 80°C to 100°C and continuing the heat retention treatment for 60 to 100 minutes while maintaining the maturation temperature at 80°C to 100°C after the initial slurry color changes, amorphous iron phosphate can undergo sufficient crystalline transformation. Here, by setting the P / Fe molar ratio of phosphoric acid to amorphous iron phosphate to 0.2:1 to 0.3:1, it is possible to avoid the problems of decreased product purity and decreased iron-to-phosphorus ratio caused by acidic salt by-products such as iron hydrogen phosphate, iron dihydrogen phosphate, and ammonium iron sulfate, which are generated under conditions of excessive phosphoric acid.

[0012] In some examples, the step of adding water and phosphoric acid to amorphous iron phosphate to form a slurry and obtain an initial slurry is, The process involves adding water to amorphous iron phosphate to form a slurry, thereby obtaining a slurry with a solid content of 15% to 30%. The process includes the step of adding phosphoric acid to a slurry with a solid content of 15% to 30% under stirring conditions to obtain an initial slurry.

[0013] In this embodiment, by adding phosphoric acid to a slurry with a solid content of 15% to 30%, a good dispersion effect can be obtained in the first slurrying process, specifically improving the degree of crystal dispersion and dispersion uniformity.

[0014] In some embodiments, the method for producing amorphous iron phosphate provided is: The process involves adding water to the titanium white by-product to dissolve it and obtain a mixed slurry, The process involves adding a second pH adjusting agent to the mixed slurry to adjust the pH value of the mixed slurry to 4.0-5.0, and then obtaining a ferrous sulfate solution by solid-liquid separation. A step of mixing ferrous sulfate solution, phosphate solution, and hydrogen peroxide solution to obtain primary amorphous iron phosphate, The process includes a step of rinsing primary amorphous iron phosphate until the conductivity of the rinse solution is 5 ms / cm or less to obtain amorphous iron phosphate.

[0015] In this embodiment, the amorphous iron phosphate produced by the above method has high purity and stability, which is advantageous for obtaining crystals of relatively uniform size and quality during the maturation process.

[0016] In some examples, the molar ratio of phosphorus in the phosphorus salt solution to iron in the ferrous sulfate solution is 1:1 to 1.1:1, and / or the molar ratio of hydrogen peroxide in hydrogen peroxide water to iron in the ferrous sulfate solution is 0.5:1 to 1:1.

[0017] In this example, controlling the dosages of the phosphate solution, hydrogen peroxide solution, and ferrous sulfate solution is advantageous in that, on the one hand, it avoids the situation where phosphorus becomes excessive and reacts with impurities in ferrous sulfate to form phosphate by-products, and on the other hand, it is advantageous in controlling the pH value during the synthesis process within a relatively appropriate range. Therefore, it is advantageous for reducing the impurity content in amorphous iron phosphate and making the particle sizes of the resulting amorphous iron phosphate consistent.

[0018] In some examples, the solute in the phosphate solution is at least one selected from ammonium dihydrogen phosphate and diammonium hydrogen phosphate, and / or the first pH adjuster is at least one selected from ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and ammonia water, and / or the second pH adjuster is at least one selected from ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and ammonia water.

[0019] In this example, since the raw materials of the phosphate solution, the first pH adjuster, and the second pH adjuster used are easily available and relatively inexpensive, it is advantageous for reducing the production cost of iron phosphate.

[0020] In some examples, the second filter cake is rinsed multiple times until the conductivity of the rinsing liquid becomes 350 μs / cm or less, and then the second filter cake is dried and calcined to obtain iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio.

[0021] In this example, by controlling the conductivity of the second filter cake within the range of 350 μs / cm or less, a cleanly washed second filter cake can be obtained, which is further advantageous for obtaining a higher purity and more consistent final iron phosphate product.

[0022] In a second aspect, the examples of this application provide iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio produced using the above method for manufacturing iron phosphate. The iron-to-phosphorus ratio of iron phosphate with low sulfur content and high iron-to-phosphorus ratio is (0.977~0.991):1, and / or the sulfur content of iron phosphate with low sulfur content and high iron-to-phosphorus ratio is 18.55 ppm~32.46 ppm, and / or the specific surface area of ​​iron phosphate with low sulfur content and high iron-to-phosphorus ratio is 10.5 m². 2 / g~12.01m 2 It is / g.

[0023] In the technical solution of the embodiment of this application, amorphous iron phosphate is provided, water and phosphoric acid are added to the amorphous iron phosphate to form a slurry, the temperature is then increased and the slurry is matured to obtain a matured slurry, the matured slurry is subjected to slurry washing and pressure filtration to obtain a first filtration cake, water and a first pH adjusting agent are added to the first filtration cake and slurry washing is performed to obtain a first slurry with a pH range of 3.0 to 5.0, the first slurry is then pressure filtered to obtain a second filtration cake, the second filtration cake is rinsed, dried, and calcined. The iron-to-phosphorus ratio obtained by this method is (0.977 to 0.991):1, the sulfur content is 18.55 ppm to 32.46 ppm, and the specific surface area is 10.50 m². 2 / g~12.01m 2 It is / g.

[0024] Therefore, the iron phosphate produced in this application overcomes the technical problems present in the prior art and has better electrochemical performance.

[0025] In a third aspect, an embodiment of the present application provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector, wherein the positive electrode active layer comprises a lithium iron phosphate material produced using iron phosphate with the above-mentioned low sulfur content and high iron-to-phosphorus ratio as a precursor.

[0026] In this embodiment, the positive electrode sheet contains a lithium iron phosphate material produced using iron phosphate with the above-mentioned low sulfur content and high iron-to-phosphorus ratio as a precursor. This lithium iron phosphate material inherits the low sulfur content and high iron-to-phosphorus ratio characteristics of iron phosphate, which is advantageous for obtaining better electrochemical performance. Therefore, a positive electrode sheet containing lithium iron phosphate material also has the advantage of excellent electrochemical performance.

[0027] In a fourth aspect, an embodiment of the present application provides a secondary battery including the above-described positive electrode sheet.

[0028] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, which has the advantage of excellent electrochemical performance.

