Layered nano-sized iron phosphate and its preparation method and application

Abstract

The present invention relates to a method for preparing layered nano-scale iron phosphate, comprising the following steps: preparing a phosphoric acid solution; using battery-grade iron phosphate as a seed crystal and mixing it with the phosphoric acid solution for reaction, followed by filtering to obtain an iron-phosphorus solution; sequentially adding hydrogen peroxide and elemental iron to the iron-phosphorus solution, reacting the mixture, and filtering to remove insoluble matter to obtain an iron phosphate solution; adding a dispersant to the iron phosphate solution, followed by evaporation concentration and spray drying to obtain a granular material; adding the granular material to pure water for aging and washing, followed by filtering and collecting the filter residue, which is iron phosphate dihydrate; washing the phosphoric acid dihydrate, flash drying, and then roasting to obtain anhydrous iron phosphate; and pulverizing the anhydrous iron phosphate to obtain layered nano-scale iron phosphate. The present invention also relates to layered nano-scale iron phosphate prepared using the above-mentioned preparation method. The present invention also relates to applications of the layered nano-scale iron phosphate.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery positive electrode material preparation, in particular to a layered nanometer-scale iron phosphate and a preparation method thereof, and applications of the layered nanometer-scale iron phosphate. Background Art

[0002] This section provides background information related to the present application which does not necessarily constitute prior art.

[0003] Iron phosphate is the raw material for synthesizing lithium iron phosphate, and its structure and morphology have a great influence on the electrochemical performance of lithium iron phosphate. Therefore, in order to prepare lithium iron phosphate with excellent performance, it is necessary to first prepare high-performance iron phosphate. Common iron phosphate has two morphologies: layered and spherical. Lithium iron phosphate prepared from iron phosphate with a layered structure is still layered. Lithium iron phosphate synthesized from iron phosphate with a layered structure can shorten the transmission distance of lithium ions during the charge and discharge process of lithium iron phosphate, thereby improving the electrochemical performance of lithium iron phosphate batteries. Lithium iron phosphate synthesized from iron phosphate with a layered structure has better comprehensive performance (including compaction density, rate performance, etc.) than lithium iron phosphate synthesized from iron phosphate with a spherical structure.

[0004] Currently, the main methods for preparing ferric phosphate include hydrothermal method, homogeneous precipitation method, sol-gel method, air oxidation method, and controlled crystallization method. Based on its size, ferric phosphate can be divided into nano-ferric phosphate, microporous ferric phosphate, and mesoporous ferric phosphate. Existing methods for preparing ferric phosphate have the following technical problems:

[0005] 1. During the preparation of iron phosphate, the iron phosphate prepared using iron salts as the iron source usually has a spherical structure, and no gas is generated during the production process; while the iron phosphate prepared using elemental iron as the iron source has a layered structure, but elemental iron usually releases hydrogen during the reaction, posing a greater safety risk.

[0006] 2. The ferric phosphate prepared by the existing preparation method is severely agglomerated, with a span (span = (D90-D10) / D50) of not less than 1.5 and a primary particle size of not less than 200 nm;

[0007] 3. The existing preparation method has many side reactions during the preparation process. The obtained ferric phosphate not only has high impurities and low product purity, but also has relatively large particles, small specific surface area and poor compaction performance. Summary of the Invention

[0008] In response to the technical problems existing in the prior art, the present invention provides a method for preparing iron phosphate. This preparation method uses elemental iron as the iron source, and the iron phosphate prepared has a layered structure and is ultrafine particles of nanometer size. At the same time, this preparation method can effectively solve the problem of elemental iron generating hydrogen during dissolution, improve safety, and has a short process flow and simple equipment, which can be industrialized. The iron phosphate prepared by this preparation method has good dispersibility, uniform particle size, a span of 0.8-1.3, and a primary particle size of 50-150nm. The iron phosphate can be used to make high-performance lithium iron phosphate positive electrode materials.

[0009] To achieve the above object, the present invention adopts the following technical solution: a method for preparing layered nano-sized iron phosphate, comprising some or all of the following steps:

[0010] Prepare phosphoric acid solution;

[0011] Using battery-grade iron phosphate as seed crystals and mixing with the phosphoric acid solution to react, and then filtering to obtain an iron-phosphorus solution;

[0012] adding hydrogen peroxide and elemental iron to the iron-phosphorus solution in sequence, mixing and reacting, filtering and removing insoluble matter to obtain an iron phosphate solution;

[0013] adding a dispersant to the ferric phosphate solution, and then sequentially performing evaporation concentration and spray drying to obtain a granular material;

[0014] adding the granular material into pure water for aging and washing, and then filtering and collecting the filter residue, wherein the filter residue is ferric phosphate dihydrate;

[0015] The dihydrated phosphoric acid is washed, flash-dried, and then calcined to obtain anhydrous ferric phosphate;

[0016] The anhydrous ferric phosphate is pulverized to obtain layered nano-sized ferric phosphate.

