Lithium iron phosphate material, preparation method and application thereof

Abstract

The application discloses a lithium iron phosphate material and a preparation method and application thereof. The preparation method of the lithium iron phosphate material comprises the following steps: S1, a first precursor is subjected to first sintering and first crushing to obtain an intermediate precursor; S2, the intermediate precursor is subjected to second sintering and second crushing to obtain the lithium iron phosphate material; wherein, in the step S1, the preparation of the first precursor comprises: grinding and drying a third mixture containing a first product and a second product to obtain the first precursor; the first product is obtained by reacting a first mixture containing an iron source and phosphoric acid; the second product is obtained by reacting a second mixture containing an organic acid and a lithium source; the iron source comprises elemental iron and / or a divalent iron compound; in the steps S1 and S2, the temperature of the first sintering is higher than that of the second sintering. The obtained lithium iron phosphate material can effectively reduce the generation of impurities while having high powder compaction density, and further used in batteries, can ensure excellent electrochemical performance of the obtained battery.

Description

Technical Field

[0001] The invention relates to a lithium iron phosphate material and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate cathode materials have become the preferred electrode materials in the field of new energy power batteries and energy storage due to their excellent safety performance, cycle stability, low cost and environmentally friendly characteristics. At present, the preparation of lithium iron phosphate materials with high powder compaction density mainly adopts two process routes: solid phase method and liquid phase method. Among them, the solid phase method uses iron phosphate, lithium carbonate, organic carbon source and dopant as raw materials, and optimizes the precursor particle size distribution through a grading process to improve the material powder compaction density. However, the existing technology has significant limitations: when the primary particle size of the iron phosphate precursor increases to a critical value (corresponding to a powder compaction density of about 2.65g / cm 3 ), further increasing the particle size will lead to the deterioration of the lithium salt diffusion kinetics during the sintering process, resulting in the degradation of the electrochemical properties of the material.

[0003] In addition, in the traditional lithium iron phosphate preparation process, its high-temperature sintering process easily induces the formation of iron-based impurities (such as magnetic byproducts such as Fe2P and Fe3P), which seriously affects the magnetic properties and cycle life of the material.

[0004] Therefore, there is an urgent need to develop a lithium iron phosphate material and its preparation process that can effectively increase the powder compaction density, reduce the generation of impurities, and thereby ensure the excellent electrochemical performance of the resulting battery. Summary of the Invention

[0005] To overcome the drawbacks of existing lithium iron phosphate material processes, such as difficulty in increasing powder compaction density and the easy formation of impurities, the present invention provides a lithium iron phosphate material, its preparation method, and its application. The lithium iron phosphate material produced by the present invention not only has a high powder compaction density but also effectively reduces the formation of impurities. Further, when used in batteries, it also ensures excellent electrochemical performance.

[0006] In order to achieve the above technical effects, the present invention provides the following technical solutions.

[0007] The present invention provides a method for preparing a lithium iron phosphate material, which comprises the following steps:

[0008] S1, the first precursor is subjected to a first sintering and a first crushing to obtain an intermediate precursor;

[0009] S2, the intermediate precursor is subjected to a second sintering and a second crushing to obtain the lithium iron phosphate material;

[0010] in,

[0011] In step S1, the preparation of the first precursor comprises: grinding and drying a third mixture containing a first product and a second product; the first product is prepared by reacting a first mixture containing an iron source and phosphoric acid; the second product is prepared by reacting a second mixture containing an organic acid and a lithium source; the iron source comprises elemental iron and / or a divalent iron compound;

[0012] In steps S1 and S2, the temperature of the first sintering is higher than the temperature of the second sintering.

[0013] In the present invention, the first product produced by reacting an iron-containing source (including elemental iron and / or a ferrous iron compound) with a first mixture of phosphoric acid is distinguished from commercially available ferric phosphate in that it is a mixture containing, among other substances, amorphous ferrous phosphate and ferrous dihydrogen phosphate (which can be considered an ferric phosphate precursor) and has a relatively small particle size. Therefore, the present invention's method for preparing a lithium iron phosphate precursor differs from prior art processes for directly preparing a lithium iron phosphate precursor from ferric phosphate.

[0014] In the preparation process of lithium iron phosphate, high-temperature sintering often helps to promote grain growth in the material, decompose organic matter and then coat carbon; however, research has found that the traditional process of directly preparing lithium iron phosphate precursors with iron phosphate is not suitable for high-temperature sintering at higher temperatures in actual operation. Even if a lower temperature is used for sintering first and then a high-temperature sintering is performed, it is difficult to avoid the formation of impurities and it is difficult to effectively control the particle size. The present invention has found that the precursor prepared based on the previous precursor preparation process can be sintered at a higher temperature to effectively reduce the formation of impurities, and then sintered at a lower temperature. Combined with two crushing steps, it can achieve an excellent particle size compounding effect and improve the powder compaction density.