[0029] The above description is merely an outline of the technical solution of this application. To better understand the technical means of this application, it can be implemented according to the specifications. Furthermore, to make the above and other objectives, features, and advantages of this application clearer and easier to understand, specific embodiments of this application are listed below. [Brief explanation of the drawing]

[0030] To more clearly illustrate the technical solution of this application, the drawings used in this application are briefly introduced below. Clearly, the drawings described below represent only some embodiments of this application, and those skilled in the art can obtain other drawings based on these without any creative effort. [Figure 1] These are the XRD patterns of iron phosphate dihydrate obtained in step (8) of Example 1 and step (5) of Comparative Example 1 of this application. [Figure 2] This figure shows the scanning electron microscope measurement results of anhydrous iron phosphate obtained in step (9) of Example 1 of this application. [Figure 3] This is a diagram showing the scanning electron microscope measurement results of anhydrous iron phosphate obtained in step (6) of Comparative Example 3 of this application. [Modes for carrying out the invention]

[0031] The following examples of the technical solution of the present invention will be described in detail with reference to the attached drawings. The following examples are provided solely to illustrate the technical solution of this application more clearly and are therefore provided only as examples and are not intended to limit the scope of protection of this application.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art relating to the present application. The terms used herein are for illustrative purposes only and are not intended to limit this application. The terms “including” and “having” and any variations thereof in the description, claims, and drawings above are intended to cover non-exclusive inclusion.

[0033] In the description of the embodiments of this application, technical terms such as “first,” “second,” etc., are used solely to distinguish different subjects and should not be understood as indicating or implying relative importance, or implicitly indicating the number, specific order, or primary-secondary relationship of the technical features presented. In the description of the embodiments of this application, “multiple” means two or more unless otherwise explicitly and specifically limited.

[0034] As used herein, “Examples” means that any particular feature, structure, or property described in combination with the Examples may be included in at least one Example of this Application. The appearance of such phrase in various places in the Specification does not necessarily refer to the same Example, nor does it refer to mutually exclusive, independent, or alternative Examples. As those skilled in the art will understand both explicitly and implicitly, the Examples described herein may be combined with other Examples.

[0035] In the description of the embodiments of this application, the term "and / or" merely describes the relationship between related objects, indicating that three relationships may exist. For example, A and / or B can mean three situations: A existing alone, A and B existing simultaneously, and B existing alone. In general, the letter " / " in this specification indicates that the preceding and following related objects are in an "or" relationship.

[0036] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple groups" refers to two or more groups (including two groups), and "multiple sheets" refers to two or more (including two sheets).

[0037] In the description of the embodiments of this application, the orientations or positional relationships indicated by technical terms such as "center," "vertical," "horizontal," "length," "width," "thickness," "top," "bottom," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inside," "outside," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" are based on the orientations or positional relationships shown in the drawings and are solely for the purpose of facilitating and simplifying the description of the embodiments of this application. They do not indicate or imply that the devices or elements mentioned must have a specific orientation, or that they must be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the embodiments of this application.

[0038] In the description of the embodiments of this application, unless otherwise explicitly stated or limited, terms such as “attach,” “connect,” “connect,” and “fix” should be understood in a broad sense, for example, whether it be a fixed connection, a removable connection, an integral connection, a mechanical connection, an electrical connection, a direct connection, an indirect connection via an intermediate medium, an internal communication between two elements, or an interaction relationship between two elements. Those skilled in the art will be able to understand the specific meaning of the above terms in the embodiments of this application, depending on the specific circumstances.

[0039] In this application, "iron-to-phosphorus ratio" always refers to the "molar ratio of iron to phosphorus." For example, in "iron phosphate with low sulfur content and high iron-to-phosphorus ratio," "iron-to-phosphorus ratio" refers to the "molar ratio of iron to phosphorus."

[0040] Furthermore, "the P / Fe molar ratio of the aforementioned phosphoric acid and the aforementioned amorphous iron phosphate" refers to "the molar ratio of the phosphorus element in the aforementioned phosphoric acid to the iron element in the aforementioned amorphous iron phosphate."

[0041] In conventional technology, sulfur content, iron-to-phosphorus ratio, specific surface area, and shape are all important performance indicators for iron phosphate. Of these, the iron-to-phosphorus ratio of iron phosphate is typically (0.95-0.97):1, the sulfur content is typically 100-500 ppm, and the specific surface area is typically 10 m². 2 The value is less than / g. Currently, the method for producing iron phosphate involves first producing amorphous iron phosphate, then adding excess phosphoric acid and aging it at high temperature to complete the crystallization transition and obtain iron phosphate. However, when there is an excess of phosphoric acid, acidic salt by-products such as iron hydrogen phosphate, iron dihydrogen phosphate, and ammonium iron sulfate tend to be formed, leading to problems such as a decrease in product purity and a decrease in the iron-to-phosphorus ratio. Furthermore, in the aging process of the above method, the material tends to aggregate easily, the particle distribution is wide, and the surface energy is too high, which causes large amounts of material to aggregate during the subsequent drying or calcination process, resulting in a small specific surface area of ​​the produced iron phosphate.

[0042] To address the technical problems inherent in conventional iron phosphate, such as its high sulfur content, low iron-to-phosphorus ratio, and small specific surface area, this application provides iron phosphate with low sulfur content and high iron-to-phosphorus ratio, as well as a method for producing the same and its applications. By using a specific production method provided in this application, which includes performing a first slurrying before maturation, a second slurrying after maturation, and adding a first pH adjusting agent during or after the second slurrying to control the pH value of the first slurry to 3.0-5.0, it is possible to produce iron phosphate with a high iron-to-phosphorus ratio, low sulfur content, and large specific surface area. Therefore, the electrochemical performance of lithium iron phosphate materials produced using it as a precursor can be improved, and furthermore, the electrochemical performance of cathode sheets and secondary batteries can also be improved.

[0043] The proposed solution for this application will be further explained below.

[0044] In a first aspect, the embodiments of this application provide a method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and this method is A process for providing amorphous iron phosphate, The process involves adding water and phosphoric acid to amorphous iron phosphate to form a slurry, then increasing the temperature to allow it to mature and obtain a matured slurry. A step to obtain a first filtered cake by performing slurry washing and pressure filtration on the aged slurry, The process involves adding water and a first pH adjuster to the first filter cake to perform slurry washing, obtaining a first slurry with a pH range of 3.0 to 5.0, and then pressurizing the first slurry to obtain a second filter cake. The process includes rinsing the second filtration cake, drying it, and calcining it to obtain iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio.

