Iron phosphate having low sulfur content and high iron-phosphorus ratio, preparation method therefor, and use thereof

Abstract

The present application relates to the technical field of battery materials, and provides iron phosphate having low sulfur content and a high iron-phosphorus ratio, a preparation method therefor, and a use thereof. The preparation method for the iron phosphate having low sulfur content and a high iron-phosphorus ratio comprises: providing amorphous iron phosphate; adding water and phosphoric acid into the amorphous iron phosphate for slurrying treatment, and then heating and aging to obtain an aged slurry; carrying out slurry washing and filter pressing on the aged slurry to obtain a first filter cake; adding water and a first pH regulator into the first filter cake for slurry washing to obtain a first slurry having a pH value range of 3.0-5.0, and carrying out filter pressing on the first slurry to obtain a second filter cake; and rinsing, drying and calcining the second filter cake to obtain iron phosphate having low sulfur content and a high iron-phosphorus ratio. The present application facilitates preparation of iron phosphate having low sulfur content, a high iron-phosphorus ratio, and a large specific surface area. The iron phosphate can be used for preparing lithium iron phosphate having good electrochemical performance, and the lithium iron phosphate can be further used for preparing a positive electrode sheet and a secondary battery having good electrochemical performance.

Description

A kind of ferric phosphate with low sulfur and high iron-phosphorus ratio and its preparation method and application

[0001] This application is based on the Chinese application with CN application number 202410354078.7 and application date March 26, 2024, and claims its priority. The disclosed content of the CN application is again introduced as a whole into this application. Technical Field

[0002] The present invention relates to the technical field of battery materials, and in particular to ferric phosphate with low sulfur and high iron-phosphorus ratio, and a preparation method and application thereof. Background Art

[0003] Iron phosphate is a relatively important precursor of lithium iron phosphate. Its sulfur content, specific surface area, morphology, and iron-to-phosphorus ratio all affect the performance of lithium iron phosphate. Currently, the preparation method of iron phosphate is to first prepare amorphous iron phosphate, and then add an excess of phosphoric acid to carry out high-temperature aging to complete the crystal transformation to obtain iron phosphate. However, in the case of excessive phosphoric acid, acid salt byproducts such as ferric hydrogen phosphate, ferric dihydrogen phosphate, and ammonium ferric sulfate are easily generated, resulting in low product purity and low iron-to-phosphorus ratio. At the same time, during the aging process of the above method, the material is prone to agglomeration, and the particle distribution is relatively wide. Excessive surface energy will also lead to a large amount of material agglomeration during the subsequent drying and calcination process, resulting in a small specific surface area of ​​the prepared iron phosphate.

[0004] Therefore, the iron phosphate prepared by the existing iron phosphate preparation method has the problems of high sulfur content, low iron-phosphorus ratio and small specific surface area.

[0005] In view of this, this application is hereby filed.

[0006] Summary of the Invention

[0007] In view of the technical problems existing in the background technology, the present application provides a low-sulfur and high iron-phosphorus ratio ferric phosphate and its preparation method and application, aiming to improve or solve the technical problems mentioned in the background technology.

[0008] In a first aspect, an embodiment of the present application provides a method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio, comprising:

[0009] Providing amorphous iron phosphate;

[0010] adding water and phosphoric acid to amorphous ferric phosphate for slurry treatment, and then heating and aging to obtain an aged slurry;

[0011] The aged slurry is subjected to slurry washing and filter pressing to obtain a first filter cake;

[0012] adding water and a first pH adjuster to the first filter cake for slurry washing to obtain a first slurry with a pH value ranging from 3.0 to 5.0, and then filtering the first slurry to obtain a second filter cake;

[0013] The second filter cake is rinsed, dried, and calcined to prepare ferric phosphate with low sulfur and high iron-phosphorus ratio.

[0014] In the technical solution of the embodiment of the present application, by adding water to the amorphous ferric phosphate for slurry treatment (first slurrying) before aging the crystals, and then after the aging crystal transformation is completed, the first filter cake is added with water for slurry treatment again (second slurrying), and the first pH regulator is added during or after the second slurrying, it is possible to avoid the first pH regulator being added under the aging high temperature state so that the material undergoes a phase transition to generate basic ammonium ferric phosphate, thereby helping to reduce the surface energy of the particles and making the particles less likely to agglomerate during the subsequent drying and calcining process. Further, in the present application, by controlling the pH value of the first slurry to 3.0-5.0, it is possible to avoid the pH value being too high, causing the ferric phosphate to hydrolyze and produce ferric hydroxide by-products, and to help avoid the problem of an excessively high iron-phosphorus ratio.

[0015] Secondly, in the technical solution of the embodiment of the present application, the above two slurrying processes can play a dispersing role, thereby making the overall distribution of particles more uniform and smaller, which is conducive to increasing the reaction area between the particles and the first pH adjuster, reducing the production of acid salts, improving the purity and iron-phosphorus ratio of the ferric phosphate, and reducing the sulfur content of the ferric phosphate. Therefore, the preparation method provided by the present application can obtain anhydrous ferric phosphate with a large specific surface area, low sulfur content, and high iron-phosphorus ratio.

[0016] In some embodiments, the step of adding water and phosphoric acid to amorphous ferric phosphate for slurry treatment, and then heating and aging to obtain an aged slurry comprises:

[0017] adding water and phosphoric acid to amorphous ferric phosphate for slurry treatment to obtain an initial slurry;

[0018] The initial slurry is heated to 80°C to 100°C, and after the initial slurry changes color, the temperature is kept for 60 minutes to 100 minutes to obtain an aged slurry;

[0019] The P / Fe molar ratio of the added phosphoric acid to the amorphous ferric phosphate is 0.2:1 to 0.3:1.

[0020] In this embodiment, by controlling the aging temperature at 80°C to 100°C, after the initial slurry changes color, the aging temperature is maintained at 80°C to 100°C and the heat treatment is continued for 60min to 100min, so that the amorphous ferric phosphate can fully undergo crystal transformation. Among them, by setting the P / Fe molar ratio of phosphoric acid to amorphous ferric phosphate to 0.2:1 to 0.3:1, the problems of low product purity and low iron-phosphorus ratio caused by acid salt byproducts such as ferric hydrogen phosphate, ferric dihydrogen phosphate, and ammonium ferric sulfate produced under conditions of excessive phosphoric acid can be avoided.

