Iron phosphate as well as preparation method and application thereof

Abstract

The invention discloses iron phosphate as well as a preparation method and application thereof. The preparation method of the iron phosphate comprises the following steps: mixing a ferrous dihydrogen phosphate solution, an oxidation solution and oxygen in a gas-liquid mixer, enabling the mixture to flow into a reaction container, heating and stirring, carrying out solid-liquid separation, drying to obtain iron phosphate dihydrate, and roasting, the feeding flow of the ferrous dihydrogen phosphate solution is 0.0030 mol / min to 0.1800 mol / min; the feeding concentration of the oxidation liquid is 3%-35%; the feeding volume flow rate of the oxidation liquid is 0.08 to 32 mL / min; and the inlet pressure of the oxygen is 0.05 to 3 Mpa. According to the method, the ferrous dihydrogen phosphate solution is jointly oxidized by the oxygen and the oxidation liquid, so that the use amount of the oxidation liquid is reduced to reduce the cost, the gas-liquid mixer is adopted to realize continuous preparation of the iron phosphate and ensure the synthesis uniformity, the obtained iron phosphate is high in quality, and the lithium iron phosphate positive electrode material subjected to subsequent processing shows excellent capacity and rate capability.

Description

Technical Field

[0001] The present invention relates to the field of chemical technology, and in particular to iron phosphate and a preparation method and application thereof. Background Art

[0002] Since Goodenough et al. first proposed the intercalated lithium polyanion compound LiMPO4 (M=Fe, Co, Ni, Mn) in 1997, lithium iron phosphate (LiFePO4) has been widely used in the new energy vehicle industry, energy storage market, uninterruptible power supply and power tool fields due to its advantages such as low raw material cost, environmental friendliness, stable structure, stable discharge platform, high specific capacity (170mAh / g), good thermal stability and outstanding cycle performance. The market prospect is very broad.

[0003] Currently, lithium iron phosphate has been widely promoted and applied, but research on its performance improvement, quality improvement and cost reduction has been ongoing. The main methods for industrially synthesizing lithium iron phosphate are: ferrous oxalate process, iron red process, full wet process, iron phosphate process, etc.

[0004] The ferrous oxalate process was the most widely used in the early days. Its main advantages are simple process and easy industrialization. The process is to grind ferrous oxalate, diammonium hydrogen phosphate and lithium carbonate in proportion and mix them evenly, and then calcine them at high temperature under the protection of inert gas to obtain lithium iron phosphate material. The materials prepared by this process have poor consistency and the process requires multiple calcinations. The synthesis cycle is long and the energy consumption is high. During the pre-burning process, waste gas will be discharged to pollute the environment, and the environmental pressure is relatively high. When the ferrous oxalate method is used to prepare LiFePO4 positive electrode materials, the ferrous oxalate precursor has a great influence on the structure and electrochemistry of its product. Since the preparation cost of ferrous oxalate is relatively high, the cost of lithium iron phosphate positive electrode materials prepared by this process is relatively high.

[0005] The iron red process uses iron oxide and lithium dihydrogen phosphate as raw materials, grinds and mixes with a carbon source, and prepares lithium iron phosphate positive electrode materials through a carbon thermal reduction method. The iron red process is relatively environmentally friendly, has abundant raw material resources, and is inexpensive, but the cost of lithium dihydrogen phosphate raw materials is high and not very stable. The particle size of lithium iron phosphate prepared by the iron red process is difficult to control, the product morphology is uneven, and the electrochemical performance is poor.

[0006] The full wet process (hydrothermal or solvent thermal) refers to a process in which ferrous sulfate, lithium hydroxide and phosphoric acid are dissolved in water or other solvents as reaction solvents in a high-temperature and high-pressure reactor to synthesize lithium iron phosphate under hydrothermal or solvent thermal conditions. Its advantages are: the synthesized material has good rate performance, small and uniform particle size, and uniform phase, but this reaction requires high-temperature and high-pressure equipment, which is costly and difficult to promote industrially.

