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

Abstract

The application discloses a lithium iron phosphate precursor, a lithium iron phosphate material and a preparation method and application 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 agglomeration of primary particles through a coating layer; and the material of the primary particles is lithium iron phosphate. The lithium iron phosphate material has good electrochemical performance and processing performance when used in a battery.

Description

Technical Field

[0001] This invention specifically relates to lithium iron phosphate precursors, lithium iron phosphate materials, their preparation methods, and applications. Background Technology

[0002] In recent years, new energy batteries such as lithium-ion batteries and sodium-ion batteries have developed rapidly. In particular, the cathode materials of lithium-ion batteries have been extensively studied, among which lithium iron phosphate (LiFePO4) and lithium manganese iron phosphate (LiMnFePO4) have received great attention. Lithium iron phosphate, in particular, has become the most attractive cathode material today due to its characteristics in terms of cycle performance, price, safety, and specific energy.

[0003] Currently, the main manufacturing processes for industrialized lithium iron phosphate (LFP) products are liquid-phase and solid-phase methods, with the solid-phase method being the dominant one. Liquid-phase methods include hydrothermal methods, sol-gel methods, and solvent evaporation methods, while solid-phase methods include high-temperature solid-phase methods, carbothermal reduction methods, and spray pyrolysis methods. Liquid-phase methods use soluble raw materials to achieve molecular-level mixing, followed by crystallization under high temperature and pressure to prepare nanoscale precursors. Solid-phase methods often use iron phosphate as a precursor, followed by physical blending and drying to obtain the lithium iron phosphate precursor. Lithium iron phosphate is then produced through sintering.

[0004] Liquid-phase and solid-phase methods each have their advantages and disadvantages. For example, lithium iron phosphate particles prepared by the liquid-phase method have uniform particle size, high specific capacity, and good cycle performance, but poor processing performance. Lithium iron phosphate prepared by the solid-phase method has good processing performance, but poor cycle performance and low specific capacity. Often, one aspect of performance is sacrificed to compensate for others, making it impossible to simultaneously achieve both processing performance and electrochemical performance. For example, commonly used modification methods in the prior art include doping, nano-sizing, and carbon coating. Nano-sizing helps improve electrical performance but is not conducive to improving processing performance. In the process of preparing lithium iron phosphate precursors, the larger the grinding particle size, the better the processing performance of the finished lithium iron phosphate, and vice versa. For example, the lithium iron phosphate precursor prepared by Chinese patent CN113896182A has a small grinding particle size and good electrochemical performance, but loses some processing performance.

[0005] Therefore, how to prepare a lithium iron phosphate material that combines processing performance and electrochemical performance is an urgent problem to be solved in the field of cathode materials. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of existing lithium iron phosphate materials in achieving both good processing performance and electrochemical performance, and provides lithium iron phosphate precursors, lithium iron phosphate materials, their preparation methods, and applications. The lithium iron phosphate material of this invention exhibits excellent electrochemical and processing performance when used in batteries.

[0007] This invention provides a lithium iron phosphate material comprising a flower cluster structure, wherein the flower cluster structure comprises secondary particles, the secondary particles being formed by the agglomeration of primary particles through a coating layer; the material of the primary particles is lithium iron phosphate.

[0008] In this invention, the number of the secondary particles comprising the primary particles can be 2-512, preferably 10-450, for example 300, 400 or 450.

[0009] The method for testing the number of primary particles contained in the secondary particles in this invention involves observing the number of primary particles on the surface using a scanning electron microscope (SEM). The number of internal primary particles is estimated by statistically analyzing 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. Based on the SEM image, the average D50 value of the secondary particles and the average D50 value of the primary particles can be measured. 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 particle size of the primary particles.

[0010] In this invention, the lithium iron phosphate material further includes free primary particles. The percentage of the number of free primary particles to the total number of secondary particles and free primary particles is 0.1%-5%, preferably 0.1%-4.5%, more preferably 0.1%-3%, and even more preferably 0.1%-2%. The free primary particles refer to primary particles that have not agglomerated into secondary particles, and their number can be determined by scanning electron microscopy.

[0011] In this invention, the coating layer can be understood as a coating layer covering part or all of the surface of the primary particles. The presence of the coating layer reduces the voids formed by the aggregation of small particles, reduces the specific surface area of ​​the material, thereby reducing the surface energy of the material, which is beneficial to improving the processing performance of lithium iron phosphate materials.

[0012] In some embodiments, the coating layer is made of carbon material, and the thickness of the coating layer is 2-20 nm, for example 4-5 nm.

[0013] In this invention, the shape of the primary particles can be one or more of the following: spherical, ellipsoidal, rod-shaped, plate-shaped, and star-shaped.

[0014] In some embodiments, the primary particles are ellipsoidal, and the D50 of the primary particles is <300nm; the D90 of the primary particles is <600nm.

[0015] In some embodiments, the primary particles have the following proportions: 50% have a particle size of 30nm-140nm, 37.76% have a particle size of 140nm-200nm, 7.14% have a particle size of 200nm-270nm, and 5.1% have a particle size of 270nm-1000nm.

[0016] In this invention, the primary particles are nanoparticles, and the particle size of the primary particles is preferably 30nm-1000nm, more preferably 100nm-800nm, for example 150nm or 200nm.