[0021] In some embodiments, the step of slurrying amorphous ferric phosphate with water and phosphoric acid to obtain an initial slurry comprises:

[0022] The amorphous ferric phosphate is slurried by adding water to obtain a slurry with a solid content of 15% to 30%;

[0023] Under stirring conditions, phosphoric acid is added to a slurry with a solid content of 15% to 30% to obtain an initial slurry.

[0024] In this embodiment, adding phosphoric acid to the slurry with a solid content of 15% to 30% is beneficial to obtaining a better dispersion effect in the first slurrying process. Specifically, the dispersion degree and dispersion uniformity of the crystals can be improved.

[0025] In some embodiments, a method for preparing amorphous ferric phosphate is provided, comprising:

[0026] Dissolving the titanium dioxide by-product in water to obtain a mixed slurry;

[0027] adding a second pH adjuster to the mixed slurry to adjust the pH value of the mixed slurry to 4.0-5.0, and then performing solid-liquid separation to obtain a ferrous sulfate solution;

[0028] Mixing ferrous sulfate solution, phosphate solution and hydrogen peroxide to obtain primary amorphous ferric phosphate;

[0029] The primary amorphous ferric phosphate is rinsed until the conductivity of the rinsing liquid is less than or equal to 5 ms / cm, thereby obtaining amorphous ferric phosphate.

[0030] In this embodiment, the amorphous ferric phosphate prepared by the above method has high purity and stability, which is conducive to obtaining crystals with relatively uniform size and quality during the aging process.

[0031] In some embodiments, the molar ratio of phosphorus in the phosphate solution to iron in the ferrous sulfate solution is 1:1 to 1.1:1; and / or the molar ratio of hydrogen peroxide in the hydrogen peroxide to iron in the ferrous sulfate solution is 0.5:1 to 1:1.

[0032] In this embodiment, by controlling the amount of phosphate solution, hydrogen peroxide and ferrous sulfate solution, on the one hand, it is beneficial to avoid excessive phosphorus from reacting with impurities in ferrous sulfate to form phosphate byproducts. On the other hand, it is beneficial to control the pH value of the synthesis process within a more appropriate range, thereby helping to reduce the impurity content in the amorphous ferric phosphate and making the particle size of the obtained amorphous ferric phosphate more consistent.

[0033] In some embodiments, the solute in the phosphate solution is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate; and / or, the first pH adjuster is selected from at least one of ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide and ammonia water; and / or, the second pH adjuster is selected from at least one of ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide and ammonia water.

[0034] In this embodiment, the raw materials of the phosphate solution, the first pH adjuster and the second pH adjuster are easily available and relatively cheap, which is beneficial to reducing the production cost of ferric phosphate.

[0035] In some embodiments, the second filter cake is rinsed multiple times until the conductivity of the rinse liquid is less than or equal to 350 μs / cm, and then the second filter cake is dried and calcined to prepare ferric phosphate with low sulfur and high iron-phosphorus ratio.

[0036] In this embodiment, by controlling the conductivity of the second filter cake to be less than or equal to 350 μs / cm, a cleanly rinsed second filter cake can be obtained, thereby facilitating the production of a finished ferric phosphate product with higher purity and better consistency.

[0037] In a second aspect, the embodiments of the present application provide a low-sulfur and high iron-phosphorus ratio ferric phosphate, which is prepared using the above-mentioned preparation method of ferric phosphate;

[0038] The iron-phosphorus ratio of the low-sulfur, high-iron-phosphorus iron phosphate is (0.977-0.991):1; and / or, the sulfur content of the low-sulfur, high-iron-phosphorus iron phosphate is 18.55ppm-32.46ppm; and / or, the specific surface area of ​​the low-sulfur, high-iron-phosphorus iron phosphate is 10.5m 2 / g~12.01m 2 / g.

[0039] In the technical solution of the embodiment of the present application, amorphous iron phosphate is provided; water and phosphoric acid are added to the amorphous iron phosphate for slurry treatment, and then the temperature is increased and aged to obtain an aged slurry; the aged slurry is slurried and washed and filtered to obtain a first filter cake; water and a first pH regulator 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, and then the first slurry is filtered to obtain a second filter cake; the second filter cake is rinsed, dried and calcined. The iron-phosphorus ratio prepared 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~12.01m 2 / g.

[0040] Therefore, the iron phosphate prepared in the present application can overcome the technical problems existing in the prior art and has better electrochemical performance.

[0041] In a third aspect, an embodiment of the present application provides a positive electrode plate, which includes a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector, and the positive electrode active layer includes a lithium iron phosphate material prepared by using the above-mentioned low-sulfur and high iron-phosphorus ratio iron phosphate as a precursor.

[0042] In this embodiment, the positive electrode plate includes a lithium iron phosphate material prepared from the above-mentioned low-sulfur and high iron-phosphorus ratio iron phosphate as a precursor. The lithium iron phosphate material can inherit the characteristics of the above-mentioned iron phosphate such as low sulfur and high iron-phosphorus ratio, which is conducive to obtaining better electrochemical properties. Therefore, the positive electrode plate including the lithium iron phosphate material also has the advantage of excellent electrochemical performance.

[0043] In a fourth aspect, an embodiment of the present application provides a secondary battery comprising the above-mentioned positive electrode plate.

[0044] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has the advantage of excellent electrochemical performance.

[0045] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0047] FIG1 is an XRD diagram of ferric phosphate dihydrate obtained in step (8) of Example 1 of the present application and step (5) of Comparative Example 1;

[0048] FIG2 is a scanning electron microscope test result diagram of anhydrous ferric phosphate obtained in step (9) of Example 1 of the present application;

[0049] FIG3 is a scanning electron microscope test result diagram of the anhydrous ferric phosphate obtained in step (6) of comparative example 3 of the present application. DETAILED DESCRIPTION

[0050] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0052] In the description of the embodiments of this application, the technical terms "first," "second," etc. are used only to distinguish different objects and should not be understood to indicate or imply relative importance or to implicitly indicate the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise specifically defined.

[0053] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0054] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0055] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0056] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they should not be understood as limiting the embodiments of the present application.

[0057] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0058] It should be noted that all "iron-phosphorus ratios" in this application document refer to the "molar ratio of iron element to phosphorus element", such as the "iron-phosphorus ratio" in "low-sulfur and high iron-phosphorus ratio iron phosphate" refers to the "molar ratio of iron element to phosphorus element".

[0059] It should be noted that the “P / Fe molar ratio of the phosphoric acid to the amorphous iron phosphate” refers to the “molar ratio of phosphorus in the phosphoric acid to iron in the amorphous iron phosphate”.

