Lithium iron phosphate precursor, lithium iron phosphate material, preparation method therefor, and use thereof

Abstract

Disclosed are a lithium iron phosphate precursor, a lithium iron phosphate material, a preparation method therefor, and a use thereof. The lithium iron phosphate material comprises a flower cluster structure, the flower cluster structure comprises secondary particles, and the secondary particles are formed by agglomerating primary particles by means of a coating layer; the material of the primary particles is lithium iron phosphate. The lithium iron phosphate material of the present invention has good electrochemical properties and processing properties when used in batteries.

Description

Lithium iron phosphate precursor, lithium iron phosphate material, preparation method and application thereof Technical Field

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

[0002] In recent years, new energy batteries such as lithium-ion batteries and sodium-ion batteries have experienced rapid development. In particular, cathode materials for lithium-ion batteries have been extensively researched, with lithium iron phosphate (LiFePO4) and lithium manganese iron phosphate (LiMnFePO4) receiving significant attention. LiFePO4, in particular, has become the most attractive cathode material today due to its advantages in cyclability, price, safety, and specific energy.

[0003] Currently, industrial lithium iron phosphate production primarily utilizes liquid-phase and solid-phase methods, with solid-phase methods predominating. Liquid-phase methods include hydrothermal, sol-gel, and solvent evaporation, while solid-phase methods include high-temperature solid-phase, carbothermal reduction, and spray pyrolysis. Liquid-phase methods utilize soluble raw materials to achieve molecular-level mixing, then crystallize under high temperature and high pressure to produce a nanoscale precursor. Solid-phase methods typically utilize iron phosphate as a precursor, then physically blend and dry to produce the lithium iron phosphate precursor. Lithium iron phosphate is then produced through sintering.

[0004] The liquid phase method and the solid phase method each have their own advantages and disadvantages. For example, the lithium iron phosphate prepared by the liquid phase method has uniform particle size, high specific capacity, and good cycle performance, but its processing performance is poor. The lithium iron phosphate prepared by the solid phase method has good processing performance, but its cycle performance is poor and its specific capacity is low. Often, the performance of a certain aspect is sacrificed to compensate for other performances, and it is impossible to take into account both processing performance and electrochemical performance at the same time. For example, the commonly used modification methods in the prior art include: doping, nano-sizing and carbon coating. Among them, nano-sizing helps to improve electrical performance, but is not conducive to improving processing performance. In the process of preparing lithium iron phosphate precursors, the larger the sand-milling particle size, the better the processing performance of the corresponding finished lithium iron phosphate, and vice versa. The better the electrical performance. For example, the lithium iron phosphate precursor prepared by Chinese patent CN113896182A has a small sand-milling particle size and good electrochemical performance, but loses some processing performance.

[0005] Therefore, how to prepare a lithium iron phosphate material with both processing performance and electrochemical properties is an urgent problem to be solved in the field of positive electrode materials.

[0006] Summary of the Invention

[0007] The present invention primarily aims to overcome the drawback of prior art lithium iron phosphate materials, which lack both good processing and electrochemical properties, by providing a lithium iron phosphate precursor, a lithium iron phosphate material, and its preparation method and application. The lithium iron phosphate material of the present invention exhibits excellent electrochemical and processing properties when used in batteries.

[0008] The present invention provides a lithium iron phosphate material, which includes a flower cluster structure. The flower cluster structure includes secondary particles. The secondary particles are formed by agglomerating primary particles through a coating layer. The material of the primary particles is lithium iron phosphate.

[0009] In the present invention, the number of the primary particles contained in the secondary particles may be 2-512, preferably 10-450, such as 300, 400 or 450.

[0010] The method for testing the number of primary particles contained in the secondary particles of the present invention is to observe the number of surface primary particles using a scanning electron microscope, and estimate the number of internal primary particles by calculating the average size of the surface primary particles. The number of primary particles contained in the secondary particles is the sum of the number of surface primary particles and the number of internal primary particles. The average D50 of the secondary particles and the average D50 of the primary particles can be measured based on the scanning electron microscope image. The number of primary particles in the secondary particles is estimated based on the ratio of the cube of the D50 particle size of the secondary particles to the cube of the D50 of the primary particles.

[0011] In the present invention, the lithium iron phosphate material further includes free primary particles, the number of which accounts for 0.1% to 5% of the total number of secondary particles and free primary particles, preferably 0.1% to 4.5%, more preferably 0.1% to 3%, and even more preferably 0.1% to 2%. The free primary particles refer to primary particles that have not aggregated into secondary particles, and their number can be determined by scanning electron microscopy.

[0012] In the present invention, the coating layer can be understood as a coating layer covering part or all of the surface of the primary particle. The presence of the coating layer reduces the voids formed by the aggregation of small particles, reduces the specific surface area of ​​the material, and thus reduces the surface energy of the material, which is beneficial to improving the processing performance of the lithium iron phosphate material.

[0013] In some embodiments, the raw material of the coating layer is a carbon material, and the thickness of the coating layer is 2-20 nm, such as 4-5 nm.

[0014] In the present invention, the shape of the primary particles may be one or more of spherical, ellipsoidal, rod-shaped, flake-shaped and star-shaped.

[0015] In some embodiments, the primary particles are ellipsoidal, and the D50 of the primary particles is less than 300 nm; and the D90 of the primary particles is less than 600 nm.

