TWI911872B - Lithium iron phosphate precursor, lithium iron phosphate material and preparation methods and applications thereof - Google Patents

Abstract

The invention discloses a lithium iron phosphate precursor, a lithium iron phosphate material and a preparation method and application thereof. The lithium iron phosphate material includes a flower cluster structure, and the flower cluster structure includes secondary particles. The secondary particles are formed by agglomeration of primary particles through 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 performance and processing performance when used in batteries.

Description

Lithium iron phosphate precursors, lithium iron phosphate materials, their preparation methods and applications

This invention specifically relates to lithium iron phosphate precursors, lithium iron phosphate materials, their preparation methods, and applications.

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.

Currently, the main industrial-scale preparation processes for lithium iron phosphate 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-state methods, carbothermal reduction methods, and spray pyrolysis methods. Liquid-phase methods use soluble raw materials to achieve molecular-level mixing, and then crystallize under high-temperature and high-pressure conditions to prepare nanoscale precursors. Solid-phase methods primarily use ferric phosphate as a precursor, which is then physically blended and dried to obtain the lithium iron phosphate precursor. Lithium iron phosphate is then produced through sintering.

Liquid-phase and solid-phase methods each have their advantages and disadvantages. For example, lithium iron phosphate prepared by the liquid-phase method has uniform particle size, high specific capacity, and good cycle performance, but its processability is poor. Lithium iron phosphate prepared by the solid-phase method has good processability, but its cycle performance is poor and its specific capacity is low. Often, one aspect of performance is sacrificed to compensate for others, making it impossible to simultaneously achieve both processability and electrochemical performance. For example, commonly used modification methods in existing technologies include doping, nano-processing, and carbon coating. Nano-processing helps improve electrical properties but is detrimental to improving processability. In the process of preparing lithium iron phosphate precursors, the larger the abrasive grain size, the better the processability of the resulting lithium iron phosphate, and vice versa. For example, the lithium iron phosphate precursor produced by Chinese patent CN113896182A has a small abrasive grain size and good electrochemical properties, but it sacrifices some machinability.

Therefore, how to prepare a lithium iron phosphate material that combines good processability and electrochemical properties is a pressing problem to be solved in the field of cathode materials.

This invention primarily aims to overcome the shortcomings of existing lithium iron phosphate materials in achieving a balance between processability and electrochemical performance. It 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 processability properties when used in batteries.

This invention provides a lithium iron phosphate material comprising a flower cluster structure, wherein the flower cluster structure includes secondary particles formed by the agglomeration of primary particles through a coating layer; the primary particles are made of lithium iron phosphate.

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

The method for testing the number of primary particles contained in secondary particles as described in this invention involves observing the number of surface primary particles using a scanning electron microscope, estimating the number of internal primary particles by statistically analyzing the average size of the surface primary particles, and determining that 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 from the scanning electron microscope image. The number of primary particles in the secondary particles is estimated by the ratio of the cube of the D50 particle size of the secondary particles to the cube of the D50 of the primary particles.

In this invention, the lithium iron phosphate material further comprises 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 observed using a scanning electron microscope.

In this invention, the coating layer can be understood as a partial or complete coverage of the primary particle surface. The presence of the coating layer reduces the voids formed by the aggregation of small particles, lowers 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 lithium iron phosphate materials.

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

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

In some embodiments, the primary particles are elliptical, and the D50 of the primary particles is <300 nm; the D90 of the primary particles is <600 nm.

In some embodiments, of the primary particles, particles with a diameter of 30nm-140nm account for 50%, particles with a diameter of 140nm-200nm account for 37.76%, particles with a diameter of 200nm-270nm account for 7.14%, and particles with a diameter of 270nm-1000nm account for 5.1%.

In this invention, the primary particles are nanoparticles, preferably with a particle size of 30nm-1000nm, more preferably 100nm-800nm, such as 150nm or 200nm.