[0017] In this invention, the D50 of the primary particles can be 100nm-300nm, and preferably the average D50 of the primary particles is 200nm.

[0018] In this invention, the crystallinity of the primary particles can be >96%.

[0019] In this invention, the D10 of the secondary particles can be ≥0.35μm.

[0020] In this invention, the D50 of the secondary particles can be 1-2.2 μm, and preferably the average D50 of the secondary particles is 1.6 μm.

[0021] In this invention, the D90 of the secondary particles can be ≤6.50μm.

[0022] In this invention, the secondary particles have a D99 < ≤ 12.50 μm.

[0023] In this invention, the particle size distribution of the secondary particles can be single-peaked, double-peaked, or multi-peaked. When the particle size distribution is tested using a laser particle size analyzer, a single peak will appear when the "general mode" is selected, while double-peaked or multi-peaked peaks will appear when the "multiple narrow peak mode" is selected.

[0024] The present invention also provides a method for preparing a lithium iron phosphate precursor, which includes the following steps:

[0025] S1. React the mixture of iron source and phosphoric acid solution, and grind the mixture after the reaction is complete to obtain product A;

[0026] A mixture of organic acid solution, lithium source, and coating source is reacted to obtain product B after the reaction is complete; the order of preparation of product A and product B is not limited;

[0027] S2. Grind the mixture of product A and product B to obtain lithium iron phosphate precursor;

[0028] The lithium iron phosphate precursor has a particle size of 800nm-2000nm.

[0029] In S2, preferably, the particle size of the lithium iron phosphate precursor is 800nm-1800nm, for example, 1000nm or 1400nm.

[0030] In this invention, the lithium iron phosphate precursor is an amorphous substance or a mixture of iron salts, lithium salts, and other substances formed by the accumulation of precipitates. Controlling the particle size of the lithium iron phosphate precursor simply changes the average size of the precipitate. The inventors accidentally discovered that controlling the particle size of the lithium iron phosphate precursor can form the lithium iron phosphate product with a secondary particle structure as described in this invention. When the particle size of the lithium iron phosphate precursor is small, the secondary particle structure is more easily destroyed in the subsequent airflow crushing process. Conversely, when the particle size of the lithium iron phosphate precursor is large, subsequent airflow crushing and other processes will not easily and completely destroy its secondary particle structure.

[0031] In this 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 this generally means that no gas is generated during the reaction.

[0032] In S1, the reaction is carried out under stirring conditions, preferably at a stirring speed of 25-50 Hz, for example, 30 Hz. The stirring speed affects the formation of precipitates, thereby affecting the particle size distribution.

[0033] In S1, as those skilled in the art will know, the present invention is a process method that includes the step of preparing iron phosphate and then preparing lithium iron phosphate. Therefore, those skilled in the art will know that the iron source mentioned here does not include iron phosphate.

[0034] Preferably, the iron source is a compound containing iron and oxygen elements, more preferably one or more of iron powder, ferric oxide, iron(II) oxide, and ferric nitrate, and even more preferably one or more of iron powder, ferric oxide, and iron(II) oxide.

[0035] Preferably, the iron content in the iron powder is 95 wt% or more, more preferably 99 wt% or more, even more preferably 99.5 wt% or more, for example 99.7 wt%.

[0036] 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.

[0037] 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.

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

[0039] In S1, preferably, the iron source has a mesh size of 100-1000 mesh, more preferably 200-500 mesh, for example 250 mesh or 300 mesh.

[0040] In S1, the phosphoric acid solution generally refers to an aqueous solution of phosphoric acid, and the mass percentage concentration of phosphoric acid in the phosphoric acid solution is preferably 20-85%, for example, 49%, 59% or 62%.

[0041] 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.

[0042] In S1, preferably, the reaction temperature of the mixture of iron source and phosphoric acid solution is 20-95°C, more preferably 30-90°C, for example 35°C, 45°C or 55°C.

[0043] In S1, preferably, the mixture of iron source and phosphoric acid solution is prepared by adding iron source to phosphoric acid solution under stirring.

[0044] In S1, preferably, the molar ratio of iron to phosphoric acid in the mixture of iron source and phosphoric acid solution is (0.94-1.05):1, more preferably (0.96-1.0):1, for example 0.98:1.

[0045] In step S1, the mixture of the iron source and the phosphoric acid solution also includes a catalyst. The catalyst has a catalytic effect during the reaction and can act as a dopant element to improve the conductivity of the lithium iron phosphate product after the reaction is complete.

[0046] Preferably, the catalyst is a titanium-based catalyst.

[0047] The catalyst is preferably added at an amount of 0.5 wt% to 2 wt% of the iron source.

[0048] The catalyst is preferably added by first mixing the catalyst with the phosphoric acid solution, and then adding the iron source.

[0049] In S1, the grinding operation and conditions can be conventional grinding operations, such as sand milling or ball milling.

[0050] 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 nanoscale horizontal sand mill), a basket mill, or a double-cone rod 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.

[0051] In S1, preferably, the viscosity of product A is 8000-20000 cps, more preferably 10000-20000 cps, for example 15000 cps.