[0045] In the technical solution of the embodiment of this application, water is added to amorphous iron phosphate to form a slurry before the crystals are aged (first slurry formation), and after the transformation of the aged crystals is complete, water is added to the first filtered cake to form a slurry again (second slurry formation). By adding the first pH adjuster during or after the second slurry formation, it is possible to avoid adding the first pH adjuster at the high temperature during aging, which would cause a phase transformation of the material and generate basic iron ammonium phosphate (the above-mentioned basic iron ammonium phosphate tends to generate an ammonia odor during the sintering process). This is advantageous in reducing the surface energy of the particles, and makes it less likely for the particles to aggregate during the subsequent drying or firing process. By controlling the pH value of the first slurry to 3.0 to 5.0, it is possible to avoid the pH value being too high, which would cause hydrolysis of iron phosphate and the generation of iron hydroxide by-products, and is also advantageous in avoiding the problem of an excessively high iron-to-phosphorus ratio.

[0046] Next, in the technical solution of the embodiments of this application, the above two slurrying processes can exert a dispersion effect, further making the overall distribution of particles more uniform and finer. This is advantageous in increasing the reaction area between the particles and the first pH adjusting agent, reducing the formation of acidic salts, improving the purity of iron phosphate and the iron-to-phosphorus ratio, and lowering the sulfur content of iron phosphate. Therefore, the manufacturing method provided by this application can yield anhydrous iron phosphate with a large specific surface area, low sulfur content, and high iron-to-phosphorus ratio.

[0047] In some embodiments, the method for producing amorphous iron phosphate provided is: The process involves adding water to the titanium white by-product to dissolve it and obtain a mixed slurry, The process involves adding a second pH adjusting agent to the mixed slurry to adjust the pH value of the mixed slurry to 4.0-5.0, and then obtaining a ferrous sulfate solution by solid-liquid separation. A step of mixing ferrous sulfate solution, phosphate solution, and hydrogen peroxide solution to obtain primary amorphous iron phosphate, The process includes a step of rinsing primary amorphous iron phosphate until the conductivity of the rinse solution is 5 ms / cm or less to obtain amorphous iron phosphate.

[0048] In this embodiment, the amorphous iron phosphate produced by the above method has high purity and stability, which is advantageous for the subsequent production of anhydrous iron phosphate.

[0049] In some embodiments, the titanium white byproduct includes ferrous sulfate. For example, Fe in the mixed slurry. 2+ The titanium white by-product can be mixed with water so that its molar concentration is between 1 mol / L and 5 mol / L.

[0050] In some embodiments, the second pH adjuster is, but is not limited to, at least one selected from ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and aqueous ammonia. Using the above substances as second pH adjusters has the advantage of readily available raw materials, relatively low cost, and is beneficial in reducing the production cost of iron phosphate.

[0051] In some examples, a mixed slurry with a pH adjusted to 4.0-5.0 can be separated into solid and liquid by pressure filtration using a pressure filter, and the separated filtrate can be allowed to stand to clarify in order to obtain an aqueous solution of ferrous sulfate. For example, a mixed slurry with a pH adjusted to 5.0 can be separated into solid and liquid by pressure filtration using a pressure filter.

[0052] In some embodiments, the process for producing primary amorphous iron phosphate may include simultaneously adding a phosphate solution and hydrogen peroxide solution dropwise to a ferrous sulfate solution, and continuing the reaction after the addition is complete to obtain primary amorphous iron phosphate.

[0053] For example, the molar ratio of phosphorus in a phosphorus salt solution to iron in a ferrous sulfate solution is 1:1 to 1.1:1, and may be, for example, 1:1, 1.05:1, or 1.1:1, or any other value within the range of 1:1 to 1.1:1. The molar ratio of hydrogen peroxide in hydrogen peroxide water to iron in a ferrous sulfate solution is 0.5:1 to 1:1, and may be, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, or 1:1, or any other value within the range of 0.5:1 to 1:1.

[0054] Controlling the dosages of the phosphate solution, hydrogen peroxide solution, and ferrous sulfate solution is advantageous on the one hand in avoiding excess phosphorus, which reacts with impurities in ferrous sulfate to form phosphate byproducts, and on the other hand in controlling the pH value during the synthesis process to a relatively appropriate range, and therefore advantageous in reducing the impurity content in amorphous iron phosphate and matching the particle size of the resulting amorphous iron phosphate.

[0055] In some embodiments, the solute in the phosphate solution is, for example, at least one selected from ammonium dihydrogen phosphate and diammonium hydrogen phosphate, but is not limited thereto. Using the above substances as solutes in the phosphate solution has the advantage that the raw materials are readily available, the price is relatively low, and it is advantageous for reducing the production cost of iron phosphate.

[0056] In some embodiments, the dropwise addition time of the phosphate solution and hydrogen peroxide solution may be 20 to 60 minutes, for example, 40 minutes, 45 minutes, 50 minutes, 55 minutes, or 60 minutes. The reaction duration after addition may be 1 to 2 hours, for example, 1 hour, 1.5 hours, or 2 hours. Exemplarily, the dropwise addition time of the phosphate solution and hydrogen peroxide solution may be 50 minutes. The reaction duration after addition may be 1 hour.

[0057] In some embodiments, the conductivity of the rinse solution after rinsing the primary amorphous iron phosphate may be, for example, 5 ms / cm, 4.5 ms / cm, 4 ms / cm, 3.5 ms / cm, 3 ms / cm, 2.5 ms / cm, 2 ms / cm, 1.5 ms / cm, 1 ms / cm, or 0.5 ms / cm. Controlling this condition is advantageous in reducing the impurity content of amorphous iron phosphate.

[0058] In some examples, the process involves adding water and phosphoric acid to amorphous iron phosphate to form a slurry, and then increasing the temperature to allow it to mature and obtain a matured slurry. The process involves adding water and phosphoric acid to amorphous iron phosphate to form a slurry and obtain an initial slurry, The process includes raising the initial slurry to 80°C to 100°C, and then, after the initial slurry changes color, maintaining the temperature for 60 to 100 minutes to obtain a matured slurry. However, the P / Fe molar ratio of the added phosphoric acid and amorphous iron phosphate is 0.2:1 to 0.3:1.