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

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

In this invention, the D10 of the secondary particles can be... 0.35μm.

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.

In this invention, the D90 of the secondary particles can be... 6.50μm.

In this invention, the secondary particles have a D99 < 12.50μm.

In this invention, the particle size distribution of the secondary particles can be unimodal, bimodal, or multimodal. When testing the particle size distribution using a laser particle size analyzer, a unimodal distribution will appear when "General Mode" is selected, while a bimodal or multimodal distribution will appear when "Multiple Narrow Peak Mode" is selected.

This invention also provides a method for preparing a lithium iron phosphate precursor, comprising the following steps: S1. reacting a mixture of an iron source and a phosphoric acid solution, and grinding the mixture 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 to obtain product B; the order of preparation of product A and product B is not limited; S2. grinding a mixture of product A and product B to obtain the lithium iron phosphate precursor; wherein the particle size of the lithium iron phosphate precursor is 800 nm-2000 nm.

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

In this invention, the lithium iron phosphate precursor is an amorphous substance or a low-crystallinity iron salt, lithium salt, or mixture thereof formed by sediment deposition. Controlling the particle size of the lithium iron phosphate precursor only changes the average size of the deposit. 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 treatments will not easily and completely destroy its secondary particle structure.

In this invention, in step S1, the reaction of the iron source and the phosphoric acid solution is completed. Those skilled in the art to which this invention pertains will know that this generally means the reaction produces no gas.

In S1, the reaction is carried out under stirred 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.

In S1, the iron source is known to those skilled in the art to which this invention pertains. This invention includes a process method for preparing lithium iron phosphate after a step of preparing ferric phosphate. Therefore, the iron source mentioned here does not include ferric phosphate.

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

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%.

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 ferric oxide has a purity of 95 wt% or higher, more preferably 99 wt% or higher, and even more preferably 99.5 wt% or higher.

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

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.

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%.

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

In step 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.

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

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.

In step S1, the mixture of iron source and phosphoric acid solution also includes a catalyst. The catalyst has a catalytic effect during the reaction and, upon completion of the reaction, can act as an dopant to improve the electrical conductivity of the lithium iron phosphate product.

Preferably, the catalyst is a titanium-based catalyst.

Preferably, the amount of catalyst added is 0.5wt%-2wt% of the amount of iron source added.

Preferably, the catalyst is added by first mixing the catalyst with the phosphoric acid solution, and then adding the iron source.

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

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 mill, or a twin-cone 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 zirconium oxide beads.

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

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

In S1, preferably, the organic acid in the organic acid solution is a carboxylic acid compound and/or ascorbic acid, wherein the carboxylic acid compound is preferably one or more selected from 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.

In S1, the organic acid can be a polymer copolymerized from unsaturated alkenes containing carboxylic acids via free radical polymerization, such as poly(meth)acrylic acid.

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

In S1, preferably, the molar ratio of lithium in the lithium source to phosphoric acid in the phosphoric acid solution is 0.98-1.05:1, for example, 1.02:1, 1.03:1, or 1.04:1.

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 condensates containing N or O elements.

The glucose derivative preferably includes at least one selected from glucose, sucrose, starch, and cyclodextrin.

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

The polyacrylic acid derivative preferably includes polyacrylate.

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

Preferably, the metal compound comprises alumina and/or zinc stannate.

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

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

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

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

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

In step S1, preferably, the amount of carbon source added accounts for 1%-60% of the mass percentage of the iron source; more preferably, it is 5%-50%; and even more preferably, it is 10%-40%.

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.

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

In S2, the mixture of product A and product B is generally obtained by simply mixing product A and product B.

In S2, the grinding operation and conditions can be those common in the art. The preferred embodiment of the grinding can be the same as in S1.

In step S2, the grinding time is conventional in this field and is 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, but may be, for example, 6-10 hours.

This invention also provides a lithium iron phosphate precursor, which is prepared by the method for preparing lithium iron phosphate precursors as described above.