[0017] In one or more embodiments, a method for preparing layered nano-sized iron phosphate includes the following steps:

[0018] Step 1: preparing a phosphoric acid solution, wherein the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 60 wt%-85 wt%, for example, the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt% or 85 wt%;

[0019] Step 2: taking homemade battery-grade iron phosphate as a seed crystal and mixing it with the phosphoric acid solution obtained in step 1 for reaction, and the mass ratio of the phosphoric acid solution to the battery-grade iron phosphate is 3-5:1, the temperature of the mixed reaction is set to 60-90°C, for example, the temperature of the mixed reaction is set to 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, the reaction time is 2-5h, for example, the reaction time is 2h, 2.5h, 3h, 3.5h, 4h, 4.5h or 5h, and then filtering and collecting the filtrate to obtain an iron-phosphorus solution, wherein the iron-phosphorus solution is rich in trivalent iron ions (Fe 3+ ), phosphate ions (PO4 3- );

[0020] Step 3: adding hydrogen peroxide to the iron-phosphorus solution obtained in step 2, wherein the mass concentration of hydrogen peroxide is 20%-30%, and the molar number of hydrogen peroxide added is the molar number of Fe in the iron-phosphorus solution. 3+ 4 / 3-2 times the molar number of ; then adding elemental iron, the elemental iron is at least one of iron powder, iron rod, and iron particles, and reacting at room temperature (the temperature range of the room temperature is 15-45°C, such as 15°C, 20°C, 25°C, 30°C, 35°C, 40°C or 45°C) for not less than 1 hour. The chemical reactions involved are as follows:

[0021] 2Fe 3+ +Fe=3Fe 2+

[0022] 2Fe 2+ +H2O2=2Fe 3+ +2OH -

[0023] The above two chemical reactions are continuously cycled until all the Fe elements in the solution are at +3 valence, thereby obtaining a mixed solution containing ferric phosphate. The mixed solution is filtered to remove insoluble matter and the filtrate is collected to obtain a ferric phosphate solution.

[0024] Step 4: The pH of the ferric phosphate solution obtained in step 3 is 0.5-1.5, for example, the pH of the ferric phosphate solution is 0.5, 0.8, 1.0, 1.2 or 1.5, and a dispersant is added to the ferric phosphate solution, wherein the mass of the added dispersant is 0.5%-3% of the mass of the ferric phosphate solution, for example, the mass of the added dispersant is 0.5%, 1.0%, 1.5%, 2.0%, 2.5% or 3.0% of the mass of the ferric phosphate solution, and the dispersant is at least one of polyethylene glycol, polyvinyl alcohol, methyl cellulose and hydroxypropyl methyl cellulose; and then the ferric phosphate solution is evaporated and concentrated until Fe in the ferric phosphate solution is 0.5%. 3+The concentration is 1.5-3 mol / L, and heating is stopped to obtain a concentrated solution; the concentrated solution is spray-dried with an air inlet temperature of 250-280°C and an air outlet temperature of 100-110°C, and the moisture content of the obtained granular material is 0.2wt%-0.5wt%;

[0025] Step five, the granular material obtained in step four is added to pure water according to a liquid-solid ratio of 4-10: 1 for aging and washing, the aging temperature is 60-90 ° C, for example, the aging temperature is 60 ° C, 65 ° C, 70 ° C, 75 ° C, 80 ° C, 85 ° C or 90 ° C, and the aging time is 2-4 h, for example, the aging time is 2 h, 2.5 h, 3 h, 3.5 h or 4 h, and washed after aging. The stirring speed during the washing process is 200-400 r / min. After washing is completed, filter press is performed, and the conductivity of the washing liquid is monitored (the conductivity of the washing liquid needs to meet not more than 800 μs / cm). If the conductivity of the washing liquid is not up to standard, pure water is continued to be added for washing again until the conductivity of the washing liquid is up to standard, and the resulting filter residue is ferric phosphate dihydrate;

[0026] Step six, flash drying the dihydrated ferric phosphate obtained in step five, the drying temperature is 110-200 ° C, and the drying time is 0.5-3 hours; then the dried ferric phosphate is calcined in an air atmosphere, the calcination temperature is 500-750 ° C, for example, the calcination temperature is 500 ° C, 550 ° C, 600 ° C, 650 ° C, 700 ° C or 750 ° C, and the calcination time is 2-4 hours, for example, the calcination time is 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, to obtain anhydrous ferric phosphate;

[0027] Step seven, the anhydrous ferric phosphate obtained in step six is ​​mechanically or air flow crushed, and the D50 of the obtained ferric phosphate agglomerates is 2-5 μm, and the crushed ferric phosphate needs to be retained as a seed crystal for the next batch of reaction to continue to participate in the reaction.