[0007] The iron phosphate process uses iron phosphate as a precursor, evenly mixes it with a lithium source and a carbon source, and then undergoes a high-temperature carbon thermal reduction reaction to synthesize lithium iron phosphate positive electrode materials. The process has loose reaction conditions and is simple to operate. It can also achieve precise control of the target product by regulating the morphology, structure, and composition of the iron phosphate precursor. Its advantages have been gradually recognized.

[0008] At present, the preparation methods of precursor ferric phosphate are mainly divided into sodium method, ammonium method and iron method. Among them, the relevant process research of producing precursor ferric phosphate by iron method is relatively small. On the one hand, there are few reports on continuous process, which can simplify the process flow, ensure product consistency and reduce preparation cost; on the other hand, Fe 2+ In the oxidation process, hydrogen peroxide, sodium hypochlorite, sodium chlorate, ammonium persulfate, etc. are often consumed in large quantities, which further increases the production cost to a certain extent. CN114644327A discloses a method for preparing intermittent iron phosphate, using ferrous dihydrogen phosphate as a raw material, and performing an oxidation precipitation reaction with hydrogen peroxide in a reactor, but due to the fast reaction rate of the iron method, high requirements for the control accuracy of mass transfer, heat transfer and reaction conditions, it is difficult to synthesize iron phosphate with stable quality and controllable particle size and morphology through traditional kettle reaction. CN116534824A discloses a method for preparing iron phosphate by a continuous oxidation process, wherein ferrous dihydrogen phosphate is continuously oxidized and overflowed in a multi-grid oxidation tank, but this method still has a large reactant concentration gradient and temperature gradient and a static dead angle of the reaction, and the nucleation and growth of the reactant cannot be accurately controlled, thereby affecting the particle size, morphology, elemental composition and calcined structure of the iron phosphate product. Liu et al. successfully prepared low-cost iron phosphate powder by introducing air into the reaction solution instead of oxidants such as hydrogen peroxide in "Preparation of FePO4 Ultrafine Particles by Air Oxidation", but the method is carried out in a reaction vessel, and it is impossible to produce continuously and ensure batch stability, and it is difficult to apply it on a large scale. CN102491302B also discloses a method for preparing iron phosphate using air as an oxidant, but this method also has the above problems. In addition, the intermediate product needs to be roasted in air to decompose and remove ammonium, hydroxide and crystal water, and the waste gas contains ammonia, which increases the cost of waste gas recovery and treatment. Summary of the invention

[0009] The technical problem to be solved by the present invention is to overcome the defects of the prior art in the preparation of iron phosphate, such as complex process, high cost, difficult particle size control, environmental pollution and poor production stability, and to provide an iron phosphate and its preparation method and application. The preparation method of iron phosphate uses oxygen and an oxidizing liquid to jointly oxidize a ferrous dihydrogen phosphate solution, reduces the amount of the oxidizing liquid to reduce costs, uses a gas-liquid mixer to achieve continuous preparation of iron phosphate and ensures synthetic uniformity, the obtained iron phosphate is of high quality, and the lithium iron phosphate positive electrode material after subsequent processing exhibits excellent capacity and rate performance.

[0010] The present invention solves the above technical problems through the following technical solutions:

[0011] The present invention provides a method for preparing ferric phosphate, which comprises the following steps:

[0012] The ferrous dihydrogen phosphate solution, the oxidizing liquid and the oxygen are mixed in a gas-liquid mixer, flowed into a reaction vessel, heated and stirred, solid-liquid separated, dried to obtain ferrous phosphate dihydrate, and roasted;

[0013] The feed flow rate of the ferrous dihydrogen phosphate solution is 0.0030 mol / min-0.1800 mol / min;

[0014] The feed concentration of the oxidizing liquid is 3%-35%, and the feed concentration is a mass percentage concentration;

[0015] The feed volume flow rate of the oxidation liquid is 0.08 mL / min-32 mL / min;

[0016] The oxygen inlet pressure is 0.05 Mpa-3 Mpa.