[0015] In the present invention, the iron source does not include iron phosphate.

[0016] In the present invention, the elemental iron may be in the form of iron powder; the purity of the iron powder is preferably 95 wt% or greater, more preferably 99 wt% or greater, and even more preferably 99.5 wt% or greater, for example 99.7 wt%. The iron powder may be one or more of primary reduced iron powder, secondary reduced iron powder, carbonyl reduced iron powder, and electrolytic iron powder.

[0017] In the present invention, the ferrous compound refers to a compound containing at least ferrous iron, such as a salt containing ferrous iron or an oxide containing ferrous iron. Specifically, the ferrous iron oxide is, for example, ferrosoferric oxide.

[0018] The purity of the ferrosoferric oxide is preferably 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more.

[0019] In some embodiments, in step S1, the iron source further comprises a ferric iron compound; the ferric iron compound refers to a compound containing ferric iron, such as a salt containing ferric iron or an oxide containing ferric iron. Specifically, the ferric iron oxide is, for example, ferric oxide.

[0020] The purity of the ferric oxide is preferably 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more.

[0021] In some embodiments, in step S1, the iron source is in powder form, and the mesh size of the iron source is preferably 200-1000 mesh, more preferably 200-500 mesh, for example, 250 mesh or 300 mesh.

[0022] In some embodiments, in step S1, the molar ratio of the iron source to the phosphoric acid, calculated as phosphorus and iron, is (0.94-1.05):1, more preferably (0.96-1.0):1, for example, 0.9625:1, 0.9675:1, 0.97:1, 0.9725:1 or 0.98:1.

[0023] In the present invention, in step S1, the phosphoric acid may be conventional phosphoric acid in the art, such as industrial-grade phosphoric acid, food-grade phosphoric acid, appliance-grade phosphoric acid, or electronic-grade phosphoric acid.

[0024] In some embodiments, in step S1, the preparation of the first mixture comprises: adding the iron source to the phosphoric acid under stirring.

[0025] In some specific embodiments, in step S1, the first mixture further comprises water. The preparation of the first mixture preferably comprises: first mixing the water and the phosphoric acid to obtain a phosphoric acid solution, and then adding the iron source while stirring. The mass percentage concentration of phosphoric acid in the phosphoric acid solution is preferably 20% to 85%, for example, 49%, 59%, or 62%.

[0026] In some embodiments, in step S1, the reaction temperature of the first mixture is 20-95°C, preferably 30-90°C, for example 35°C, 45°C or 55°C.

[0027] In some embodiments, in step S1, after the reaction of the first mixture, a grinding step is further included; the grinding operation can be a conventional grinding operation in the art, such as ball milling or sand milling.

[0028] The sand milling device is preferably a sand mill, which is preferably a vertical sand mill, a horizontal sand mill (such as a nano-scale horizontal sand mill), a basket sand mill or a double-cone rod sand mill.

[0029] The grinding media used in the sand grinding process are preferably zirconium oxide grinding beads; the particle size of the zirconium oxide grinding beads is preferably 0.1-3.3 mm, such as 0.3 or 0.4 mm.

[0030] In some embodiments, in step S1, the organic acid is a carboxylic acid compound and / or ascorbic acid.

[0031] The carboxylic acid compound is preferably one or more of formic acid, acetic acid, propionic acid, sorbic acid, oxalic acid, salicylic acid, citric acid, malic acid and tartaric acid, such as acetic acid and / or tartaric acid.

[0032] In some embodiments, in step S1, the amount of the organic acid added is 30%-150%, for example, 115%; the percentage is the percentage of the mass of the organic acid to the mass of the phosphoric acid.

[0033] In some embodiments, in step S1, the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium nitrate, and lithium acetate, preferably one or more of lithium carbonate, lithium hydroxide monohydrate, and lithium acetate. The lithium carbonate is, for example, industrial-grade lithium carbonate or battery-grade lithium carbonate.

[0034] In some embodiments, in step S1, the molar ratio of the lithium source to the phosphoric acid is (0.98-1.05):1, for example, 1.025:1, 1.03:1 or 1.04:1, calculated based on phosphorus and lithium.

[0035] In some embodiments, in step S1, the preparation of the second mixture comprises: adding the lithium source to an organic acid under stirring.