[0059] In some embodiments, amorphous iron phosphate may be first dispersed in water, and then phosphoric acid may be added. Dispersion may be carried out under stirring conditions, with a stirring speed of, for example, 400 rpm to 600 rpm, such as 400 rpm, 450 rpm, 500 rpm, 550 rpm, or 600 rpm. The dispersion time may be, for example, 30 minutes to 50 minutes, such as 30 minutes, 35 minutes, 40 minutes, 45 minutes, or 50 minutes.

[0060] In some embodiments, the P / Fe molar ratio of phosphoric acid to amorphous iron phosphate can be set to 0.2:1, 0.25:1, or 0.3:1, or any other value within the range of 0.2:1 to 0.3:1. Exemplarily, the P / Fe molar ratio of phosphoric acid to amorphous iron phosphate is 0.3:1.

[0061] By setting the P / Fe molar ratio of phosphoric acid to amorphous iron phosphate to 0.2:1 to 0.3:1, it is possible to avoid problems such as a decrease in product purity and a decrease in the iron-to-phosphorus ratio caused by acidic salt byproducts such as iron hydrogen phosphate, iron dihydrogen phosphate, and ammonium iron sulfate, which are produced under conditions of excess phosphoric acid. Here, setting the P / Fe molar ratio of phosphoric acid to amorphous iron phosphate to 0.3:1 yields even better results.

[0062] In some examples, the step of adding water and phosphoric acid to amorphous iron phosphate to form a slurry and obtain an initial slurry is, The process involves adding water to amorphous iron phosphate to form a slurry, thereby obtaining a slurry with a solid content of 15% to 30%. The process includes the step of adding phosphoric acid to a slurry with a solid content of 15% to 30% under stirring conditions to obtain an initial slurry.

[0063] The solid content of the slurry is 15% to 30%, and may be, for example, 15%, 18%, 20%, 22%, 25%, 28%, or 30%, or any other value within the range of 15% to 30%.

[0064] Adding phosphoric acid to a slurry with a solid content of 15% to 30% is advantageous in obtaining a good dispersion effect during the first slurrying process, for example, improving the degree of dispersion and dispersion uniformity of large and small crystals.

[0065] In some embodiments, the maturation temperature can be controlled to 80°C to 100°C, that is, the initial slurry can be heated to 80°C, 85°C, 90°C, 95°C, or 100°C, or any other value within the range of 80°C to 100°C. By controlling the maturation temperature within the above range, the crystalline transition of amorphous iron phosphate is achieved. Exemplarily, the maturation temperature may be 90°C.

[0066] In some embodiments, the incubation time after the initial slurry color changes may be 60, 65, 70, 75, 80, 85, 90, 95, or 100 minutes, or any other value within the range of 60 to 100 minutes. Incubating for 60 to 100 minutes after the initial slurry color changes ensures sufficient crystalline transformation of amorphous iron phosphate, thereby improving the yield. Exemplarily, the slurry is incubated for 100 minutes after the initial slurry color changes.

[0067] In some embodiments, the number of times the matured slurry is slurried may be one, or it may be multiple times, for example, two, three, four or more times. Accordingly, one pressure filtration can be performed after each slurried washing. Exemplarily, the solid content of the slurried washing may be 15% to 30%. Exemplarily, the slurried washing can be performed under stirring conditions (stirring speed may be 400 rpm to 600 rpm). Exemplarily, the duration of the slurried washing may be 30 to 50 minutes.

[0068] To make it easier to understand, performing slurry washing on a matured slurry is advantageous in two ways: on the one hand, it is beneficial for particle dispersion, and on the other hand, it is beneficial for washing away impurities.

[0069] In some embodiments, after performing slurry washing and pressure filtration on the matured slurry, the mother liquor obtained by pressure filtration can be recovered and reused.

[0070] In some embodiments, in the process of obtaining the first slurry, the ratio of the first filter cake to water satisfies the condition that the solid content of the first slurry is 15% to 30%, for example, 15%, 18%, 20%, 22%, 25%, 28%, or 30%.

[0071] In some embodiments, the first filter cake may be dispersed in water first, and then the first pH adjuster may be added. Dispersion of the first filter cake in water may be carried out under stirring conditions, with a stirring speed of, for example, 400 rpm to 600 rpm, such as 400 rpm, 450 rpm, 500 rpm, 550 rpm, or 600 rpm. The dispersion time may be, for example, 30 minutes to 50 minutes, such as 30 minutes, 35 minutes, 40 minutes, 45 minutes, or 50 minutes.

[0072] In some embodiments, the first pH adjuster is, but is not limited to, at least one selected from ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and aqueous ammonia. The above-mentioned first pH adjuster is advantageous because its raw materials are readily available, its price is relatively low, and it helps reduce the production cost of iron phosphate.

[0073] In some embodiments, the pH value of the first slurry may be 3.0, 3.5, 4.0, 4.5, or 5.0, or any other value within the range of 3.0 to 5.0. Controlling the pH value of the first slurry to 3.0 to 5.0 is advantageous in avoiding excessively high pH values ​​which would cause hydrolysis of iron phosphate and the formation of iron hydroxide by-products, and also in avoiding excessively high iron-to-phosphorus ratios. Exemplarily, the pH value of the first slurry can be controlled to 3.5 to 4.5.

[0074] In some embodiments, the second filter cake may be rinsed only once, or multiple times, for example, two, three, four or more times. The conductivity of the rinse solution after rinsing should be 350 μs / cm or less.

[0075] By controlling the conductivity of the second filter cake to a range of 350 μs / cm or less, a cleanly washed second filter cake can be obtained, which is further advantageous in obtaining a final iron phosphate product with high purity and good consistency.

[0076] In some embodiments, drying the second filter cake after rinsing can be carried out at a temperature of 100°C to 200°C (for example, 100°C, 120°C, 150°C, 180°C, or 200°C, etc.) for 0.5 hours to 1 hour (for example, 0.5 hours, 0.8 hours, or 1 hour, etc.). Specifically, methods such as flash drying can be used. The calcination after drying can be carried out under conditions of 500°C to 700°C (for example, 500°C, 550°C, 600°C, 650°C, or 700°C, etc.) for 2 hours to 3 hours (for example, 2 hours, 2.5 hours, or 3 hours, etc.).

[0077] In a second aspect, the examples of the present application provide iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio produced using the above-described method for producing iron phosphate. Here, the iron-to-phosphorus ratio of the iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio is (0.977 to 0.991):1, and / or the sulfur content is 18.55 ppm to 32.46 ppm, and / or the specific surface area is 10.50 m 2 / g to 12.01 m 2 / g.

