Ferrous phosphate and its preparation method and application

Abstract

The present application provides a ferrous phosphate, a preparation method thereof, and an application thereof, belonging to the technical field of lithium batteries. The preparation method of the ferrous phosphate includes the following steps: using an iron plate as the anode, an inert metal plate as the cathode, and a mixed solution of hydrogen phosphate, phosphoric acid, buffer, complexing agent, and antioxidant as the electrolyte to carry out an electrolysis reaction, and filtering to obtain a precipitate containing Fe3(PO4)2 . 8H2O, washing, drying, and calcining the precipitate in sequence to obtain ferrous phosphate; wherein, the pH value of the electrolyte is controlled at 7.0 - 7.4. In the present application, the electrolysis method is used to prepare ferrous phosphate, and ferrous phosphate with high purity, small particle size, and uniform particle size distribution can be stably and continuously obtained, thereby improving the electrochemical performance of the lithium iron phosphate cathode material prepared by using the ferrous phosphate.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to ferrous phosphate and a preparation method and application thereof. Background Art

[0002] Ferrous phosphate is an inorganic compound with the chemical formula Fe3(PO4)2. It is a white-blue monoclinic crystal with a specific gravity of 2.58g / L. It is soluble in dilute acid but insoluble in water, acetic acid, and alcohol. It exists in the form of blue iron ore in nature and is mostly obtained by the reaction of ferrous salt solution with phosphate. It can be used to produce lithium iron phosphate battery materials, as a catalyst, and to manufacture ceramics.

[0003] At present, the preparation method of ferrous phosphate is similar to that of ferric phosphate, that is, ferrous phosphate is obtained by liquid phase precipitation of ferrous salt and phosphate; however, this process is complicated, and it is necessary to prevent the oxidation of ferrous ions, and at the same time, it is also necessary to control crystallization and growth to avoid the formation of hydroxides. The existing preparation method leads to low purity of ferrous phosphate, excessively large and uneven particle size, which ultimately affects the electrochemical properties of lithium iron phosphate. At the same time, this method also produces a large amount of wastewater and has poor environmental protection.

[0004] Therefore, there is an urgent need to provide a new method for preparing ferrous phosphate to improve the purity, particle size and uniformity of ferrous phosphate, thereby improving the electrochemical performance of lithium iron phosphate. Summary of the invention

[0005] In view of the technical problems existing in the background technology, the present application provides a ferrous phosphate and a preparation method and application thereof, aiming to solve the technical problem of how to improve the purity, particle size and uniformity of ferrous phosphate.

[0006] In the first aspect, the present invention provides a method for preparing ferrous phosphate, comprising the following steps: using an iron plate as an anode, an inert metal plate as a cathode, and a mixed solution of hydrogen phosphate, phosphoric acid, a buffer, a chelating agent, and an antioxidant as an electrolyte, performing an electrolytic reaction, filtering to obtain a solution containing Fe3(PO4)2 . 8H2O, washing, drying and calcining the precipitate in sequence to obtain ferrous phosphate; wherein the pH value of the electrolyte is controlled at 7.0-7.4.

[0007] In the technical solution of the embodiment of the present application, the process of preparing ferrous phosphate by electrolysis is as follows: the anode iron plate reacts with the hydrogen phosphate in the electrolyte to generate an electrolyte containing Fe3(PO4)2 . 8H2O precipitation, hydrogen and strong alkali, the strong alkali reacts with phosphoric acid to form hydrogen phosphate, the generated hydrogen phosphate can be recycled as an electrolyte component, and the obtained Fe3(PO4)2 . The precipitate of 8H2O is washed with water to remove impurities and then dehydrated to obtain ferrous phosphate (anhydrous ferrous phosphate).

[0008] Specifically, the advantages of using electrolyte electrolysis to prepare ferrous phosphate in the present application are: (1) The electrolysis method of the present application does not require the preparation of a ferrous salt solution in advance, but directly uses an iron plate to prepare ferrous phosphate, which is low in cost, produces less wastewater, and has a short process. In addition, in the electrolysis of the present application, impurities will not be electrolyzed and enter the electrolyte, so the obtained ferrous phosphate has high purity; (2) The electrolyte of the present application can be recycled and continuously electrolyzed, and the electrolyte is consistent and uniform, and the pH value of the electrolysis environment fluctuates little, so that ferrous phosphate can be stably generated. Under the cooperation of the electrolyte of the present application and the pH value electrolysis environment, the obtained ferrous phosphate has high particle size uniformity, large BET, small primary particle size, and high purity; (3) The chelate contained in the electrolyte of the present application The mixture can improve the dispersibility of ferrous phosphate, reduce the agglomeration index, and obtain ferrous phosphate with a smaller particle size. The buffer contained in it is used to stabilize the pH value of the electrolytic environment and alleviate local changes in the electrolytic process, so that the preparation environment of ferrous phosphate is highly consistent and the uniformity of the obtained product is good. The role of the antioxidant contained in it is to improve the reducing environment and avoid the oxidation of ferrous iron. At the same time, the hydrogen generated also has the effect of reducing the oxidation of ferrous ions, so that the obtained ferrous phosphate has high purity. In addition to providing phosphate radicals to enable the reaction to proceed smoothly, phosphoric acid and hydrogen phosphate also have the function of consuming hydroxide radicals generated by electrolysis, avoiding the generation of impurities and maintaining a stable pH value. At the same time, phosphoric acid also replenishes water for the electrolytic reaction to achieve continuous production of ferrous phosphate.