[0060] In the prior art, sulfur content, iron-phosphorus ratio, specific surface area and morphology are all important performance indicators of iron phosphate. Among them, the iron-phosphorus ratio of iron phosphate is usually (0.95-0.97):1, the sulfur content is usually 100ppm-500ppm, and the specific surface area is usually less than 10m 2 / g. Currently, the preparation method of ferric phosphate is to first prepare amorphous ferric phosphate, then add an excess of phosphoric acid for high-temperature aging to complete crystal transformation to obtain ferric phosphate. However, when there is an excess of phosphoric acid, acid salt byproducts such as ferric hydrogen phosphate, ferric dihydrogen phosphate, and ammonium ferric sulfate are easily generated, resulting in low product purity and a low iron-phosphorus ratio. At the same time, during the aging process of the above method, the material is prone to agglomeration, resulting in a wide particle distribution. The excessively high surface energy also causes a large amount of material agglomeration during the subsequent drying and calcination process, resulting in a small specific surface area of ​​the prepared ferric phosphate.

[0061] In order to solve the technical problems of high sulfur content, low iron-phosphorus ratio and small specific surface area in the prior art iron phosphate, the present application provides a low-sulfur and high iron-phosphorus ratio iron phosphate and its preparation method and application, wherein, by adopting the specific preparation method provided by the present application, including performing a first slurrying before aging, performing a second slurrying after aging, and adding a first pH regulator during or after the second slurrying, and controlling the pH value of the first slurry at 3.0-5.0, iron phosphate with a higher iron-phosphorus ratio, lower sulfur content and larger specific surface area can be prepared, thereby improving the electrochemical performance of the lithium iron phosphate material prepared using it as a precursor, and thereby improving the electrochemical performance of the positive electrode sheet and the secondary battery.

[0062] The solution of this application is further described below.

[0063] In a first aspect, the present invention provides a method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio, which comprises:

[0064] Providing amorphous iron phosphate;

[0065] adding water and phosphoric acid to amorphous ferric phosphate for slurry treatment, and then heating and aging to obtain an aged slurry;

[0066] The aged slurry is subjected to slurry washing and filter pressing to obtain a first filter cake;

[0067] adding water and a first pH adjuster to the first filter cake for slurry washing to obtain a first slurry with a pH value ranging from 3.0 to 5.0, and then filtering the first slurry to obtain a second filter cake;

[0068] The second filter cake is rinsed, dried, and calcined to prepare ferric phosphate with low sulfur and high iron-phosphorus ratio.

[0069] In the technical solution of the embodiment of the present application, by adding water to the amorphous ferric phosphate for slurry treatment (first slurrying) before aging the crystals, and then after the aging crystal transformation is completed, the first filter cake is added with water for slurry treatment again (second slurrying), and the first pH regulator is added during or after the second slurrying, it is possible to avoid the first pH regulator being added under the aging high temperature state so that the material undergoes a phase transformation to generate basic ammonium ferric phosphate (the above-mentioned basic ammonium ferric phosphate is prone to produce ammonia smell during the sintering process), which is beneficial to reducing the surface energy of the particles and making the particles less likely to agglomerate during the subsequent drying and calcining process. By controlling the pH value of the first slurry to 3.0-5.0, it is possible to avoid the pH value being too high, which causes the ferric phosphate to hydrolyze and produce iron hydroxide by-products, and it is beneficial to avoid the problem of an excessively high iron-phosphorus ratio.

[0070] Secondly, in the technical solution of the embodiment of the present application, the above two slurrying processes can play a dispersing role, thereby making the overall distribution of particles more uniform and smaller, which is conducive to increasing the reaction area between the particles and the first pH adjuster, reducing the production of acid salts, improving the purity and iron-phosphorus ratio of the ferric phosphate, and reducing the sulfur content of the ferric phosphate. Therefore, the preparation method provided by the present application can obtain anhydrous ferric phosphate with a large specific surface area, low sulfur content, and high iron-phosphorus ratio.

[0071] In some embodiments, a method for preparing amorphous ferric phosphate is provided, comprising:

[0072] Dissolving the titanium dioxide by-product in water to obtain a mixed slurry;

[0073] adding a second pH adjuster to the mixed slurry to adjust the pH value of the mixed slurry to 4.0-5.0, and then performing solid-liquid separation to obtain a ferrous sulfate solution;

[0074] Mixing ferrous sulfate solution, phosphate solution and hydrogen peroxide to obtain primary amorphous ferric phosphate;

[0075] The primary amorphous ferric phosphate is rinsed until the conductivity of the rinsing liquid is less than or equal to 5 ms / cm, thereby obtaining amorphous ferric phosphate.

[0076] In this embodiment, the amorphous ferric phosphate prepared by the above method has high purity and stability, which is beneficial to the subsequent preparation of anhydrous ferric phosphate.

[0077] In some embodiments, the titanium dioxide byproduct includes ferrous sulfate. 2+ The titanium dioxide by-product is mixed with water at a molar concentration of 1 mol / L to 5 mol / L in the mixed slurry.

[0078] In some embodiments, the second pH adjuster is illustratively but not exclusively selected from at least one of ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and aqueous ammonia. Using such substances as the second pH adjuster has the advantages of readily available raw materials and relatively low prices, thereby reducing the production cost of ferric phosphate.

[0079] In some embodiments, the mixed slurry after adjusting the pH value to 4.0-5.0 can be subjected to solid-liquid separation by means of a filter press, and the filtrate obtained by separation can be allowed to stand for clarification to obtain a ferrous sulfate solution. For example, the mixed slurry after adjusting the pH value to 5.0 can be subjected to solid-liquid separation by means of a filter press.

[0080] In some embodiments, the preparation process of primary amorphous ferric phosphate may include: simultaneously dropping a phosphate salt solution and hydrogen peroxide into a ferrous sulfate solution, and continuing the reaction after the dropwise addition is completed to obtain primary amorphous ferric phosphate.

[0081] For example, the molar ratio of phosphorus in the phosphate solution to iron in the ferrous sulfate solution can be 1:1 to 1.1:1, such as 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 to iron in the ferrous sulfate solution can be 0.5:1 to 1:1, such as 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.

