Lithium iron phosphate precursor, preparation method thereof, lithium iron phosphate and preparation method and application thereof

Abstract

The application discloses a lithium iron phosphate precursor, a preparation method thereof, a lithium iron phosphate and a preparation method and application thereof. The preparation method of the lithium iron phosphate precursor comprises the following steps: S1. reacting a mixture I comprising an iron source and a phosphoric acid solution, and grinding the mixture I to obtain product A after the reaction is completed; reacting a mixture II comprising a first organic acid solution, a lithium source and a coating source, and obtaining product B after the reaction is completed; S2. dividing a mixture of the product A and the product B into two parts, grinding the two parts respectively to obtain component I and component II, and mixing the component I and the component II to obtain the lithium iron phosphate precursor; wherein the average particle size of the component I is 800-1600 nm; the average particle size of the component II is 50-600 nm; and the mass ratio of the component I to the component II is (0.25-9):1. The lithium iron phosphate prepared through subsequent steps of the lithium iron phosphate precursor has high tap density and good electrical properties.

Description

TECHNICAL FIELD

[0001] The present application relates to a lithium iron phosphate precursor and a preparation method thereof, a lithium iron phosphate and a preparation method and application thereof. BACKGROUND

[0002] As a positive material of lithium ion battery, lithium iron phosphate has the advantages of good cycle performance and high safety when used as a power battery, but its energy density is low and cannot meet the requirements of long endurance.

[0003] In order to solve the problem of endurance anxiety, the existing research technology uses lithium manganese iron phosphate as the next generation of lithium iron phosphate material, but lithium manganese iron phosphate has the problems of lower electrical conductivity and poor cycle performance caused by the Jiang Taylor effect.

[0004] In the prior art, although the powder compaction density of lithium iron phosphate is improved to improve the energy density of lithium iron phosphate battery. However, the powder compaction density of the industrialized lithium iron phosphate product on the market is at most 2.60 g / cm 3 However, these preparation methods are complex and unstable for industrial production, and it is difficult to realize stable industrial production of high powder compaction density lithium iron phosphate. SUMMARY

[0005] The present application mainly overcomes the defects of low compaction density and complex preparation method of lithium iron phosphate material in the prior art, and provides a lithium iron phosphate precursor and a preparation method thereof, a lithium iron phosphate and a preparation method and application thereof. The lithium iron phosphate prepared by the present application has high compaction density and good electrical performance.

[0006] In order to overcome the above technical problems, the present application provides the following technical solutions.

[0007] The present application provides a preparation method of a lithium iron phosphate precursor, which comprises the following steps:

[0008] S1. Reacting a mixture I comprising an iron source and a phosphoric acid solution, and grinding the product A after the reaction is completed;

[0009] Reacting a mixture II comprising a first organic acid solution, a lithium source and a coating source, and obtaining a product B after the reaction is completed; the preparation sequence of the product A and the product B is not limited;

[0010] S2. Dividing the mixture of the product A and the product B into two parts, respectively grinding to obtain components I and II, and then mixing the components I and II to obtain a lithium iron phosphate precursor; wherein the average particle size of the component I is 800-1600 nm; the average particle size of the component II is 50-600 nm; and the mass ratio of the component I to the component II is (0.25-9):1.

[0011] In S1, the reaction is completed when there is no gas generated, which is known to those skilled in the art.

[0012] In S1, the reaction is carried out under stirring, preferably at a speed of 25-50 Hz, for example 30 Hz. The stirring speed affects the formation of precipitates, thereby affecting the particle size distribution.

[0013] In S1, the iron source does not include iron phosphate. Those skilled in the art know that the present application is a process for preparing lithium phosphate after a step of preparing iron phosphate.

[0014] In S1, the iron source is a compound containing iron and oxygen elements, more preferably one or more of iron powder, ferrous oxide, ferric oxide, and ferric nitrate, and further more preferably one or more of iron powder, ferrous oxide, and ferric oxide.

[0015] Preferably, the iron content in the iron powder is more than 95 wt%, more preferably more than 99 wt%, and further more preferably more than 99.5 wt%, for example 99.7 wt%.