This invention also provides a method for preparing lithium iron phosphate material, comprising the following steps: sequentially subjecting the lithium iron phosphate precursor as described above to spray drying, sintering, and crushing.

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

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

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

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, thereby disrupting the secondary particle structure.

The crushing conditions may include: processing the sintered product using an airflow crushing device to obtain the target finished product, lithium iron phosphate cathode material, with a particle size D50 of 1.5-3 μm.

This invention also provides a lithium iron phosphate material prepared by the method described above.

This invention also provides an application of the lithium iron phosphate material, as described above, as a cathode material in lithium-ion batteries.

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

All reagents and raw materials used in this invention are commercially available.

The significant improvement of this invention lies in the fact that the lithium iron phosphate material provided by this invention possesses a unique cluster structure, while also exhibiting excellent processing and recycling properties.

Figure 1 shows the cycling performance of the lithium iron phosphate materials prepared in Example 1 and Comparative Examples 1-2.

Figure 2 shows the microstructure of the lithium iron phosphate material prepared in Example 1.

Figure 3 shows the microstructure of the lithium iron phosphate material prepared in Comparative Example 1.

Figure 4 shows the microstructure of the lithium iron phosphate material prepared in Comparative Example 2.

Figure 5 shows the dimensional distribution of the lithium iron phosphate material obtained in Example 1.

Figure 6 shows the size distribution of the lithium iron phosphate material prepared in Comparative Example 1.

Figure 7 shows the size distribution of the lithium iron phosphate material prepared in Comparative Example 2.

Figure 8 shows a transmission electron microscope (TEM) image of the lithium iron phosphate material prepared in Example 1.

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

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

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

Figure 12 shows the transmission electron microscope (TEM) dimensions of the lithium iron phosphate material prepared in Comparative Example 1.

Figure 13 shows the transmission electron microscope (TEM) dimensions of the lithium iron phosphate material prepared in Comparative Example 2.

The invention is further illustrated below by way of examples, but this does not limit the invention to the scope of the examples described. Experimental methods in the following examples that do not specify specific conditions are based on conventional methods and conditions, or as selected according to the product instructions.

Implementation Example 1

(1) Based on a 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%. While stirring (at 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 fed into a sand mill for grinding. The grinding beads in the sand mill were 0.3 mm zirconium oxide beads. During the grinding process, the viscosity gradually increased to 15000 cps, and the color gradually turned pure white, yielding product A.

(2) Based on a lithium to phosphorus molar ratio of 1.04:1, 4 kg of citric acid was dissolved in 3.25 kg of deionized water to prepare a solution. While stirring, 2.3 kg of battery-grade lithium carbonate and 1 kg of sucrose were gradually added to the solution. The reaction was carried out at 40°C. During the reaction, a large amount of gas was generated. The reaction continued until no more gas was generated, forming a transparent solution, yielding product B.

(3) Add product B to product A, mix and stir. The viscosity of the system rapidly decreases to 1000 cps. Continue grinding for 6-7 hours. When the particle size is ground to approximately 1800 nm (D50), the reaction ends, yielding a slurry with a solid content of 50%.

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

The spray drying conditions are as follows: inlet temperature 280℃, outlet temperature 110℃. The calcination conditions are: under a nitrogen atmosphere of 99.999% purity, the temperature is 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 are: the sintered product is processed by an airflow crushing device to obtain lithium iron phosphate material with a target particle size D50 of 1.5-3μm and a flower-like structure.

Implementation Example 2

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.

Implementation Example 3

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 stirring.

Comparative Example 1

Liquid phase products

Comparative Example 2

Solid-phase products

Comparative Example 3

Compared to Example 1, Comparative Example 3 involved a grinding time of 8-10 hours and a sand particle size of 200 nm. Due to the reduced particle size, the slurry viscosity increased, and the slurry solid content was adjusted to 35%, while other parameters remained largely the same.