[0082] By controlling the amounts of phosphate solution, hydrogen peroxide, and ferrous sulfate solution, it is beneficial to prevent excessive phosphorus from reacting with impurities in ferrous sulfate to form phosphate byproducts. On the other hand, it is beneficial to control the pH value of the synthesis process within a more appropriate range, thereby reducing the impurity content in the amorphous ferric phosphate and ensuring that the particle size of the obtained amorphous ferric phosphate is more consistent.

[0083] In some embodiments, the solute in the phosphate salt solution is illustratively but not exclusively selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate. Using such substances as the solute in the phosphate salt solution has the advantages of readily available raw materials and relatively low prices, thereby reducing the production cost of ferric phosphate.

[0084] In some embodiments, the phosphate salt solution and hydrogen peroxide solution can be added dropwise for 20 to 60 minutes, such as 40 minutes, 45 minutes, 50 minutes, 55 minutes, or 60 minutes. The reaction can be continued for 1 to 2 hours after the addition is complete, such as 1 hour, 1.5 hours, or 2 hours. For example, the phosphate salt solution and hydrogen peroxide solution can be added dropwise for 50 minutes. The reaction can be continued for 1 hour after the addition is complete.

[0085] In some embodiments, the conductivity of the rinsing liquid after rinsing the primary amorphous ferric phosphate can 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. By controlling this condition, the impurity content of the amorphous ferric phosphate can be reduced.

[0086] In some embodiments, the step of adding water and phosphoric acid to amorphous ferric phosphate for slurry treatment, and then heating and aging to obtain an aged slurry comprises:

[0087] adding water and phosphoric acid to amorphous ferric phosphate for slurry treatment to obtain an initial slurry;

[0088] The initial slurry is heated to 80°C to 100°C, and after the initial slurry changes color, the temperature is kept for 60 minutes to 100 minutes to obtain an aged slurry;

[0089] The P / Fe molar ratio of the added phosphoric acid to the amorphous ferric phosphate is 0.2:1 to 0.3:1.

[0090] In some embodiments, amorphous ferric phosphate can be first dispersed in water, and then phosphoric acid can be added. Dispersion can be performed under stirring conditions, and the stirring speed can be illustratively 400 rpm to 600 rpm, such as 400 rpm, 450 rpm, 500 rpm, 550 rpm, or 600 rpm. The dispersion time can be illustratively 30 min to 50 min, such as 30 min, 35 min, 40 min, 45 min, or 50 min.

[0091] In some embodiments, the P / Fe molar ratio of phosphoric acid to amorphous ferric 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. For example, the P / Fe molar ratio of phosphoric acid to amorphous ferric phosphate is 0.3:1.

[0092] By setting the P / Fe molar ratio of phosphoric acid to amorphous ferric phosphate to 0.2:1 to 0.3:1, it is possible to avoid the problems of low product purity and low iron-to-phosphorus ratio caused by acid salt byproducts such as ferric hydrogen phosphate, ferric dihydrogen phosphate, and ammonium ferric sulfate produced under conditions of excessive phosphoric acid. In particular, setting the P / Fe molar ratio of phosphoric acid to amorphous ferric phosphate to 0.3:1 can achieve even better results.

[0093] In some embodiments, the step of slurrying amorphous ferric phosphate with water and phosphoric acid to obtain an initial slurry comprises:

[0094] The amorphous ferric phosphate is slurried by adding water to obtain a slurry with a solid content of 15% to 30%;

[0095] Under stirring conditions, phosphoric acid is added to a slurry with a solid content of 15% to 30% to obtain an initial slurry.

[0096] The solid content of the slurry may be 15% to 30%, such as 15%, 18%, 20%, 22%, 25%, 28% or 30%, or any other value within the range of 15% to 30%.

[0097] By adding phosphoric acid to the slurry with a solid content of 15% to 30%, it is beneficial to obtain a better dispersion effect in the first slurrying process, such as improving the dispersion degree and dispersion uniformity of large particle crystals and small particle crystals.

[0098] In some embodiments, the aging temperature can be controlled at 80°C to 100°C, that is, the initial slurry is 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 aging temperature within the above range, the crystal transformation of amorphous iron phosphate is achieved. For example, the aging temperature can be 90°C.

[0099] In some embodiments, the holding time after the initial slurry changes color can be 60 minutes, 65 minutes, 70 minutes, 75 minutes, 80 minutes, 85 minutes, 90 minutes, 95 minutes, or 100 minutes, or any other value within the range of 60 to 100 minutes. By holding the slurry for an additional 60 to 100 minutes after the initial slurry changes color, sufficient crystal transformation of the amorphous ferric phosphate can be ensured, thereby improving the yield. For example, the initial slurry is held for an additional 100 minutes after the initial slurry changes color.

[0100] In some embodiments, the slurry washing of the aged slurry can be performed once or multiple times, for example, twice, three times, four times or more. Accordingly, each slurry washing can be followed by a corresponding filter press. Exemplarily, the solid content of the slurry washing can be 15% to 30%. Exemplarily, the slurry washing can be performed under stirring conditions (the stirring speed can be 400 rpm to 600 rpm). Exemplarily, the slurry washing time can be 30 min to 50 min.

[0101] It can be understood that slurry washing of the aged slurry is beneficial to the dispersion of particles on the one hand and to the removal of impurities on the other hand.

[0102] In some embodiments, after the aged slurry is subjected to slurry washing and filter pressing, the mother liquor obtained from the filter pressing can be recovered for recycling.

[0103] In some embodiments, in the process of obtaining the first slurry, the amount of the first filter cake and water is such that the solid content of the first slurry is 15% to 30%, such as 15%, 18%, 20%, 22%, 25%, 28% or 30%.

[0104] In some embodiments, the first filter cake can be first dispersed in water before the first pH adjuster is added. The first filter cake can be dispersed in water under stirring. The stirring speed can be illustratively 400 rpm to 600 rpm, such as 400 rpm, 450 rpm, 500 rpm, 550 rpm, or 600 rpm. The dispersion time can be illustratively 30 min to 50 min, such as 30 min, 35 min, 40 min, 45 min, or 50 min.

[0105] In some embodiments, the first pH adjuster is illustratively but not limitedly selected from at least one of ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide, and ammonia. The raw materials of the first pH adjuster are readily available and relatively inexpensive, which helps reduce the production cost of ferric phosphate.

[0106] In some embodiments, the pH value of the first slurry can 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. By controlling the pH value of the first slurry to be between 3.0 and 5.0, it is possible to avoid the hydrolysis of ferric phosphate to produce ferric hydroxide byproducts due to excessively high pH values, and to help avoid excessively high iron-phosphorus ratios. For example, the pH value of the first slurry can be controlled to be between 3.5 and 4.5.