[0009] In some embodiments, the hydrogen phosphate includes at least one of dipotassium hydrogen phosphate and disodium hydrogen phosphate.

[0010] In this embodiment, the selected hydrogen phosphate can generate sodium hydroxide or potassium hydroxide after electrolysis reaction with iron. Sodium ions and potassium ions can promote the electrolysis reaction. At the same time, they have good water solubility and will not introduce other impurities into the product.

[0011] In some embodiments, the buffer includes at least one of sodium citrate and potassium citrate.

[0012] In this embodiment, the selected buffer can stabilize the pH of the electrolysis at 7.0-7.4 without introducing other impurities.

[0013] In some embodiments, the complexing agent includes at least one of EDTA and EDTA sodium salt.

[0014] In this embodiment, the selected complexing agent can improve the dispersibility of ferrous phosphate, reduce the agglomeration index, improve the uniformity of ferrous phosphate, and reduce its particle size.

[0015] In some embodiments, the antioxidant includes at least one of ascorbic acid, hydrazine hydrate, and sodium sulfite.

[0016] In this embodiment, the selected antioxidant can prevent the oxidation of ferrous ions and has good water solubility and is easy to remove impurities.

[0017] In some embodiments, the concentration of hydrogen phosphate in the electrolyte is 2-3 mol / L.

[0018] In this embodiment, if the concentration of hydrogen phosphate is too high or too low, it will cause the pH value to fluctuate, making the electrolysis environment unstable, and further affecting the purity, particle size and uniformity of ferrous phosphate.

[0019] In some embodiments, the concentration of phosphoric acid is 4-6 mol / L.

[0020] In this embodiment, if the concentration of phosphoric acid is too high or too low, it will cause the pH value to fluctuate, making the electrolysis environment unstable, and further affecting the purity, particle size and uniformity of ferrous phosphate.

[0021] In some embodiments, the concentration of the buffer is 0.05-0.1 mol / L.

[0022] In this embodiment, if the concentration of the buffer is too high or too low, it will cause the pH value to fluctuate, making the electrolysis environment unstable, and further affecting the purity, particle size and uniformity of ferrous phosphate.

[0023] In some embodiments, the concentration of the complexing agent is 0.02-0.05 mol / L.

[0024] In this embodiment, if the concentration of the complexing agent is too low, the effect is not obvious, and if the concentration is too high, the raw materials are wasted.

[0025] In some embodiments, the concentration of the antioxidant is 0.02-0.05 mol / L.

[0026] In this embodiment, if the concentration of the antioxidant is too low, the effect is not obvious, and if the concentration is too high, the raw materials are wasted.

[0027] In some embodiments, during the electrolysis step, the cell voltage is 0.15-0.35 V and the current density is 200-340 A / m 2 , the temperature is 35-45℃, and the distance between cathode and anode is 8-15cm.

[0028] In this embodiment, within the limited range, the cell voltage can prevent impurities in the iron plate from being electrolytically precipitated, further improving the purity of ferrous phosphate. However, if the cell voltage exceeds the limited range, it will be unfavorable for improving the purity of ferrous phosphate. Under the current density limit of the present application, the primary particle size of ferrous phosphate is further reduced. If the current density exceeds the limited range of the present application, the primary particle size of ferrous phosphate will increase.

[0029] In some embodiments, the precipitate is washed, dried and calcined in sequence to obtain ferrous phosphate, comprising: acid washing the precipitate until the phosphorus content in the acid solution after washing is less than 50 ppm, solid-liquid separation, and obtaining a first filter residue; drying the first filter residue at 100-150°C until the water content of the first filter residue is less than 1.5% to obtain a second filter residue; calcining the second filter residue in an atmosphere containing an inert gas to obtain ferrous phosphate.

[0030] In this embodiment, the purpose of washing the precipitate is to remove impurities and avoid the subsequent introduction of impurities, while the purpose of drying and calcining is to remove Fe3(PO4)2 . 8H2O combined water, finally get ferrous phosphate; if the drying temperature is too low, it will not have the drying effect, if the drying temperature is too high, it will waste energy; the purpose of using inert atmosphere in the calcination process is to avoid oxidation of ferrous phosphate and reduce its purity.

[0031] In some embodiments, the step of calcining the second filter residue in an atmosphere containing an inert gas to obtain ferrous phosphate includes: calcining the second filter residue at 400-500°C for 3-6h at a heating rate of 1-3°C / h in an atmosphere containing an inert gas, and maintaining the oxygen content less than or equal to 100ppm during the calcination process to obtain a calcined material; cooling the calcined material to 100°C or below in an inert atmosphere, and then sieving it through 60-100 mesh to obtain ferrous phosphate.