[0016] In some embodiments, among the primary particles, the particle size of 30nm-140nm accounts for 50%, the particle size of 140nm-200nm accounts for 37.76%, the particle size of 200nm-270nm accounts for 7.14%, and the particle size of 270nm-1000nm accounts for 5.1%.

[0017] In the present invention, the primary particles are nanoparticles, and the particle size of the primary particles is preferably 30 nm-1000 nm, more preferably 100 nm-800 nm, such as 150 nm or 200 nm.

[0018] In the present invention, the D50 of the primary particles may be 100 nm to 300 nm, and preferably the average value of the D50 of the primary particles is 200 nm.

[0019] In the present invention, the crystallinity of the primary particles can be greater than 96%.

[0020] In the present invention, the D10 of the secondary particles may be ≥0.35 μm.

[0021] In the present invention, the D50 of the secondary particles may be 1-2.2 μm, and preferably the average value of the D50 of the secondary particles is 1.6 μm.

[0022] In the present invention, the D90 of the secondary particles may be ≤6.50 μm.

[0023] In the present invention, D99 of the secondary particles is less than or equal to 12.50 μm.

[0024] In the present invention, the particle size distribution of the secondary particles can be unimodal, bimodal, or multimodal. When the particle size distribution is measured using a laser particle size analyzer, a unimodal distribution appears when the "general mode" is selected, and a bimodal or multimodal distribution appears when the "multiple narrow peaks mode" is selected.

[0025] The present invention also provides a method for preparing a lithium iron phosphate precursor, which comprises the following steps:

[0026] S1. reacting a mixture of an iron source and a phosphoric acid solution, and grinding the mixture after the reaction to obtain a product A;

[0027] reacting a mixture of an 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;

[0028] S2. Grinding the mixture of the product A and the product B to obtain a lithium iron phosphate precursor;

[0029] Wherein, the particle size of the lithium iron phosphate precursor is 800nm-2000nm.

[0030] In S2, preferably, the particle size of the lithium iron phosphate precursor is 800 nm-1800 nm, for example, 1000 nm or 1400 nm.

[0031] In the present invention, the lithium iron phosphate precursor is an amorphous material or low-crystallinity iron salt, lithium salt, or mixture thereof formed by the accumulation of precipitates. Controlling the particle size of the lithium iron phosphate precursor merely changes the average size of the precipitate. The inventors discovered through research that controlling the particle size of the lithium iron phosphate precursor can produce the lithium iron phosphate product of the present invention having a secondary particle structure. When the particle size of the lithium iron phosphate precursor is small, the secondary particle structure is more easily destroyed during the subsequent air flow crushing process. Conversely, when the particle size of the lithium iron phosphate precursor is large, subsequent air flow crushing and other processes will not easily and completely destroy its secondary particle structure.

[0032] In the present invention, in step S1, the mixing reaction of the iron source and the phosphoric acid solution is completed. Those skilled in the art know that generally, the reaction is completed when no gas is generated.

[0033] In S1, the reaction is carried out under stirring, and the stirring speed is preferably 25-50 Hz, for example 30 Hz. The stirring speed will affect the formation of the precipitate, thereby affecting the particle size distribution.

[0034] In S1, those skilled in the art know that the iron source herein is a process for preparing lithium iron phosphate after comprising the steps of preparing iron phosphate. Therefore, those skilled in the art know that the iron source here does not include iron phosphate.

[0035] Preferably, the iron source is a compound containing iron and oxygen, more preferably one or more of iron powder, ferric oxide, ferroferric oxide and ferric nitrate, and even more preferably one or more of iron powder, ferric oxide and ferroferric oxide.

[0036] Preferably, the iron content in the iron powder is 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more, for example 99.7 wt %.

[0037] 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.

[0038] Preferably, the purity of the ferric oxide is 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more.

[0039] Preferably, the purity of the ferrosoferric oxide is 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more.

[0040] In S1, preferably, the mesh size of the iron source is 100-1000 mesh, more preferably 200-500 mesh, for example, 250 mesh or 300 mesh.

[0041] In S1, the phosphoric acid solution generally refers to a phosphoric acid aqueous solution. The mass percentage concentration of phosphoric acid in the phosphoric acid solution is preferably 20-85%, for example, 49%, 59% or 62%.

[0042] In S1, the phosphoric acid in the phosphoric acid solution can be conventional phosphoric acid in the art, such as industrial-grade phosphoric acid, food-grade phosphoric acid, electrical-grade phosphoric acid, or electronic-grade phosphoric acid. The electrical-grade phosphoric acid can be purchased from Guangxi Qinzhou Chengxing Chemical Technology Co., Ltd.

[0043] In S1, preferably, the reaction temperature of the mixture of the iron source and the phosphoric acid solution is 20-95°C, more preferably 30-90°C, for example 35°C, 45°C or 55°C.

[0044] In S1, preferably, the mixture of the iron source and the phosphoric acid solution is prepared by adding the iron source to the phosphoric acid solution under stirring.

[0045] In S1, preferably, in the mixture of the iron source and the phosphoric acid solution, the molar ratio of the iron element to the phosphoric acid is (0.94-1.05):1, more preferably (0.96-1.0):1, for example 0.98:1.