[0107] In some embodiments, the second filter cake may be rinsed only once, or multiple times, such as 2, 3, 4 or more times, as long as the conductivity of the rinse liquid after rinsing is less than or equal to 350 μs / cm.

[0108] By controlling the conductivity of the second filter cake to be less than or equal to 350 μs / cm, a cleanly rinsed second filter cake can be obtained, which is conducive to obtaining a finished ferric phosphate product with higher purity and better consistency.

[0109] In some embodiments, the rinsed second filter cake can be dried at a temperature of 100° C. to 200° C. (e.g., 100° C., 120° C., 150° C., 180° C., or 200° C.) for 0.5 to 1 hour (e.g., 0.5, 0.8, or 1 hour), specifically by flash drying. After drying, calcination can be performed at a temperature of 500° C. to 700° C. (e.g., 500° C., 550° C., 600° C., 650° C., or 700° C.) for 2 to 3 hours (e.g., 2 hours, 2.5 hours, or 3 hours).

[0110] In a second aspect, the present invention provides a low-sulfur, high-iron-phosphorus ratio iron phosphate, which is prepared by the above-mentioned preparation method of iron phosphate. The iron-phosphorus ratio of the low-sulfur, high-iron-phosphorus ratio iron phosphate is (0.977-0.991):1, and / or the sulfur content is 18.55ppm-32.46ppm, and / or the specific surface area is 10.50m 2 / g~12.01m 2 / g.

[0111] In the technical solution of the embodiment of the present application, amorphous iron phosphate is provided; water and phosphoric acid are added to the amorphous iron phosphate for slurry treatment, and then the temperature is increased and aged to obtain an aged slurry; the aged slurry is slurried and washed and filtered to obtain a first filter cake; water and a first pH regulator 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, and then the first slurry is filtered to obtain a second filter cake; the second filter cake is rinsed, dried and calcined. The iron-phosphorus ratio prepared 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~12.01m 2 / g.

[0112] Therefore, the iron phosphate prepared in the present application can overcome the technical problems existing in the prior art and has better electrochemical performance.

[0113] Furthermore, in some embodiments, the obtained ferric phosphate has not only a higher iron-to-phosphorus ratio, a lower sulfur content, and a larger specific surface area, but is also pure-phase ferric phosphate with higher purity.

[0114] In a third aspect, an embodiment of the present application provides a positive electrode plate, which includes a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector, and the positive electrode active layer includes a lithium iron phosphate material prepared by using the above-mentioned low-sulfur and high iron-phosphorus ratio iron phosphate as a precursor.

[0115] In this embodiment, the positive electrode plate includes a lithium iron phosphate material prepared from the above-mentioned low-sulfur and high iron-phosphorus ratio iron phosphate as a precursor. The lithium iron phosphate material can inherit the characteristics of low sulfur and high iron-phosphorus ratio of the above-mentioned iron phosphate, thereby facilitating the acquisition of better electrochemical performance. Therefore, the positive electrode plate including the lithium iron phosphate also has the advantage of excellent electrochemical performance.

[0116] In a fourth aspect, an embodiment of the present application provides a secondary battery comprising the above-mentioned positive electrode plate.

[0117] In this embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has the advantage of excellent electrochemical performance.

[0118] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0119] 1. Preparation method

[0120] Example 1

[0121] This embodiment provides a low-sulfur and high iron-phosphorus ratio ferric phosphate, the preparation method of which includes:

[0122] Step (1): dissolve ferrous sulfate, a by-product of titanium dioxide, in water to obtain Fe 2+ A mixed slurry having a molar concentration of ions of 1.5 mol / L;

[0123] Step (2): adding ammonium bicarbonate to the mixed slurry to adjust the pH value of the mixed slurry to 5, then filtering the mixed slurry through a filter press, and standing the filtrate obtained by the filtration to clarify, thereby obtaining a ferrous sulfate solution;

[0124] Step (3): adding ammonium dihydrogen phosphate and hydrogen peroxide to the ferrous sulfate solution simultaneously over 50 minutes, wherein the molar ratio of phosphorus in the phosphate solution to iron in the ferrous sulfate solution is 1:1, and the molar ratio of hydrogen peroxide in the hydrogen peroxide to iron in the ferrous sulfate solution is 1:1. After the addition is completed, the reaction is continued for 1 hour to obtain primary amorphous iron phosphate;

[0125] Step (4): filter-pressing and rinsing the primary amorphous ferric phosphate until the conductivity of the rinsing liquid is 5 ms / cm to obtain amorphous ferric phosphate;

[0126] Step (5): adding water to the rinsed amorphous ferric phosphate at a solid content of 20%, slurrying and dispersing the mixture at a stirring speed of 500 rpm for 40 minutes (the first slurrying process), then adding phosphoric acid at a molar ratio of P:Fe = 0.3:1, heating the mixture to 90°C, and keeping the temperature for 100 minutes after the mixture changes color to obtain an aged slurry;

[0127] Step (6): The aged slurry is subjected to filter pressing, and the filter cake after the filter pressing is slurried and washed by adding water at a solid content of 20%. The slurry washing process is carried out at a stirring speed of 500 rpm for 40 minutes. After washing, the slurry is returned to the filter press for filter pressing to obtain a first filter cake and a mother liquor, wherein the mother liquor obtained by the filter pressing can be collected for recycling;

[0128] Step (7): adding water to the first filter cake at a solid content of 20%, and slurrying and dispersing the mixture for 40 minutes at a stirring speed of 500 rpm (second slurrying process), followed by adding ammonium bicarbonate to obtain a first slurry having a pH of 4; returning the first slurry to a filter press for filtration to obtain a second filter cake;

[0129] Step (8): rinsing the second filter cake multiple times until the conductivity of the rinsing liquid is 350 μs / cm, thereby obtaining ferric phosphate dihydrate;

[0130] Step (9): drying the rinsed ferric phosphate dihydrate at 150° C. for 1 h to obtain a dried product; and calcining the dried product at 600° C. for 2 h to obtain anhydrous ferric phosphate.

[0131] Example 2

[0132] This embodiment provides a low-sulfur and high iron-phosphorus ratio ferric phosphate, the preparation method of which includes:

[0133] Steps (1) to (4): same as steps (1) to (4) in Example 1;

[0134] Step (5): adding water to the rinsed amorphous ferric phosphate at a solid content of 15%, slurrying and dispersing the mixture for 30 minutes at a stirring speed of 400 rpm (the first slurrying process), and then adding phosphoric acid at a molar ratio of P:Fe = 0.3:1. The mixture was heated to 90° C. and kept warm for 100 minutes after color change to obtain an aged slurry.