[0016] Preferably, the iron powder is one or more of primary reduced iron powder, secondary reduced iron powder, carbonyl reduced iron powder, and electrolytic iron powder.

[0017] Preferably, the purity of the ferrous oxide is more than 95 wt%, more preferably more than 99 wt%, and further more preferably more than 99.5 wt%.

[0018] Preferably, the purity of the ferric oxide is more than 95 wt%, more preferably more than 99 wt%, and further more preferably more than 99.5 wt%.

[0019] In S1, the mesh number of the iron source can be 100-1000 mesh, more preferably 200-500 mesh, for example 250 mesh or 300 mesh.

[0020] In S1, the phosphoric acid solution generally refers to an aqueous phosphoric acid solution, and the mass percentage concentration of phosphoric acid in the phosphoric acid solution is preferably 20-85%, for example 49%, 59%, or 62%.

[0021] In S1, the phosphoric acid in the phosphoric acid solution can be conventional phosphoric acid in the art, for example industrial-grade phosphoric acid, food-grade phosphoric acid, appliance-grade phosphoric acid, or electronic-grade phosphoric acid.

[0022] The appliance-grade phosphoric acid can be purchased from Guangxi Qinzhou Chengxing Chemical Technology Co., Ltd.

[0023] In S1, the reaction temperature of the mixture I can be 20-95°C, preferably 30-90°C, for example 35°C, 45°C, 55°C or 85°C.

[0024] In S1, the mixture I can be prepared by adding the iron source into the phosphoric acid solution under stirring.

[0025] In S1, the molar ratio of iron element to phosphoric acid in the mixture I can be (0.94-1.05):1, preferably (0.96-1.0):1, for example 0.98:1 or 0.995:1.

[0026] In S1, the mixture I can further comprise a catalyst. The catalyst has catalytic effect during the reaction, and can improve the conductivity of the lithium iron phosphate product after the reaction.

[0027] Preferably, the catalyst is a titanium-based catalyst.

[0028] Preferably, the molar ratio of titanium element in the catalyst to iron element in the iron source is (0.5-1):100, for example 1.15:100.

[0029] Preferably, the catalyst is mixed with the phosphoric acid solution before the iron source is added.

[0030] In S1, the grinding operation and conditions can be conventional grinding operation, which can be sand milling or ball milling.

[0031] Preferably, the grinding is performed by a sand mill.

[0032] The sand mill is preferably a vertical sand mill, a horizontal sand mill, a basket sand mill or a double-cone rod sand mill; the horizontal sand mill is preferably a nanometer horizontal sand mill.

[0033] The particle size of the grinding beads used in the sand mill is preferably 0.1-3.0mm, for example 0.3mm or 0.4mm.

[0034] The grinding beads used in the sand mill are preferably zirconia beads.

[0035] In S1, the viscosity of the product A can be 8000-20000cps, preferably 10000-20000cps, for example 15000cps.

[0036] In S1, the first organic acid solution generally refers to an aqueous solution of the first organic acid, and the mass percentage concentration of the first organic acid in the first organic acid solution is preferably 5-98%, for example 30%, 55%, 62% or 72%.

[0037] In S1, the first organic acid in the first organic acid solution is one or more of a carboxylic compound, ascorbic acid, and a polymer acid, and the carboxylic compound is preferably one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid, and malic acid.

[0038] In S1, the first organic acid in the first organic acid solution is one or more of a carboxylic compound, ascorbic acid, and a polymer acid, and the carboxylic compound is preferably one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid, and malic acid.

[0039] In S1, the first organic acid in the first organic acid solution is one or more of a carboxylic compound, ascorbic acid, and a polymer acid, and the carboxylic compound is preferably one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid, and malic acid.

[0040] In some embodiments, the organic acid is one or more of citric acid, oxalic acid, malic acid, tartaric acid, and polyacrylic acid.

[0041] In S1, the lithium source is a lithium-containing compound commonly used in the art, and is preferably one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, and lithium acetate, such as lithium hydroxide monohydrate and / or lithium carbonate; and the lithium carbonate is preferably industrial-grade lithium carbonate or battery-grade lithium carbonate.