[0017] In this scheme, through the co-oxidation reaction of oxygen and oxidizing liquid, the usage of oxidizing liquid is reduced, the production cost is reduced, and the continuous preparation of iron phosphate is achieved and the synthesis uniformity is guaranteed.

[0018] Preferably, the ferrous dihydrogen phosphate solution and the oxidizing liquid enter from a feed inlet on one side of the gas-liquid mixer respectively, and the oxygen enters from a feed inlet on the other side of the gas-liquid mixer.

[0019] More preferably, the feeding temperature of the ferrous dihydrogen phosphate solution is 80°C-100°C, for example 90°C.

[0020] Preferably, the preparation method of the ferrous dihydrogen phosphate solution is to mix an iron source with a phosphoric acid solution, react, and filter.

[0021] Further preferably, the iron source is an iron element with a purity of 99% or more, wherein % is the mass percentage, the concentration of the phosphoric acid solution is 12%-35%, more preferably 20%-30%, for example 25%, and the concentration is the mass percentage concentration.

[0022] More preferably, the molar ratio of the iron source to the phosphoric acid in the phosphoric acid solution is 1:(2.2-3.5), and even more preferably 1:(2.5-3.5), for example 1:3.

[0023] More preferably, the reaction temperature is 50°C-95°C, and even more preferably 60°C-90°C.

[0024] More preferably, the reaction time is 1 h-10 h, and even more preferably 1 h-2 h.

[0025] More preferably, the reaction is carried out in a ferric oxide reactor.

[0026] Preferably, the feed flow rate of the ferrous dihydrogen phosphate solution is 0.0036 mol / min-0.1736 mol / min, for example 0.0850 mol / min.

[0027] Preferably, the feed volume flow rate of the ferrous dihydrogen phosphate solution is 5 mL / min-150 mL / min, more preferably 80 mL / min-120 mL / min, for example 100 mL / min-120 mL / min; the feed concentration of the ferrous dihydrogen phosphate solution is 0.75 mol / L-1.1 mol / L, for example 0.85 mol / L-1.1 mol / L.

[0028] Preferably, the oxidizing liquid is a combination of one or more of sodium hypochlorite, hydrogen peroxide, potassium chlorate and sodium chlorate, such as hydrogen peroxide.

[0029] Preferably, the feed concentration of the oxidizing solution is 5%-20%, for example 10%, and the feed concentration is a mass percentage concentration.

[0030] Preferably, the feed volume flow rate of the oxidizing liquid is 5 mL / min-10 mL / min, for example 6.5 mL / min.

[0031] Preferably, when the oxidizing liquid is hydrogen peroxide, the feed molar flow ratio of the ferrous dihydrogen phosphate solution and the hydrogen peroxide is (28-58):1, for example 44.5:1.

[0032] Preferably, the oxygen intake pressure is 0.3 Mpa-0.6 Mpa.

[0033] In this scheme, the oxygen pressure affects the increase of the oxidation rate in the reaction. Oxygen participates in the full oxidation of divalent iron ions and promotes nucleation, reducing the particle size. Therefore, the oxygen inlet pressure should be maintained at 0.3 Mpa-0.6 Mpa to ensure the high efficiency of the reaction and the stability of the particle size.

[0034] Preferably, the reaction vessel is an aging kettle.

[0035] Preferably, the heating and stirring temperature is 50°C-98°C.

[0036] Preferably, the stirring rate of the heating stirring is 10 rpm-1500 rpm.

[0037] Preferably, the heating and stirring time is 10 min-12 h.

[0038] More preferably, the temperature of the heating and stirring is 60°C-95°C, for example 90°C.

[0039] Preferably, the stirring rate of the heating stirring is 50 rpm-900 rpm.

[0040] Preferably, the heating and stirring time is 30 min-4 h, for example 1 h.

[0041] Preferably, the solid-liquid separation method is suction filtration.

[0042] Preferably, a water washing step is included after the solid-liquid separation and before the drying.

[0043] More preferably, the end point of the water washing is that the conductivity of the washed product is ≤0.5 us / cm.

[0044] Preferably, the drying temperature is 100°C-160°C, more preferably 105°C-140°C, for example 120°C.