[0032] In this embodiment, the temperature is slowly raised to remove bound water and the oxygen content is kept low to make Fe3(PO4)2 . The bound water of 8H2O is removed smoothly and the formation of impurities is avoided; if the calcination temperature is too high, energy is wasted, and if the calcination temperature is too low, the bound water cannot be completely removed.

[0033] In some embodiments, the iron content of the iron plate is greater than or equal to 96%, wherein the nickel, chromium, copper and zinc content is less than 30 ppm.

[0034] In this embodiment, the greater the iron content of the iron plate, the more conducive it is to improving the purity of the product ferrous phosphate and avoiding the mixing of impurities. If the iron content is too low, the purity of the obtained ferrous phosphate is low.

[0035] In some embodiments, the method further includes continuously replenishing phosphoric acid solution into the electrolyte.

[0036] In this embodiment, the continuous addition of phosphoric acid can not only replenish the phosphate and hydrogen ions consumed by the production of ferrous phosphate precipitation, but also replenish water, because water will be consumed during the electrolysis process, and part of the water will also evaporate, and the volatilization of hydrogen will also take away part of the water, so that the electrolysis reaction can proceed continuously. Furthermore, the continuous addition of phosphoric acid can maintain the stability of the electrolysis pH environment, so that the electrolysis reaction can proceed smoothly and stably.

[0037] In a second aspect, the present invention provides a ferrous phosphate prepared by the aforementioned preparation method, wherein the BET of the ferrous phosphate is 9.67-19.56 m 2 / g, D10 is 0.59-0.98μm, D50 is 2.88-3.89μm, D90 is 9.74-11.24μm, and the purity is greater than or equal to 98%.

[0038] In the technical solution of the embodiment of the present application, ferrous phosphate has the characteristics of uniform distribution, small particle size and high purity.

[0039] In a third aspect, an embodiment of the present application provides a lithium iron phosphate positive electrode material, which is prepared using the aforementioned ferrous phosphate as a raw material.

[0040] In this embodiment, the lithium iron phosphate positive electrode material is made from the above-mentioned ferrous phosphate, and thus has better electrochemical properties.

[0041] 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

[0042] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0043] Figure 1 This is a SEM image of the ferrous phosphate material in Example 1 of the present application;

[0044] Figure 2 This is the XRD diagram of the ferrous phosphate material in Example 1 of the present application. DETAILED DESCRIPTION

[0045] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field 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" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0047] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0048] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0049] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0050] 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).

[0051] 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, which 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, and therefore should not be understood as a limitation on the embodiments of the present application.

[0052] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0053] At present, the preparation method of ferrous phosphate is similar to that of ferric phosphate, that is, ferrous phosphate is obtained by liquid phase precipitation of ferrous salt and phosphate; however, this process is complicated, and it is necessary to prevent the oxidation of ferrous ions, and at the same time, it is also necessary to control crystallization and growth to avoid the formation of hydroxides. The existing preparation method leads to low purity of ferrous phosphate, excessively large and uneven particle size, which ultimately affects the electrochemical properties of lithium iron phosphate. At the same time, this method also produces a large amount of wastewater and has poor environmental protection.

[0054] In order to solve the technical problems of low purity, large particle size and low uniformity of ferrous phosphate, the present application provides a ferrous phosphate and a preparation method thereof and a lithium iron phosphate battery to improve the purity, particle size and uniformity of ferrous phosphate, thereby achieving an improvement in the electrochemical performance of lithium iron phosphate.

[0055] In the first aspect, the present invention provides a method for preparing ferrous phosphate, comprising the following steps: using an iron plate as an anode, an inert metal plate as a cathode, and a mixed solution of hydrogen phosphate, phosphoric acid, a buffer, a chelating agent, and an antioxidant as an electrolyte, performing an electrolytic reaction, filtering to obtain a solution containing Fe3(PO4)2 . 8H2O, washing, drying and calcining the precipitate in sequence to obtain ferrous phosphate; wherein the pH value of the electrolyte is controlled at 7.0-7.4.

[0056] In the technical solution of the embodiment of the present application, the process of preparing ferrous phosphate by electrolysis is as follows: the anode iron plate reacts with the hydrogen phosphate in the electrolyte to generate an electrolyte containing Fe3(PO4)2 . 8H2O precipitation, hydrogen and strong alkali, the strong alkali reacts with phosphoric acid to form hydrogen phosphate, the generated hydrogen phosphate can be recycled as an electrolyte component, and the obtained Fe3(PO4)2 . The precipitate of 8H2O is washed with water to remove impurities and then dehydrated to obtain ferrous phosphate (anhydrous ferrous phosphate).