[0042] In S1, the lithium source is a lithium-containing compound commonly used in the art, and is preferably one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, and lithium acetate, such as lithium hydroxide monohydrate and / or lithium carbonate; and the lithium carbonate is preferably industrial-grade lithium carbonate or battery-grade lithium carbonate.

[0043] In S1, the coating source is one or more of a carbon material, a metal compound, and a conductive polymer. The coating source can complex with metal ions, which helps to achieve the special structure of lithium iron phosphate.

[0044] In S1, the lithium source is a lithium-containing compound commonly used in the art, and is preferably one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, and lithium acetate, such as lithium hydroxide monohydrate and / or lithium carbonate; and the lithium carbonate is preferably industrial-grade lithium carbonate or battery-grade lithium carbonate.

[0045] The glucose derivative preferably includes at least one of sucrose, starch, and cyclodextrin.

[0046] The second organic acid preferably includes at least one of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid, and malic acid.

[0047] The polyacrylic acid derivative preferably includes polyacrylate.

[0048] The heterocyclic polymer with N or O elements preferably includes polyvinylpyrrolidone.

[0049] Preferably, the metal compound comprises aluminum oxide and / or zinc stannate.

[0050] Preferably, the conductive polymer comprises one or more of polyaniline, polystyrene thiol, polyacetylene, lithium carbonate and polycarbonate.

[0051] In some embodiments, the coating source is polyethylene glycol.

[0052] In some embodiments, the coating source is sucrose.

[0053] In some embodiments, the coating source is citric acid and polyacrylic acid.

[0054] In some embodiments, the coating source is lithium carbonate, sucrose and polyvinylpyrrolidone.

[0055] In some embodiments, the coating source is a mixture of polyvinyl alcohol, cyclodextrin and polyethylene glycol.

[0056] In S1, the amount of the coating source can be 1%-60%, preferably 5%-50%, more preferably 10%-40%, for example 15%, the percentage refers to the mass percentage of the coating source in the mass of the phosphoric acid.

[0057] In S1, the reaction temperature of the mixture II can be 20-95°C, preferably 30-90°C, for example 35°C, 40°C, 45°C or 55°C.

[0058] In S1, the mixture II can be prepared by adding the lithium source and the coating source into the organic acid solution under stirring.

[0059] In S2, the mixture of the product A and the product B can be prepared by simply mixing the product A and the product B.

[0060] In S2, the operation and conditions of the grinding can be conventional grinding operation, which can be sand milling or ball milling.

[0061] Preferably, the grinding is performed by using a sand mill.

[0062] The sand mill is preferably a vertical sand mill, a horizontal sand mill, a basket sand mill or a double-cone rod sand mill; the horizontal sand mill is preferably a nanometer horizontal sand mill.

[0063] The particle size of the grinding beads used in the sand mill is preferably 0.1-3.0mm, for example 0.3mm or 0.4mm.

[0064] The grinding beads used in the sand mill are preferably zirconium oxide beads.

[0065] In S2, the milling time can be conventional in the art, and can be affected by the size of the zirconium beads, the feeding speed, and the abrasion of the zirconium beads, etc. The average particle size of each component is controlled, and the milling is stopped when the sand milling reaches the target particle size. The corresponding time is not fixed, for example, 6-10 h.

[0066] In S2, the average particle size of the component I can be 1000-1500 nm, for example, 1200 nm.

[0067] In S2, the average particle size of the component II can be 100-300 nm, for example, 150 nm.

[0068] In S2, the mass ratio of the component I to the component II can be (0.25-9):1, preferably (1-5):1, for example, 7:3.

[0069] The present application provides a lithium iron phosphate precursor prepared by the preparation method as described above.

[0070] In the present application, the lithium iron phosphate precursor is an amorphous substance or a low-crystallinity iron salt, lithium salt, and mixture thereof formed by the accumulation of precipitates, and the particle size of the lithium iron phosphate precursor is controlled only by changing the average size of the accumulation.

[0071] The present application provides a lithium iron phosphate precursor prepared by the preparation method as described above.