[0052] In S1, preferably, the organic acid solution generally refers to an aqueous solution of organic acid, and the mass percentage concentration of organic acid in the organic acid solution is preferably 5-98%, for example 55%, 62% or 72%.

[0053] 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.

[0054] In S1, the organic acid can be a polymer copolymerized from unsaturated olefins containing carboxylic acids through free radical polymerization, such as poly(meth)acrylic acid.

[0055] 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.

[0056] In S1, preferably, 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.

[0057] In S1, the coating source can be one or more of carbon materials, metal compounds, and conductive polymers. The coating source can complex metal ions, which helps to achieve the unique structure of lithium iron phosphate. Preferably, the carbon material is 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 condensation polymers containing N or O elements.

[0058] The glucose derivative preferably includes at least one of glucose, sucrose, starch, and cyclodextrin.

[0059] The organic acid preferably includes at least one selected from formic acid, acetic acid, oxalic acid, citric acid, tartaric acid, and malic acid.

[0060] The polyacrylic acid derivative preferably includes polyacrylate.

[0061] The heterocyclic polymer containing N or O elements preferably includes polyvinylpyrrolidone.

[0062] The metal compound preferably includes aluminum oxide and / or zinc stannate.

[0063] Preferably, the conductive polymer includes one or more of polyaniline, polystyrene thiol, polyacetylene, lithium carbonate, and polycarbonate.

[0064] In some embodiments, the coating source is lithium carbonate and sucrose.

[0065] In some embodiments, the coating source is citric acid and polyacrylic acid.

[0066] In some embodiments, the coating source is lithium carbonate, sucrose, and polyvinylpyrrolidone.

[0067] In some embodiments, the coating source is a mixture of polyvinyl alcohol, cyclodextrin, and polyethylene glycol.

[0068] In S1, preferably, the amount of carbon source added accounts for 1%-60% of the mass percentage of the iron source, more preferably 5%-50%, and even more preferably 10%-40%.

[0069] In S1, preferably, the reaction temperature of the mixture of organic acid, lithium source and carbon source is 20-95°C, more preferably 30-90°C, for example 35°C, 40°C or 45°C.

[0070] In S1, preferably, the mixture of organic acid, lithium source and carbon source is prepared by adding lithium source and carbon source to organic acid solution under stirring.

[0071] In S2, the mixture of product A and product B is generally simply a mixture of product A and product B.

[0072] In S2, the grinding operation and conditions can be conventional in the art. The preferred embodiment of the grinding can be the same as in S1.

[0073] 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 grinding is stopped when the target particle size is reached. The corresponding time is not fixed, for example, 6-10 hours.

[0074] The present invention also provides a lithium iron phosphate precursor, which is prepared by the method for preparing lithium iron phosphate precursor as described above.

[0075] This invention also provides a method for preparing lithium iron phosphate material, which includes the following steps:

[0076] The lithium iron phosphate precursor, as described above, was sequentially spray-dried, sintered, and crushed.

[0077] In this invention, both the synthesis process and the post-processing process of the lithium iron phosphate precursor affect the formation of the secondary particle structure.

[0078] In the spray drying process, the air inlet temperature can be 280°C.

[0079] In the spray drying process, the outlet temperature can be 130°C.

[0080] The sintering conditions can be as follows: under a nitrogen atmosphere of 99.999% purity, the temperature is gradually increased from room temperature to 650℃-750℃ at a heating rate of 5℃ / min, and then held at 650℃-750℃ for 5-20 hours, followed by cooling to obtain the sintered product. Higher sintering temperatures make it easier for primary particles to fuse, thus disrupting the secondary particle structure.

[0081] The crushing conditions may be as follows: the sintered product is processed by an airflow crushing device to obtain the target finished positive electrode lithium iron phosphate material with a particle size D50 = 1.5-3 μm.

[0082] The present invention also provides a lithium iron phosphate material prepared by the preparation method described above.

[0083] The present invention also provides an application of the lithium iron phosphate material as a cathode material in lithium-ion batteries as described above.

[0084] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0085] The reagents and raw materials used in this invention are all commercially available.

[0086] The positive and progressive effects of this invention are as follows:

[0087] The lithium iron phosphate material provided by this invention has a special flower cluster structure and also has good processing performance and recycling performance. Attached Figure Description

[0088] Figure 1 The graph shows the cycle performance of the lithium iron phosphate materials prepared in Example 1 and Comparative Examples 1-2.

[0089] Figure 2 This is a microscopic morphology diagram of the lithium iron phosphate material prepared in Example 1.

[0090] Figure 3 The image shows the microstructure of the lithium iron phosphate material prepared in Comparative Example 1.

[0091] Figure 4 The image shows the microstructure of the lithium iron phosphate material prepared in Comparative Example 2.

[0092] Figure 5This is a size distribution diagram of the lithium iron phosphate material prepared in Example 1.

[0093] Figure 6 This is a size distribution diagram of the lithium iron phosphate material prepared in Comparative Example 1.

[0094] Figure 7 This is a size distribution diagram of the lithium iron phosphate material prepared in Comparative Example 2.

[0095] Figure 8 The image shows the transmission electron microscope (TEM) morphology of the lithium iron phosphate material prepared in Example 1.