[0046] In S1, the mixture of the iron source and the phosphoric acid solution further includes a catalyst. The catalyst has a catalytic effect during the reaction process and can serve as a doping element to improve the conductivity of the lithium iron phosphate product after the reaction is completed.

[0047] The catalyst is preferably a titanium-based catalyst.

[0048] Wherein, the added amount of the catalyst is preferably 0.5wt%-2wt% of the added amount of the iron source.

[0049] The catalyst is preferably added by first mixing the catalyst with the phosphoric acid solution and then adding the iron source.

[0050] In S1, the grinding operation and conditions can be conventional grinding operations, such as sand milling or ball milling.

[0051] Preferably, the grinding is performed using a sand mill. The sand mill is preferably a vertical sand mill, a horizontal sand mill (e.g., a nano-scale horizontal sand mill), a basket sand mill, or a double-cone rod sand mill. The particle size of the grinding beads used in the sand mill is preferably 0.1-3.0 mm, for example, 0.3 mm or 0.4 mm. The grinding beads used in the sand mill are preferably zirconia beads.

[0052] In S1, preferably, the viscosity of the product A is 8000-20000 cps, more preferably 10000-20000 cps, for example 15000 cps.

[0053] In S1, preferably, the organic acid solution generally refers to an organic acid aqueous solution, and the mass percentage concentration of the organic acid in the organic acid solution is preferably 5-98%, for example, 55%, 62% or 72%.

[0054] In S1, preferably, the organic acid in the organic acid solution is a carboxylic acid compound and / or ascorbic acid, and the carboxylic acid compound is preferably one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid; the organic acid is, for example, citric acid and / or oxalic acid, or malic acid and / or tartaric acid.

[0055] In S1, the organic acid may be a high molecular weight polymer obtained by copolymerizing an unsaturated olefin containing a carboxylic acid through free radical polymerization, such as poly(meth)acrylic acid.

[0056] In S1, preferably, the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate, more preferably lithium hydroxide monohydrate and / or lithium acetate; the lithium carbonate is preferably industrial grade lithium carbonate or battery grade lithium carbonate.

[0057] In S1, preferably, the molar ratio of the lithium element in the lithium source to the phosphoric acid in the phosphoric acid solution is 0.98-1.05, for example, 1.02, 1.03 or 1.04.

[0058] In S1, the coating source may be one or more of a carbon material, a metal compound, and a conductive polymer. The coating source can complex metal ions, helping to achieve the unique structure of lithium iron phosphate. The carbon material is preferably glucose, a glucose derivative, an organic acid, an organic acid derivative, a phenolic resin, polyethylene, polyethylene glycol, polyvinyl alcohol, a polyvinyl alcohol derivative, polyacrylic acid, a polyacrylic acid derivative, or a heterocyclic polymer or condensation polymer containing nitrogen or oxygen elements.

[0059] The glucose derivative preferably includes at least one of glucose, sucrose, starch and cyclodextrin.

[0060] The organic acid preferably includes at least one of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid.

[0061] The polyacrylic acid derivative preferably includes polyacrylate.

[0062] The heterocyclic polymer containing N or O elements preferably includes polyvinyl pyrrolidone.

[0063] Wherein, the metal compound preferably includes aluminum oxide and / or zinc stannate.

[0064] The conductive polymer preferably includes one or more of polyaniline, polystyrene thiol, polyacetylene, lithium carbonate and polycarbonate.

[0065] In some embodiments, the coating source is lithium carbonate and sucrose.

[0066] In some embodiments, the coating source is citric acid and polyacrylic acid.

[0067] In some embodiments, the coating source is lithium carbonate, sucrose, and polyvinyl pyrrolidone.

[0068] In some embodiments, the coating source is a mixture of polyvinyl alcohol, cyclodextrin, and polyethylene glycol.

[0069] In S1, preferably, the amount of the carbon source added accounts for 1% to 60% by mass of the iron source, more preferably 5% to 50% by mass, and even more preferably 10% to 40% by mass.

[0070] In S1, preferably, the reaction temperature of the mixture of the organic acid, the lithium source and the carbon source is 20-95°C, more preferably 30-90°C, for example 35°C, 40°C or 45°C.

[0071] In S1, preferably, the mixture of the organic acid, the lithium source and the carbon source is prepared by adding the lithium source and the carbon source to a stirred organic acid solution.

[0072] In S2, the mixture of the product A and the product B is generally prepared by mixing the product A and the product B.

[0073] In S2, the grinding operation and conditions can be conventional in the art. The preferred embodiment of the grinding can be the same as S1.

[0074] In S2, the grinding time can be conventional in the art and will be affected by factors such as the size of the zirconium beads, the feed rate, and the wear of the zirconium beads. Specifically, the average particle size of the slurry is controlled, and the sanding is stopped when the target particle size is reached. The corresponding time is not fixed, for example, 6-10 hours.

[0075] The present invention also provides a lithium iron phosphate precursor, which is prepared by the above-mentioned preparation method of the lithium iron phosphate precursor.

[0076] The present invention also provides a method for preparing a lithium iron phosphate material, which comprises the following steps:

[0077] The lithium iron phosphate precursor as described above is spray dried, sintered and crushed in sequence.