[0078] In the technical solution of the examples of the present application, amorphous iron phosphate is provided. Water and phosphoric acid are added to the amorphous iron phosphate to form a slurry, and then the temperature is raised for aging to obtain an aged slurry. The aged slurry is subjected to slurry washing and pressure filtration to obtain a first filter cake. Water and a first pH adjuster are added to the first filter cake for slurry washing to obtain a first slurry with a pH value range of 3.0 to 5.0. Then, the first slurry is pressure filtered to obtain a second filter cake. The second filter cake is rinsed, dried, and calcined. The iron-to-phosphorus ratio obtained by this method is (0.977 to 0.991):1, the sulfur content is 18.55 ppm to 32.46 ppm, and the specific surface area is 10.50 m 2 / g to 12.01 m 2 / g.

[0079] Therefore, the iron phosphate produced in the present application can overcome the technical problems existing in the prior art and has better electrochemical performance.

[0080] Furthermore, in some embodiments, the resulting iron phosphate is also pure iron phosphate with high purity, in addition to having a high iron-to-phosphorus ratio, low sulfur content, and large specific surface area.

[0081] In a third aspect, an embodiment of the present application provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector, wherein the positive electrode active layer comprises a lithium iron phosphate material produced using iron phosphate with the above-mentioned low sulfur content and high iron-to-phosphorus ratio as a precursor.

[0082] In this embodiment, the positive electrode sheet contains a lithium iron phosphate material produced using iron phosphate with the low sulfur content and high iron-to-phosphorus ratio described above as a precursor. This lithium iron phosphate material inherits the characteristics of iron phosphate, such as its low sulfur content and high iron-to-phosphorus ratio, which is advantageous for obtaining better electrochemical performance. Therefore, the positive electrode sheet containing this lithium iron phosphate also has the advantage of excellent electrochemical performance.

[0083] In a fourth aspect, an embodiment of the present application provides a secondary battery including the above-described positive electrode sheet.

[0084] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, which has the advantage of excellent electrochemical performance.

[0085] The following are some specific examples, but please note that these examples are illustrative and used only to interpret the present invention and should not be understood as limiting it. Where specific technical or conditional details are not explicitly stated in the examples, the technical or conditional details described in the relevant art literature or product descriptions shall apply. Unless otherwise specified, all reagents and equipment used are commercially available, standard products.

[0086] 1. Manufacturing method (Example 1) This example provides iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and the method for producing it is as follows: Dissolve ferrous sulfate, a by-product of titanium white, in water, and Fe 2+ Step (1) to obtain a mixed slurry with an ion molar concentration of 1.5 mol / L, (2) The above mixed slurry is mixed with ammonium bicarbonate to adjust the pH of the mixed slurry to 5, then pressure filtered using a pressure filter, and the filtrate obtained by pressure filtration is allowed to stand and clarified to obtain an aqueous ferrous sulfate solution. Step (3) involves simultaneously adding ammonium dihydrogen phosphate and hydrogen peroxide solution dropwise to the above ferrous sulfate solution for 50 minutes, ensuring a molar ratio of phosphorus in the phosphorus solution to iron in the ferrous sulfate solution of 1:1, and a molar ratio of hydrogen peroxide in the hydrogen peroxide solution to iron in the ferrous sulfate solution of 1:1, and continuing the reaction for 1 hour after the dropwise addition is complete to obtain primary amorphous iron phosphate. (4) The process involves pressure filtration of primary amorphous iron phosphate and rinsing until the conductivity of the rinse solution reaches 5 ms / cm to obtain amorphous iron phosphate, The process involves adding water to the rinsed amorphous iron phosphate at a solid content of 20%, performing slurry dispersion at a stirring speed of 500 rpm for 40 minutes (first slurry process), then adding phosphoric acid in a molar ratio of P:Fe=0.3:1, raising the temperature to 90°C, and maintaining the temperature for 100 minutes after discoloration to obtain a matured slurry (5), and Step (6) involves pressure filtering the matured slurry, adding water to the filtered cake after pressure filtering in a proportion that results in a solid content of 20% to perform slurry washing, carrying out the slurry washing process for 40 minutes under conditions of stirring speed of 500 rpm, returning to the pressure filter after washing and pressure filtering to obtain the first filtered cake and mother liquor (the mother liquor obtained by pressure filtering can be collected and reused), The process involves adding water to the first filter cake at a ratio of 20% solids content, performing slurry dispersion at a stirring speed of 500 rpm for 40 minutes (second slurry process), then adding ammonium bicarbonate to obtain a first slurry with a pH of 4, returning the first slurry to a pressure filter for pressure filtration to obtain a second filter cake (7), and The second filtration cake is rinsed multiple times until the conductivity of the rinsing solution reaches 350 μs / cm to obtain iron phosphate dihydrate (8), The process includes (9) drying the rinsed iron phosphate dihydrate at 150°C for 1 hour to obtain a dried product, and then calcining the dried product at 600°C for 2 hours to obtain anhydrous iron phosphate.

[0087] (Example 2) This example provides iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and the method for producing it is as follows: Steps (1) to (4) are the same as steps (1) to (4) of Example 1, The process involves adding water to the rinsed amorphous iron phosphate at a solid content of 15%, performing slurry dispersion at a stirring speed of 400 rpm for 30 minutes (first slurry process), then adding phosphoric acid in a molar ratio of P:Fe=0.3:1, raising the temperature to 90°C, and maintaining the temperature for 100 minutes after discoloration to obtain a matured slurry (5), and Step (6) involves pressure filtering the matured slurry, adding water to the filtered cake after pressure filtering in a proportion that results in a solid content of 15% to perform slurry washing, carrying out the slurry washing process for 30 minutes under conditions of stirring speed of 400 rpm, returning to the pressure filter after washing and pressure filtering to obtain the first filtered cake and mother liquor (the mother liquor obtained by pressure filtering can be collected and reused), The process involves adding a certain amount of water to the first filter cake at a ratio of 15% solids content, performing slurry dispersion for 30 minutes under conditions of stirring speed of 400 rpm (second slurry process), then adding ammonium bicarbonate to obtain a first slurry with a pH of 5, returning the first slurry to a pressure filter for pressure filtration to obtain a second filter cake (7), and The process includes steps (8) and (9) which are the same as those in Example 1, and finally anhydrous iron phosphate is obtained.