[0057] Taking the hydrogen phosphate as disodium hydrogen phosphate as an example, the reaction process of the present application is as follows:

[0058] 6Fe+24H2O+4Na2HPO4----2Fe3(PO4)2.8H2O↓+8NaOH+3H2↑

[0059] 2NaOH+H3PO4---Na2HPO4+2H2O

[0060] Specifically, the advantages of using electrolyte electrolysis to prepare ferrous phosphate in the present application are: (1) The electrolysis method of the present application does not require the preparation of a ferrous salt solution in advance, but directly uses an iron plate to prepare ferrous phosphate, which is low in cost. The total cost per ton of product of this process is 10-15% lower than that of the traditional liquid phase co-precipitation process, and less wastewater is generated, the process is short, and in the electrolysis of the present application, impurities will not be electrolyzed and enter the electrolyte, so the obtained ferrous phosphate has high purity; (2) The electrolyte of the present application can be recycled and continuously electrolyzed, and the electrolyte is consistent and uniform, and the pH value of the electrolysis environment fluctuates little, so that ferrous phosphate can be generated continuously and stably. With the cooperation of the electrolyte of the present application and the pH value electrolysis environment, the obtained ferrous phosphate has high particle size uniformity, large BET, and a primary particle size. Small, high purity; (3) In the electrolyte of the present application, the chelating agent can improve the dispersibility of ferrous phosphate, reduce the agglomeration index, and make the particle size of the obtained ferrous phosphate small. The buffer is used to stabilize the pH value of the electrolysis environment and alleviate local changes in the electrolysis process, so that the preparation environment of ferrous phosphate is highly consistent and the uniformity of the obtained product is good. The role of the antioxidant is to improve the reducing environment and avoid the oxidation of ferrous iron. At the same time, the hydrogen produced also has the effect of reducing the oxidation of ferrous ions, so that the obtained ferrous phosphate has high purity. In addition to providing phosphate to enable the reaction to proceed smoothly, phosphoric acid and hydrogen phosphate also have the function of consuming the hydroxide produced by electrolysis, avoiding the generation of impurities and maintaining a stable pH value. At the same time, phosphoric acid also replenishes water for the electrolysis reaction to achieve continuous production of ferrous phosphate.

[0061] Specifically, the pH value of the electrolyte is controlled at 7.0, 7.1, 7.2, 7.3, 7.4 or any value in the range of 7.0 to 7.4.

[0062] In some embodiments, the hydrogen phosphate includes at least one of dipotassium hydrogen phosphate and disodium hydrogen phosphate.

[0063] In some embodiments, the buffer includes at least one of sodium citrate and potassium citrate.

[0064] In some embodiments, the complexing agent includes at least one of EDTA and EDTA sodium salt.

[0065] In some embodiments, the antioxidant includes at least one of ascorbic acid, hydrazine hydrate, and sodium sulfite.

[0066] In the embodiments of the present application, the selected hydrogen phosphate can generate sodium hydroxide or potassium hydroxide after electrolysis reaction with iron. The sodium ions and potassium ions can promote the electrolysis reaction. At the same time, they have good water solubility and will not introduce other impurities into the product. The selected buffer solution can stabilize the pH of the electrolysis at 7.0-7.4 without introducing other impurities. The selected chelating agent can improve the dispersibility of ferrous phosphate, reduce the agglomeration index, improve the uniformity of ferrous phosphate, and reduce its particle size.

[0067] In some embodiments, the concentration of hydrogen phosphate in the prepared electrolyte is 2-3 mol / L; for example, the concentration of hydrogen phosphate is 2 mol / L, 2.5 mol / L, 3 mol / L or any value between 2-3 mol / L.

[0068] In some embodiments, the concentration of phosphoric acid is 4-6 mol / L; for example, the concentration of phosphoric acid is 4 mol / L, 5 mol / L, 6 mol / L, or any value between 4-6 mol / L.

[0069] In some embodiments, the concentration of the buffer is 0.05-0.1 mol / L; for example, the concentration of the buffer is 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, or any value between 0.05-0.1 mol / L.

[0070] In some embodiments, the concentration of the complexing agent is 0.02-0.05 mol / L; for example, the concentration of the complexing agent is 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, 0.05 mol / L, or any value between 0.02-0.05 mol / L.

[0071] In some embodiments, the concentration of the antioxidant is 0.02-0.05 mol / L; the concentration of the antioxidant is 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, 0.05 mol / L or any value between 0.02-0.05 mol / L.

[0072] In the embodiments of the present application, if the concentration of the chelating agent or antioxidant is too low, the effect is not obvious, and if the concentration is too high, the raw materials are wasted. In addition, if the concentration of hydrogen phosphate, phosphate, or buffer is too high or too low, it will cause fluctuations in the pH value, making the electrolysis environment unstable and affecting the purity, particle size, and uniformity of ferrous phosphate.

[0073] In some embodiments, during the electrolysis step, the cell voltage is 0.15-0.35 V, for example, the cell voltage is 0.15 V, 0.2 V, 0.25 V, 0.3 V, 0.35 V or any value between 0.15-0.35 V, and the current density is 200-340 A / m 2, for example, the current density is 200A / m 2 , 250A / m 2 、300A / m 2 、340A / m 2 or 200-340A / m 2 The temperature is 35-45°C, for example, the temperature is 35°C, 40°C, 45°C or any value between 35-45°C, and the spacing between the cathode and the anode is 8-15cm, for example, the spacing between the cathode and the anode is 8cm, 10cm, 12 cm, 15 cm or any value between 8-15cm.