[0135] Step (6): The aged slurry is subjected to filter pressing, and the filter cake after the filter pressing is slurried and washed by adding water at a solid content of 15%. The slurry washing process is carried out at a stirring speed of 400 rpm for 30 minutes. After washing, the slurry is returned to the filter press for filter pressing to obtain a first filter cake and a mother liquor, wherein the mother liquor obtained by the filter pressing can be collected for recycling;

[0136] Step (7): adding a fixed amount of water to the first filter cake at a solid content of 15%, slurrying and dispersing the mixture at a stirring speed of 400 rpm for 30 minutes (second slurrying process), and then adding ammonium bicarbonate to obtain a first slurry with a pH value of 5; returning the first slurry to the filter press for filtration to obtain a second filter cake;

[0137] Steps (8) and (9): The same as steps (8) and (9) of Example 1, finally anhydrous ferric phosphate is obtained.

[0138] Example 3

[0139] This embodiment provides a low-sulfur and high iron-phosphorus ratio ferric phosphate, the preparation method of which includes:

[0140] Steps (1) to (4): same as steps (1) to (4) in Example 1;

[0141] Step (5): adding water to the rinsed amorphous ferric phosphate at a solid content of 30%, slurrying and dispersing the mixture for 50 minutes at a stirring speed of 600 rpm (the first slurrying process), and then adding phosphoric acid at a molar ratio of P:Fe = 0.3:1. The mixture was heated to 90°C and kept warm for 100 minutes after discoloration to obtain an aged slurry.

[0142] Step (6): The aged slurry is subjected to filter pressing, and the filter cake after the filter pressing is slurried and washed by adding water at a solid content of 30%. The slurry washing process is carried out at a stirring speed of 600 rpm for 50 minutes. After washing, the slurry is returned to the filter press for filter pressing to obtain a first filter cake and a mother liquor, wherein the mother liquor obtained by the filter pressing can be collected for recycling;

[0143] Step (7): adding water to the first filter cake at a solid content of 30%, slurrying and dispersing the mixture at a stirring speed of 600 rpm for 50 minutes (second slurrying process), and then adding ammonium bicarbonate to obtain a first slurry with a pH value of 3; returning the first slurry to the filter press for filtration to obtain a second filter cake;

[0144] Steps (8) and (9): The same as steps (8) and (9) of Example 1, finally anhydrous ferric phosphate is obtained.

[0145] Example 4

[0146] 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 filter cake, and finally anhydrous ferric phosphate is prepared.

[0147] Example 5

[0148] 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 filter cake, and finally anhydrous ferric phosphate is prepared.

[0149] Example 6

[0150] 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 filter cake, and finally anhydrous ferric phosphate is prepared.

[0151] Example 7

[0152] 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 filter cake, and finally anhydrous ferric phosphate is prepared.

[0153] Example 8

[0154] 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 filter cake, and finally anhydrous ferric phosphate is prepared.

[0155] Example 9

[0156] The difference between this embodiment and embodiment 1 is that in step (5), the P / Fe molar ratio of the added phosphoric acid to the amorphous ferric phosphate is 0.2:1, and an aged slurry is obtained, and anhydrous ferric phosphate is finally prepared.

[0157] Example 10

[0158] The difference between this embodiment and embodiment 1 is that in step (5), the P / Fe molar ratio of the added phosphoric acid to the amorphous ferric phosphate is 0.15:1, and an aged slurry is obtained, and anhydrous ferric phosphate is finally prepared.

[0159] Example 11

[0160] The difference between this embodiment and embodiment 1 is that in step (5), the P / Fe molar ratio of the added phosphoric acid to the amorphous ferric phosphate is 0.35:1, and an aged slurry is obtained, and anhydrous ferric phosphate is finally prepared.

[0161] Example 12

[0162] The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the iron element in the ferrous sulfate solution is 0.5:1, thereby obtaining primary amorphous ferric phosphate and finally obtaining anhydrous ferric phosphate.

[0163] Example 13

[0164] The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the iron element in the ferrous sulfate solution is 0.8:1, thereby obtaining primary amorphous ferric phosphate and finally obtaining anhydrous ferric phosphate.

[0165] Example 14

[0166] The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the iron element in the ferrous sulfate solution is 0.48:1, thereby obtaining primary amorphous ferric phosphate and finally obtaining anhydrous ferric phosphate.

[0167] Example 15

[0168] The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the iron element in the ferrous sulfate solution is 1.1:1, thereby obtaining primary amorphous ferric phosphate and finally obtaining anhydrous ferric phosphate.

[0169] Example 16

[0170] The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of phosphorus in the phosphate solution to iron in the ferrous sulfate solution is 1.1:1.

[0171] Example 17

[0172] The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of phosphorus in the phosphate solution to iron in the ferrous sulfate solution is 0.9:1.

[0173] Example 18

[0174] 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 value of 3.5; the first slurry is returned to the filter press for filtration to obtain a second filter cake, and finally anhydrous ferric phosphate is obtained.

[0175] Example 19

[0176] 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 value of 4.5; the first slurry is returned to the filter press for filtration to obtain a second filter cake, and finally anhydrous ferric phosphate is obtained.

[0177] Example 20

[0178] 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 value of 3; the first slurry is returned to the filter press for filtration to obtain a second filter cake, and finally anhydrous ferric phosphate is obtained.

[0179] Example 21

[0180] 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 value of 5; the first slurry is returned to the filter press for filtration to obtain a second filter cake, and finally anhydrous ferric phosphate is obtained.

[0181] Comparative Example 1

[0182] This comparative example provides a low-sulfur and high iron-phosphorus ratio ferric phosphate, the preparation method of which comprises:

[0183] Steps (1) to (4): the same as steps (1) to (4) in Example 1;

[0184] Step (5): adding water to the rinsed amorphous ferric phosphate at a solid content of 20%, slurrying and dispersing the mixture at a stirring speed of 500 rpm for 40 minutes (the first slurrying process), then adding phosphoric acid at a molar ratio of P:Fe = 0.3:1, heating the mixture to 90°C, and keeping the temperature for 100 minutes after the mixture changes color. Then, adding ammonium bicarbonate to adjust the pH to 4, and after aging, filter pressing and washing the mixture until the conductivity reaches 350 μs / cm to obtain ferric phosphate dihydrate;

[0185] Step (6): Same as step (9) in Example 1, finally anhydrous ferric phosphate is obtained.