[0036] In some preferred embodiments, in step S1, the second mixture further comprises a metal oxide, which can serve as a dopant to improve the ionic and electronic conductivity of the obtained lithium iron phosphate, thereby improving the electrical performance.

[0037] The metal oxide is preferably one or more of titanium oxide, vanadium oxide and niobium oxide, such as titanium dioxide.

[0038] Wherein, calculated on the basis of phosphorus element and metal element, the molar ratio of the metal oxide to the phosphoric acid is preferably (0.02-0.03):1, for example, 0.0225:1, 0.025:1 or 0.027:1.

[0039] In some preferred embodiments, in step S1, the preparation of the second mixture comprises: adding the lithium source and the metal oxide to the organic acid under stirring.

[0040] In some preferred embodiments, in step S1, the second mixture further comprises water. The preparation of the second mixture preferably comprises: firstly mixing the water and the organic acid to obtain an organic acid solution, and then adding the lithium source under stirring.

[0041] In some more preferred embodiments, in step S1, the second mixture further includes water and a metal oxide, and the preparation of the second mixture preferably includes: first mixing the water and the organic acid to obtain an organic acid solution, and then adding the lithium source and the metal oxide under stirring.

[0042] In some embodiments, in step S1, the reaction temperature of the second mixture is 20-95°C, more preferably 30-90°C, for example 35°C, 40°C or 45°C.

[0043] In some embodiments, in step S1, the solid content of the third mixture is 40%-50%, for example, 45%, where the percentage refers to the percentage of the mass of the solid in the third mixture to the total mass of the third mixture.

[0044] In some embodiments, in step S1, the average particle size of the solid particles in the third mixture is 800-2300 nm, preferably 1400-2300 nm, for example, 1500 nm or 2000 nm.

[0045] In some embodiments, in step S1, the viscosity of the third mixture is 500 cps-1000 cps.

[0046] In the present invention, in step S1, the drying and grinding operations can be conventional in the art.

[0047] In some embodiments, in step S1, the drying is spray drying.

[0048] Wherein, the air inlet temperature of the spray drying is preferably 250-280°C.

[0049] The outlet temperature of the spray drying is preferably 100-110°C, for example 105°C.

[0050] In some embodiments, in step S1, the grinding method is ball milling or sand milling.

[0051] The sand milling device is preferably a sand mill, which is preferably a vertical sand mill, a horizontal sand mill (such as a nano-scale horizontal sand mill), a basket sand mill or a double-cone rod sand mill.

[0052] The grinding media used in the sand grinding process are preferably zirconium oxide grinding beads; the particle size of the zirconium oxide grinding beads is preferably 0.1-3.3 mm, such as 0.3 or 0.4 mm.

[0053] In some embodiments, in steps S1 and S2, the temperature difference between the first sintering and the second sintering is 50°C or more, preferably 100-200°C, such as 150°C, 170°C or 190°C.

[0054] In some embodiments, in step S1, the temperature of the first sintering is 780°C-870°C, for example, 830°C or 850°C.

[0055] In some embodiments, in step S1, the first sintering time is 10-15 hours.

[0056] In some embodiments, in step S2, the temperature of the second sintering is 680°C-780°C.

[0057] In some embodiments, in step S2, the second sintering time is 2-5 hours.

[0058] In some embodiments, in steps S1 and S2, the first sintering and the second sintering are each independently performed in an inert atmosphere, such as nitrogen.

[0059] In some embodiments, in steps S1 and S2, the heating rate of heating to the first sintering temperature and the heating rate of heating to the second sintering temperature are each independently 1-5°C / min, for example 3°C / min.

[0060] In the present invention, in step S1 and step S2, the operations and conditions of the first crushing and the second crushing can be conventional in the art.

[0061] In some embodiments, in steps S1 and S2, the first crushing and the second crushing are each independently mechanical crushing and / or air flow crushing, such as air flow crushing.

[0062] The classifying wheel speed frequency of the pneumatic crushing is preferably 70-80 Hz, for example 75 Hz.

[0063] The crushing pressure of the pneumatic crushing is preferably 0.5-1.0 MPa, for example 0.8 MPa.

[0064] In some embodiments, in step S1, the particle size D50 of the intermediate precursor is 1.5-5 μm, for example, 1.76 μm, 1.79 μm, 1.82 μm, 1.84 μm, 1.86 μm or 1.87 μm.

[0065] In some embodiments, in step S2, before the second sintering, the intermediate precursor and water are further mixed to obtain a slurry, and then dried.

[0066] The solid content of the slurry is preferably 20%-50%, for example 40%; the percentage refers to the percentage of the mass of the solid in the slurry to the total mass of the slurry.