Comparative Example 4

Compared to Example 1, Comparative Example 3 had a grinding time of 4-5 hours, an abrasive grain size of 2500 nm, and other parameters remained essentially the same.

Implementation Examples

I. Electrochemical Performance

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

1. Discharge specific capacity

Using a charge/discharge transducer (Blue Electric CT3002A) 1.1 within a charge/discharge range of 2.0V-4.2V, charge/discharge was performed at 0.1C, 0.2C, 0.5C, 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. (Data can be rounded; the unit of test data is mAh/g.)

2. Power charge/discharge test

Power charge/discharge tests were conducted using a charge/discharge motor (Landian CT3002A) within a voltage range of 2.5V-3.75V.

Power multiplier test: Charge and discharge cycles were performed at 0.1P power, 0.33P power, 1P power charging to 0.33P power discharging, 2P power charging to 0.33P power discharging, 5P power charging to 0.33P power discharging, 7P power charging to 0.33P power discharging, and 10P power charging to 0.33P power discharging. Each cycle was repeated twice. The test results are shown in Table 2.

Power discharge test: Charge-discharge cycles were performed at 0.1P, 0.33P, 0.33P charge-discharge-1P, 0.33P charge-2P discharge, 0.33P charge-5P discharge, 0.33P charge-7P discharge, and 0.33P charge-10P discharge rates. Each cycle was repeated twice. The test results are shown in Table 3. (Data may be rounded; the unit of test data is mAh/g.)

3. Circulation performance

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

Preparation of the positive electrode sheet: 94g of lithium iron phosphate, the positive electrode active material prepared in the examples and the 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 with a size of 540*43.5mm² ^. The weight of the active material lithium iron phosphate was about 7.3g. Preparation of the 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. 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* ^44mm² , containing 3.6g of natural graphite active material.

Battery assembly

The above-mentioned positive electrode, negative electrode, and polyethylene separator were wound into a square lithium-ion battery core. Lithium hexafluorophosphate ( _LiPF6 ) was then dissolved at a concentration of 1 mol/L in a mixed solvent of EC/EMC/DEC = 1:1:1 as the electrolyte. This electrolyte was 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 was approximately 1050 mAh.

(1) Cyclic performance test at 25℃

The aforementioned lithium-ion soft-pack batteries were charged at a constant current and voltage rate of 1C in a 25°C constant-temperature chamber. The cutoff voltage was 4.5V. After 30 minutes, the batteries were discharged from 4.5V to 2.0V at a current of 1C. This cycle test was repeated, 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 shown in Figure 1 and Table 4.

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; and in Comparative Example 4, which used a larger phosphate precursor particle size, the decay to 80% required only 294 cycles. In contrast, the samples from Examples 1-3 decayed to 80% with over 1031 cycles. This demonstrates the significant advantage of the samples from Examples 1-3.

(2) Low-temperature capacity retention

The aforementioned lithium-ion soft-pack batteries were charged and discharged at 0.2C using a 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 the 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 high (low) temperature chamber temperature 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 discharge ceased. The ratio of the discharge specific capacity to the discharge specific capacity at 0.2C under the 23±2℃ condition was calculated.

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 actual examples, indicating that the lithium iron phosphate product of this invention has significant advantages in low-temperature applications.

(3) Self-discharge rate

At 25℃, after 30 minutes of storage, the charging cutoff voltage is 4.2V, the constant voltage charging cutoff current is 0.05C, and the discharge cutoff voltage is 2.0V. The discharge capacity is obtained by charging and discharging at 0.5C. The device is then placed at 55℃ for 7 days. The discharge specific capacity is obtained by discharging at 0.5C. The difference between the discharge specific capacity value measured at 0.5C and the initial capacity, divided by the initial capacity, is the self-discharge rate.

As can be seen from the data in Table 6, the lithium iron phosphate products of Examples 1-3 have lower self-discharge rates than those of Comparative Examples 1, 2, and 4, exhibiting good capacity retention performance.