[0045] Preferably, the drying time is 30 min-120 min, more preferably 30 min-90 min, for example 60 min.

[0046] Preferably, the grain size of the ferric phosphate dihydrate is 22 nm-30 nm, for example, 22.9 nm-23.7 nm or 23.7 nm-29.2 nm.

[0047] Preferably, the calcination temperature is 480°C-800°C, more preferably 500°C-650°C, for example 600°C.

[0048] Preferably, the calcination time is 2h-6h, more preferably 2h-4h.

[0049] The present invention also provides an iron phosphate, which is prepared by the above-mentioned preparation method of iron phosphate.

[0050] Preferably, the particle size D50 of the iron phosphate is 4µm-13µm, for example, 4.35µm-4.48µm or 4.48µm-12.31µm.

[0051] The present invention also provides a use of the above-mentioned iron phosphate in the preparation of lithium iron phosphate positive electrode material.

[0052] The positive and progressive effects of the present invention are:

[0053] (1) By using oxygen and oxidizing liquid to oxidize the ferrous dihydrogen phosphate solution, the amount of oxidizing liquid can be reduced, thereby reducing costs. In addition, the participation of oxygen promotes the full oxidation of divalent iron, increases the nucleation rate, and effectively reduces the particle size.

[0054] (2) The gas-liquid mixer can realize continuous preparation of iron phosphate and ensure the uniformity of synthesis. The iron-phosphorus molar ratio of the iron phosphate particles prepared by the present invention meets the industry standard of 0.96-1.02, ensuring the quality of the material.

[0055] (3) The iron phosphate provided by the present invention is of high quality. The lithium iron phosphate positive electrode material obtained through subsequent processing has excellent electrochemical properties. In battery applications, it shows a first coulombic efficiency of more than 98%, and the first charge and discharge specific capacity and rate performance are better than those of samples not using a gas-liquid mixer, especially better than those prepared by the hydrogen peroxide oxidation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 Flow chart of the method for preparing iron phosphate particles in Example 1 and Comparative Example 3. DETAILED DESCRIPTION

[0057] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods in the following examples without specifying specific conditions are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0058] In the following examples, the test method for the conductivity of the washed material is to take the washed material into a beaker, insert the Leici DDSJ-318T conductivity meter under the liquid surface of the washed material for testing, and read the reading.

[0059] In the following examples, the iron-making kettle is a 2L glass beaker purchased from Titan Technology Exploration Platform.

[0060] In the following examples, iron powder comes from the Titan Technology Exploration Platform.

[0061] In the following examples, the gas-liquid mixer is from Microfluidics Technology (Changzhou) Co., Ltd., model MF-X3.

[0062] In the following examples, the aging kettle is from Shanghai Yuhua Instrument Co., Ltd., model number is S212-2L.

[0063] Example 1

[0064] A kind of iron phosphate particles, the flow chart of the preparation method is shown in Figure 1 , using a gas-liquid mixer, the preparation method comprises the following steps:

[0065] (1) Preparation of ferrous dihydrogen phosphate solution: Place iron powder with a purity of >99% in an iron-making kettle, and add a dilute phosphoric acid solution with a concentration of 25% so that the molar ratio of the iron source to the phosphoric acid in the phosphoric acid solution is 1:3. Ensure that the temperature during the reaction is 60°C and the reaction time is 2 h. Filter the remaining residue to make the solution clear and transparent to obtain a 0.85 mol / L ferrous dihydrogen phosphate solution.

[0066] (2) The feed temperature of ferrous dihydrogen phosphate solution was set to 90 °C, the feed temperature of hydrogen peroxide was set to room temperature, the feed volume flow rates of ferrous dihydrogen phosphate solution and 10% hydrogen peroxide solution were 100 mL / min (converted to molar flow rate, the feed flow rate of ferrous dihydrogen phosphate solution was 0.085 mol / min) and 6.5 mL / min, respectively, and the oxygen inlet pressure was 0.3 MPa. The ferrous dihydrogen phosphate solution and the oxidizing liquid were respectively introduced into the gas-liquid mixer from the lower feed port, and the oxygen was introduced into the gas-liquid mixer from the upper feed port.