[0072] The present application provides a preparation method of lithium iron phosphate, which comprises the following steps: sequentially performing spray drying, sintering, and crushing on the lithium iron phosphate precursor as described above to obtain the lithium iron phosphate.

[0073] In the present application, the lithium iron phosphate precursor is an amorphous substance or a low-crystallinity iron salt, lithium salt, and mixture thereof formed by the accumulation of precipitates, and the particle size of the lithium iron phosphate precursor is controlled only by changing the average size of the accumulation.

[0074] The particle size of the upper layer of the crushed material satisfies 1.1 μm≤D50≤1.80 μm.

[0075] The particle size of the lower layer of the crushed material satisfies 0.8 μm≤D50≤1.45 μm.

[0076] In the present application, the D50 particle size of the upper layer of the broken screening material can be 1.1-1.6nm, such as 1.59nm, 1.53nm, 1.52nm, 1.49nm, 1.47nm, 1.41nm, 1.38nm, 1.34nm, 1.32nm, 1.31nm, 1.27nm or 1.13nm.

[0077] In the present application, the D50 particle size of the lower layer of the broken screening material can be 0.8-1.4nm, such as 1.34nm, 1.24nm, 1.21nm, 1.16nm, 1.13nm, 1.12nm, 1.1nm, 1.09nm, 1.09nm, 1.02nm, 0.99nm, 0.98nm or 0.82nm.

[0078] In the present application, the mass percentage of the particles of the lower layer of the broken screening material in the lithium iron phosphate can be 10wt%-40wt%, such as 40wt%, 38wt%, 35wt%, 34wt%, 33wt%, 32wt%, 31wt%, 29wt%, 28wt%, 27wt%, 26wt%, 25wt%, 24wt%, 21wt% or 10wt%.

[0079] In the present application, the particle size of the upper layer of the broken screening material can satisfy D10≤0.54μm.

[0080] In the present application, the particle size of the upper layer of the broken screening material can satisfy D99≤12.5μm.

[0081] In the present application, the particle size of the upper layer of the broken screening material can satisfy Dmax≤18μm.

[0082] In the present application, the particle size of the lower layer of the broken screening material can satisfy D10≤0.47μm.

[0083] In the present application, the particle size of the lower layer of the broken screening material can satisfy D99≤5.05μm.

[0084] In the present application, the particle size of the lower layer of the broken screening material can satisfy Dmax≤6.02μm.

[0085] In the present application, the inlet temperature of the spray drying can be 280℃.

[0086] In the present application, the outlet temperature of the spray drying can be 130℃.

[0087] In the present application, the sintering temperature can be 730℃-800℃, such as 730℃, 760℃ or 800℃.

[0088] In the present application, the heating rate of the sintering can be 3-8℃ / min, for example 5℃ / min.

[0089] In the present application, the time of the sintering can be 5h-20h, for example 12h.

[0090] In the present application, the sintering can be carried out under inert atmosphere, for example nitrogen, the purity of the nitrogen preferably being 99.999%.

[0091] In the present application, the mesh size of the screen used in the sieving can be 150-400 mesh, for example 300 mesh.

[0092] In the present application, during the crushing of the upper layer of the sieved material, the frequency of the airflow crushing can be 100-140HZ, for example 100HZ, 120HZ or 140HZ.

[0093] In the present application, during the crushing of the lower layer of the sieved material, the frequency of the airflow crushing can be 140-180HZ, for example 140HZ, 160HZ or 180HZ.

[0094] The present application provides a lithium iron phosphate prepared by the preparation method as described above.

[0095] The present application provides the use of the lithium iron phosphate as described above in the positive electrode material of a lithium ion battery,

[0096] On the basis of common general knowledge in the art, the above-mentioned preferred conditions can be combined arbitrarily, thus obtaining various preferred examples of the present application.

[0097] The reagents and raw materials used in the present application are commercially available.