[0028] Determine the primary particle size of the prepared iron phosphate material, is an important characterization method for predicting and evaluating the rate performance and low temperature performance of the lithium iron phosphate synthesized by the iron phosphate, and is also a key test item for the physical and chemical characteristics of the iron phosphate material. The present invention adopts scanning electron microscope (SEM) to test the primary particle size of the prepared iron phosphate material, and adopts laser particle size analyzer to test the agglomerate particle size of the prepared iron phosphate material. The primary particle size of the iron phosphate material prepared by the preparation method provided by the present invention is 50-150nm, and the surface energy of the powder is relatively high, so the primary particles of the iron phosphate material will spontaneously agglomerate (soft agglomerate) to form soft agglomerate secondary particles with lower system surface energy, and in addition, in the roasting process of step six, some of the primary particles of the iron phosphate will appear locally melted together (hard agglomerate) to form hard agglomerate secondary particles, which can not be completely deagglomerated by the pulverization process of step seven (even if deagglomeration occurs, the nano single particles of the obtained iron phosphate will also form soft agglomerate secondary particles). The prepared ferric phosphate agglomerates include soft and hard secondary particles. Therefore, the laser particle size analyzer measures the particle size of the secondary particles (i.e., agglomerates) of the prepared ferric phosphate material. The D50 of the ferric phosphate agglomerates is 2-5 μm. Using a scanning electron microscope (SEM), the prepared primary particles of the ferric phosphate material were observed under multiple fields of view and the individual particle sizes were measured. The average of the obtained primary particle sizes was calculated to determine the primary particle size of the ferric phosphate material to be 50-150 nm.

[0029] A layered nano-scale iron phosphate is prepared by the above preparation method.

[0030] An application of layered nano-scale iron phosphate, wherein the layered nano-scale iron phosphate is used to prepare lithium iron phosphate batteries.

[0031] In one or more embodiments, the layered nano-sized iron phosphate is prepared into lithium iron phosphate by the following steps:

[0032] Step 1: Weighing layered nano-sized iron phosphate as an iron source, adding a lithium source, preferably lithium carbonate, at a molar ratio of Li / Fe = 0.95-1.05, and then adding a carbon source to obtain a raw material mixture; wherein the carbon source is a carbohydrate chemical substance, and the amount of the added carbon source is 1-20wt% of the total amount;

[0033] Step 2: The raw material mixture prepared in step 1 is first mixed in a mixing tank at a stirring speed of not less than 150 r / min and a mixing time of not less than 20 min, and then ground on three sand mills connected in series to control the particle size to 0.1-2.5 μm; the mixture after ball milling is filtered through an 800-mesh filter and spray-dried to obtain a precursor;

[0034] Step three, pre-sintering the precursor prepared in step two in a vacuum sintering furnace at a temperature of 150-300 ° C for 2-4 hours, and then sintering it at a high temperature in a vacuum sintering furnace protected by nitrogen, with the sintering temperature controlled at 600-1000 ° C for 8-15 hours to allow the coated organic carbon to fully decompose and graphitize. Then, cool it to below 500 ° C, remove it from the furnace, crush it, and package it to obtain the finished lithium iron phosphate composite material.

[0035] In one or more embodiments, the lithium iron phosphate (LiFePO4) prepared using the layered nano-sized iron phosphate has a particle size of 0.3-30 μm, a high tap density, and a compression molded body density of 2.0 g / cm 3 The filling capacity is improved, and the energy density per unit volume can be increased. The BET specific surface area is 10-35m 2 / g, and Fe 3+ The phase content is low, the impurities are few, the amount of impurity crystal phase Li3PO4 is less than 5wt%, the crystallite size is 30-300nm, the agglomerated particle size is 1-20μm, and the resistivity of lithium iron phosphate is 1-1.0×10 5 Ω·cm, and the charge-discharge cycle characteristics are improved.

[0036] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects:

[0037] 1. The present invention provides a method for preparing layered nano-sized iron phosphate. This method solves the problems of gas production, safety hazards, slow or incomplete reaction, and low production capacity associated with using elemental iron to prepare layered iron phosphate. This method achieves ultrafast reaction at room temperature, reduces energy consumption, and increases production capacity. It also reduces safety risks compared to existing technical solutions and offers technical advantages.