[0067] (3) The mixed slurry naturally flows into the aging kettle, the heating and stirring temperature is 90°C, the heating and stirring speed is 900 rpm, and the heating and stirring time is 1 h.

[0068] (4) Water washing: The aged mixed slurry is filtered and the filter cake is washed with water. The end point of the water washing is that the conductivity of the washed material is 0.5 us / cm.

[0069] (5) Drying and calcining: The obtained product was dried at 120°C for 60 min to obtain dihydrate iron phosphate particles, and then the temperature was raised to 600°C and calcined for 2 h to obtain iron phosphate particles.

[0070] Example 2

[0071] An iron phosphate particle, which differs from Example 1 only in that, in step (2), the oxygen inlet pressure is set to 0.05 MPa, and the other steps and conditions are the same as those in Example 1.

[0072] Example 3

[0073] An iron phosphate particle, which differs from Example 1 only in that, in step (2), the oxygen inlet pressure is set to 0.6 MPa, and the other steps and conditions are the same as those in Example 1.

[0074] Comparative Example 1

[0075] A method for preparing iron phosphate particles comprises the following steps:

[0076] (1) Preparation of ferrous dihydrogen phosphate solution: Place iron powder with a purity of >99% in an iron-making kettle, and add a dilute phosphoric acid solution with a concentration of 25% so that the molar ratio of the iron source to the phosphoric acid in the phosphoric acid solution is 1:3. Ensure that the temperature during the reaction is 60°C and the reaction time is 2 h. Filter the remaining residue to make the solution clear and transparent to obtain a 0.85 mol / L ferrous dihydrogen phosphate solution.

[0077] (2) Directly introduce ferrous dihydrogen phosphate solution and 10% hydrogen peroxide solution into the aging kettle, with the feed flow rates of ferrous dihydrogen phosphate solution and oxidant being 100 mL / min and 15.6 mL / min, respectively. Ensure that the heating and stirring temperature in the aging kettle is 90°C, the stirring rate is 900 rpm, and the heating and stirring time is 1 h.

[0078] (3) Water washing: The aged mixed slurry is filtered and the filter cake is washed with water. The end point of the water washing is that the conductivity of the washed material is 0.5 us / cm.

[0079] (4) Drying and calcining: The obtained product was dried at 120°C for 60 min to obtain iron phosphate dihydrate particles, and then the temperature was raised to 600°C and calcined for 2 h to obtain iron phosphate particles.

[0080] Comparative Example 2

[0081] A method for preparing iron phosphate particles comprises the following steps:

[0082] (1) Preparation of ferrous dihydrogen phosphate solution: Place iron powder with a purity of >99% in an iron-making kettle, and add a dilute phosphoric acid solution with a concentration of 25% so that the molar ratio of the iron source to the phosphoric acid in the phosphoric acid solution is 1:3. Ensure that the temperature during the reaction is 60°C and the reaction time is 2 h. Filter the remaining residue to make the solution clear and transparent to obtain a 0.85 mol / L ferrous dihydrogen phosphate solution.

[0083] (2) The ferrous dihydrogen phosphate solution and 10% hydrogen peroxide solution were directly introduced into the aging kettle. The feed flow rates of the ferrous dihydrogen phosphate solution and the oxidant were 100 mL / min and 6.5 mL / min, respectively. Oxygen was continuously introduced into the reactor to ensure that the temperature in the aging kettle was 90°C, the stirring rate was 900 rpm, and the reaction time was 1 h.

[0084] (3) Water washing: The aged mixed slurry is filtered and the filter cake is washed with water. The end point of the water washing is that the conductivity of the washed material is 0.5 us / cm.

[0085] (4) Drying and calcining: The obtained product was dried at 120°C for 60 min to obtain iron phosphate dihydrate particles, and then the temperature was raised to 600°C and calcined for 2 h to obtain iron phosphate particles.