[0098] The positive progress effect of the present application is that:

[0099] The lithium iron phosphate precursor provided by the present application has high tap density and good electrical properties. When it is applied to a battery, the charge specific capacity and discharge specific capacity of the battery are close under the same rate, which indicates that the energy loss of the battery during the charging and discharging process is small, and the performance of the battery is good. Moreover, the preparation method provided by the present application is simple and can realize stable industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0100] Figure 1 SEM image of the lithium iron phosphate prepared in Example 1.

[0101] Figure 2 SEM image of the lithium iron phosphate prepared in Comparative Example 4. DETAILED DESCRIPTION

[0102] The application will be further described in the following by way of examples without thereby limiting the application to the examples described. The experimental methods in the following examples, for which no specific conditions are indicated, are selected in accordance with the usual methods and conditions, or in accordance with the instructions of the commercial suppliers.

[0103] Preparation method of examples and comparative examples

[0104] (1) A solution of phosphoric acid with a concentration of 62% and titanium dioxide are mixed, and then reduced iron powder is added, and stirring is carried out at 85°C to make it react, and gas is generated during the reaction, and then the material is put into a sand mill to be sand milled, the grinding beads in the sand mill are 0.3mm zirconia beads, to obtain product A; wherein the molar ratio of iron element to phosphoric acid is 0.995:1; wherein the molar ratio of titanium element to iron element is 1.15:100;

[0105] (2) A solution of malic acid with a concentration of 30%, lithium carbonate and polyethylene glycol are mixed, and reaction is carried out at 55°C until no gas is generated, to obtain product B; wherein the molar ratio of lithium element to phosphoric acid is 1.03:1; the mass of polyethylene glycol is 15%, the percentage is the mass percentage of polyethylene glycol in the mass of phosphoric acid; the addition amount of malic acid is 32%, the percentage is the mass percentage of malic acid in the mass of phosphoric acid;

[0106] (3) The product A and the product B are mixed, and then the mixture is divided into two parts, and sand milling is carried out respectively to obtain component I and component II, and then component I and component II are mixed to obtain a lithium iron phosphate precursor;

[0107] (4) The lithium iron phosphate precursor is sequentially subjected to spray drying and sintering, and then the material obtained by sintering is sieved by using a 300-mesh sieve to obtain upper layer sieved material and lower layer sieved material, and then the upper layer sieved material and the lower layer sieved material are respectively crushed, and then mixed, to obtain lithium iron phosphate.

[0108] Example 14

[0109] The difference from example 1 is that no catalyst titanium dioxide is added.

[0110] Example 15

[0111] The difference from example 1 is that the reduced iron powder in step (1) is replaced by diiron trioxide.

[0112] Example 16

[0113] The difference from example 1 is that the molar ratio of iron element to phosphoric acid in step (1) is replaced by 0.98:1.

[0114] Example 17

[0115] The difference from Example 1 is that the malic acid solution in step (2) is replaced by polyacrylic acid.

[0116] Example 18

[0117] The difference from Example 1 is that the lithium carbonate in step (2) is replaced by lithium hydroxide monohydrate.

[0118] Example 19

[0119] The difference from Example 1 is that the polyethylene glycol in step (2) is replaced by sucrose.

[0120] Example 20

[0121] The difference from Example 1 is that the molar ratio of lithium element to phosphoric acid in step (2) is replaced by 1.01:1.

[0122] Other specific process parameters are shown in Table 1.

[0123] Effect Example 1

[0124] 1. Particle size

[0125] The particle size was tested by a laser particle size analyzer with a refractive index of 1.8%, and the equipment model was Malvern 3000. The test results are shown in Table 2.

[0126] 2. Resistivity

[0127] The resistivity of the lithium iron phosphate powder obtained in all examples and comparative examples was tested by a powder resistivity tester (model: ST2742B). The test method was four-probe testing, and the test pressure was 8 MPa. The test results are shown in Table 2.

[0128] 3. Carbon content

[0129] The total carbon content of steel and alloy was determined by infrared absorption method after induction furnace combustion according to the national standard GBT 223.86-2009. The equipment manufacturer was Ke'gu Instrument, and the model was HCS-500. The test results are shown in Table 2.