[0096] Figure 9 The image shows the transmission electron microscope (TEM) morphology of the lithium iron phosphate material prepared in Comparative Example 1.

[0097] Figure 10 The image shows the transmission electron microscope (TEM) morphology of the lithium iron phosphate material prepared in Comparative Example 2.

[0098] Figure 11 This is a transmission electron microscope (TEM) image of the lithium iron phosphate material prepared in Example 1.

[0099] Figure 12 This is a transmission electron microscope (TEM) image of the lithium iron phosphate material prepared in Comparative Example 1.

[0100] Figure 13 The image shows the transmission electron microscope (TEM) dimensions of the lithium iron phosphate material prepared in Comparative Example 2. Detailed Implementation

[0101] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0102] Example 1

[0103] (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 diluted to a concentration of 49%. Under stirring (30 Hz), 3.25 kg of 200 mesh, 99% pure secondary reduced iron powder was slowly added. The reaction was carried out at 45°C. During the reaction, some gas was generated, and the color of the reactant gradually changed from grayish-black to grayish-white. When no more gas was generated, the material was put into a sand mill for sand milling. The grinding beads in the sand mill were 0.3 mm zirconia beads. During the sand milling process, its viscosity gradually increased to 15000 cps, and the color gradually turned pure white, yielding product A.

[0104] (2) According to the molar ratio of lithium to phosphorus of 1.04:1, 4 kg of citric acid was dissolved in 3.25 kg of deionized water to prepare a solution. Under stirring, 2.3 kg of battery-grade lithium carbonate and 1 kg of sucrose were gradually added to the solution and reacted at 40°C. During the reaction, a large amount of gas was generated. The reaction continued until no gas was generated and a transparent solution was formed, and product B was obtained.

[0105] (3) Add product B to product A, mix and stir, and the viscosity of the system will drop rapidly to 1000 cps. Continue grinding for 6-7 hours. When the particle size of the material is ground to D50 of about 1800 nm, the reaction ends and a slurry with a solid content of 50% is obtained.

[0106] (4) The reaction product slurry is spray-dried, sintered and crushed to obtain lithium iron phosphate cathode material.

[0107] The spray drying conditions were as follows: inlet temperature 280℃ and outlet temperature 110℃. The calcination conditions were as follows: under a nitrogen atmosphere of 99.999% purity, the temperature was gradually increased from room temperature to 650℃ at a rate of 5℃ / min, held at 650℃ for 10 hours, and then cooled to obtain the sintered product. The crushing conditions were as follows: the sintered product was processed by an airflow crusher to obtain lithium iron phosphate material with a flower-like structure and a particle size D50 of 1.5-3µm.

[0108] Example 2

[0109] 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.

[0110] Example 3

[0111] The difference between Example 3 and Example 1 is that in step (2), 2.3 kg of battery-grade lithium carbonate, 0.5 kg of sucrose, and 0.3 kg of polyvinylpyrrolidone are gradually added to the solution while it is being stirred.

[0112] Comparative Example 1

[0113] Liquid phase products

[0114] Comparative Example 2

[0115] solid phase products

[0116] Comparative Example 3

[0117] Compared to Example 1, Comparative Example 3 had a grinding time of 8-10 hours and a sand mill particle size of 200 nm. Due to the reduced particle size, the slurry viscosity increased, and the slurry solid content was adjusted to 35%. Other aspects remained basically the same.

[0118] Comparative Example 4

[0119] Compared with Example 1, Comparative Example 3 had a grinding time of 4-5 hours, a sand particle size of 2500nm, and other aspects remained basically the same.

[0120] Effect Example

[0121] I. Electrochemical Performance

[0122] The lithium iron phosphate materials prepared in the examples and comparative examples were mixed with carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 80:10:10, coated onto aluminum foil, dried, and made into suitable positive electrode test pieces. These were then combined with lithium metal to form a 2032 button cell.

[0123] 1. Discharge specific capacity

[0124] Using a charge / discharge tester (Landian CT3002A) 1.1 within the charge / discharge range of 2.0V-4.2V, tests were conducted at 0.1C, 0.2C, and 0.5C rates, as well as at 0.5C charge-to-1C discharge, 0.5C charge-to-2C discharge, 0.5C charge-to-5C discharge, 0.5C charge-to-10C discharge, and 0.5C charge-to-20C rates. (Data may be rounded; the unit of test data is mAh / g.)

[0125] Table 1

[0126] sample 0.1C 0.2C 0.5C 1C 2C 5C 10C 20C Example 1 161 158 153 147 139 123 108 87 Example 2 161 159 153 147 138 124 110 89 Example 3 160 158 153 146 137 122 108 89 Comparative Example 1 161 158 151 144 135 117 101 79 Comparative Example 2 160 159 154 148 138 119 98 66 Comparative Example 3 161 158 154 147 139 124 113 35 Comparative Example 4 153 148 143 134 130 115 89 72

[0127] 2. Power charge / discharge test

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

[0129] Power doubling test: Charge and discharge at 0.1P power, charge and discharge at 0.33P power, charge and discharge at 1P power and discharge at 0.33P power, charge at 2P power and discharge at 0.33P power, charge at 5P power and discharge at 0.33P power, charge at 7P power and discharge at 0.33P power, and charge at 10P power and discharge at 0.33P power, and perform a rate doubling test according to the above steps. Each cycle is tested twice. The test results are shown in Table 2.