[0086] Comparative Example 3

[0087] A kind of iron phosphate particles, the flow chart of the preparation method is shown in Figure 1 , using a gas-liquid mixer, the preparation method comprises the following steps:

[0088] (1) Preparation of ferrous dihydrogen phosphate solution: Place iron powder with a purity of >99% in an iron-making kettle, and add a dilute phosphoric acid solution with a concentration of 25% so that the molar ratio of the iron source to the phosphoric acid in the phosphoric acid solution is 1:3. Ensure that the temperature during the reaction is 60°C and the reaction time is 2 h. Filter the remaining residue to make the solution clear and transparent to obtain a 0.85 mol / L ferrous dihydrogen phosphate solution.

[0089] (2) The feed temperature of ferrous dihydrogen phosphate solution was set to 90 °C, the feed temperature of hydrogen peroxide was set to room temperature, the feed flow rates of ferrous dihydrogen phosphate solution and 10% hydrogen peroxide solution were 100 mL / min and 15.6 mL / min, respectively, the inlet pressure of oxygen was 0 MPa, and the ferrous dihydrogen phosphate solution and oxidizing liquid were respectively introduced into the gas-liquid mixer from the lower feed port.

[0090] (3) The mixed slurry naturally flows into the aging kettle, the heating and stirring temperature is 90°C, the heating and stirring speed is 900 rpm, and the heating and stirring time is 1 h.

[0091] (4) Water washing: The aged mixed slurry is filtered and the filter cake is washed with water. The end point of the water washing is that the conductivity of the washed material is 0.5 us / cm.

[0092] (5) Drying and calcining: The obtained product was dried at 120°C for 60 min to obtain dihydrate iron phosphate particles, and then the temperature was raised to 600°C and calcined for 2 h to obtain iron phosphate particles.

[0093] Effect Example 1

[0094] (1) The particle size D50 of the iron phosphate particles obtained in Examples 1-3 and Comparative Examples 1-3 was measured using a Malvern 3000 laser particle size analyzer. The measurement results are shown in Table 1.

[0095] (2) The iron-phosphorus molar ratio of the iron phosphate particles obtained in Examples 1-3 and Comparative Examples 1-3 was measured using an inductively coupled plasma spectrometer PerkinElmer 8300. The measurement results are shown in Table 1.

[0096] (3) The XRD diffraction patterns of the iron phosphate dihydrate particles of Examples 1-3 and Comparative Examples 1-3 were measured using Rigaku's Smartlab X-ray diffractometer, with a scanning rate of 5°C / min and a scanning range of 15-80°. The grain size was calculated using the Scherrer formula D=Kλ / (βcosθ). The test results are shown in Table 1.

[0097] (4) The iron phosphate particles obtained in Example 1-3 and Comparative Example 1-3 were mixed with glucose and lithium carbonate and poured into deionized water, wherein the molar ratio of iron phosphate to lithium carbonate was 1:1.05, the amount of glucose added was 8.5% of the total mass of iron phosphate and lithium carbonate, and the amount of deionized water added was 66% of all solids. The mixture was sand-milled for 40 min using a horizontal sand mill (Changzhou Longxin Intelligent Equipment Co., Ltd., WSP-0.5), and the obtained slurry was granulated using a spray dryer (Shanghai Shunyi Experimental Equipment Co., Ltd., SP1500). The lithium iron phosphate (LiFePO4) prepared by carbothermal reduction was mixed with the above powders in a mass ratio of LiFePO4: conductive carbon black: PVDF = 8:1:1 in a box furnace (Shanghai Jingzhao Machinery Equipment Co., Ltd. QLYF-4-1200) under a flowing nitrogen atmosphere at 400 °C for 2 h and then raised to 730 °C for 4 h. N-methylpyrrolidone was added to prepare a positive electrode slurry with a solid content of 55%, which was evenly coated on a carbon-coated aluminum foil, vacuum dried, taken out, rolled, and punched into an area of ​​2.01 cm 2 Electrode. Button cells were assembled in a glove box (high-purity Ar atmosphere), the electrolyte used was a commercial product (SS-SHRG004, Dongguan Shanshan Battery Materials Co., Ltd.), the negative electrode used a metal lithium sheet, and the separator used a Celgard2400PE film. At 25°C and 0.1C, the button cell was subjected to constant current charge and discharge cycle test, with a charge and discharge voltage of 2.0 to 3.7V, and the first coulomb efficiency was calculated. There was no special test method, and the first coulomb efficiency = first discharge capacity / first charge specific capacity; at 25°C and 2C, the button cell rate was tested, and the test results are shown in Table 1.