[0130] 4. Compaction density

[0131] The compaction density of the lithium iron phosphate powder was tested by an electronic pressure testing machine (model UTM7305Z09) with pressure parameters of 10 kN, 20 kN and 30 kN. The powder compaction density under a pressure of 30 kN was taken as the test result. The test results are shown in Table 2.

[0132] Effect Example 2

[0133] Preparation of button cells

[0134] (1) The lithium iron phosphate material, conductive carbon black and polyvinylidene fluoride of the examples and comparative examples were dispersed in NMP solution at a mass ratio of 90:5:5, and stirred in a vacuum stirrer for 3 h, to control the slurry solid content to 50%, to prepare a positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil, and then placed in a vacuum drying oven and dried at 120°C for 12 h, and then punched into a circular sheet with a diameter of 12 mm as a positive electrode sheet.

[0135] 2) A lithium metal sheet was used as a negative electrode, Celgard 2400 microporous membrane was used as a separator, and a 1.0 mol / L LiPF6 solution was used as an electrolyte, the solvent of the electrolyte was prepared by mixing ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) at a volume ratio of 1:1:1. The above-mentioned positive electrode sheet, negative electrode sheet, separator and electrolyte were assembled into a CR2016 button cell in an argon-filled glove box.

[0136] The button cell prepared by the above method was tested for electrical performance. During the test, the cutoff voltage of the charging process was 3.75 V, and the cutoff voltage of the discharging process was 2.0 V. Equipment manufacturer: Xiwel Electronics Co., Ltd., equipment specifications: 5V / 10mA

[0137] Constant current charging at a current corresponding to 0.1C to a voltage of 3.75V, constant voltage charging to a current of 0.02C, constant current discharging at a current corresponding to 0.1C to a voltage of 2.0V, and recording the 0.1C charge specific capacity and the 0.1C discharge specific capacity as the test results.

[0138] Constant current charging at a current corresponding to 1C to a voltage of 3.75V, constant voltage charging to a current of 0.02C, constant current discharging at a current corresponding to 1C to a voltage of 2.0V, and recording the 1C discharge specific capacity as the test results. The test results are shown in Table 2.

[0139] Table 1

[0140]

[0141] Table 2

[0142]

[0143] As can be seen from Table 2, the lithium iron phosphate prepared by the preparation method provided by the application has low resistivity (6.14-9.14Ω•cm), high compaction density (2.561-2.696 g / cm 3The advantages of the lithium iron phosphate prepared in the application are that when the lithium iron phosphate is applied to a battery, the specific charge capacity of the battery at 0.1C is 160.1-162.3 mAh / g, and the specific discharge capacity of the battery at 0.1C is 157.1-159.4 mAh / g. It can be seen that the specific charge capacity and the specific discharge capacity are close at 0.1C, which indicates that the energy loss of the battery in the charging and discharging process is small, and the performance of the battery is good.

[0144] Figure 1 SEM image of the lithium iron phosphate prepared for Example 1. Figure 2 The SEM image of the lithium iron phosphate prepared for Comparative Example 4. It can be seen from the data in Tables 1 and 2 that improving the powder compaction density is crucial to the primary large particles of the lithium iron phosphate. In the presence of the large particles, the small particles are uniformly distributed in the gaps between the large particles, thereby effectively improving the powder compaction density. It is difficult to effectively grade the material obtained by sintering by using a one-step crushing process. Since the crushing process is constant, the primary large particles will be broken into multiple primary small particles by strong airflow. Such primary small particles have many exposed lithium iron phosphate sections, thereby forming defects. Meanwhile, the crushing process cannot break the agglomerated primary small particles. These agglomerated small particles cannot effectively fill the gaps between the large particles, thereby resulting in an inability to improve the powder compaction density. When the powder compaction density is low, there are a large number of voids between the primary particles of the lithium iron phosphate, the filling rate per unit volume is low, the transmission path of lithium ions and electrons is long, and the electrical performance of the material is reduced. Improving the powder compaction density can improve the electrical performance and also significantly improve the energy density.