[0074] In this embodiment, within the limited range, the cell voltage can prevent impurities in the iron plate from being electrolytically precipitated, further improving the purity of ferrous phosphate. However, if the cell voltage exceeds the limited range, it will be unfavorable for improving the purity of ferrous phosphate. Under the current density limit of the present application, the primary particle size of ferrous phosphate is further reduced. If the current density exceeds the limited range of the present application, the primary particle size of ferrous phosphate will increase.

[0075] Further, in some embodiments, the step of washing, drying and calcining the precipitate in sequence to obtain ferrous phosphate comprises:

[0076] The precipitate is acid-washed until the phosphorus content in the acid solution after washing is less than 50ppm, and the solid-liquid is separated to obtain a first filter residue; the acid used includes but is not limited to citric acid, and the concentration of citric acid is 0.01-0.02mol / L. The washing water after solid-liquid separation can be returned to prepare the phosphoric acid solution; the first filter residue is dried at 100-150°C until the water content of the first filter residue is less than 1.5% to obtain a second filter residue; the second filter residue is calcined in an atmosphere containing an inert gas to obtain ferrous phosphate.

[0077] In this embodiment, the purpose of washing the precipitate is to remove impurities and avoid the subsequent introduction of impurities, while the purpose of drying and calcining is to remove Fe3(PO4)2 . 8H2O combined water, finally get ferrous phosphate, if the drying temperature is too low, it will not have the drying effect, if the drying temperature is too high, it will waste energy. The purpose of using inert atmosphere in the calcination process is to avoid oxidation of ferrous phosphate and reduce its purity.

[0078] Furthermore, in some embodiments, the step of calcining the second filter residue in an atmosphere containing an inert gas to obtain ferrous phosphate includes: calcining the second filter residue at 400-500°C for 3-6h at a heating rate of 1-3°C / h in an atmosphere containing an inert gas, and maintaining the oxygen content less than or equal to 100ppm during the calcination process to obtain a calcined material; cooling the calcined material to 100°C or below in an inert atmosphere, and then sieving it through 60-100 mesh to obtain ferrous phosphate.

[0079] In this embodiment, the temperature is slowly raised to remove bound water and the oxygen content is kept low to make Fe3(PO4)2 . The bound water of 8H2O is removed smoothly and the formation of impurities is avoided; if the calcination temperature is too high, energy is wasted, and if the calcination temperature is too low, the bound water is not removed completely.

[0080] In some embodiments, the iron content of the iron plate is greater than or equal to 96%, wherein the nickel, chromium, copper and zinc content is less than 30 ppm; suitably but not restrictively, the electrolysis of the present application adopts diaphragm electrolysis, that is, an electrolysis diaphragm bag is put on the anode sleeve.

[0081] In this embodiment, the greater the iron content of the iron plate, the more conducive it is to improving the purity of the product ferrous phosphate and avoiding the mixing of impurities. If the iron content is too low, the purity of the obtained ferrous phosphate is low.

[0082] In some embodiments, the method further includes continuously replenishing phosphoric acid solution into the electrolyte.

[0083] In this embodiment, the continuous addition of phosphoric acid can not only replenish the phosphate and hydrogen ions consumed by the production of ferrous phosphate precipitation, but also replenish water, because water will be consumed during the electrolysis process, and part of the water will also evaporate, and the volatilization of hydrogen will also take away part of the water, so that the electrolysis reaction can proceed continuously. Furthermore, the continuous addition of phosphoric acid can maintain the stability of the electrolysis pH environment, so that the electrolysis reaction can proceed smoothly and stably.

[0084] In some embodiments, the specific operation of the ferrous phosphate preparation method is as follows: in an electrolytic cell, an iron plate is an anode, a titanium plate is a cathode, an electrolyte is a mixture of hydrogen phosphate, phosphoric acid, a buffer, a chelating agent and an antioxidant, and the pH of the electrolyte is 7.0-7.4, and Fe3(PO4)2 is obtained by electrolysis. . 8H2O precipitate, the obtained Fe3(PO4)2 .8H2O precipitates to the bottom of the electrolytic cell, which is then pumped out and filtered. The filter residue is washed, dried and calcined in turn to obtain ferrous phosphate, i.e. anhydrous ferrous phosphate. The filtrate is returned to the electrolytic cell. At the same time, during the electrolysis process, 4-6 mol / L phosphoric acid solution is continuously added to maintain the pH of the electrolyte unchanged. The circulation rate of the filtrate is 1 / 15-1 / 8 of the volume of the electrolyte in the electrolytic cell. The process of electrolyte circulation is to extract Fe3(PO4)2 from the bottom of the electrolytic cell. . 8H2O is precipitated and then filtered, and the filtrate flows into the electrolytic cell from the upper part of the electrolytic cell; the filtration is performed by a centrifuge, a filter press or a ceramic membrane filter.