[0067] The drying method is preferably spray drying; the air inlet temperature of the spray drying is preferably 250-280°C; the air outlet temperature of the spray drying is preferably 100-110°C, for example, 105°C.

[0068] Preferably, the step of demagnetization is further included before drying; the demagnetization is preferably performed by a demagnetizer; the magnetic field strength of the demagnetizer is preferably 6000Gs-9000Gs, for example 8000Gs.

[0069] The present invention also provides a lithium iron phosphate material, which is prepared by the above-mentioned preparation method of the lithium iron phosphate material.

[0070] In some embodiments, the particle size D10 of the lithium iron phosphate material is 0.4-0.5 μm, for example, 0.45 μm, 0.46 μm, or 0.48 μm.

[0071] In some embodiments, the particle size D50 of the lithium iron phosphate material is 1.3-1.7 μm, for example, 1.34 μm, 1.38 μm, 1.41 μm, 1.47 μm, 1.58 μm, 1.59 μm, or 1.61 μm.

[0072] In some embodiments, the particle size D90 of the lithium iron phosphate material is 5-12 μm, for example, 5.75 μm, 5.95 μm, 6.26 μm, 9.72 μm, 10.47 μm, 10.59 μm, or 11.21 μm.

[0073] In some embodiments, the carbon content of the lithium iron phosphate material is 1.3%-1.5%, for example, 1.33%, 1.37%, 1.38%, 1.39%, 1.43% or 1.44%, 1.38%.

[0074] In some embodiments, the resistivity of the lithium iron phosphate material is 6-18 Ω·cm, for example, 6.14 Ω·cm, 7.65 Ω·cm, 7.87 Ω·cm, 8.21 Ω·cm, 8.58 Ω·cm, 9.47 Ω·cm, or 17.14 Ω·cm.

[0075] In some embodiments, the powder compaction density of the lithium iron phosphate material is 2.5-2.7 g / cm 3 , for example 2.58g / cm 3 , 2.59g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 or 2.67g / cm 3 .

[0076] In some embodiments, the specific surface area of ​​the lithium iron phosphate material is 12-14 m 2 / g, for example 12.26m 2 / g, 12.47m 2 / g, 12.64m 2 / g, 12.74m 2 / g, 12.84m 2 / g, 13.1m 2 / g or 13.20m 2 / g.

[0077] The present invention also provides a use of the lithium iron phosphate material as described above in a lithium ion battery.

[0078] On the basis of conforming to the common sense in this field, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

[0079] The reagents and raw materials used in the present invention are commercially available.

[0080] The positive progress effect of the present invention is:

[0081] The lithium iron phosphate material preparation method of the present invention is based on the synergistic effect of a precursor preparation process, a two-step sintering process, and a two-step crushing process. It can produce a lithium iron phosphate material with a high powder compaction density and effectively solve the problem of impurity control caused by high-temperature sintering. The content of impurity phases such as iron phosphide in the obtained material can be lower than 0.0001%. After further use in batteries, the excellent electrochemical performance of the obtained battery can be effectively guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Figure 1 This is the SEM image of the lithium iron phosphate material obtained in Example 5.

[0083] Figure 2 This is the particle size distribution diagram of the lithium iron phosphate material obtained in Example 5. DETAILED DESCRIPTION

[0084] The present invention is further illustrated by way of examples below, but the present invention is not limited to the scope of the examples. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0085] Example 1

[0086] Step S1: Preparation of intermediate precursor:

[0087] (1) According to the molar ratio of iron to phosphorus of 0.9725:1, 6.895 kg of 85% industrial-grade phosphoric acid was added to 5 L of deionized water and stirred to dilute the solution to obtain a phosphoric acid solution. 3.314 kg of 200-mesh, 99% purity secondary reduced iron powder was slowly added under stirring to obtain a first mixture. The mixture was then reacted at 45°C. During the reaction, some gas was generated and the color of the reactant gradually changed from gray-black to yellow-white. When no gas was generated during the reaction, the material was placed in a sand mill for sand grinding. The grinding beads in the sand mill were 0.3 mm zirconium oxide beads. During the sand grinding process, the viscosity gradually increased to 4500 cps and the color remained yellow-white, thereby obtaining the first product.

[0088] (2) According to the molar ratio of lithium to phosphorus being 1.025:1, 2.964 kg of acetic acid was dissolved in 3.792 kg of tartaric acid and 3.25 kg of deionized water to prepare an organic acid solution. 2.27 kg of battery-grade lithium carbonate and 108.05 g of titanium dioxide were gradually added to the organic acid solution under stirring to obtain a second mixture. The mixture was then reacted at 40°C, during which a large amount of gas was generated. The reaction was continued until no gas was generated to obtain a second product.