[0145] The difference between Examples 1-5 and Comparative Examples 2-3 lies in the particle sizes of the component I and the component II in the lithium iron phosphate precursor. It can be seen that changing the particle sizes of the component I and the component II will affect the particle sizes of the upper and lower sieved materials obtained in the subsequent steps. Changing the particle sizes of the component I and the component II will have a certain effect on the compaction density and the electrical performance of the lithium iron phosphate powder, but the technical effects of the application can be achieved within the scope of the application. Outside the scope of the application, the compaction density and the electrical performance are greatly affected.

[0146] The difference between Examples 1, 6-7 and Comparative Example 1 lies in the mass ratio of the component I and the component II in the lithium iron phosphate precursor. It can be seen from the data in Table 2 that the mass ratio will have a great effect on the powder compaction density. This may be because the proportion of the large particles and the small particles directly determines the filling degree of the large particle gaps. It is necessary to be within a suitable range to have good effects.

[0147] The difference between Example 1 and Examples 8-9 lies in the sintering temperature. The sintering temperature will affect the fusion of the particles. Within a certain range, increasing the sintering temperature is beneficial to the fusion of the particles to obtain a lithium iron phosphate product with a size particle size.

[0148] Examples 1, 10-13 and Comparative Examples 6-7 differ in the crushing frequency of the upper layer of the sieved material and the crushing frequency of the lower layer of the sieved material, and the particle size of the crushed material is also different. It can be seen that, within the scope of the present application, the material obtained by using different crushing frequencies is mixed, and the particle size meets the present application, and good compacting density and electrical properties can be achieved. If the particle size is not within the scope of the present application, the compacting density and electrical properties will be significantly reduced.

[0149] Examples 1 and 14-20 differ in the types of raw materials used or the proportions of the raw materials, but good compacting density and electrical properties can be obtained.

[0150] Examples 1 and 21-24 differ in the proportion of the particles of the lower layer of the sieved material after crushing in the final lithium iron phosphate product, which will have some effect on the results.

[0151] Comparative Examples 4-5 use the traditional one-step crushing method, and as described above, the compacting density and electrical properties of the powder are both poor.

[0152] The above-described examples are only better examples of the present application, and facilitate the understanding and use of the present application by those skilled in the art. Obviously, any skilled person in the art can make slight modifications or changes to the present examples without creative labor and apply them to other examples. Therefore, the present application is not limited to the above examples, and any equivalent changes, simple modifications and modifications within the scope of the present application still fall within the scope of the present application.

Claims

1. A method for preparing lithium iron phosphate, characterized in that: The method comprises the following steps: spray drying, sintering and crushing a lithium iron phosphate precursor in sequence to obtain the lithium iron phosphate; The preparation method of the lithium iron phosphate precursor comprises the following steps: S1. A mixture comprising an iron source and a phosphoric acid solution is reacted I, and after completion of the reaction, the mixture is ground to obtain a product A; reacting a mixture II comprising a first organic acid solution, a lithium source, and a coating source to obtain a product B after the reaction is completed; the order of preparing the product A and the product B is not limited; S2. Dividing the mixture of Product A and Product B into two parts, grinding each part to obtain a component I and a component II, and then mixing the components I and II to obtain a lithium iron phosphate precursor; wherein the average particle size of the component I is 800-1600 nm; the average particle size of the component II is 50-600 nm; and the mass ratio of the components I to the components II is (0.25-9):1; The crushing includes screening the sintered material to obtain an upper screened material and a lower screened material, and then crushing the upper screened material and the lower screened material separately, and then mixing them to obtain the lithium iron phosphate; The particle size of the upper screening material after crushing satisfies the following requirements: 1.1 μm ≤ D50 ≤ 1.80 μm; The particle size of the lower layer screening material after crushing satisfies the following conditions: 0.8 μm≤D50≤1.45 μm.

2. The method for preparing lithium iron phosphate according to claim 1, wherein: It satisfies one or more of the following conditions (a)-(h): (a) the average particle size of component I is 1000-1500 nm; (b) the average particle size of component II is 100-300 nm; (c) the mass ratio of the component I to the component II is (1-5):1; (d) the reaction temperature of the mixture I is 20-95°C; (e) In the mixture I, the molar ratio of iron to phosphoric acid is (0.94-1.05):1; (f) the molar ratio of lithium in the lithium source to phosphoric acid in the phosphoric acid solution is (0.98-1.05):1; (g) the coating source is used in an amount of 1% to 60%, where the percentage refers to the mass percentage of the coating source to the mass percentage of the phosphoric acid; (h) The reaction temperature of the mixture II is 20-95°C.