[0038] 2. The present invention provides a method for preparing layered nano-sized iron phosphate, which uses battery-grade iron phosphate with low impurity content as a seed crystal, dissolves the seed crystal in a phosphoric acid solution to prepare a layered nano-sized iron phosphate. 3+ and PO4 3- A solution of hydrogen peroxide and elemental iron can be added to the solution to react at room temperature, solve the gas production problem and increase the reaction rate;

[0039] 3. The present invention provides a method for preparing layered nano-scale iron phosphate. The preparation method adopts a phosphorus source and elemental iron to prepare layered nano-scale iron phosphate. Compared with the traditional method of preparing iron phosphate, the impurity content in iron phosphate is effectively reduced, the purity of the product is improved, and the particle morphology of the prepared iron phosphate is small, the specific surface area is large, and it is easy to compact. When it is used in lithium-ion batteries, the specific capacity of the battery is also effectively improved, and the electrochemical performance of the lithium-ion battery is greatly improved.

[0040] 4. The present invention provides a method for preparing layered nano-scale iron phosphate. The layered nano-scale iron phosphate prepared by this preparation method has the characteristics of small particles, large specific surface area, and high compaction density, and ensures its electrochemical performance in lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings further illustrate the present invention, but the embodiments in the accompanying drawings do not constitute any limitation to the present invention.

[0042] Figure 1 This is a schematic flow chart of the method for preparing layered nano-sized iron phosphate provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0043] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0044] The present invention provides a method for preparing layered nano-sized iron phosphate. In order to better understand the technical solution of the present application, the preparation method of the layered nano-sized iron phosphate of the present application is further explained below.

[0045] [Example 1]

[0046] See also Figure 1 The preparation method of the layered nano-sized iron phosphate provided in this embodiment includes the following steps:

[0047] S100, preparing a phosphoric acid solution, wherein the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 60 wt %. It is understood that the phosphoric acid solution can be regarded as a phosphorus source for synthesizing iron phosphate, and the pH of the phosphoric acid solution can be adjusted by controlling the mass percentage concentration of phosphoric acid;

[0048] S200, take homemade battery-grade iron phosphate as a seed crystal and mix it with the phosphoric acid solution obtained in step 1, and the mass ratio of the phosphoric acid solution to the battery-grade iron phosphate is 3:1. The temperature of the mixed reaction is set to 60°C, the reaction time is 2 hours, and then the filtrate is filtered and collected to obtain an iron-phosphorus solution. It can be understood that battery-grade iron phosphate can be regarded as an iron source. Mixing the iron source and the phosphorus source in advance can better control the Fe content in the iron-phosphorus solution. 3+ and PO4 3- The content range of iron phosphate is to ensure the stable generation of subsequent iron phosphate;

[0049] S300, hydrogen peroxide is added to the iron-phosphorus solution, the mass concentration of hydrogen peroxide is 20%, and the molar number of hydrogen peroxide added is the Fe 3+4 / 3 times the molar number of ; then add elemental iron, elemental iron is iron powder, and react at room temperature for 1 hour to obtain a mixed solution containing iron phosphate, the mixed solution is filtered to remove insoluble matter and the filtrate is collected to obtain an iron phosphate solution. It can be understood that hydrogen peroxide can be regarded as an oxidant and elemental iron can be regarded as an iron source;

[0050] S400, by controlling the mass percentage concentration of phosphoric acid in the phosphoric acid solution so that the pH of the ferric phosphate solution is 1.5, the effective generation of ferric phosphate is better ensured, and a dispersant is added to the ferric phosphate solution, the mass of the added dispersant is 0.5% of the mass of the ferric phosphate solution, and the dispersant is polyethylene glycol; and then the ferric phosphate solution is evaporated and concentrated until the Fe in the ferric phosphate solution is 1.5. 3+ The concentration was 1.5 mol / L, and the heating was stopped to obtain a concentrated solution. The concentrated solution was spray-dried with an air inlet temperature of 250°C and an air outlet temperature of 100°C. The moisture content of the obtained granular material was 0.5 wt%. It can be understood that the iron phosphate in the iron phosphate solution after the addition of the dispersant was not fully transformed into iron phosphate crystals, that is, the iron phosphate was not fully transformed into iron phosphate crystals, resulting in a low iron phosphate content in the iron phosphate solution. Therefore, the iron phosphate solution after the addition of the dispersant was concentrated and dried to obtain a solid solution. By controlling the Fe content in the iron phosphate solution during the evaporation and concentration process, the iron phosphate content in the iron phosphate solution was reduced. 3+ The concentration increases the content of iron phosphate crystals in the concentrated solution, ensures the initial formation of iron phosphate crystals in the concentrated solution, and further promotes the effect of crystal nucleation induction, thereby accelerating the formation rate of iron phosphate crystals during the spray drying operation;