In summary, the lithium iron phosphate product of this invention exhibits superior electrochemical properties compared to lithium iron phosphate products prepared by solid-phase or liquid-phase methods or using precursors with larger particle sizes.

II. Machining Performance Testing Machining Performance Testing

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 embodiments and comparative examples. The mixture was stirred at 2000 rpm under vacuum for 5 hours. The slurry was then filtered through a 200-mesh sieve, and its viscosity, solid content, fineness, and electrode compaction density were tested.

Table 7

As shown in Table 7, the lithium iron phosphate product produced by this invention maintains a high solids content while exhibiting low viscosity during processing, which helps reduce energy consumption in subsequent processes and facilitates further processing.

In summary, the lithium iron phosphate material produced by this invention simultaneously possesses excellent cycle performance, capacity retention, and processability. Furthermore, the preparation method is simple, environmentally friendly, generates 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 produced by this invention also exhibits good energy density.

III. Structural Characterization

1. Specific surface area test

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

The isothermal adsorption-desorption curve test results show that the pore structure of the lithium iron phosphate product of this invention is consistent with that 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 morphologies.

As shown in Table 8, the specific surface area of the multi-point BET in Example 1 is 11.20 ^m² /g, the specific surface area of the multi-point BET in Comparative Example 1 (prepared by liquid phase method) is 10.94 ^m² /g, and the specific surface area of the multi-point BET in Comparative Example 2 (prepared by solid phase method) is 12.26 ^m² /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.

2. Microscopic morphology comparison

Microscopic morphology testing was performed using a field-emission high-resolution scanning electron microscope (model: Tescan Mira 3 XH, manufacturer: Tescan, Czech Republic). Figures 2, 3, and 4 show the microscopic morphology of the lithium iron phosphate materials prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively. Figures 5, 6, and 7 show the size distribution of the 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, with primary particles forming secondary particles through the coating layer.

3. XRD and crystallinity measurement

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.

Based on the data in Table 10, it can be seen that the XRD test results show that the exemplary and comparative examples are all lithium iron phosphate products with an olivine crystal structure, corresponding to PDF card number 40-1499. The XRD crystal forms of the three are consistent, but their crystallinity (Xc) differs.

4. Raman spectroscopy test

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

ID corresponds to disordered carbon structure, and IG corresponds to ordered graphitized carbon structure. A smaller ID/IG ratio indicates a higher degree of graphitization. Combining the data in Table 11, it can be seen that 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 for lithium ion transport.

5. Transmission electron microscope testing

The microstructure of lithium iron phosphate was tested using a transmission electron microscope (TEM). Figures 8, 9, and 10 show the TEM morphology of the lithium iron phosphate materials prepared in Example 1, Comparative Example 1, and Comparative Example 2. Figures 11, 12, and 13 show the TEM dimensions of the lithium iron phosphate materials prepared 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 prepared in Examples 1-3 and Comparative Examples 1-4 is between 4-10 nm, indicating a carbon-coated structure.

The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this invention. It should be understood that the above descriptions are merely specific embodiments of this invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can 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 limited by the scope of the appended patent applications.

Claims (10)