[0088] (Example 3) This example provides iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and the method for producing it is as follows: Steps (1) to (4) are the same as steps (1) to (4) of Example 1, The process involves adding water to the rinsed amorphous iron phosphate at a solid content of 30%, performing slurry dispersion at a stirring speed of 600 rpm for 50 minutes (first slurry process), then adding phosphoric acid in a molar ratio of P:Fe=0.3:1, raising the temperature to 90°C, and maintaining the temperature for 100 minutes after discoloration to obtain a matured slurry (5), and Step (6) involves pressure filtering the matured slurry, adding water to the filtered cake after pressure filtering in a proportion that results in a solid content of 30% to perform slurry washing, carrying out the slurry washing process for 50 minutes under conditions of stirring speed of 600 rpm, returning to the pressure filter after washing and pressure filtering to obtain the first filtered cake and mother liquor (the mother liquor obtained by pressure filtering can be collected and reused), The process involves adding water to the first filter cake at a ratio of 30% solids content, performing slurry dispersion at a stirring speed of 600 rpm for 50 minutes (second slurry process), then adding ammonium bicarbonate to obtain a first slurry with a pH of 3, returning the first slurry to a pressure filter for pressure filtration to obtain a second filter cake (7), and The process includes steps (8) and (9) which are the same as those in Example 1, and finally anhydrous iron phosphate is obtained.

[0089] (Example 4) The difference between this embodiment and Embodiment 1 is that the ammonium bicarbonate in step (7) is replaced with sodium hydroxide to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0090] (Example 5) The difference between this embodiment and Embodiment 1 is that the ammonium bicarbonate in step (7) is replaced with aqueous ammonia to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0091] (Example 6) The difference between this embodiment and Embodiment 1 is that the ammonium bicarbonate in step (7) is replaced with ammonium carbonate to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0092] (Example 7) The difference between this embodiment and Embodiment 1 is that the ammonium bicarbonate in step (7) is replaced with sodium bicarbonate to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0093] (Example 8) The difference between this embodiment and Embodiment 1 is that the ammonium bicarbonate in step (7) is replaced with sodium carbonate to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0094] (Example 9) The difference between this embodiment and Example 1 is that in step (5), the P / Fe molar ratio of the added phosphoric acid and amorphous iron phosphate is set to 0.2:1 to obtain a matured slurry, and finally anhydrous iron phosphate is obtained.

[0095] (Example 10) The difference between this embodiment and Example 1 is that in step (5), the P / Fe molar ratio of the added phosphoric acid and amorphous iron phosphate is set to 0.15:1 to obtain a matured slurry, and finally anhydrous iron phosphate is obtained.

[0096] (Example 11) The difference between this embodiment and Example 1 is that in step (5), the P / Fe molar ratio of the added phosphoric acid and amorphous iron phosphate is set to 0.35:1 to obtain a matured slurry, and finally anhydrous iron phosphate is obtained.

[0097] (Example 12) The difference between this embodiment and Example 1 is that in step (3), the molar ratio of hydrogen peroxide in hydrogen peroxide solution to iron element in ferrous sulfate solution is set to 0.5:1 to obtain primary amorphous iron phosphate, and finally anhydrous iron phosphate is obtained.

[0098] (Example 13) The difference between this embodiment and Example 1 is that in step (3), the molar ratio of hydrogen peroxide in hydrogen peroxide to iron element in ferrous sulfate solution is set to 0.8:1 to obtain primary amorphous iron phosphate, and finally anhydrous iron phosphate is obtained.

[0099] (Example 14) The difference between this embodiment and Example 1 is that in step (3), the molar ratio of hydrogen peroxide in hydrogen peroxide solution to iron element in ferrous sulfate solution is set to 0.48:1 to obtain primary amorphous iron phosphate, and finally anhydrous iron phosphate is obtained.

[0100] (Example 15) The difference between this embodiment and Example 1 is that in step (3), the molar ratio of hydrogen peroxide in hydrogen peroxide solution to iron element in ferrous sulfate solution is set to 1.1:1 to obtain primary amorphous iron phosphate, and finally anhydrous iron phosphate is obtained.

[0101] (Example 16) The difference between this example and Example 1 is that in step (3), the molar ratio of phosphorus in the phosphorus salt solution to iron in the ferrous sulfate solution is 1.1:1.

[0102] (Example 17) The difference between this example and Example 1 is that in step (3), the molar ratio of phosphorus in the phosphorus salt solution to iron in the ferrous sulfate solution is 0.9:1.

[0103] (Example 18) The difference between this embodiment and Example 1 is that in step (7), ammonium bicarbonate is added to obtain a first slurry with a pH of 3.5, the first slurry is returned to a pressure filter and pressure filtered to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0104] (Example 19) The difference between this embodiment and Example 1 is that in step (7), ammonium bicarbonate is added to obtain a first slurry with a pH of 4.5, the first slurry is returned to a pressure filter and pressure filtered to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0105] (Example 20) The difference between this embodiment and Embodiment 1 is that in step (7), ammonium bicarbonate is added to obtain a first slurry with a pH of 3, the first slurry is returned to a pressure filter and pressure filtered to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0106] (Example 21) The difference between this embodiment and Example 1 is that in step (7), ammonium bicarbonate is added to obtain a first slurry with a pH of 5, the first slurry is returned to a pressure filter and pressure filtered to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0107] (Comparative Example 1) This comparative example provides iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and its manufacturing method is as follows: Steps (1) to (4) are the same as steps (1) to (4) of Example 1, The process involves adding water to the rinsed amorphous iron phosphate at a solid content of 20%, performing slurry dispersion at a stirring speed of 500 rpm for 40 minutes (first slurry process), then adding phosphoric acid in a molar ratio of P:Fe=0.3:1, raising the temperature to 90°C, maintaining the temperature for 100 minutes after discoloration, adding ammonium bicarbonate to adjust the pH to 4, pressurizing filtration after maturation is complete, washing until the conductivity is 350 μs / cm, and obtaining iron phosphate dihydrate (5). The process includes step (6), which is the same as step (9) in Example 1, and finally anhydrous iron phosphate is obtained.