[0085] In this embodiment, the ferrous phosphate precipitate is extracted from the bottom and then filtered, and the filtrate is added to the electrolytic cell from the upper part of the electrolytic cell, thereby filtering out the ferrous phosphate precipitate and realizing the up-and-down circulation of the electrolyte.

[0086] In a second aspect, the present invention provides a ferrous phosphate prepared by the aforementioned preparation method, wherein the BET of the ferrous phosphate is 9.67-19.56 m 2 / g, D10 is 0.59-0.98μm, D50 is 2.88-3.89μm, D90 is 9.74-11.24μm, and the purity is greater than or equal to 98%.

[0087] In the technical solution of the embodiment of the present application, the ferrous phosphate is prepared by the aforementioned preparation method of ferrous phosphate, and has the characteristics of uniform distribution, small particle size, high purity, etc.

[0088] In a third aspect, an embodiment of the present application provides a lithium iron phosphate positive electrode material, which is prepared using the aforementioned ferrous phosphate as a raw material.

[0089] In this embodiment, the lithium iron phosphate positive electrode material is made from the above-mentioned ferrous phosphate, and thus has better electrochemical properties.

[0090] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used without specifying the manufacturer are all conventional products that can be obtained commercially.

[0091] 1. Preparation method

[0092] Example 1

[0093] A method for preparing ferrous phosphate comprises the following steps:

[0094] (1) In the electrolytic cell, an iron plate with an iron content of 97.3% and a nickel, chromium, copper and zinc content of 7.7 ppm was used as the anode, a titanium plate was used as the cathode, and a mixed solution of 2.5 mol / L disodium hydrogen phosphate, 5 mol / L phosphoric acid, 0.08 mol / L sodium citrate, 0.04 mol / L EDTA and 0.03 mol / L ascorbic acid was used as the electrolyte. The pH was maintained at 7.02, the cell voltage was 0.25 V, and the electrolysis current density was 280 A / m 2 The electrolyte temperature was 40°C, the distance between the cathode plate and the anode plate was 12 cm, and the electrolysis reaction was carried out under the condition that 5 mol / L phosphoric acid was continuously added. The obtained product precipitated to the bottom of the electrolytic cell, and then it was pumped out by a pump and filtered to obtain Fe3(PO4)2 . 8H2O precipitate, the obtained filtrate is returned from the upper part of the electrolytic cell to the electrolytic cell to continue the electrolytic reaction, and the circulation rate of the filtrate is 1 / 12 of the total volume of the electrolyte in the electrolytic cell;

[0095] (2) washing the precipitate with a 0.015 mol / L citric acid solution until the phosphorus content in the acid solution after washing is less than 50 ppm, separating the solid and the liquid to obtain a first filter residue, and returning the washing water to prepare the phosphoric acid solution; drying the first filter residue at 150° C. until the water content of the first filter residue is less than 1.5%, thereby obtaining a second filter residue;

[0096] (3) In an atmosphere containing an inert gas, the second filter residue is calcined at a final calcination temperature of 450°C at a heating rate of 2°C / h for 5 hours, and the oxygen content is maintained at 100 ppm or less during calcination to obtain a calcined material; the calcined material is cooled to 100°C in an inert atmosphere, and then sieved through 80 mesh, and after iron removal by an electromagnetic iron remover, vacuum packed to obtain ferrous phosphate. The SEM image of the ferrous phosphate material obtained in Example 1 is as follows: Figure 1 As shown; the XRD pattern of the ferrous phosphate material obtained in Example 1 is as shown Figure 2 shown.

[0097] Example 2

[0098] A method for preparing ferrous phosphate comprises the following steps:

[0099] (1) In the electrolytic cell, an iron plate with an iron content of 97.3% and a nickel, chromium, copper and zinc content of 7.7 ppm was used as the anode, a titanium plate was used as the cathode, and a mixed solution of 3 mol / L disodium hydrogen phosphate, 4 mol / L phosphoric acid, 0.05 mol / L sodium citrate, 0.04 mol / L disodium EDTA and 0.05 mol / L hydrazine hydrate was used as the electrolyte. The pH was maintained at 7.2, the cell voltage was 0.35 V, and the electrolysis current density was 200 A / m 2The electrolyte temperature was 35°C, the distance between the cathode plate and the anode plate was 8 cm, and the electrolysis reaction was carried out under the condition of continuous addition of 5 mol / L phosphoric acid. The obtained product precipitated to the bottom of the electrolytic cell, and then it was pumped out by a pump and filtered to obtain Fe3(PO4)2 . 8H2O precipitate, the obtained filtrate is returned from the upper part of the electrolytic cell to the electrolytic cell to continue the electrolytic reaction, and the circulation rate of the filtrate is 1 / 15 of the total volume of the electrolyte in the electrolytic cell;

[0100] (2) washing the precipitate with a 0.01 mol / L citric acid solution until the phosphorus content in the acid solution after washing is less than 50 ppm, separating the solid and the liquid to obtain a first filter residue, and returning the washing water to prepare the phosphoric acid solution; drying the first filter residue at 100° C. until the water content of the first filter residue is less than 1.5%, thereby obtaining a second filter residue;

[0101] (3) In an atmosphere containing an inert gas, calcining the second filter residue at a final calcination temperature of 400° C. at a heating rate of 1° C. / h for 6 h, and maintaining the oxygen content during calcination to be less than or equal to 100 ppm, to obtain a calcined material; cooling the calcined material to 100° C. in an inert atmosphere, and then sieving it through a 60-mesh sieve, removing iron through an electromagnetic iron remover, and vacuum packaging to obtain ferrous phosphate.