[0130] Power discharge test: Charge and discharge cycles were performed at 0.1P power, 0.33P power, 0.33P power charging to 1P power discharging, 0.33P power charging to 2P power discharging, 0.33P power charging to 5P power discharging, 0.33P power charging to 7P power discharging, and 0.33P power charging to 10P power discharging. Each cycle was tested twice. The test results are shown in Table 3. (Data may be rounded; the unit of test data is mAh / g.)

[0131] Table 2

[0132]

[0133] Table 3

[0134]

[0135] 3. Cyclic performance

[0136] The lithium iron phosphate materials prepared in the examples and comparative examples were assembled into pouch cells for testing. The specific steps are as follows:

[0137] ① Preparation of positive electrode sheet: 94g of lithium iron phosphate, the positive electrode active material prepared in the examples and comparative examples, 4g of polyvinylidene fluoride (PVDF) binder and 4g of acetylene black conductive agent were added to 80g of N-methylpyrrolidone to prepare a uniform positive electrode slurry. The slurry was coated on both sides of an aluminum foil with a thickness of 16μm, then dried at 120℃, rolled, and cut to prepare a positive electrode sheet of 540*43.5mm. The weight of the active material lithium iron phosphate was about 7.3g.

[0138] Preparation of negative electrode sheet: 94g of negative electrode active ingredient 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. The slurry was coated on both sides of a copper foil with a thickness of 8μm, dried at 90℃, rolled and cut to obtain a negative electrode sheet with a size of 400*44mm, which contains 3.6g of natural graphite active material.

[0139] ② Battery assembly

[0140] The above-mentioned positive electrode, negative electrode and polyethylene separator are wound into a square lithium-ion battery core. Then, lithium hexafluorophosphate (LiPF6) is dissolved at a concentration of 1 mol / L in a mixed solvent of EC / EMC / DEC = 1:1:1 as an electrolyte. The electrolyte is injected into the aluminum battery shell at a rate 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 is about 1050 mAh.

[0141] (1) Cyclic performance test at 25℃

[0142] The above-mentioned lithium-ion pouch batteries were charged at a constant current and constant voltage rate of 1C in a 25°C constant temperature chamber, with a cutoff voltage of 4.5V. After resting for 30 minutes, they were discharged from 4.5V to 2.0V at a current of 1C. Cyclic testing was performed, and the initial discharge capacity was recorded. The test was stopped when the remaining capacity was below 80%, and the number of cycles was recorded. The results are as follows: Figure 1 As shown in Table 4.

[0143] In Comparative Example 1, the liquid phase product decayed to 80% with 691 cycles. In Comparative Example 2, the solid phase product decayed to 80% with 402 cycles. In Comparative Example 4, which used a larger particle size of the phosphate precursor, the decayed to 80% with only 294 cycles. In contrast, the samples from Examples 1-3 decayed to 80% with over 1031 cycles. It can be seen that the samples from Examples 1-3 have a significant advantage.

[0144] Table 4

[0145]

[0146]

[0147] (2) Low-temperature capacity retention

[0148] The aforementioned lithium-ion pouch battery was charged and discharged at 0.2C under constant current and constant voltage charging mode at 23±2℃. Charging was continued at 0.2C until the limiting voltage of 4.2V was reached, then constant voltage charging was switched to 0.05C until the charging current was less than or equal to the 0.02C current value, at which point charging was stopped. The temperature of the high (low) temperature constant temperature chamber was adjusted to -10℃ and maintained for 3 hours. Discharge was then carried out at 0.2C until the termination voltage of 2V was reached, at which point the discharge was terminated. The ratio of the discharge specific capacity to the discharge specific capacity at 0.2C under 23±2℃ conditions was calculated.

[0149] The data in Table 5 show that the low-temperature capacity retention rates of Comparative Examples 1, 2, and 4 are all worse than those of the Example, indicating that the lithium iron phosphate product of the present invention has significant advantages in low-temperature applications.

[0150] Table 5

[0151] sample Low temperature retention rate (-10℃) Example 1 66.2％ Example 2 67.2％ Example 3 69.5％ Comparative Example 1 51.3％ Comparative Example 2 48.6％ Comparative Example 3 68.4％ Comparative Example 4 41.7％

[0152] (3) Self-discharge rate

[0153] At 25℃, after 30 minutes of resting, the charging cutoff voltage is 4.2V, the constant voltage charging cutoff current is 0.05C, and the discharging cutoff voltage is 2.0V. The discharge capacity is obtained by charging and discharging at 0.5C. After being placed at 55℃ for 7 days, the discharge specific capacity is obtained by discharging at 0.5C. The self-discharge rate is calculated by dividing the difference between the self-discharge rate and the initial capacity after 7 days at 55℃ by the initial capacity.

[0154] Table 6

[0155] sample Self-discharge rate Example 1 4.18％ Example 2 3.58％ Example 3 3.87％ Comparative Example 1 5.18％ Comparative Example 2 7.12％ Comparative Example 3 4.02％ Comparative Example 4 7.44％

[0156] As can be seen from the data in Table 6, the self-discharge rate of the lithium iron phosphate products in Examples 1-3 is lower than that of Comparative Examples 1, 2 and 4, and they have good capacity retention performance.