[0089] (3) The second product is added to the first product, and the mixture is mixed and stirred to obtain a third mixture, and the viscosity of the system is rapidly reduced to 500 cps-1000 cps, and the solid content is 45%.

[0090] (4) Grinding the obtained third mixture, and when the material particle size D50 is ground to about 800 nm, spray drying is performed to obtain the first precursor.

[0091] The spray drying conditions are as follows: the air inlet temperature is 280°C and the air outlet temperature is 105°C.

[0092] (5) The obtained first precursor is subjected to a first sintering and a first crushing to obtain an intermediate precursor, the average particle size of which is 1.87 μm.

[0093] The first sintering condition is: heating from room temperature to 850° C. at a heating rate of 3° C. / min for 10 h.

[0094] Among them, the first crushing condition is: the classifying wheel speed frequency is 75Hz, and the crushing pressure is 0.8MPa.

[0095] Step S2: Preparation of lithium iron phosphate material:

[0096] (1) The intermediate precursor and deionized water were mixed to form a slurry with a solid content of 40%, and the slurry was demagnetized in a slurry remover with a magnetic field strength of 8000 Gs to remove magnetic substances and iron phosphide impurities, and the intermediate precursor was graded at the same time.

[0097] (2) The demagnetized slurry is dried, sintered, and crushed to obtain lithium iron phosphate material.

[0098] The drying conditions are as follows: the air inlet temperature is 280°C and the air outlet temperature is 105°C.

[0099] The second sintering condition is: heating from room temperature to 680° C. at a heating rate of 3° C. / min for 5 h.

[0100] Among them, the second crushing condition is: the classifying wheel speed frequency is 75Hz, and the crushing pressure is 0.8MPa.

[0101] Examples 2-7 and Comparative Examples 1-4

[0102] With reference to the contents of Example 1, lithium iron phosphate materials of Examples 2-7 and Comparative Examples 1-4 were prepared according to the process parameters in Table 1.

[0103] Table 1

[0104]

[0105] In Table 1, the calculation method of each raw material feed ratio is as follows:

[0106] (1) Based on the molar number of phosphorus in 85% phosphoric acid, the phosphoric acid feed ratio, iron powder feed ratio, lithium carbonate feed ratio (calculated as lithium element) and titanium dioxide (calculated as titanium element) feed ratio are obtained;

[0107] (2) Based on the total mass of 85% phosphoric acid, the acetic acid feed ratio and the tartaric acid feed ratio are obtained.

[0108] The only difference between Comparative Examples 1 and 2 compared to Example 1 is that ferric phosphates with iron-to-phosphorus ratios of 0.9725 and 0.9700, respectively, were first prepared using a conventional liquid-phase precipitation method in the art, and then subsequent treatment was performed with reference to steps S1 (2) to (5) and S2 of Example 1. The specific steps of the liquid-phase precipitation method are as follows:

[0109] (1) Preparation of iron phosphate with an iron-phosphorus ratio of 0.9725:

[0110] ①Solution preparation

[0111] Weigh 392.79 g of Fe(NO3)3·9H2O, dissolve it in about 500 mL of deionized water, and stir until it is completely dissolved to obtain solution A.

[0112] Weigh 115.29 g of 85% (w / w) H3PO4, carefully dilute with about 300 mL of deionized water, and stir well to obtain Solution B.

[0113] Pour an appropriate amount of 25%-28% (w / w) ammonia water (about 100-200 mL) into the dropping funnel and set aside.

[0114] ②Precipitation reaction

[0115] Add solution A dropwise to solution B. Simultaneously, add aqueous ammonia via a dropping funnel to adjust the pH to between 1.8 and 2.0. Maintain the reaction temperature at 50 ± 2°C. After the addition of solution A is complete, continue to maintain temperature, stirring, and pH control, and age for 1.5 hours. Filter, wash, and dry to obtain a white or light yellow iron phosphate powder.

[0116] (2) Preparation of iron phosphate with an iron-phosphorus ratio of 0.9700:

[0117] Refer to step 2 above, only changing the amount of ferric nitrate added to 391.88 g.

[0118] The only difference between Comparative Example 3 and Example 1 is that the conditions of the first sintering and the second sintering (only the sintering temperature and time) are swapped.

[0119] The only difference between Comparative Example 4 and Example 1 is that it does not involve the second sintering and second crushing steps, that is, the intermediate precursor obtained by the first crushing is the final lithium iron phosphate material.