[0102] Example 3

[0103] A method for preparing ferrous phosphate comprises the following steps:

[0104] (1) In the electrolytic cell, an iron plate with an iron content of 98.1% and a nickel, chromium, copper and zinc content of 7.7 ppm was used as the anode, a titanium plate was used as the cathode, and a mixed solution of 3 mol / L disodium hydrogen phosphate, 6 mol / L phosphoric acid, 0.1 mol / L sodium citrate, 0.02 mol / L EDTA and 0.02 mol / L sodium sulfite was used as the electrolyte. The pH was maintained at 7.4, the cell voltage was 0.15 V, and the electrolysis current density was 200 A / m 2 The electrolyte temperature was 45°C, the distance between the cathode plate and the anode plate was 15 cm, and the electrolysis reaction was carried out under the condition that 4 mol / L phosphoric acid was continuously added. The obtained product precipitated to the bottom of the electrolytic cell, and then it was pumped out by a pump and filtered to obtain Fe3(PO4)2 . 8H2O precipitate, the obtained filtrate is returned from the upper part of the electrolytic cell to the electrolytic cell to continue the electrolytic reaction, and the circulation rate of the filtrate is 1 / 8 of the total volume of the electrolyte in the electrolytic cell;

[0105] (2) washing the precipitate with a 0.02 mol / L citric acid solution until the phosphorus content in the acid solution after washing is less than 50 ppm, separating the solid and the liquid to obtain a first filter residue, and returning the washing water to prepare the phosphoric acid solution; drying the first filter residue at 120° C. until the water content of the first filter residue is less than 1.5%, thereby obtaining a second filter residue;

[0106] (3) In an atmosphere containing an inert gas, calcining the second filter residue at a final calcination temperature of 500° C. at a heating rate of 3° C. / h for 6 h, and maintaining the oxygen content during calcination to be less than or equal to 100 ppm, to obtain a calcined material; cooling the calcined material to 100° C. in an inert atmosphere, and then sieving it through a 100-mesh sieve, removing iron through an electromagnetic iron remover, and vacuum packaging to obtain ferrous phosphate.

[0107] Comparative Example 1

[0108] A method for preparing ferrous phosphate, which is different from Example 2 only in that the pH value of electrolysis is maintained at 6.8.

[0109] Comparative Example 2

[0110] A method for preparing ferrous phosphate, which is different from Example 2 only in that the pH value of electrolysis is maintained at 7.6.

[0111] Furthermore, in the present application, the ferrous phosphate prepared in the embodiment and the comparative example is used as a raw material to prepare a lithium iron phosphate positive electrode material, and the preparation method includes: adding ferrous phosphate to phosphoric acid and lithium carbonate, and then adding glucose; the molar ratio of ferrous phosphate, phosphoric acid, lithium carbonate and glucose is 1:1.52:1.01:0.035, after adding pure water to slurry, grinding to a particle size of 405nm, and then spray drying and calcining, the calcination temperature is 760°C, and the calcination time is 10h to obtain a lithium iron phosphate positive electrode material.

[0112] 2. Test Method

[0113] 1. Performance testing of ferrous phosphate materials

[0114] The ferrous phosphate obtained in the embodiment and the comparative example was subjected to performance tests, wherein: the element content was determined according to the method specified in HG / T 4701-2021; the particle size (D10, D50 and D90) was determined according to GB / T 19077-2016; the specific surface area was determined according to the method specified in GB / T 19587; the tap density was determined according to the method specified in HG / T 4701-2021; the moisture content was determined according to the method specified in GB / T 6283; the trivalent iron ion was determined according to the method specified in GB / T33828-2017; the content of magnetic substances was determined according to the method specified in HG / T 4701-2021.

[0115] The results are shown in Table 1.

[0116] 2. Performance testing of lithium iron phosphate cathode materials

[0117] The electrochemical properties of the lithium iron phosphate cathode material obtained by using the ferrous phosphate prepared in the examples and comparative examples as raw materials were tested, wherein the method specified in GB / T 42161-2022 was followed. The results are shown in Table 2.

[0118] III. Analysis of test results of various embodiments and comparative examples

[0119] Depend on Figure 1 It can be seen that the primary particle size of the ferrous phosphate obtained in Example 1 is 300 nm, and it is relatively loose, the particles are spherical, and the dispersibility is good. Figure 2 It can be seen that the obtained ferrous phosphate has high crystallinity, indicating that the ferrous phosphate obtained in Example 1 has high purity.