[0078] In the present invention, both the synthesis process and the post-treatment process of the lithium iron phosphate precursor will have an impact on the formation of the secondary particle structure.

[0079] Wherein, during the spray drying, the air inlet temperature may be 280°C.

[0080] Wherein, during the spray drying, the outlet temperature may be 130°C.

[0081] The sintering conditions may include gradually increasing the temperature from room temperature to 650°C-750°C at a rate of 5°C / min in a 99.999% pure nitrogen atmosphere, maintaining the temperature at 650°C-750°C for 5-20 hours, and then cooling to obtain a sintered product. The higher the sintering temperature, the more likely the primary particles are to fuse, thereby destroying the secondary particle structure.

[0082] The crushing conditions may be as follows: the sintered product is processed by air flow crushing equipment to obtain a target finished positive electrode lithium iron phosphate material with a particle size of D50=1.5-3um.

[0083] The present invention also provides a lithium iron phosphate material prepared by the above-mentioned preparation method.

[0084] The present invention also provides a use of the lithium iron phosphate material as described above as a positive electrode material in a lithium ion battery.

[0085] 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.

[0086] The reagents and raw materials used in the present invention are commercially available.

[0087] The positive progress effect of the present invention is:

[0088] The lithium iron phosphate material provided by the present invention has a special flower cluster structure and has both good processing performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] FIG1 is a graph showing the cycle performance of the lithium iron phosphate materials prepared in Example 1 and Comparative Examples 1-2.

[0090] FIG2 is a microscopic morphology of the lithium iron phosphate material prepared in Example 1.

[0091] FIG3 is a microscopic morphology of the lithium iron phosphate material prepared in Comparative Example 1.

[0092] FIG4 is a microscopic morphology of the lithium iron phosphate material prepared in Comparative Example 2.

[0093] FIG5 is a size distribution diagram of the lithium iron phosphate material prepared in Example 1.

[0094] FIG6 is a size distribution diagram of the lithium iron phosphate material prepared in Comparative Example 1.

[0095] FIG7 is a size distribution diagram of the lithium iron phosphate material prepared in Comparative Example 2.

[0096] FIG8 is a transmission electron microscope image of the lithium iron phosphate material prepared in Example 1.

[0097] FIG9 is a transmission electron microscope image of the lithium iron phosphate material prepared in Comparative Example 1.

[0098] FIG10 is a transmission electron microscope image of the lithium iron phosphate material prepared in Comparative Example 2.

[0099] FIG11 is a transmission electron microscope dimensional diagram of the lithium iron phosphate material prepared in Example 1.

[0100] FIG12 is a transmission electron microscope dimensional diagram of the lithium iron phosphate material prepared in Comparative Example 1.

[0101] FIG13 is a transmission electron microscope dimensional diagram of the lithium iron phosphate material prepared in Comparative Example 2. DETAILED DESCRIPTION

[0102] 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.

[0103] Example 1

[0104] (1) According to the molar ratio of iron to phosphorus of 0.96:1, 6.895 kg of 85% industrial-grade phosphoric acid was added to 5 L of deionized water and stirred to dilute to a concentration of 49%. Under stirring (rotation speed of 30 Hz), 3.25 kg of 200 mesh, 99% purity secondary reduced iron powder was slowly added. The reaction was carried out at 45°C. Some gas was generated during the reaction, and the color of the reactant gradually changed from gray-black to gray-white. When no gas was generated during the reaction, the material was put into 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 15,000 cps and the color gradually changed to pure white, thereby obtaining product A.

[0105] (2) According to the molar ratio of lithium to phosphorus being 1.04:1, 4 kg of citric acid was dissolved in 3.25 kg of deionized water to prepare a solution. 2.3 kg of battery-grade lithium carbonate and 1 kg of sucrose were gradually added to the solution under stirring. The solution was reacted at 40°C. During the reaction, a large amount of gas was generated. The reaction was continued until no gas was generated and a transparent solution was formed to obtain product B.

[0106] (3) Product B was added to product A. After mixing, the viscosity of the system rapidly decreased to 1000 cps. Sand milling was continued for 6-7 hours. When the particle size of the material was ground to D50 of about 1800 nm, the reaction was terminated to obtain a slurry with a solid content of 50%.

[0107] (4) The reaction product slurry is spray-dried, sintered, and crushed to obtain lithium iron phosphate positive electrode material.

[0108] The spray drying process was conducted under the following conditions: an inlet temperature of 280°C and an outlet temperature of 110°C. The calcination process was conducted under a 99.999% pure nitrogen atmosphere, with the temperature gradually increased from room temperature to 650°C at a rate of 5°C / min, maintained at 650°C for 10 hours, and then cooled to obtain a sintered product. The crushing process involved the sintered product being subjected to air flow crushing equipment to obtain the target finished product, a lithium iron phosphate material with a flower cluster structure and a particle size D50 of 1.5-3 μm.

[0109] Example 2

[0110] The difference between Example 2 and Example 1 is that in step (2), 3 kg of citric acid and 2 kg of polyacrylic acid are dissolved in 3.25 kg of deionized water to prepare a solution.

[0111] Example 3

[0112] Example 3 differs from Example 1 in that, in step (2), 2.3 kg of battery-grade lithium carbonate, 0.5 kg of sucrose, and 0.3 kg of polyvinyl pyrrolidone are gradually added to the solution under stirring.