[0120] Effect Example 1 Physical and Chemical Parameters Test

[0121] The following tests were performed on the lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4:

[0122] (1) Resistivity test

[0123] The resistivity of the lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4 was tested using a powder resistivity tester (model: ST2742B) using a four-probe test at a pressure of 8 MPa.

[0124] (2) Carbon Content Test The lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4 were tested respectively, and the test was carried out in accordance with the national standard: GBT 223.86-2009 Determination of total carbon content of steel and alloys by infrared absorption method after induction furnace combustion.

[0125] (3) Powder compaction density test

[0126] The lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4 were tested respectively. The compaction density of the lithium iron phosphate material powder was tested using an electronic pressure testing machine (model UTM7305Z09) with a pressure parameter of 30 kN.

[0127] (4) Particle size test

[0128] The intermediate precursors and lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4 were tested respectively using a laser particle size analyzer with a refractive index of 1.8% and an equipment model: Malvern 3000.

[0129] (5) Specific surface area BET

[0130] The lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4 were tested, respectively, according to the national standard GB / T33822-2017 for measuring specific surface area per unit mass. The degassing temperature was 220°C and the degassing time was 1 hour. (Equipment manufacturer: Beijing Jingwei Gaobo, model: JW-TB400).

[0131] (6) Magnetic material content

[0132] The intermediate precursors and lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4 were tested respectively, with reference to the industry standard: SJ / T 11795-2022.

[0133] Effect Example 2 Pole piece compaction density test

[0134] For the lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4, pole pieces were prepared according to the following steps, and then the pole piece compaction density was tested:

[0135] The lithium iron phosphate material, conductive carbon black and polyvinylidene fluoride obtained in Examples 1-7 and Comparative Examples 1-4 were respectively dissolved in N-methylpyrrolidone (NMP) solution in a mass ratio of 97:1.2:1.8, and stirred in a vacuum mixer for 3 hours. The solid content of the slurry was controlled to be 60% and the viscosity was 6000-8000 mPa·s to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on aluminum foil and then placed in a vacuum drying oven at 60°C for 4 hours. After drying, it was punched into square positive electrode sheets with a width of 5 cm and a length of 10 cm. The surface density of the electrode sheet was controlled to be 40 mg / cm 2, and roll the pole piece on a roller press to test the pole piece compaction density. Under the condition of an elongation of 0.6%, the pole piece will not peel off when folded forward and backward, and light transmission and breakage are used as standards to test the pole piece compaction density.

[0136] Effect Example 3 Battery Performance Test

[0137] For the lithium iron phosphate materials obtained in Examples 1-7 and Comparative Examples 1-4, button batteries were prepared according to the following steps, and then the relevant electrical properties of the obtained batteries were tested:

[0138] (1) Positive electrode production

[0139] The lithium iron phosphate material, conductive carbon black, and polyvinylidene fluoride obtained in Examples 1-7 and Comparative Examples 1-4 were dissolved in an NMP solution at a mass ratio of 90:5:5, and stirred in a vacuum mixer for 3 hours. The solid content of the slurry was controlled to 50% to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on aluminum foil and then dried in a vacuum drying oven at 120°C for 12 hours. After drying, the slurry was punched into 12 mm diameter discs to serve as the positive electrode sheets.

[0140] (2) Production of button batteries

[0141] A lithium metal sheet was used as the negative electrode, a Celgard 2400 microporous membrane as the separator, and a 1.0 mol / L LiPF6 solution as the electrolyte. The solvent was a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a 1:1:1 volume ratio. The positive electrode sheet, negative electrode sheet, separator, and electrolyte were assembled into a CR2016 button cell in an argon-filled glove box.

[0142] (3) The electrical performance of the button battery prepared using the above method was tested.

[0143] During the test, the cut-off voltage of the charging process was 3.75V, and the cut-off voltage of the discharging process was 2.0V.

[0144] Charge at a constant current of 0.1C to a voltage of 3.75V, then switch to constant voltage charging to a current of 0.02C, and discharge at a constant current of 0.1C to a voltage of 2.0V. Take the 0.1C charge and discharge specific capacity as the test result.

[0145] Furthermore, the ratio of the 0.1C first discharge specific capacity to the 0.1C first charge specific capacity is taken as the 0.1C first efficiency.

[0146] Charge at a constant current of 1C to a voltage of 3.75V, then switch to constant voltage charging to a current of 0.02C, and discharge at a constant current of 1C to a voltage of 2.0V. Take the 1C discharge specific capacity as the test result.

[0147] The results are shown in Table 2-Table 3 and Figure 1-Figure 2 shown.