[0120] Table 1 Performance test results of various ferrous phosphates

[0121]

[0122] As shown in Table 1, the BET of ferrous phosphate obtained by the scheme of the present application is 9.67-19.56m 2 / g, D10 is 0.59-0.98μm, D50 is 2.88-3.89μm, D90 is 9.74-11.24μm, the primary particle size is controlled at 187-205nm, and the purity is greater than or equal to 98%; while the electrolysis pH value of comparative example 1 is lower than the specified range of the present application, its BET is lower than that of embodiment 2, the primary particle size is increased, and the purity is reduced, and the electrolysis pH value of comparative example 2 is higher than the specified range of the present application, its BET is increased compared with embodiment 2, but the primary particle size is increased, and the purity is greatly reduced; it shows that only under the scheme of the present application can ferrous phosphate with large BET, small primary particle size and high purity be obtained.

[0123] Table 2 Performance test of various lithium iron phosphate

[0124]

[0125] It can be seen from Table 2 that the 0.1C first charge capacity of the lithium iron phosphate positive electrode material prepared by the ferrous phosphate of the present application is 161.2-161.8 mAh / g, the 0.1C first discharge efficiency is 99.1-99.4%, the 0.2C first discharge capacity is 159.8-160.9 mAh / g, the 0.5C discharge capacity is 155.3-158.5 mAh / g, and the 1C discharge capacity is 148.1-149.2 mAh / g, while the electrolysis pH values ​​in Comparative Examples 1 and 2 are respectively too low or too high, which causes a decrease in the 0.1C first charge capacity of the prepared lithium iron phosphate positive electrode material, a decrease in the 0.1C first discharge efficiency, a decrease in the 0.2C first discharge capacity, a decrease in the 0.5C discharge capacity, and a decrease in the 1C discharge capacity. Therefore, the lithium iron phosphate positive electrode material prepared from ferrous phosphate of the present application has better electrochemical performance, which is beneficial to the improvement of the electrochemical performance of the secondary battery prepared using the lithium iron phosphate positive electrode material.

[0126] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and playing the same role within the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing ferrous phosphate, characterized in that: The following steps are involved: An iron plate is used as an anode, an inert metal plate is used as a cathode, and a mixed solution of hydrogen phosphate, phosphoric acid, a buffer, a chelating agent, and an antioxidant is used as an electrolyte to perform an electrolytic reaction, and a precipitate containing Fe3(PO4)2·8H2O is obtained by filtering, and the precipitate is sequentially washed, dried, and calcined to obtain the ferrous phosphate; The step of washing, drying and calcining the precipitate in sequence to obtain the ferrous phosphate comprises: The precipitate is acid-washed until the phosphorus content in the acid solution after washing is less than 50 ppm, and the solid-liquid separation is performed to obtain a first filter residue; Drying the first filter residue at 100-150° C. until the water content of the first filter residue is less than 1.5%, thereby obtaining a second filter residue; calcining the second filter residue in an atmosphere containing an inert gas to obtain ferrous phosphate; The pH value of the electrolyte is controlled at 7.0-7.4, and the hydrogen phosphate includes at least one of dipotassium hydrogen phosphate and disodium hydrogen phosphate. The buffer comprises at least one of sodium citrate and potassium citrate, The complexing agent includes at least one of EDTA and EDTA sodium salt, The antioxidant includes at least one of ascorbic acid, hydrazine hydrate and sodium sulfite.

2. The method for preparing ferrous phosphate according to claim 1, wherein In the electrolyte, the concentration of hydrogen phosphate is 2-3 mol / L, and / or The concentration of phosphoric acid is 4-6 mol / L, and / or The concentration of the buffer is 0.05-0.1 mol / L, and / or The concentration of the complexing agent is 0.02-0.05 mol / L, and / or The concentration of the antioxidant is 0.02-0.05 mol / L.

3. The method for preparing ferrous phosphate according to claim 1, characterized in that: In the electrolysis step, the cell voltage is 0.15-0.35V and the current density is 200-340 A / m 2 , the temperature is 35-45°C, and the distance between the cathode and the anode is 8-15 cm.

4. The method for preparing ferrous phosphate according to claim 3, characterized in that: The step of calcining the second filter residue in an atmosphere containing an inert gas to obtain ferrous phosphate comprises: In an atmosphere containing an inert gas, calcining the second filter residue at 400-500° C. for 3-6 hours at a heating rate of 1-3° C. / h, and maintaining an oxygen content of less than or equal to 100 ppm during the calcination process to obtain a calcined material; Under an inert atmosphere, the calcined material is cooled to 100° C. or below, and then sieved through a 60-100 mesh screen to obtain the ferrous phosphate.

5. The method for preparing ferrous phosphate according to claim 1, characterized in that: The iron content of the iron plate is greater than or equal to 96%, wherein the content of nickel, chromium, copper and zinc is less than 30 ppm.

6. The method for preparing ferrous phosphate according to claim 1, characterized in that: The method also includes continuously replenishing phosphoric acid solution into the electrolyte.

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