[0051] S500, add the granular material to pure water at a liquid-solid ratio of 4:1 for aging and washing, the aging temperature is 60 ° C, the aging time is 2h, and the aging is carried out after washing. The stirring speed during the washing process is 200r / min. After washing, it is filtered through a filter press and the conductivity of the washing liquid is monitored (the conductivity of the washing liquid needs to be no more than 800μs / cm). If the conductivity of the washing liquid is not up to standard, continue to add pure water and wash again until the conductivity of the washing liquid meets the standard. The resulting filter residue is ferric phosphate dihydrate. It can be understood that by using pure water to age and wash the granular material, the impurity content of the prepared ferric phosphate dihydrate can be effectively reduced and the sufficient crystallization of the ferric phosphate can be effectively ensured. The ferric phosphate dihydrate is ferric phosphate containing crystal water and free water;

[0052] S600, flash drying the dihydrated ferric phosphate at a drying temperature of 110°C for 0.5 hours; then calcining the dried ferric phosphate in an air atmosphere at a temperature of 500°C for 4 hours to obtain anhydrous ferric phosphate. It can be understood that flash drying combined with calcination can effectively ensure the complete removal of crystalline water and free water from the ferric phosphate crystals;

[0053] S700, mechanically crushing the anhydrous ferric phosphate obtained in step 6, and obtaining ferric phosphate agglomerates with a D50 of 2 μm. The crushed ferric phosphate needs to be retained as a seed crystal for the next batch of reactions to continue participating in the reaction of step S200.

[0054] [Example 2]

[0055] The preparation method of layered nano-sized iron phosphate provided in this embodiment comprises the following steps:

[0056] S100, preparing a phosphoric acid solution, wherein the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 85 wt%;

[0057] S200, taking homemade battery-grade iron phosphate as a seed crystal and mixing it with the phosphoric acid solution obtained in step 1, wherein the mass ratio of the phosphoric acid solution to the battery-grade iron phosphate is 5:1, the temperature of the mixed reaction is set to 90° C., the reaction time is 5 hours, and then filtering and collecting the filtrate to obtain an iron-phosphorus solution;

[0058] S300, hydrogen peroxide is added to the iron-phosphorus solution, the mass concentration of hydrogen peroxide is 30%, and the molar number of hydrogen peroxide added is the Fe 3+ Then, elemental iron is added, and the elemental iron is an iron rod, and the reaction is carried out at room temperature for 2 hours to obtain a mixed solution containing iron phosphate, and the mixed solution is filtered to remove insoluble matter and the filtrate is collected to obtain an iron phosphate solution;

[0059] S400, by controlling the mass percentage concentration of phosphoric acid in the phosphoric acid solution so that the pH of the ferric phosphate solution is 0.5, adding a dispersant to the ferric phosphate solution, the mass of the added dispersant is 3% of the mass of the ferric phosphate solution, and the dispersant is polyvinyl alcohol; then evaporating and concentrating the ferric phosphate solution until the Fe content in the ferric phosphate solution is less than 0.5. 3+ The concentration was 3 mol / L, and heating was stopped to obtain a concentrated solution; the concentrated solution was spray-dried with an air inlet temperature of 280°C and an air outlet temperature of 110°C, and the moisture content of the obtained granular material was 0.2 wt%;

[0060] S500, adding the granular material to pure water at a liquid-to-solid ratio of 10:1 for aging and washing, the aging temperature being 90° C. and the aging time being 4 hours, and washing after aging, with the stirring speed during the washing process being 200-400 r / min. After washing, filtering is performed through a filter press, and the conductivity of the washing liquid is monitored (the conductivity of the washing liquid needs to be no greater than 800 μs / cm). If the conductivity of the washing liquid does not meet the standard, pure water is continued to be added for further washing until the conductivity of the washing liquid meets the standard. The resulting filter residue is ferric phosphate dihydrate;

[0061] S600, flash drying the dihydrated ferric phosphate at a drying temperature of 200° C. for 3 hours; then calcining the dried ferric phosphate in an air atmosphere at a calcination temperature of 750° C. for 2 hours to obtain anhydrous ferric phosphate;

[0062] S700, the anhydrous ferric phosphate obtained in step 6 is subjected to air flow pulverization, and the D50 of the obtained ferric phosphate agglomerates is 5 μm. The pulverized ferric phosphate needs to be retained as a seed crystal for the next batch of reaction to continue to participate in the reaction of step S200.