[0113] Comparative Example 1

[0114] Liquid phase products

[0115] Comparative Example 2

[0116] Solid phase method products

[0117] Comparative Example 3

[0118] In Comparative Example 3, compared with Example 1, the grinding time is 8-10 hours, the sanding particle size is controlled to 200 nm, the slurry viscosity increases due to the reduction in particle size, the slurry solid content is adjusted to 35%, and the other parameters remain basically the same.

[0119] Comparative Example 4

[0120] Compared with Example 1, in Comparative Example 3, the grinding time is 4-5 hours, the sanding particle size is controlled to 2500 nm, and the other aspects are basically the same.

[0121] Effect embodiment

[0122] 1. Electrochemical performance

[0123] The lithium iron phosphate materials prepared in the examples and comparative examples were mixed with carbon black and polyvinylidene fluoride (PVDF) at a mass ratio of 80:10:10, coated on aluminum foil, and dried to form appropriate positive electrode test pieces. The mixture was then combined with lithium metal to form a 2032 button-type battery.

[0124] 1. Discharge specific capacity

[0125] Using a charger (Blue Electric CT3002A) 1.1 in the 2.0V-4.2V range, the battery was charged and discharged at rates of 0.1C, 0.2C, 0.5C, 0.5C charge and discharge, 1C discharge, 2C discharge, 5C charge and discharge, 10C discharge, and 20C charge. (The data may be rounded off, and the unit of test data is mAh / g.)

[0126] Table 1

[0127] 2. Power charge and discharge test

[0128] Power charge and discharge tests were performed using a charger and discharge machine (Blue Power CT3002A) within the voltage range of 2.5V-3.75V.

[0129] Power double charge test: charge and discharge at 0.1P power, charge and discharge at 0.33P power, charge and discharge at 1P power, discharge at 0.33P power, charge and discharge at 2P power, charge and discharge at 0.33P power, charge and discharge at 5P power, discharge and discharge at 0.33P power, charge and discharge at 7P power, discharge and discharge at 10P power, perform rate charge and discharge cycle test according to the above steps, test twice for each cycle, the test results are shown in Table 2.

[0130] Power discharge test: 0.1P power charge and discharge, 0.33P power charge and discharge, 0.33P power charge and discharge 1P power discharge, 0.33P charge and discharge 2P power discharge, 0.33P charge and discharge 5P power discharge, 0.33P charge and discharge 7P power discharge, 0.33P power charge and discharge 10P power discharge, follow the above steps to perform rate charge and discharge cycle test, each cycle test twice, test results are shown in Table 3. (Data can be rounded off, the unit of test data is mAh / g)

[0131] Table 2

[0132] Table 3

[0133] 3. Cycle performance

[0134] The lithium iron phosphate materials prepared in the embodiment and the comparative example were assembled into soft-pack batteries for testing. The specific steps are as follows:

[0135] ① Preparation of the positive electrode sheet: 94 g of the positive electrode active material lithium iron phosphate prepared in the embodiment and comparative example, 4 g of the binder polyvinylidene fluoride (PVDF), and 4 g of the conductive agent acetylene black were added to 80 g of N-methylpyrrolidone to prepare a uniform positive electrode slurry. The slurry was coated on both sides of a 16 μm thick aluminum foil, and then dried at 120°C, roll-pressed, and cut to prepare a 540*43.5 mm positive electrode sheet, wherein the active material lithium iron phosphate weighed about 7.3 g.

[0136] Preparation of negative electrode sheet: 94g of natural graphite, 1.4g of CMC, and 2g of conductive carbon black were added to 125g of deionized water, and 1.6g of SBR was added to prepare a uniform negative electrode slurry, which was coated on both sides of a copper foil with a thickness of 8μm, dried at 90°C, rolled, and cut to obtain a negative electrode sheet with a size of 400*44mm, which contained 3.6g of natural graphite active material.

[0137] ②Battery assembly

[0138] The above-mentioned positive electrode sheet, negative electrode sheet and polyethylene separator were respectively wound into the pole core of the square lithium-ion battery, and then lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent of EC / EMC / DEC = 1:1:1 at a concentration of 1 mol / L as an electrolyte. The electrolyte was injected into the battery aluminum shell at a dosage of 3.25 g / Ah and sealed to prepare a lithium-ion soft-pack battery. The discharge capacity of the lithium-ion soft-pack battery under 0.2C conditions was about 1050 mAh.

[0139] (1) Cyclic performance test at 25°C

[0140] The above-mentioned lithium-ion soft-pack battery was charged at a constant current and constant voltage rate of 1C in a constant temperature box at 25°C, with a cut-off voltage of 4.5V. After being left for 30 minutes, it was discharged from 4.5V to 2.0V at a current of 1C. The cycle test was performed and the first discharge capacity was recorded. The test was stopped when the remaining capacity was less than 80%, and the number of cycles was recorded. The results are shown in Figure 1 and Table 4.

[0141] In Comparative Example 1, when the liquid phase product decays to 80%, the number of cycles is 691; in Comparative Example 2, when the solid phase method decays to 80%, the number of cycles is 402; in Comparative Example 4, a larger particle size of the phosphoric acid precursor is selected, and when it decays to 80%, the number of cycles is only 294; when the samples of Examples 1-3 decay to 80%, the number of cycles is more than 1031. It can be seen that the samples of Examples 1-3 have obvious advantages.