[0108] (Comparative Example 2) The difference between this comparative example and Example 1 is that in step (7) of Example 1, the pH value of the first slurry is adjusted to 6 to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0109] (Comparative Example 3) This comparative example provides iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, and its manufacturing method is as follows: Steps (1) to (4) are the same as steps (1) to (4) of Example 1, The process involves adding water to the rinsed amorphous iron phosphate at a solid content of 20%, performing slurry dispersion at a stirring speed of 500 rpm for 40 minutes (first slurry process), then adding phosphoric acid in a molar ratio of P:Fe=0.3:1, raising the temperature to 90°C, maintaining the temperature for 100 minutes after discoloration, and after maturation is complete, performing pressure filtration and washing until the conductivity reaches 350 μs / cm to obtain iron phosphate dihydrate (5), The process includes step (6), which is the same as step (9) in Example 1, and finally anhydrous iron phosphate is obtained.

[0110] (Comparative Example 4) The difference from Example 1 is that in step (7), ammonium bicarbonate is added to obtain a first slurry with a pH of 2, the first slurry is returned to a pressure filter and pressure filtered to obtain a second filtration cake, and finally anhydrous iron phosphate is obtained.

[0111] 2. Test Methods and Test Results 1. Characteristic testing of iron phosphate materials XRD measurements were performed on the iron phosphate dihydrate obtained in step (8) of Example 1 and the iron phosphate dihydrate obtained in Comparative Example 1, and the results are shown in Figure 1.

[0112] As can be seen from Figure 1, in Example 1, after maturation is complete, the mixture is first pressure filtered and washed to form a slurry. After pressure filtering, it is made into a slurry and ammonium bicarbonate is added to adjust the pH to 4. Then, pressure filtering is continued and washing is performed until the conductivity reaches 350 μs / cm. The iron phosphate dihydrate obtained at this point is the pure phase FePO4·2H2O. In contrast, in Comparative Example 1, ammonium bicarbonate is added at a high temperature during the maturation process to adjust the pH to 4. At this time, the temperature is very high and crystallization transitions are likely to occur in an alkaline environment. Therefore, the iron phosphate dihydrate obtained at this point is the pure phase (NH4)Fe2(PO4)2(OH)·2H2O. Iron phosphate dihydrate in this phase has an ammonia odor when sintered.

[0113] Furthermore, scanning electron microscope measurements were performed on the anhydrous iron phosphate obtained in step (9) of Example 1 and the anhydrous iron phosphate obtained in step (6) of Comparative Example 3, and the results (SEM images) are shown in Figures 2 and 3.

[0114] Comparing Figure 2 and Figure 3, it can be seen that the anhydrous iron phosphate obtained in Example 1 has more uniform particles and pores between the particles compared to the anhydrous iron phosphate obtained in Comparative Example 3, while the particles of the anhydrous iron phosphate obtained in Comparative Example 3 are clearly aggregated and dense. This corresponds to the specific surface area in Table 1 below, and the specific surface area of ​​the anhydrous iron phosphate obtained in Example 1 is larger than that of the anhydrous iron phosphate obtained in Comparative Example 3. It was found that after maturation is complete, pressure filtration and slurry washing, and further slurry dispersion after pressure filtration is advantageous for uniformly dispersing the particles. At the same time, adjusting the pH to 4 by adding alkali reduces the formation of acidic salts, lowers the surface energy, and effectively reduces aggregates between particles, resulting in anhydrous iron phosphate with uniform, loose, porous particles and a large specific surface area. Furthermore, the electrochemical performance of the lithium iron phosphate cathode material produced using the anhydrous iron phosphate obtained in Example 1 is also improved.

[0115] Furthermore, in this application, the elemental content and specific surface area of ​​the anhydrous iron phosphate obtained in Examples 1 to 21 and Comparative Examples 1 to 4 were measured, and the results are shown in Table 1.

[0116] Here, sulfur content was detected using an infrared carbon-sulfur analyzer, specific surface area was detected using a specific surface area meter and nitrogen adsorption method, iron content was detected using potassium dichromate titration, and phosphorus content was detected using quinoline phosphomolybdate gravimetric method. [Table 1]

[0117] The following can be seen from Table 1. (1) The iron-to-phosphorus ratio of the iron phosphate obtained in the examples was in the range of (0.977~0.991):1, which was higher than that of Comparative Examples 1 and 3. This indicates that adding alkali after maturation, pressure filtration, and rinsing can effectively neutralize acids and salts and improve the iron-to-phosphorus ratio.

[0118] (2) The iron-to-phosphorus ratio of the iron phosphate obtained in the example is lower than that of Comparative Example 2, and the iron-to-phosphorus ratio data for the iron phosphate obtained in Comparative Example 2 is greater than 1, suggesting that the iron is not simply the iron in the iron phosphate, but also contains small amounts of other iron compounds, possibly iron hydroxide. This indicates that when the pH value is adjusted to 6, the pH value is too high, making it easy for hydrolysis of iron phosphate to occur and generating iron hydroxide byproducts.

[0119] (3) The specific surface area of ​​the iron phosphate obtained in the example was 10.50 m². 2 / g~12.01m 2 The values ​​were in the range of / g, which are all higher than those of Comparative Examples 1 and 3. This is because, on the one hand, the two slurrying processes disperse larger particles, resulting in a more uniform particle distribution, and on the other hand, by adding alkali to adjust the composition, acidic salts are removed, the surface energy of the particles is reduced, and they are less likely to aggregate during the drying and calcination processes.

[0120] (4) The specific surface area of ​​iron phosphate obtained in Comparative Example 2 was 13.61 m². 2The value is very large at / g, mainly because when the pH value is too high, hydrolysis of iron phosphate easily occurs, producing iron hydroxide by-products, and the colloidal particles of iron oxide are very small.

[0121] (5) The final iron phosphate products obtained in Examples and Comparative Example 2 all had low sulfur content, which indicates that two slurries can disperse large particles and reduce particle size, and that adding alkali can effectively remove acidic salts and reduce sulfur adsorption.

[0122] 2. Characteristic testing of a CR2032 button cell half-cell assembled with a positive electrode sheet. The anhydrous iron phosphate obtained in Examples 1 to 21 and Comparative Examples 1 to 4 were each manufactured into positive electrode sheets according to the following method. Then, the obtained positive electrode sheets were each assembled into CR2032 button-type half-cells, and characteristic tests were performed on the resulting CR2032 button-type half-cells. Manufacturing method: First, the iron phosphate and lithium sources obtained in each of the above examples and comparative examples are sintered to obtain the corresponding lithium iron phosphate cathode material. Next, the obtained lithium iron phosphate cathode materials are mixed with conductive carbon powder and PVDF binder in a mass ratio of 90:5:5, respectively, homogenized, coated onto aluminum foil, dried at 100°C, rolled with a double roller machine, and then a sheet with a diameter of 14 mm is produced using a sheet punching machine. The sheet is weighed and the mass of the aluminum foil is subtracted to obtain the mass of the active material.