3. The method for preparing lithium iron phosphate according to claim 1, wherein: It satisfies one or more of the following conditions (a)-(g): (a) the iron source is one or more of iron powder, ferric oxide, ferroferric oxide and ferric nitrate; (b) the mesh size of the iron source is 100-1000 mesh; (c) the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 20-85%; (d) the phosphoric acid in the phosphoric acid solution is industrial grade phosphoric acid, food grade phosphoric acid, electrical grade phosphoric acid or electronic grade phosphoric acid; (e) the first organic acid in the first organic acid solution is one or more of a carboxylic acid compound, ascorbic acid, and a polymer acid; when the first organic acid comprises a carboxylic acid compound, the carboxylic acid compound is one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid, and malic acid; when the first organic acid comprises a polymer acid, the polymer acid is preferably polyacrylic acid or polymethacrylic acid; (f) the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate; (g) the coating source is one or more of a carbon material, a metal compound, and a conductive polymer; Wherein, when the coating source includes a carbon material, the carbon material is glucose, a glucose derivative, a second organic acid, an organic acid derivative, a phenolic resin, polyethylene, polyethylene glycol, polyvinyl alcohol, a polyvinyl alcohol derivative, polyacrylic acid, a polyacrylic acid derivative, a heterocyclic polymer or a condensation polymer containing N or O elements; Wherein, when the coating source comprises a metal compound, the metal compound comprises aluminum oxide and / or zinc stannate; Wherein, when the coating source includes a conductive polymer, the conductive polymer includes one or more of polyaniline, polystyrene thiol, polyacetylene, lithium carbonate and polycarbonate.

4. The method for preparing lithium iron phosphate according to claim 1, wherein: It satisfies one or more of the following conditions (a)-(d): (a) The mixture I may further include a catalyst, wherein the catalyst is a titanium-based catalyst; (b) the mass percentage concentration of the first organic acid in the first organic acid solution is 5-98%; (c) the coating source is used in an amount of 1% to 60%, where the percentage refers to the mass percentage of the coating source to the mass percentage of the phosphoric acid; (d) The grinding method is sand milling or ball milling.

5. The method for preparing lithium iron phosphate according to claim 1, wherein: It satisfies one or more of the following conditions (a)-(e): (a) The D50 particle size of the upper screening material after crushing is 1.1-1.6 nm; (b) The D50 particle size of the lower layer screened material after crushing is: 0.8-1.4nm; (c) In the lithium iron phosphate, the mass percentage of the crushed particles of the lower layer of screened material is 10wt%-40wt%; (d) The particle size of the upper screening material after crushing satisfies the following requirements: D10≤0.54μm, D99≤12.5μm, Dmax≤18μm; (e) The particle size of the lower layer screening material after crushing satisfies the following requirements: D10≤0.47μm; D99≤5.05μm; Dmax≤6.02μm.

6. The method for preparing lithium iron phosphate according to claim 1, wherein: It satisfies one or more of the following conditions (a)-(g): (a) the sintering temperature is 730°C-800°C; (b) the sintering temperature rise rate is 3-8°C / min; (c) the sintering time is 5h-20h; (d) the sintering is carried out under an inert atmosphere; (e) The mesh size of the sieve used for the screening is 150-400 mesh; (f) During the crushing process of the upper screened material, the air flow crushing frequency is 100 Hz to 140 Hz; (g) During the crushing process of the lower layer screening material, the air flow crushing frequency is 140HZ-180HZ.

7. A lithium iron phosphate, characterized in that It is prepared by the preparation method of lithium iron phosphate according to any one of claims 1 to 6.

8. Use of the lithium iron phosphate as claimed in claim 7 in a positive electrode material for lithium-ion batteries.

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