[0142] Table 4

[0143] (2) Low temperature capacity retention rate

[0144] The lithium-ion soft-pack battery was charged and discharged at 0.2C in a constant current and constant voltage mode at 23±2°C. Charge at 0.2C to a limit voltage of 4.2V. Then, switch to a constant voltage mode of 0.05C until the charge current is less than or equal to the 0.02C current value, at which point charging was stopped. The high (low) temperature incubator was adjusted to -10°C for 3 hours. Discharge was terminated at 0.2C to a cutoff voltage of 2V. Calculate the ratio of the discharge specific capacity to the 0.2C discharge specific capacity at 23±2°C.

[0145] It can be seen from the data in Table 5 that the low-temperature capacity retention rates of Comparative Examples 1, 2 and 4 are all worse than those of the embodiment. The lithium iron phosphate product of the present invention has obvious advantages in application at low temperatures.

[0146] Table 5

[0147] (3) Self-discharge rate

[0148] After 30 minutes at 25°C, the charge cutoff voltage was 4.2V, the constant voltage charge cutoff current was 0.05C, and the discharge cutoff voltage was 2.0V. The discharge capacity was obtained by charging and discharging at 0.5C. The battery was then placed at 55°C for 7 days. A discharge rate of 0.5C was performed to obtain the specific discharge capacity. After 7 days at 55°C, the specific discharge capacity was measured at 0.5C. The difference between the specific discharge capacity and the initial capacity, divided by the initial capacity, was the self-discharge rate.

[0149] Table 6

[0150] According to the data in Table 6, it can be seen that the self-discharge rates of the lithium iron phosphate products of Examples 1-3 are lower than those of Comparative Examples 1, 2 and 4, and have good capacity retention performance.

[0151] In summary, the lithium iron phosphate product of the present invention has better electrochemical performance than lithium iron phosphate products obtained by solid-phase method and liquid-phase method or using precursors with larger particle sizes.

[0152] 2. Processing performance test

[0153] Battery slurry preparation: 1200g of NMP, 75g of PVDF binder, and 64g of carbon black were added to the lithium iron phosphate materials prepared in Example and Comparative Example, respectively. The mixture was stirred at 2000 rpm under vacuum for 5 hours. The slurry was filtered through a 200-mesh filter and tested for viscosity, solids content, fineness, and electrode compaction density.

[0154] Table 7

[0155] According to the effect data in Table 7, it can be seen that the lithium iron phosphate product prepared by the present invention can maintain a high solid content while having a low viscosity during processing, which is beneficial to reducing the energy consumption of subsequent processes and facilitating subsequent processing.

[0156] In summary, the lithium iron phosphate material produced by the present invention can simultaneously achieve excellent cycle performance, capacity retention, and processing performance. Furthermore, the preparation method is simple, environmentally friendly, and free of waste, waste, and waste gas emissions, all at low cost. Furthermore, the multiplication of energy density, specific capacity, and electrode compaction density shows a positive correlation. The greater the electrode compaction density, the more lithium iron phosphate cathode material can be placed per unit volume, resulting in a greater capacity. This indicates that the lithium iron phosphate material produced by the present invention also has excellent energy density.

[0157] 3. Structural Characterization

[0158] 1. Specific surface area test

[0159] The specific surface area was measured using a surface area meter (model: TB400, manufacturer: Beijing Jingwei Gaobo). After degassing at 220°C for 2 hours, a certain amount of sample was weighed and the specific surface area and isothermal adsorption-desorption curve were measured.

[0160] The results of the isothermal adsorption-desorption curve test show that the pore structure of the lithium iron phosphate product of the present invention is consistent with the structure of the products prepared in Comparative Examples 1 and 2, and both are narrow slit structures, indicating that the materials exist in a stacked manner and the secondary particles do not have microporous or porous morphology.

[0161] As shown in Table 8, the multi-point BET specific surface area of ​​Example 1 is 11.20 m 2 / g, the multi-point BET specific surface area of ​​Comparative Example 1 (liquid phase preparation) is 10.94m 2 / g, the multi-point BET specific surface area of ​​comparative example 2 (prepared by solid phase method) is 12.26m 2 The specific surface area of ​​Example 1 is at a medium level. Combined with the results of microscopic morphology analysis, it is shown that carbon coating of primary particles to form secondary particles can effectively reduce the specific surface area.

[0162] Table 8

[0163] 2. Microscopic morphology comparison

[0164] Micromorphology testing was performed using a field-emission high-resolution scanning electron microscope (Tescan Mira 3XH, manufacturer: Czech Tescan). Figures 2, 3, and 4 show the micromorphologies of the lithium iron phosphate materials produced in Example 1, Comparative Example 1, and Comparative Example 2, respectively. Figures 5, 6, and 7 show the size distributions of the lithium iron phosphate materials produced in Example 1, Comparative Example 1, and Comparative Example 2, respectively. It can be seen that Example 1 has a narrower particle size distribution and smaller particles than Comparative Examples 1 and 2, with the primary particles forming secondary particles through the coating layer.