[0123] After drying the positive electrode sheet, it was assembled in a Braun UNlab-type inert gas glove box in Germany to form a CR2032 button cell half-cell. The assembly was carried out in the following order: negative electrode case, lithium sheet, electrolyte, separator, electrolyte, positive electrode sheet, gasket, elastic piece, and positive electrode case. Using the Wuhan Blue-Electric CT2001A battery test system, the electrochemical performance of the above CR2032 button cell half-cell was tested over a voltage range of 2.0 to 4.6V, and the test results are shown in Table 2. [Table 2]

[0124] As can be seen from Table 2, the CR2032 button cell corresponding to iron phosphate provided by the above embodiments of this application can have a higher 0.1C charge ratio capacity and 0.1C discharge ratio capacity than the CR2032 button cell corresponding to iron phosphate provided by the comparative example.

[0125] In summary, this application is advantageous for producing iron phosphate with low sulfur content, a high iron-to-phosphorus ratio, a large specific surface area, and high purity. This iron phosphate can be used to produce lithium iron phosphate with good electrochemical performance, and lithium iron phosphate can further be used to produce cathode sheets and secondary batteries with good electrochemical performance.

[0126] Furthermore, this application is not limited to the embodiments described above. The embodiments described above are merely illustrative, and any embodiment that has substantially the same configuration as the technical idea and exhibits the same effects within the scope of the technical solution of this application is included in the scope of this application. In addition, any modifications to the embodiments that a person skilled in the art could conceive, or other forms constructed by combining some of the components of the embodiments, are also included in the scope of this application, as long as they do not depart from the spirit of this application.

Claims

1. A method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, A process for providing amorphous iron phosphate, The process involves adding water and phosphoric acid to the amorphous iron phosphate to form a slurry, and then increasing the temperature to allow it to mature and obtain a matured slurry. The process involves performing slurry washing and pressure filtration on the aged slurry to obtain a first filtered cake, The process involves adding water and a first pH adjusting agent to the first filter cake and performing slurry washing to obtain a first slurry with a pH range of 3.0 to 5.0, and then pressurizing the first slurry to obtain a second filter cake. A method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, characterized by comprising the steps of rinsing the second filtration cake, drying it, and calcining it to obtain iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio.

2. The process of adding water and phosphoric acid to the amorphous iron phosphate to form a slurry, and then raising the temperature to mature it and obtain a matured slurry, is as follows: The process involves adding water and phosphoric acid to the amorphous iron phosphate to form a slurry and obtain an initial slurry. The process includes raising the temperature of the initial slurry to 80°C to 100°C, and then keeping it warm for 60 to 100 minutes after the color of the initial slurry changes to obtain the matured slurry. The method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to claim 1, characterized in that the P / Fe molar ratio of the added phosphoric acid to the amorphous iron phosphate is 0.2:1 to 0.3:

1.

3. The above step of adding water and phosphoric acid to amorphous iron phosphate to form a slurry and obtain an initial slurry is as follows: The process involves adding water to the amorphous iron phosphate to form a slurry, thereby obtaining a slurry with a solid content of 15% to 30%. A method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to claim 2, characterized by comprising the step of adding phosphoric acid to the slurry having a solid content of 15% to 30% under stirring conditions to obtain the initial slurry.

4. The method for producing amorphous iron phosphate provided is: The process involves adding water to the titanium white by-product to dissolve it and obtain a mixed slurry, The process involves adding a second pH adjusting agent to the mixed slurry to adjust the pH value of the mixed slurry to 4.0 to 5.0, and then obtaining a ferrous sulfate solution by solid-liquid separation. The process involves mixing the ferrous sulfate solution, the phosphate solution, and hydrogen peroxide solution to obtain primary amorphous iron phosphate. A method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to claim 1, characterized by comprising the step of rinsing the primary amorphous iron phosphate until the conductivity of the rinsing solution is 5 ms / cm or less to obtain the amorphous iron phosphate.

5. The molar ratio of phosphorus in the phosphorus salt solution to iron in the ferrous sulfate solution is 1:1 to 1.1:

1. The method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to claim 4, characterized in that the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to iron elements in the ferrous sulfate solution is 0.5:1 to 1:1, and / or the method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to claim 4.

6. The solute in the phosphate solution is at least one selected from ammonium dihydrogen phosphate and diammonium hydrogen phosphate. and / or the first pH adjusting agent is at least one selected from ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and aqueous ammonia. The method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to claim 4 or 5, characterized in that the second pH adjusting agent is at least one selected from ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and aqueous ammonia.

7. A method for producing iron phosphate with low sulfur content and a high iron-to-phosphorus ratio according to any one of claims 1 to 6, characterized in that the second filter cake is rinsed multiple times until the conductivity of the rinsing solution is 350 μs / cm or less, and then the second filter cake is dried and calcined to obtain iron phosphate with low sulfur content and a high iron-to-phosphorus ratio.

8. A method for producing iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio according to any one of claims 1 to 7, characterized in that the pH value of the first slurry is 3.5 to 4.

5.

9. Iron phosphate with a low sulfur content and a high iron-to-phosphorus ratio, manufactured using the method for producing iron phosphate described in any one of claims 1 to 8. The iron-to-phosphorus ratio of the iron phosphate with the low sulfur content and high iron-to-phosphorus ratio is (0.977 to 0.991):

1. and / or, the sulfur content of the iron phosphate with the low sulfur content and high iron-to-phosphorus ratio is 18.55 ppm to 32.46 ppm, and / or, the specific surface area of ​​iron phosphate with the low sulfur content and high iron-to-phosphorus ratio is 10.50 m². ２ / g ~ 12.01m ２ Iron phosphate characterized by a low sulfur content and a high iron-to-phosphorus ratio, having a value of / g.

10. A positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector, wherein the positive electrode active layer comprises a lithium iron phosphate material produced using iron phosphate with a low sulfur content and high iron-to-phosphorus ratio as described in claim 9 as a precursor.

11. A secondary battery characterized by including the positive electrode sheet described in claim 10.

Images