[0098] Table 1

[0099]

[0100] As can be seen from Table 1, the iron phosphate particles obtained in Examples 1-3 and Comparative Examples 1-3 have an iron-to-phosphorus molar ratio that meets the industry standard of an iron-to-phosphorus ratio of 0.96 to 1.02.

[0101] The data of Example 1, Example 2, and Example 3 show that as the oxygen inlet pressure increases from 0.05 to 0.3 MPa, the particle size of the product decreases, mainly because a large amount of oxygen participates in the oxidation to fully oxidize the divalent iron to trivalent iron, and the nucleation rate increases, resulting in a decrease in particle size. Sufficient oxygen completely oxidizes the divalent iron, and further increasing the oxygen inlet pressure to 0.6 MPa has no obvious effect on the product particle size.

[0102] The data of Example 1 and Comparative Examples 1 and 2 show that the particle size of the product is greatly reduced. This is because the use of a gas-liquid mixer can ensure sufficient mixing with the components to increase the nucleation rate, greatly reduce the particle size of the iron phosphate particles, and the ferrous dihydrogen phosphate solution, oxidant and oxygen achieve spontaneous fluid cutting, shearing, rotation and remixing in the mixing chamber, achieving the purpose of good dispersion and mixing between the fluids. At the same time, the product is continuously discharged, further reducing the risk of blockage, and achieving continuous and low-cost production of iron phosphate.

[0103] The data of Example 1 and Comparative Example 3 show that the introduction of oxygen into the gas-liquid mixer can reduce the grain size of the generated iron phosphate dihydrate particles and improve the rate performance and charge-discharge capacity of the material.

[0104] The lithium iron phosphate positive electrode material obtained by subsequent processing of the iron phosphate obtained by the continuous sampling method using a gas-liquid mixer in Examples 1-3, when used in a battery, has an initial coulombic efficiency of more than 98%, and an initial charge and discharge specific capacity and rate performance that are better than the samples of Comparative Examples 1-2 without using a mixer and Comparative Example 3 using a mixer without oxygen, and is significantly better than the sample obtained by the hydrogen peroxide oxidation method, and has excellent electrochemical properties, indicating that the lithium iron phosphate prepared by the iron phosphate provided by the present invention has good electrical properties.

[0105] Although the specific embodiments of the present invention are described above, it should be understood by those skilled in the art that this is only for illustration and the protection scope of the present invention is defined by the appended claims. Those skilled in the art may make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, but these changes and modifications all fall within the protection scope of the present invention.

Claims

1. A method for preparing ferric phosphate, characterized in that: It includes the following steps: The ferrous dihydrogen phosphate solution, the oxidizing liquid and the oxygen are mixed in a gas-liquid mixer, flowed into a reaction vessel, heated and stirred, solid-liquid separated, dried to obtain ferrous phosphate dihydrate, and roasted; The feed flow rate of the ferrous dihydrogen phosphate solution is 0.0030 mol / min-0.1800 mol / min; The feed concentration of the oxidizing liquid is 3%-35%, and the feed concentration is a mass percentage concentration; The feed volume flow rate of the oxidation liquid is 0.08 mL / min-32 mL / min; The oxygen inlet pressure is 0.05 Mpa-3 Mpa.