[0165] Table 9

[0166] 3. XRD and crystallinity measurement

[0167] The crystal form and crystallinity of the powder were tested using an X-ray diffractometer (model: Ultima, manufacturer: Rigaku Corporation) with a copper target and a wavelength of 0.154 nm.

[0168] According to the data in Table 10, it can be seen that the XRD test results show that the embodiment and the comparative example are all lithium iron phosphate products with an olivine structure. The corresponding PDF card numbers are 40-1499. The XRD crystal forms of the three are consistent, but there are differences in crystallinity (Xc).

[0169] Table 10

[0170] 4. Raman spectroscopy test

[0171] The graphitization degree of lithium iron phosphate was tested three times using a Raman spectrometer (model: LABRAM HR800, manufacturer: HORIBA, USA), and the average value was taken.

[0172] ID corresponds to disordered carbon, IG corresponds to ordered graphitized carbon. The smaller the ratio of ID / IG, the higher the degree of graphitization. Combined with the data in Table 11, it can be seen that the lithium iron phosphate material of Example 1 has a small proportion of disordered carbon and a high degree of graphitization, which is conducive to the transmission of lithium ions.

[0173] Table 11

[0174] 5. Transmission electron microscopy test

[0175] The micromorphology of lithium iron phosphate was tested using a transmission electron microscope. Figures 8, 9, and 10 are transmission electron microscope morphology images of the lithium iron phosphate materials obtained in Example 1, Comparative Example 1, and Comparative Example 2. Figures 11, 12, and 13 are transmission electron microscope dimension images of the lithium iron phosphate materials obtained in Example 1, Comparative Example 1, and Comparative Example 2. Combined with the data in Table 12, it can be seen that the carbon layer thickness of the lithium iron phosphate products obtained in Examples 1-3 and the lithium iron phosphate products obtained in Comparative Examples 1-4 is between 4-10 nm, indicating that they have a carbon-coated structure.

[0176] Table 12

[0177] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0178] Although the above describes specific embodiments of the present invention, it should be understood by those skilled in the art that these are merely illustrative and that various changes or modifications may be made to these embodiments without departing from the principles and essence of the present invention. Therefore, the scope of protection of the present invention is defined by the appended claims.

Claims

1. A lithium iron phosphate material, characterized in that: It comprises a flower cluster structure, wherein the flower cluster structure comprises secondary particles, wherein the secondary particles are formed by agglomerating primary particles through a coating layer; and the material of the primary particles is lithium iron phosphate.

2. The lithium iron phosphate material according to claim 1, characterized in that: The number of the secondary particles including the primary particles is 2-512, preferably 10-450, for example 300, 400 or 450; And / or, the lithium iron phosphate material further includes free primary particles, the number of the free primary particles accounts for 0.1%-5% of the total number of secondary particles and free primary particles, preferably 0.1%-4.5%, more preferably 0.1%-3%, and further more preferably 0.1%-2%; And / or, the coating layer has a thickness of 2-20 nm, such as 4-5 nm.

3. The lithium iron phosphate material according to claim 1 or 2, characterized in that: The primary particles are nanoparticles, and the particle size of the primary particles is preferably in the range of 30 nm to 1000 nm, more preferably in the range of 100 nm to 800 nm, such as 150 nm or 200 nm; Preferably, the D50 of the primary particles is 100nm-300nm, more preferably 200nm; and / or, the crystallinity of the primary particles is >96%; and / or, the shape of the primary particles is one or more of spherical, ellipsoidal, rod-shaped, flake-shaped and star-shaped; Preferably, the primary particle is ellipsoidal, and the D50 of the primary particle is <300nm; the D90 of the primary particle is <600nm; Preferably, among the primary particles, the particle size of 30nm-140nm accounts for 50%, 140nm-200nm accounts for 37.76%, 200nm-270nm accounts for 7.14%, and 270nm-1000nm accounts for 5.1%; And / or, the particle size of the secondary particles satisfies at least one of the following conditions (1)-(4): (1) The D10 of the secondary particles is ≥ 0.35 μm; (2) The D50 of the secondary particles is 1-2.2 μm, preferably 1.6 μm; (3) The D90 of the secondary particles is ≤6.50 μm; (4) The D99 of the secondary particles is <≤12.50 μm.

4. A method for preparing a lithium iron phosphate precursor, characterized in that: It includes the following steps: S1. reacting a mixture of an iron source and a phosphoric acid solution, and grinding after the reaction to obtain a product A; reacting a mixture of an organic acid solution, a lithium source and a coating source, and obtaining a product B after the reaction is completed; the order of preparing the product A and the product B is not limited; S2. Grinding the mixture of the product A and the product B to obtain a lithium iron phosphate precursor; Wherein, the particle size of the lithium iron phosphate precursor is 800nm-2000nm.