[0063] [Example 3]

[0064] The preparation method of layered nano-sized iron phosphate provided in this embodiment comprises the following steps:

[0065] S100, preparing a phosphoric acid solution, wherein the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 80 wt%;

[0066] S200, taking homemade battery-grade iron phosphate as a seed crystal and mixing it with the phosphoric acid solution obtained in step 1, wherein the mass ratio of the phosphoric acid solution to the battery-grade iron phosphate is 4:1, the temperature of the mixed reaction is set to 80° C., the reaction time is 4 hours, and then filtering and collecting the filtrate to obtain an iron-phosphorus solution;

[0067] S300, hydrogen peroxide is added to the iron-phosphorus solution, the mass concentration of hydrogen peroxide is 25%, and the molar number of hydrogen peroxide added is the Fe 3+ 1.5 times the molar number of ; and then adding elemental iron, the elemental iron is iron particles, and reacting at room temperature for 1 hour to obtain a mixed solution containing iron phosphate, filtering the mixed solution to remove insoluble matter and collecting the filtrate to obtain an iron phosphate solution;

[0068] S400, by controlling the mass percentage concentration of phosphoric acid in the phosphoric acid solution so that the pH of the ferric phosphate solution is 1.2, adding a dispersant to the ferric phosphate solution, the mass of the added dispersant is 2% of the mass of the ferric phosphate solution, and the dispersant is prepared by mixing methyl cellulose and hydroxypropyl methyl cellulose in a mass ratio of 1:1; then evaporating and concentrating the ferric phosphate solution until the Fe in the ferric phosphate solution is 0. 3+ The concentration was 2 mol / L, and heating was stopped to obtain a concentrated solution; the concentrated solution was spray-dried with an inlet air temperature of 270°C and an outlet air temperature of 105°C, and the moisture content of the obtained granular material was 0.4 wt%;

[0069] S500, adding the granular material to pure water at a liquid-to-solid ratio of 7:1 for aging and washing, the aging temperature being 80° C. and the aging time being 3 h, and washing after aging, with a stirring speed of 300 r / min during the washing process. After washing, filtering is performed through a filter press, and the conductivity of the washing liquid is monitored (the conductivity of the washing liquid needs to meet no more than 800 μs / cm). If the conductivity of the washing liquid does not meet the standard, pure water is continued to be added for further washing until the conductivity of the washing liquid meets the standard. The resulting filter residue is ferric phosphate dihydrate;

[0070] S600, flash drying the dihydrated ferric phosphate at a drying temperature of 180° C. for 2 hours; then calcining the dried ferric phosphate in an air atmosphere at a calcination temperature of 700° C. for 3 hours to obtain anhydrous ferric phosphate;

[0071] S700, the anhydrous ferric phosphate obtained in step 6 is subjected to air flow pulverization, and the D50 of the obtained ferric phosphate agglomerates is 4 μm. The pulverized ferric phosphate needs to be retained as a seed crystal for the next batch of reaction to continue to participate in the reaction of step S200.

[0072] [Preparation of lithium iron phosphate]

[0073] The lithium iron phosphate positive electrode material was prepared by using the iron phosphate prepared by the preparation methods provided in Examples 1-3, respectively. The specific steps are as follows:

[0074] S100, preparing 150 g of iron phosphate and lithium hydroxide in a molar ratio of Li / Fe = 1.03, adding 3 kg of butanol and deionized water to fully disperse for 6 hours, then ball milling for 8 hours, and spray drying to obtain a granular material;

[0075] S200, placing the granular material into a granulator for secondary granulation, and then sintering it in an inert atmosphere sintering furnace at 750° C. for 9 hours to obtain an initial lithium iron phosphate positive electrode material;

[0076] S300, crushing the initial lithium iron phosphate positive electrode material to obtain a lithium iron phosphate positive electrode material.

[0077] Table 1 shows the electrochemical performance test of lithium iron phosphate positive electrode materials synthesized by the iron phosphate prepared by the preparation methods provided in Examples 1-3.

[0078] Table 1

[0079]

[0080] The results in Table 1 indicate that the iron phosphate prepared by the preparation method provided by the present invention meets the standards of battery-grade anhydrous iron phosphate for lithium iron phosphate, and when used to prepare lithium iron phosphate batteries, can better ensure the initial charge and discharge capacity and discharge efficiency.