[0157] In summary, the lithium iron phosphate product of the present invention has better electrochemical performance than lithium iron phosphate products obtained by solid-phase and liquid-phase methods or by using precursors with larger particle sizes.

[0158] II. Processing Performance Test

[0159] Battery slurry preparation: 1200g of NMP, 75g of PVDF binder, and 64g of carbon black were added sequentially to the lithium iron phosphate materials prepared in the examples and comparative examples. The mixture was stirred at 2000r / min under vacuum for 5h. The slurry was filtered through a 200-mesh filter, and its viscosity, solid content, fineness, and electrode compaction density were tested.

[0160] Table 7

[0161]

[0162] As can be seen from the effect data in Table 7, the lithium iron phosphate product prepared by this 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.

[0163] In summary, the lithium iron phosphate material prepared by this invention simultaneously possesses excellent cycle performance, capacity retention, and processing performance. Furthermore, the preparation method is simple, environmentally friendly, produces no waste, and is low-cost. Moreover, the product of energy density and specific capacity with electrode compaction density shows a positive correlation; the higher the electrode compaction density, the more lithium iron phosphate cathode material can be placed per unit volume, resulting in a larger capacity. This demonstrates that the lithium iron phosphate material prepared by this invention also exhibits good energy density.

[0164] III. Structural Characterization

[0165] 1. Specific surface area test

[0166] The specific surface area was measured using a specific surface area meter (model: TB400, manufacturer: Beijing Jingwei Gaobo). After degassing at 220℃ for 2 hours, a certain mass of the sample was weighed, and the specific surface area and isothermal adsorption-desorption curves were measured respectively.

[0167] 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 micropores or porous morphology.

[0168] As shown in Table 8, the specific surface area of ​​the multi-point BET in Example 1 is 11.20 m². 2 / g, Comparative Example 1 (prepared by liquid phase method) has a multi-point BET specific surface area of ​​10.94m². 2 / g, Comparative Example 2 (prepared by solid-state method) has a multi-point BET specific surface area of ​​12.26m². 2 / g. The specific surface area of ​​Example 1 is at a moderate level. Combined with the microstructure analysis results, this indicates that the formation of secondary particles by primary carbon coating can effectively reduce the specific surface area.

[0169] Table 8

[0170]

[0171]

[0172] 2. Microscopic morphology comparison

[0173] Microscopic morphology was tested using a field emission high-resolution scanning electron microscope (model: Tescan Mira 3XH, manufacturer: Tescan, Czech Republic). Figure 2 , Figure 3 and Figure 4 The images show the microstructures of the lithium iron phosphate materials prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively. Figure 5 , Figure 6 and Figure 7 The figures show the size distribution of lithium iron phosphate materials prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively. It can be seen that compared with Comparative Examples 1 and 2, the particles in Example 1 have a narrower particle size distribution and smaller particles, and the primary particles form secondary particles through the coating layer.

[0174] Table 9

[0175] sample The number of primary particles in secondary particles The proportion of free primary particles Example 1 2-300 ＜2％ Example 2 2-400 ＜3％ Example 3 2-400 ＜4.5％ Comparative Example 1 2-50 ＞95％ Comparative Example 2 2-100 ＞98％ Comparative Example 3 2-300 ＜4％ Comparative Example 4 2-450 ＜2％

[0176] 3. XRD and crystallinity measurement

[0177] 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.

[0178] According to the data in Table 10, the XRD test results show that the examples and comparative examples are all lithium iron phosphate products with an olivine crystal structure and corresponding PDF card numbers of 40-1499. The XRD crystal structures of the three are consistent, but the crystallinity (Xc) is different.

[0179] Table 10

[0180]

[0181]

[0182] 4. Raman spectroscopy test

[0183] The degree of graphitization of lithium iron phosphate was measured three times using a Raman spectrometer (model: LABRAM HR800, manufacturer: HORIBA, USA), and the average value was taken.

[0184] ID corresponds to disordered carbon structure, and IG corresponds to ordered graphitized carbon structure. The smaller the ratio of ID / IG, the higher the degree of graphitization. According to the data in Table 11, the lithium iron phosphate material in Example 1 has a low proportion of disordered carbon structure and a high degree of graphitization, which is beneficial to the transport of lithium ions.

[0185] Table 11

[0186] sample first The second The third ID / IG Example 1 0.9150 0.9317 0.8878 0.9115 Example 2 1.0015 0.9938 0.9722 0.9892 Example 3 0.9893 0.9849 0.9817 0.9853 Comparative Example 1 0.9757 0.9912 0.9589 0.9753 Comparative Example 2 1.0687 0.9724 0.9815 1.0075 Comparative Example 3 0.9939 0.950 0.9783 0.9743 Comparative Example 4 1.0393 1.0801 1.0041 1.041

[0187] 5. Transmission electron microscopy test

[0188] The microstructure of lithium iron phosphate was tested using transmission electron microscopy. Figure 8 , Figure 9 and Figure 10 Transmission electron microscope (TEM) images of the lithium iron phosphate materials prepared in Example 1, Comparative Example 1, and Comparative Example 2. Figure 11 , Figure 12 and Figure 13 The images show transmission electron microscopy (TEM) dimensions of the lithium iron phosphate materials prepared in Examples 1, 1, and 2. Combined with the data in Table 12, it can be seen that the carbon layer thickness of the lithium iron phosphate products prepared in Examples 1-3 and 1-4 is between 4-10 nm, indicating that they possess a carbon-coated structure.