5. The method for preparing a lithium iron phosphate precursor according to claim 4, characterized in that: The particle size of the lithium iron phosphate precursor is 800nm-1800nm, for example, 1000nm or 1400nm; And / or, the coating source is one or more of a carbon material, a metal compound and a conductive polymer; Wherein, the carbon material is preferably glucose, glucose derivatives, organic acids, organic acid derivatives, phenolic resins, polyethylene, polyethylene glycol, polyvinyl alcohol, polyvinyl alcohol derivatives, polyacrylic acid, polyacrylic acid derivatives, heterocyclic polymers or polycondensates containing N or O elements; The glucose derivative preferably includes at least one of glucose, sucrose, starch and cyclodextrin; The organic acid preferably includes at least one of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid; The polyacrylic acid derivative preferably includes polyacrylate; The heterocyclic polymer with N or O elements preferably includes polyvinyl pyrrolidone; Wherein, the metal compound preferably includes aluminum oxide and / or zinc stannate; Wherein, the conductive polymer preferably includes one or more of polyaniline, polystyrene thiol, polyacetylene, lithium carbonate and polycarbonate; Preferably, the coating source is lithium carbonate and sucrose; Preferably, the coating source is citric acid and polyacrylic acid; Preferably, the coating source is lithium carbonate, sucrose and polyvinyl pyrrolidone.

6. The method for preparing a lithium iron phosphate precursor according to claim 4 or 5, characterized in that: The iron source is a compound containing iron and oxygen, preferably one or more of iron powder, ferric oxide, ferroferric oxide and ferric nitrate, more preferably one or more of iron powder, ferric oxide and ferroferric oxide; Preferably, the iron content in the iron powder is 95wt% or more, more preferably 99wt% or more, and further preferably 99.5wt% or more, for example 99.7wt%. 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; Preferably, the purity of the ferric oxide is 95wt% or more, more preferably 99wt% or more, and even more preferably 99.5wt% or more; Preferably, the purity of the ferrosoferric oxide is 95wt% or more, more preferably 99wt% or more, and even more preferably 99.5wt% or more; And / or, the mesh size of the iron source is 100-1000 mesh, preferably 200-500 mesh, for example 250 mesh or 300 mesh; And / or, the phosphoric acid solution 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%; And / or, 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; and / or, the reaction temperature of the mixture of the iron source and the phosphoric acid solution is 20-95° C., preferably 30-90° C., for example 35° C., 45° C. or 55° C.; And / or, in the mixture of the iron source and the phosphoric acid solution, the molar ratio of the iron element to the phosphoric acid is (0.94-1.05):1, preferably (0.96-1.0):1, for example 0.98:1; And / or, the mixture of the iron source and the phosphoric acid solution further comprises a catalyst; the type of the catalyst is preferably a titanium-based catalyst; wherein the amount of the catalyst added is preferably 0.5wt%-2wt% of the amount of the iron source added; wherein the catalyst is preferably added by first mixing the catalyst with the phosphoric acid solution and then adding the iron source; And / or, the mass percentage concentration of the organic acid in the organic acid solution is 5-98%, for example, 55%, 62% or 72%; And / or, the organic acid in the organic acid solution is a carboxylic acid compound and / or ascorbic acid, and the carboxylic acid compound is preferably one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid; the organic acid is, for example, citric acid and / or oxalic acid, or malic acid and / or tartaric acid; And / or, the organic acid is a high molecular weight polymer obtained by free radical polymerization of unsaturated olefins containing carboxylic acid, such as poly(meth)acrylic acid or polyacrylic acid; And / or, the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate, preferably lithium hydroxide monohydrate and / or lithium acetate; the lithium carbonate is preferably industrial grade lithium carbonate or battery grade lithium carbonate; and / or, the molar ratio of lithium in the lithium source to phosphoric acid in the phosphoric acid solution is 0.98-1.05, for example, 1.02, 1.03 or 1.04; And / or, the added amount of the coating source accounts for 1%-60% by mass of the iron source, preferably 5%-50%, more preferably 10%-40% by mass; and / or, the reaction temperature of the mixture of the organic acid, the lithium source and the coating source is 20-95° C., more preferably 30-90° C., for example 35° C., 40° C. or 45° C.; And / or, the mixture of the organic acid, the lithium source and the coating source is prepared by adding the lithium source and the coating source into a stirred organic acid solution; And / or, in S1, the reaction is carried out under stirring, and the stirring speed is preferably 25-50 Hz, for example 30 Hz; And / or, in S2, the grinding is sand milling or ball milling; preferably, the grinding is performed using a sand mill, and the particle size of the grinding beads used in the sand mill is preferably 0.1-3.0 mm, for example, 0.3 mm or 0.4 mm; the grinding beads used in the sand mill are preferably zirconia beads; And / or, in S2, the grinding time is 6-10 hours.

7. A lithium iron phosphate precursor, characterized in that: It is obtained by the preparation method according to at least one of claims 4 to 6.

8. A method for preparing a lithium iron phosphate material, characterized in that: The method comprises the following steps: sequentially drying, sintering and crushing the lithium iron phosphate precursor as claimed in claim 7 to obtain the lithium iron phosphate material; Preferably, the drying method is spray drying; the air inlet temperature of the spray drying is preferably 280°C; the outlet temperature of the spray drying is preferably 130°C; Preferably, the sintering conditions are: in a 99.999% pure nitrogen atmosphere, gradually heating from room temperature to 650°C-750°C at a heating rate of 5°C / min, maintaining a constant temperature at 650°C-750°C for 5-20h, and then cooling; Preferably, the crushing method is air flow crushing.

9. A lithium iron phosphate material, characterized in that: It is prepared by the preparation method as claimed in claim 8.

10. Use of the lithium iron phosphate material according to at least one of claims 1 to 3 and 9 as a positive electrode material in a lithium ion battery.

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