[0081] [Comparative Example 1]

[0082] This comparative example provides a method for preparing ferric phosphate, which comprises the following steps:

[0083] S100, weighing the required ferrous sulfate heptahydrate and phosphoric acid at a molar ratio of Fe to P of 3:1.5, and preparing them into solutions;

[0084] S200, placing a ferrous sulfate solution in a reactor, adjusting the pH value of the ferrous sulfate solution to 8 with a sodium hydroxide solution, then adding an ammonium phosphate solution and sodium lignin sulfonate to the ferrous sulfate solution for mixing, and controlling the weight ratio of ferrous sulfate, phosphoric acid, ammonium phosphate and sodium lignin sulfonate to be 70:15:1 / 200:3 / 1000;

[0085] S300, adding the prepared phosphoric acid dropwise to the ferrous sulfate solution while stirring, controlling the addition rate to be 1 L / h and the stirring rate to be 200 r / min, to obtain ferrous phosphate slurry after the reaction;

[0086] S400, then naturally cooling the ferrous phosphate slurry to room temperature, and then washing with water, adding phenethyl ether during the washing process, and then filtering and drying to obtain a ferrous phosphate product;

[0087] S500, roasting the ferrous phosphate product, controlling the roasting temperature to be 200° C. and the roasting time to be 8 hours, and finally obtaining a battery-grade ferric phosphate product.

[0088] [Comparative Example 2]

[0089] This comparative example provides a method for preparing ferric phosphate, which comprises the following steps:

[0090] S100, preparing an acid solution containing 30 wt% of an inorganic mixed acid and 1 wt% of an organic acid, wherein the inorganic mixed acid is a mixture of phosphoric acid and sulfuric acid, more specifically, a mixture of 85 wt% phosphoric acid and 98 wt% concentrated sulfuric acid in a mass ratio of 10:1, and the organic acid is phytic acid;

[0091] S200, placing elemental iron (elemental iron is iron particles) into the acid solution, heating to 95° C., and fully reacting for 20 hours. When the pH is 3, filtering, demagnetizing, and removing impurities to obtain a pure ferrous phosphate solution, and adjusting the concentration of ferrous phosphate in the ferrous phosphate solution to 170 g / L using deionized water;

[0092] S300, 500L of ferrous phosphate solution and 50kg of hydrogen peroxide (mass concentration of hydrogen peroxide is 25%) are fully mixed and reacted, followed by filter pressing, demagnetization, washing, and impurity removal, and then a phosphoric acid solution (the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 60wt%) is added to form a slurry and the pH of the slurry is adjusted to less than 3. After being fully dispersed, the slurry is heated to 95°C and the reaction time is 5h. Soluble impurities are removed by a rinsing process, and a filter cake is formed using a compressor. Finally, the filter cake is spray-dried to obtain layered ferric phosphate.

[0093] [Comparative Example 3]

[0094] This comparative example provides a method for preparing ferric phosphate, which comprises the following steps:

[0095] S100, weigh 834g of ferrous sulfate and dissolve it in 1.5L of water, add 58.8g of sulfuric acid and 270g of hydrogen peroxide, after the ferrous sulfate is completely oxidized to ferric sulfate, dilute to 2L to obtain a ferric sulfate solution;

[0096] S200, weighing 36.24 g of sodium aluminate and dissolving it in 500 ml of water to obtain a sodium aluminate solution;

[0097] S300, weighing 345.09 g of ammonium dihydrogen phosphate and dissolving it in water, adding the sodium metaaluminate solution to the ammonium dihydrogen phosphate solution, and adding water to make the volume to 2 L to obtain a mixed solution;

[0098] S400, adding ferric sulfate solution into the reactor as the reaction base liquid, setting the stirring speed to 200 r / min, and pumping the mixed solution into the reactor with a peristaltic pump, and the feeding time is 30 min;

[0099] S500, add 51.88g of phosphoric acid to the reactor, stir evenly and then start heating, set the insulation temperature to 92°C, and keep it warm for 3 hours to obtain iron phosphate slurry;

[0100] S600, washing the iron phosphate slurry until the conductivity of the washed filtrate is no more than 500 μS / cm, then placing it in an oven at 105° C. for 12 hours, and naturally cooling it to obtain layered iron phosphate.

[0101] [Comparative Example 4]

[0102] This comparative example provides a commercially available iron phosphate.

[0103] Table 2 shows the test results of various physical and chemical indicators of the iron phosphate prepared by the preparation methods provided in Examples 1-3, the iron phosphate prepared by the preparation methods provided in Comparative Examples 1-3, and the iron phosphate provided in Comparative Example 4.