[0186] Comparative Example 2

[0187] The difference between this comparative example and Example 1 is that the pH value of the first slurry in step (7) of Example 1 is adjusted to 6 to obtain a second filter cake, and finally anhydrous ferric phosphate is prepared.

[0188] Comparative Example 3

[0189] This comparative example provides a low-sulfur and high iron-phosphorus ratio ferric phosphate, the preparation method of which comprises:

[0190] Steps (1) to (4): the same as steps (1) to (4) in Example 1;

[0191] Step (5): adding water to the rinsed amorphous ferric phosphate at a solid content of 20%, slurrying and dispersing the mixture at a stirring speed of 500 rpm for 40 minutes (the first slurrying process), then adding phosphoric acid at a molar ratio of P:Fe = 0.3:1, heating the mixture to 90°C, and keeping the temperature for 100 minutes after the mixture changes color. After aging, the mixture is filter-pressed and washed until the conductivity reaches 350 μs / cm to obtain ferric phosphate dihydrate;

[0192] Step (6): Same as step (9) in Example 1, finally anhydrous ferric phosphate is obtained.

[0193] Comparative Example 4

[0194] The difference from Example 1 is that, in step (7), ammonium bicarbonate is added to obtain a first slurry with a pH value of 2; the first slurry is returned to the filter press for filtration to obtain a second filter cake, and finally anhydrous ferric phosphate is obtained.

[0195] 2. Test methods and test results

[0196] 1. Property test of iron phosphate material

[0197] The ferric phosphate dihydrate obtained in step (8) of Example 1 and the ferric phosphate dihydrate obtained in Comparative Example 1 were subjected to XRD tests, and the results are shown in FIG1 .

[0198] As can be seen from Figure 1, after the aging is completed in Example 1, the slurry is first filtered and washed, and after the slurry is filtered, ammonium bicarbonate is added to the slurry to adjust the pH value to 4, and then the filtration is continued to be filtered and rinsed until the conductivity is 350 μs / cm. At this time, the ferric phosphate dihydrate obtained is a pure phase FePO4·2H2O; compared with Comparative Example 1, ammonium bicarbonate is added at high temperature during the aging process to adjust the pH to 4. At this time, the temperature is very high, and crystal transformation is easy to occur in an alkaline environment. Therefore, the ferric phosphate dihydrate obtained at this time is a pure phase (NH4)Fe2(PO4)2(OH)·2H2O. The sintering process of ferric phosphate dihydrate of this phase has an ammonia smell.

[0199] In addition, the anhydrous ferric phosphate obtained in step (9) of Example 1 and the anhydrous ferric phosphate obtained in step (6) of Comparative Example 3 were subjected to scanning electron microscopy testing, and the results (SEM images) are shown in Figures 2 and 3.

[0200] As can be seen from the comparison of Figures 2 and 3, the anhydrous ferric phosphate obtained in Example 1 is more uniform than the anhydrous ferric phosphate obtained in Comparative Example 3, and there are holes between the particles, while the particles of the anhydrous ferric phosphate obtained in Comparative Example 3 are significantly agglomerated and denser. This corresponds to the specific surface area in Table 1 below. The specific surface area of ​​the anhydrous ferric phosphate obtained in Example 1 is larger than that of the anhydrous ferric phosphate obtained in Comparative Example 3. It can be seen that after aging is completed, the slurry is washed by pressure filtration, and the slurry is continued to be dispersed after the pressure filtration, which is conducive to uniform dispersion of the particles. At the same time, alkali is added to adjust the pH to 4, which reduces the formation of acid salts, reduces surface energy, and effectively reduces the agglomeration between the particles. It is possible to obtain anhydrous ferric phosphate with uniform particles, loose and porous particles, and a large specific surface area. The electrochemical performance of the lithium iron phosphate positive electrode material prepared by the anhydrous ferric phosphate obtained in Example 1 is also improved.

[0201] Furthermore, in this application, the anhydrous ferric phosphate obtained in Examples 1 to 21 and Comparative Examples 1 to 4 was tested for element content and specific surface area, and the results are shown in Table 1.

[0202] Among them, the sulfur content is detected by an infrared carbon-sulfur meter, the specific surface area is detected by a nitrogen adsorption method using a specific surface area tester, the iron content is detected by potassium dichromate titration, and the phosphorus content is detected by a quinoline phosphomolybdate weight method.

[0203] Table 1 Performance indicators of anhydrous ferric phosphate in different embodiments and comparative examples

[0204] From Table 1 we can see that:

[0205] (1) The iron-phosphorus ratio of the ferric phosphate prepared in Example 1 is in the range of (0.977-0.991):1, which is higher than that of Comparative Examples 1 and 3. This indicates that the addition of alkali after aging, filter pressing, and rinsing can effectively neutralize the acid and salt, thereby increasing the iron-phosphorus ratio.

[0206] (2) The iron-phosphorus ratio of the iron phosphate prepared in Example 2 is lower than that in Comparative Example 2. The iron-phosphorus ratio of the iron phosphate prepared in Comparative Example 2 is greater than 1. It is speculated that the iron is not simply the iron in the iron phosphate, but also contains a small amount of other iron compounds, which may be iron hydroxide. This indicates that when the pH value is adjusted to 6, the high pH value easily causes the iron phosphate to hydrolyze and produce iron hydroxide by-products;

[0207] (3) The specific surface area of ​​the iron phosphate prepared in the embodiment is 10.50 m 2 / g~12.01m 2 / g range, which is higher than that of Comparative Examples 1 and 3. On the one hand, the two slurryings disperse the large particles and the particle distribution is more uniform. On the other hand, the addition of alkali eliminates the acid salt and reduces the surface energy of the particles, making it less likely to agglomerate during the drying and calcining process.

[0208] (4) The specific surface area of ​​the iron phosphate prepared in Comparative Example 2 is very large, i.e., 13.61 m 2 / g, mainly because the high pH value easily leads to the hydrolysis of iron phosphate, producing iron hydroxide by-product, and the colloidal particles of iron oxide are very small;

[0209] (5) The sulfur content of the finished ferric phosphate products obtained in Example 1 and Comparative Example 2 is relatively low, indicating that the two slurrying processes can disperse large particles and reduce the particles. The addition of alkali can effectively eliminate acid salts and reduce sulfur adsorption.