[0189] Table 12

[0190] sample carbon layer structure carbon layer thickness Example 1 Carbon coating 4-5nm Example 2 carbon coating 4-5nm Example 3 Carbon coating 4-5nm Comparative Example 1 Carbon coating 4-5nm Comparative Example 2 Carbon coating 5-9nm Comparative Example 3 carbon coating 4-5nm Comparative Example 4 carbon coating 6-10nm

[0191] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are 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 within the protection scope of the present invention.

Claims

1. A lithium iron phosphate material, characterized in that, The application relates to a lithium iron phosphate material, which comprises a flower cluster structure, wherein the flower cluster structure comprises secondary particles formed by agglomeration of primary particles through a coating layer; the material of the primary particles is lithium iron phosphate; the D50 of the primary particles is 100-300 nm; the D50 of the secondary particles is 1-2.2 mu m; the lithium iron phosphate material further comprises free primary particles, and the percentage of the number of the free primary particles in the total number of the secondary particles and the free primary particles is 0.1-5%. The preparation method of the lithium iron phosphate material comprises the following steps: sequentially drying, sintering and crushing a lithium iron phosphate precursor to obtain the lithium iron phosphate material. The preparation method of the lithium iron phosphate precursor comprises the following steps: S1. reacting a mixture of an iron source and a phosphoric acid solution, and grinding the product after the reaction to obtain product A; reacting a mixture of an organic acid solution, a lithium source and a coating source, and obtaining product B after the reaction; the preparation sequence of the product A and the product B is not limited; S2. grinding a 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 800-2000 nm; In step S1, the coating source is sucrose and polyacrylic acid, or sucrose and polyvinylpyrrolidone; the addition amount of the coating source accounts for 1-60% of the mass percentage of the iron source; and the reaction temperature of the mixture of the organic acid, the lithium source and the coating source is 20-95 DEG C.

2. The lithium iron phosphate material of claim 1, wherein, The number of the primary particles contained in the secondary particles is 2-512; And / or, the thickness of the coating layer is 2-20 nm.

3. The lithium iron phosphate material of claim 2, wherein, The number of the primary particles contained in the secondary particles is 10-450; And / or, the percentage of the number of the free primary particles in the total number of the secondary particles and the free primary particles is 0.1-4.5%; And / or, the thickness of the coating layer is 4-5 nm.

4. The lithium iron phosphate material of claim 3, wherein, The number of the primary particles contained in the secondary particles is 300, 400 or 450; And / or, the percentage of the number of the free primary particles in the total number of the secondary particles and the free primary particles is 0.1-3%.

5. The lithium iron phosphate material of claim 4, wherein, The percentage of the number of the free primary particles in the total number of the secondary particles and the free primary particles is 0.1-2%.

6. The lithium iron phosphate material of claim 1, wherein, The primary particles are nanoparticles; 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, flaky and star-shaped; 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 mu m; (2) the D50 of the secondary particles is 1.6 mu m; (3) the D90 of the secondary particles is <=6.50 mu m; (4) the D99 of the secondary particles is <=12.50 mu m.

7. The lithium iron phosphate material of claim 6, wherein, The particle size of the primary particles ranges from 30 nm to 1000 nm; And / or, the primary particles are ellipsoidal, the D50 of the primary particles is <300 nm, and the D90 of the primary particles is <600 nm; And / or, in the primary particles, the proportion of particles with a particle size of 30 nm-140 nm is 50%, the proportion of particles with a particle size of 140 nm-200 nm is 37.76%, the proportion of particles with a particle size of 200 nm-270 nm is 7.14%, and the proportion of particles with a particle size of 270 nm-1000 nm is 5.1%.

8. The lithium iron phosphate material of claim 7, wherein, The particle size of the primary particles is 100 nm-800 nm. And / or, the D50 of the primary particles is 200 nm.

9. The lithium iron phosphate material of claim 8, wherein, The particle size of the primary particles is 150 nm or 200 nm.

10. The lithium iron phosphate material of claim 1, wherein, The particle size of the lithium iron phosphate precursor is 800 nm-1800 nm.

11. The lithium iron phosphate material of claim 1, wherein, The particle size of the lithium iron phosphate precursor is 1000 nm or 1400 nm.

12. The lithium iron phosphate material of claim 1, wherein, The iron source is a compound containing iron and oxygen elements; And / or, the mesh number of the iron source is 100-1000 mesh; And / or, the phosphoric acid solution refers to an aqueous phosphoric acid solution; 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℃; And / or, in the mixture of the iron source and the phosphoric acid solution, the molar ratio of iron to phosphoric acid is (0.94-1.05):1; And / or, the mixture of the iron source and the phosphoric acid solution further comprises a catalyst; And / or, the mass percentage concentration of organic acid in the organic acid solution is 5-98%; And / or, the organic acid in the organic acid solution is a carboxylic acid compound and / or ascorbic acid; And / or, the organic acid is a high molecular polymer formed by free radical polymerization and copolymerization of an unsaturated olefin containing a carboxylic acid; And / or, the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate; And / or, the molar ratio of lithium in the lithium source to phosphoric acid in the phosphoric acid solution is 0.98-1.05; 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 the organic acid solution under stirring; And / or, in S1, the reaction is carried out under stirring; And / or, in S2, the grinding is sand grinding or ball grinding; And / or, in S2, the grinding time is 6-10 h.