[0104] Table 2

[0105]

[0106] The preparation method of iron phosphate provided in Comparative Example 1 is to add a soluble phosphorus source solution by stirring while adding dropwise to generate amorphous ferrous phosphate. The preparation method of iron phosphate provided in Comparative Example 2 is to first prepare ferrous hydroxide with uniform particles by a homogeneous precipitation method, and then add phosphoric acid and hydrogen peroxide to achieve oxidation and precipitation conversion to obtain layered iron phosphate. The preparation method of iron phosphate provided in Comparative Example 3 is that metaaluminate forms Al (OH) 3 colloid during the synthesis of iron phosphate, and Al (OH) 3 colloid selectively adsorbs the initial crystal nucleus formed by iron phosphate, and the hydroxyl group of Al (OH) 3 colloid adsorbs the specific crystal face of the crystal nucleus, thereby reducing the surface energy of the crystal face, thereby causing the growth rate of the crystal face to decrease, while other crystal faces grow at a normal rate to form a two-dimensional sheet of iron phosphate. As can be seen from Table 2, compared to the iron phosphate prepared by the preparation method provided in Comparative Examples 1-3 and the iron phosphate provided in Comparative Example 4, the iron phosphate prepared by the preparation method provided in Example 1-3 has a larger specific surface area, a more concentrated particle size distribution, a smaller span, and better consistency.

[0107] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0108] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for preparing layered nano-sized iron phosphate, characterized in that: The steps include: Step 1, preparing a phosphoric acid solution; Step 2: using battery-grade iron phosphate as seed crystals and mixing with the phosphoric acid solution to react, and then filtering to obtain an iron-phosphorus solution; Step 3, adding hydrogen peroxide and elemental iron to the iron-phosphorus solution in sequence, mixing and reacting, filtering and removing insoluble matter to obtain an iron phosphate solution; Step 4, adding a dispersant to the ferric phosphate solution, and then sequentially performing evaporation concentration and spray drying to obtain a granular material; Step 5: adding the granular material to pure water for aging and washing, and then filtering and collecting the filter residue, wherein the filter residue is ferric phosphate dihydrate; Step 6: washing the dihydrated phosphoric acid, flash drying it, and then roasting it to obtain anhydrous ferric phosphate; Step seven, crushing anhydrous ferric phosphate to obtain the layered nano-sized ferric phosphate; In the step 3, the mass concentration of the hydrogen peroxide is 20%-30%, and the molar number of the added hydrogen peroxide is the Fe 3+ 4 / 3-2 times the molar number of the elemental iron, and reacting at room temperature for not less than 1 hour; In the step 4, the pH of the ferric phosphate solution is 0.5-1.5, the mass of the added dispersant is 0.5%-3% of the mass of the ferric phosphate solution, and the dispersant is at least one of polyethylene glycol, polyvinyl alcohol, methyl cellulose, and hydroxypropyl methyl cellulose; the ferric phosphate solution is evaporated and concentrated until the Fe in the ferric phosphate solution is 3+ The concentration is 1.5-3 mol / L to obtain a concentrated solution; the concentrated solution is spray-dried with an air inlet temperature of 250-280°C and an air outlet temperature of 100-110°C, and the moisture content of the obtained granular material is 0.2wt%-0.5wt%; In the step five, the granular material is added to pure water at a liquid-to-solid ratio of 4-10:1 for aging and washing. The aging temperature is 60-90°C and the aging time is 2-4 hours. After aging, the granular material is washed. The stirring speed during the washing process is 200-400 r / min. After washing, the granular material is filtered and the resulting filter residue is ferric phosphate dihydrate.

2. The preparation method according to claim 1, characterized in that In the step 1, the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 60wt%-85wt%.

3. The preparation method according to claim 1, characterized in that In the step 2, the mass ratio of the phosphoric acid solution to the battery-grade iron phosphate is 3-5:1, the temperature of the mixing reaction is set to 60-90° C., and the reaction time is 2-5 hours.

4. The preparation method according to claim 1, characterized in that In step six, the dihydrated ferric phosphate is flash dried at a drying temperature of 110-200° C. for a drying time of 0.5-3 h; the dried ferric phosphate is then calcined in an air atmosphere at a temperature of 500-750° C. for a drying time of 2-4 h to obtain anhydrous ferric phosphate.

5. The preparation method according to claim 1, characterized in that In the step seven, the anhydrous ferric phosphate is pulverized mechanically or by air flow.

6. The layered nano-sized iron phosphate prepared by the preparation method according to any one of claims 1 to 5.

7. The layered nano-scale iron phosphate according to claim 6 is used in the preparation of lithium iron phosphate batteries.

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