[0148] Table 2

[0149]

[0150] Table 3

[0151]

[0152] From the data in Tables 1 to 3, it can be seen that the lithium iron phosphate materials obtained in the embodiments of the present invention can meet the following requirements while having good particle size distribution: magnetic material content is less than 0.4ppm, carbon content is 1.33%-1.44%, resistivity is not higher than 18Ω·cm, specific surface area is 12-14m 2 / g, and the powder compaction density is as high as 2.58g / cm 3 And after actual use in batteries, it can simultaneously guarantee: 0.1C charge specific capacity is not less than 158.4mAh / g, 0.1C discharge specific capacity is not less than 156.9mAh / g, 1C discharge specific capacity is not less than 142.3mAh / g, and 0.1C first efficiency is not less than 99.05%.

[0153] Further, Figure 1 and Figure 2 The following are SEM images and particle size distribution diagrams of the lithium iron phosphate material obtained in Example 5. The results in these figures also show that the lithium iron phosphate material contains large primary particles (7 μm in size), medium particles, and small particles. The good gradation of these particles of different sizes can further increase the powder compaction density of the material while also achieving better electrical performance.

[0154] Compared with Example 2, and compared with Example 5, in the case of preparing iron phosphate by liquid phase precipitation method, even if the component formula and subsequent sintering and crushing processes are the same, the content of magnetic substance (i.e., impurity phase) is significantly increased in both the intermediate precursor obtained by the first crushing and the lithium iron phosphate material obtained by the second crushing; among them, even if the powder compaction density of Comparative Example 1 and Example 2 is equivalent, their D50, D99 and impurity phase content are higher. After finally being used in the battery, the charge and discharge performance is significantly reduced due to the difference in the diffusion of lithium in the precursor during the sintering process; and compared with Example 5, the powder compaction density of Comparative Example 2 is lower, and the D50, D99 and impurity phase content are higher. After finally being used in the battery, the charge and discharge performance is significantly reduced.

[0155] Compared with Example 1, Comparative Example 3 adopts a sintering process of first low temperature and then high temperature. Even though the compaction density of the powder is equivalent, its D10, D50, D99 and resistivity are significantly increased; and even though the content of magnetic substance in the intermediate precursor obtained by the first crushing is reduced, the magnetic substance in the final lithium iron phosphate material is significantly increased, and the electrical performance of the final battery is also significantly reduced.

[0156] Compared with Example 1, Comparative Example 4 only involves the first spray drying, the first sintering and the first crushing. Even though the powder compaction density is equivalent, its D10, D50, D99, magnetic material content and resistivity are significantly increased, and the electrical performance of the battery finally obtained is also significantly reduced.

[0157] In summary, in the present invention, based on a specific precursor preparation process and a two-step sintering process of high temperature followed by low temperature and a two-step crushing process, the magnetic material content and average particle size of the obtained lithium iron phosphate material can be significantly reduced, and a better particle size distribution can be obtained; after being used in batteries, better electrical performance can also be guaranteed.

Claims

1. A method for preparing a lithium iron phosphate material, characterized in that: The preparation method of the lithium iron phosphate material comprises the following steps: S1, the first precursor is subjected to a first sintering and a first crushing to obtain an intermediate precursor; S2, the intermediate precursor is subjected to a second sintering and a second crushing to obtain the lithium iron phosphate material; in, In step S1, the preparation of the first precursor comprises: grinding and drying a third mixture containing a first product and a second product; the first product is prepared by reacting a first mixture containing an iron source and phosphoric acid; the second product is prepared by reacting a second mixture containing an organic acid, a lithium source, and a metal oxide; the iron source comprises elemental iron and / or a divalent iron compound; In step S1, the organic acid is acetic acid and / or tartaric acid; the amount of the organic acid added is 115%-150%; the percentage is the percentage of the mass of the organic acid to the mass of the phosphoric acid; in step S1, the metal oxide is one or more of titanium oxide, vanadium oxide, and niobium oxide; the molar ratio of the metal oxide to the phosphoric acid is (0.02-0.03):1, calculated based on phosphorus element and metal element; In step S1, the particle size D50 of the intermediate precursor is 1.5-5 μm; In steps S1 and S2, the temperature of the first sintering is higher than the temperature of the second sintering; the temperature of the first sintering is 780°C-870°C; the temperature of the second sintering is 680°C-780°C.

2. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In steps S1 and S2, the temperature difference between the first sintering and the second sintering is greater than 50°C; (2) In step S1, the first sintering time is 10-15 hours; (3) In step S2, the second sintering time is 2-5 hours; (4) In steps S1 and S2, the heating rate to the first sintering temperature and the heating rate to the second sintering temperature are each independently 1-5°C / min; (5) In steps S1 and S2, the first crushing and the second crushing are each independently mechanical crushing and / or air flow crushing; (6) In step S2, before the second sintering, the intermediate precursor and water are mixed to obtain a slurry, and then dried.

3. The method for preparing the lithium iron phosphate material according to claim 2, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In steps S1 and S2, the temperature difference between the first sintering and the second sintering is 100-200°C; (2) In steps S1 and S2, in the first crushing and the second crushing, the classifying wheel speed frequency of the air flow crushing is independently 70-80 Hz; (3) In steps S1 and S2, in the first crushing and the second crushing, the crushing pressure of the pneumatic crushing is independently 0.5-1.0 MPa; (4) In step S2, the solid content of the slurry in the step of "mixing to obtain a slurry and then drying" is 20%-50%; the percentage refers to the percentage of the mass of the solids in the slurry to the total mass of the slurry; (5) In step S2, the drying method in the step of "mixing to obtain a slurry and then drying" is spray drying; (6) In step S2, the step of "mixing to obtain a slurry and then drying" also includes a demagnetization step before drying.

4. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the divalent iron compound includes ferrosoferric oxide; (2) In step S1, the iron source further includes a trivalent iron compound; (3) In step S1, the iron source is in powder form; (4) In step S1, the molar ratio of the iron source to the phosphoric acid is (0.94-1.05):1, calculated based on phosphorus and iron. (5) In step S1, the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium nitrate and lithium acetate; (6) In step S1, the molar ratio of the lithium source to the phosphoric acid is (0.98-1.05):1, calculated based on phosphorus element and lithium element.

5. The method for preparing the lithium iron phosphate material according to claim 4, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the mesh size of the iron source is 200-1000 mesh; (2) In step S1, the trivalent iron compound includes ferric oxide; (3) In step S1, the molar ratio of the iron source to the phosphoric acid is (0.96-1.0):1, calculated based on phosphorus and iron. (4) In step S1, the lithium source is one or more of lithium carbonate, lithium hydroxide monohydrate and lithium acetate.

6. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the preparation of the first mixture includes: adding the iron source to the phosphoric acid under stirring; (2) In step S1, the first mixture further includes water; (3) In step S1, the reaction temperature of the first mixture is 20-95°C; (4) In step S1, after the reaction of the first mixture, a grinding step is further included; (5) In step S1, the preparation of the second mixture includes: adding the lithium source to the organic acid under stirring; (6) In step S1, the second mixture further includes water; (8) In step S1, the reaction temperature of the second mixture is 20-95°C.

7. The method for preparing the lithium iron phosphate material according to claim 6, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the preparation of the first mixture includes: first mixing the water and phosphoric acid to obtain a phosphoric acid solution, and then adding the iron source under stirring; (2) In step S1, the reaction temperature of the first mixture is 30-90°C; (3) In step S1, the preparation of the second mixture includes: adding the lithium source and the metal oxide to the organic acid under stirring; Alternatively, the water and the organic acid are first mixed to obtain an organic acid solution, and then the lithium source and the metal oxide are added under stirring; (4) In step S1, the reaction temperature of the second mixture is 30-90°C.

8. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the solid content of the third mixture is 40%-50%, where the percentage refers to the percentage of the mass of the solid in the third mixture to the total mass of the third mixture; (2) In step S1, the average particle size of the solid particles in the third mixture is 800-2300 nm; (3) In step S1, the drying method is spray drying; (4) In step S1, the grinding method is ball milling or sand milling.

9. A lithium iron phosphate material, characterized in that: The lithium iron phosphate material is prepared by the preparation method of the lithium iron phosphate material according to any one of claims 1 to 8.

10. The lithium iron phosphate material according to claim 9, characterized in that The lithium iron phosphate material meets one or more of the following conditions: (1) The particle size D10 of the lithium iron phosphate material is 0.4-0.5 μm; (2) The particle size D50 of the lithium iron phosphate material is 1.3-1.7 μm; (3) The particle size D99 of the lithium iron phosphate material is 5-12 μm; (4) The carbon content of the lithium iron phosphate material is 1.3%-1.5%; (5) The resistivity of the lithium iron phosphate material is 6-18Ω·cm; (6) The powder compaction density of the lithium iron phosphate material is 2.5-2.7 g / cm 3 ; (7) The specific surface area of ​​the lithium iron phosphate material is 12-14m 2 / g.

11. Use of the lithium iron phosphate material according to claim 9 or 10 in a lithium ion battery.

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