2. The method for preparing ferric phosphate according to claim 1, characterized in that: The ferrous dihydrogen phosphate solution and the oxidizing liquid are respectively introduced from a feed port on one side of the gas-liquid mixer, and the oxygen is introduced from a feed port on the other side of the gas-liquid mixer; Preferably, the feed temperature of the ferrous dihydrogen phosphate solution is 80°C-100°C, for example 90°C; And / or, the preparation method of the ferrous dihydrogen phosphate solution is to mix an iron source with a phosphoric acid solution, react, and filter; Preferably, the iron source is an iron element with a purity of 99% or more, wherein % is a mass percentage, the concentration of the phosphoric acid solution is 12%-35%, more preferably 20%-30%, for example 25%, and the concentration is a mass percentage concentration; Preferably, the molar ratio of the iron source to the phosphoric acid in the phosphoric acid solution is 1:(2.2-3.5), more preferably 1:(2.5-3.5), for example 1:3; Preferably, the reaction temperature is 50°C-95°C, more preferably 60°C-90°C; Preferably, the reaction time is 1h-10h, more preferably 1h-2h; Preferably, the reaction is carried out in a ferric oxide reactor.

3. The method for preparing ferric phosphate according to claim 1, characterized in that: The feed rate of the ferrous dihydrogen phosphate solution is 0.0036 mol / min-0.1736 mol / min, for example 0.0850 mol / min; And / or, the feed volume flow rate of the ferrous dihydrogen phosphate solution is 5 mL / min-150 mL / min, preferably 80 mL / min-120 mL / min, for example 100 mL / min-120 mL / min; the feed concentration of the ferrous dihydrogen phosphate solution is 0.75 mol / L-1.1 mol / L, for example 0.85 mol / L-1.1 mol / L; And / or, the oxidizing liquid is a combination of one or more of sodium hypochlorite, hydrogen peroxide, potassium chlorate and sodium chlorate, such as hydrogen peroxide; And / or, the feed concentration of the oxidizing solution is 5%-20%, for example 10%, and the feed concentration is a mass percentage concentration; and / or, the feed volume flow rate of the oxidation liquid is 5 mL / min-10 mL / min, for example 6.5 mL / min; And / or, when the oxidizing liquid is hydrogen peroxide, the feed molar flow ratio of the ferrous dihydrogen phosphate solution and the hydrogen peroxide is (28-58):1, for example 44.5:1; And / or, the oxygen intake pressure is 0.3 Mpa-0.6 Mpa.

4. The method for preparing ferric phosphate according to claim 1, characterized in that: The reaction vessel is an aging kettle; And / or, the temperature of the heating and stirring is 50°C-98°C; And / or, the stirring rate of the heating stirring is 10rpm-1500rpm; And / or, the heating and stirring time is 10 min-12 h.

5. The method for preparing ferric phosphate according to claim 4, characterized in that: The heating and stirring temperature is 60°C-95°C, for example 90°C; And / or, the stirring rate of the heating stirring is 50 rpm-900 rpm; And / or, the heating and stirring time is 30 min-4 h, for example 1 h.

6. The method for preparing ferric phosphate according to claim 1, characterized in that: The solid-liquid separation method is suction filtration; And / or, a washing step is further included after the solid-liquid separation and before the drying; Preferably, the end point of the water washing is that the conductivity of the washed product is ≤0.5us / cm.

7. The method for preparing ferric phosphate according to claim 1, characterized in that: The drying temperature is 100°C-160°C, preferably 105°C-140°C, for example 120°C; And / or, the drying time is 30 min-120 min, preferably 30 min-90 min, for example 60 min; And / or, the grain size of the ferric phosphate dihydrate is 22nm-30nm, for example, 22.9nm-23.7nm or 23.7nm-29.2nm.

8. The method for preparing ferric phosphate according to claim 1, characterized in that: The calcination temperature is 480°C-800°C, preferably 500°C-650°C, for example 600°C; And / or, the calcination time is 2h-6h, preferably 2h-4h.

9. A kind of iron phosphate, characterized in that: The iron phosphate is prepared by the preparation method of the iron phosphate according to any one of claims 1 to 8; Preferably, the particle size D50 of the iron phosphate is 4µm-13µm, for example, 4.35µm-4.48µm or 4.48µm-12.31µm.

10. Use of the iron phosphate as claimed in claim 9 in preparing lithium iron phosphate positive electrode material.

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