13. The lithium iron phosphate material of claim 12, wherein, The iron source is one or more of iron powder, diiron trioxide, triiron tetroxide and ferric nitrate; And / or, the mesh number of the iron source is 200-500 mesh; And / or, the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 20-85%; And / or, the reaction temperature of the mixture of the iron source and the phosphoric acid solution is 30-90℃; And / or, in the mixture of the iron source and the phosphoric acid solution, the molar ratio of iron to phosphoric acid is (0.96-1.0):1; And / or, the type of the catalyst is a titanium-based catalyst; And / or, the addition amount of the catalyst is 0.5wt%-2wt% of the addition amount of the iron source; And / or, the catalyst is added by first mixing the catalyst with the phosphoric acid solution and then adding the iron source; And / or, the mass percentage concentration of organic acid in the organic acid solution is 55%, 62% or 72%. And / or, the carboxylic acid compound is one or more of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid; And / or, the organic acid is polymethylacrylic acid or polyacrylic acid; And / or, the lithium source is lithium hydroxide monohydrate and / or lithium acetate; And / or, the lithium carbonate is 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 1.02, 1.03 or 1.04; And / or, the amount of the coating source added accounts for 5%-50% of the mass percentage of the iron source; And / or, the reaction temperature of the mixture of the organic acid, lithium source and coating source is 30-90℃; And / or, in S1, the stirring speed is 25-50Hz; And / or, in S2, the grinding is performed by using a sand mill.

14. The lithium iron phosphate material of claim 13, wherein, The iron source is one or more of iron powder, diiron trioxide and ferroferric oxide; And / or, the mesh number of the iron source is 250 mesh or 300 mesh; And / or, the mass percentage concentration of phosphoric acid in the phosphoric acid solution is 49%, 59% or 62%; And / or, the reaction temperature of the mixture of the iron source and the phosphoric acid solution is 35℃, 45℃ or 55℃; And / or, in the mixture of the iron source and the phosphoric acid solution, the molar ratio of iron to phosphoric acid is 0.98:1; And / or, the organic acid is citric acid and / or oxalic acid, or malic acid and / or tartaric acid; And / or, the amount of the coating source added accounts for 10%-40% of the mass percentage of the iron source; And / or, the reaction temperature of the mixture of the organic acid, lithium source and coating source is 35℃, 40℃ or 45℃; And / or, in S1, the stirring speed is 30Hz; And / or, in S2, the particle size of the grinding beads used in the sand mill is 0.1-3.0mm; And / or, the grinding beads used in the sand mill are zirconium oxide beads.

15. The lithium iron phosphate material of claim 14, wherein, In S2, the particle size of the grinding beads used in the sand mill is 0.3mm or 0.4mm.

16. The lithium iron phosphate material of claim 13 or 14, wherein, The content of iron in the iron powder is more than 95wt%; And / or, the iron powder is one or more of primary reduced iron powder, secondary reduced iron powder, carbonyl reduced iron powder and electrolytic iron powder; And / or, the purity of the diiron trioxide is more than 95wt%; And / or, the purity of the ferroferric oxide is more than 95wt%.

17. The lithium iron phosphate material of claim 16, wherein, The content of iron in the iron powder is more than 99wt%; And / or, the purity of the diiron trioxide is more than 99wt%; And / or, the purity of the ferroferric oxide is more than 99wt%.

18. The lithium iron phosphate material of claim 17, wherein, The content of iron in the iron powder is more than 99.5wt%; And / or, the purity of the diiron trioxide is more than 99.5wt%; And / or, the purity of the ferroferric oxide is more than 99.5wt%.

19. The lithium iron phosphate material of claim 18, wherein, The content of iron in the iron powder is more than 99.7wt%.

20. A method of producing the lithium iron phosphate material according to any one of claims 1 to 19, characterized in that, It comprises the following steps: drying, sintering and crushing the lithium iron phosphate precursor in sequence to obtain the lithium iron phosphate material.

21. The method of claim 20, wherein the lithium iron phosphate material is prepared by the steps of: The drying method is spray drying. ​ And / or, the sintering condition is: under the atmosphere of nitrogen gas with the purity of 99.999%, gradually increasing the temperature from room temperature to 650-750℃ at the rate of 5℃ / min, keeping the temperature of 650-750℃ for 5-20h, and then decreasing the temperature; And / or, the breaking method is air flow breaking.

22. The method for preparing lithium iron phosphate material as described in claim 21, characterized in that, The inlet temperature of the spray drying is 280℃; And / or, the outlet temperature of the spray drying is 130℃.

23. The application of the lithium iron phosphate material as claimed in any one of claims 1-19 as the positive electrode material in lithium ion battery.

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