[0210] 2. Property test of CR2032 button half-cell assembled with positive electrode sheets

[0211] The anhydrous ferric phosphate obtained in Examples 1 to 21 and Comparative Examples 1 to 4 was prepared into positive electrode sheets according to the following method, and then the obtained positive electrode sheets were assembled into CR2032 button half-cells, and the performance of the obtained CR2032 button half-cells was tested.

[0212] Preparation method: First, the iron phosphate and lithium source prepared in the above embodiments and comparative examples are sintered to obtain the corresponding lithium iron phosphate positive electrode material. Thereafter, the prepared lithium iron phosphate positive electrode material is mixed with conductive carbon powder and PVDF binder in a mass ratio of 90:5:5, and after homogenization, it is coated on aluminum foil, dried at 100°C and rolled with a double-roller machine. Then, a pole piece with a diameter of 14 mm is prepared using a punch, and the mass of the active material is obtained by weighing and deducting the mass of the aluminum foil.

[0213] After drying the positive electrode sheet, the CR2032 button-type half-cell was assembled in a Braun UNlab inert gas glove box in Germany. The assembly sequence was negative electrode shell, lithium sheet, electrolyte, separator, electrolyte, positive electrode sheet, gasket, spring, and positive electrode shell. The electrochemical performance of these CR2032 button-type half-cells was tested using a Wuhan Blue Electric CT2001A battery testing system over a voltage range of 2.0 to 4.6 V. The test results are shown in Table 2.

[0214] Table 2 Test results of CR2032 button half-cells in different embodiments and comparative examples

[0215] As can be seen from Table 2, the CR2032 button half-cell corresponding to the iron phosphate provided in the above embodiments of the present application can have higher 0.1C charge specific capacity and 0.1C discharge specific capacity than the CR2032 button half-cell corresponding to the iron phosphate provided in the comparative example.

[0216] In summary, the present application facilitates the preparation of iron phosphate with low sulfur content, high iron-phosphorus ratio, large specific surface area, and high purity. This iron phosphate can be used to prepare lithium iron phosphate with excellent electrochemical performance, which can be further used to prepare positive electrode sheets and secondary batteries with excellent electrochemical performance.

[0217] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are included within the technical scope of the present application. In addition, without departing from the scope of the present application, any modifications that can be conceived by those skilled in the art to the embodiments, or other methods constructed by combining some of the constituent elements in the embodiments are also included within the scope of the present application.

Claims

1. A method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio, characterized in that: include: Providing amorphous iron phosphate; The amorphous ferric phosphate is added with water and phosphoric acid to be slurried, and then aged at elevated temperature to obtain an aged slurry; The aged slurry is subjected to slurry washing and filter pressing to obtain a first filter cake; adding water and a first pH adjuster to the first filter cake for slurry washing to obtain a first slurry with a pH value ranging from 3.0 to 5.0, and then filtering the first slurry to obtain a second filter cake; The second filter cake is rinsed, dried, and calcined to prepare the low-sulfur and high-iron-phosphorus iron phosphate.

2. The method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio according to claim 1, characterized in that: The step of adding water and phosphoric acid to the amorphous ferric phosphate for slurry treatment, and then heating and aging to obtain an aged slurry comprises: Adding water and phosphoric acid to the amorphous ferric phosphate for slurry treatment to obtain an initial slurry; Heating the initial slurry to 80° C. to 100° C., and keeping the temperature for 60 to 100 minutes after the initial slurry changes color, to obtain the aged slurry; Wherein, 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 method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio according to claim 2, characterized in that: The step of adding water and phosphoric acid to the amorphous ferric phosphate for slurry treatment to obtain an initial slurry comprises: adding water to the amorphous ferric phosphate for slurry treatment to obtain a slurry with a solid content of 15% to 30%; Under stirring conditions, phosphoric acid is added to the slurry with a solid content of 15% to 30% to obtain the initial slurry.

4. The method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio according to claim 1, characterized in that: The preparation method of the amorphous ferric phosphate provided includes: Dissolving the titanium dioxide by-product in water to obtain a mixed slurry; A second pH regulator is added to the mixed slurry to adjust the pH value of the mixed slurry to 4.0~5.0, then, through solid-liquid separation, ferrous sulfate solution is obtained; Mixing the ferrous sulfate solution, the phosphate solution and the hydrogen peroxide to obtain primary amorphous ferric phosphate; The primary amorphous ferric phosphate is rinsed until the conductivity of the rinsing liquid is less than or equal to 5 ms / cm to obtain the amorphous ferric phosphate.

5. The method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio according to claim 4, characterized in that: The molar ratio of phosphorus in the phosphate solution to iron in the ferrous sulfate solution is 1:1 to 1.1:1; And / or, the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the iron element in the ferrous sulfate solution is 0.5:1 to 1:

1.

6. The method for preparing ferric phosphate with low sulfur and high iron-phosphorus ratio according to claim 4 or 5, characterized in that: The solute in the phosphate solution is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate; and / or, the first pH regulator is selected from at least one of ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide and ammonia water; And / or, the second pH adjuster is selected from at least one of ammonium bicarbonate, ammonium carbonate, sodium bicarbonate, sodium carbonate, sodium hydroxide and ammonia water.

7. The method for preparing ferric phosphate with low sulfur and high iron-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 rinse liquid is less than or equal to 350 μs / cm. Thereafter, the second filter cake is dried and calcined to prepare the low-sulfur and high-iron-phosphorus ratio ferric phosphate.

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

5.

9. A low-sulfur and high iron-phosphorus ratio ferric phosphate, characterized in that: Prepared by the preparation method of ferric phosphate according to any one of claims 1 to 8; wherein, The iron-phosphorus ratio of the low-sulfur and high-iron-phosphorus ratio ferric phosphate is (0.977-0.991):1; And / or, the sulfur content of the low-sulfur and high iron-phosphorus ratio ferric phosphate is 18.55 ppm to 32.46 ppm; And / or, the specific surface area of ​​the low-sulfur and high-iron-phosphorus ratio iron phosphate is 10.50m 2 / g~12.01m 2 / g.

10. A positive electrode plate, characterized in that: The invention comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector, wherein the positive electrode active layer comprises a lithium iron phosphate material prepared by using the low-sulfur and high iron-phosphorus ratio iron phosphate according to claim 9 as a precursor.

11. A secondary battery, characterized in that: The secondary battery includes the positive electrode sheet according to claim 10.

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