A lithium iron phosphate material, comprising a cluster structure, the cluster structure comprising 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; the preparation method of the lithium iron phosphate material includes 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 includes the following steps: S1. reacting a mixture of an iron source and a phosphoric acid solution, and grinding the mixture after the reaction is complete 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 is complete; the order of preparation of product A and product B is not limited; S2. mixing product A and product B... The mixture is ground to obtain a lithium iron phosphate precursor; wherein the lithium iron phosphate precursor has a particle size of 800nm-2000nm; in step S1, the organic acid in the organic acid solution is citric acid; the lithium source is lithium carbonate; the coating source is sucrose and polyacrylic acid, or sucrose and polyvinylpyrrolidone. The lithium iron phosphate material as described in claim 1, wherein the number of the secondary particles comprising the primary particles is 2-512; or, the lithium iron phosphate material further comprises free primary particles, the number of which accounts for 0.1%-5% of the total number of secondary particles and free primary particles; or, the thickness of the coating layer is 2-20 nm. The lithium iron phosphate material as described in claim 1 or 2, wherein the primary particles are nanoparticles with a particle size range of 30nm-1000nm; the D50 of the primary particles is 100nm-300nm; or, the crystallinity of the primary particles is >96%; or, the primary particles are elliptical with a D50 of <300nm; the D90 of the primary particles is <600nm; in the primary particles, the proportion of particles with a size of 30nm-140nm is 50%, the proportion of particles with a size of 140nm-200nm is 37.76%, the proportion of particles with a size of 200nm-270nm is 7.14%, and the proportion of particles with a size of 270nm-1000nm is 5.1%; Alternatively, 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 ≥0.35μm; (2) the D50 of the secondary particles is 1-2.2μm; (3) the D90 of the secondary particles is ≤6.50μm; (4) the D99 of the secondary particles is ≤12.50μm. A method for preparing a lithium iron phosphate precursor, comprising the following steps: S1. Reacting a mixture of an iron source and a phosphoric acid solution, and grinding the mixture after the reaction is complete 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 is complete; the order of preparation of product A and product B is not limited; S2. Grinding the mixture of product A and product B to obtain the lithium iron phosphate precursor; wherein the particle size of the lithium iron phosphate precursor is 800nm-2000nm; in step S1, the organic acid in the organic acid solution is citric acid; the lithium source is lithium carbonate; the coating source is sucrose and polyacrylic acid, or sucrose and polyvinylpyrrolidone. The method for preparing the lithium iron phosphate precursor as described in claim 4, wherein the particle size of the lithium iron phosphate precursor is 800 nm-1800 nm. The method for preparing the lithium iron phosphate precursor as described in claim 4 or 5, wherein the iron source is iron powder; the iron content in the iron powder is 95 wt% or more, and the iron powder is one or more of primary reduced iron powder, secondary reduced iron powder, carbonyl reduced iron powder, and electrolytic iron powder; or, the mesh size of the iron source is 100-1000 mesh; or, the phosphoric acid solution refers to an aqueous solution of phosphoric acid; 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; or, the reaction temperature of the mixture of the iron source and the phosphoric acid solution is 20-95℃; 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; Alternatively, the organic acid solution contains 5-98% by mass; or the lithium carbonate is industrial-grade or battery-grade; or the molar ratio of lithium in the lithium source to phosphoric acid in the phosphoric acid solution is 0.98-1.05:1; or the amount of the coating source added is 1%-60% by mass of the iron source; or the reaction temperature of the mixture of organic acid, lithium source, and coating source is 20-95°C; or the mixture of organic acid, lithium source, and coating source is prepared by adding the lithium source and coating source to an organic acid solution under stirring; or, in S1, the reaction is carried out under stirring conditions, and the stirring speed is 25-50 Hz. Alternatively, in S2, the grinding is sand milling; the grinding is carried out using a sand mill, and the grinding beads used in the sand mill have a particle size of 0.1-3.0 mm; the grinding beads used in the sand mill are zirconium oxide beads; or, in S2, the grinding time is 6-10 hours. A lithium iron phosphate precursor, wherein it is prepared by the preparation method described in any one of claims 4-6. A method for preparing lithium iron phosphate material, comprising the following steps: sequentially drying, sintering and crushing the lithium iron phosphate precursor as described in claim 7 to obtain the lithium iron phosphate material. The method for preparing lithium iron phosphate material as described in claim 8, wherein the drying method is spray drying; the inlet temperature of the spray drying is 280°C; and the outlet temperature of the spray drying is 130°C; or, the sintering conditions are: under a nitrogen atmosphere of 99.999% purity, gradually increasing the temperature from room temperature to 650°C-750°C at a heating rate of 5°C/min, maintaining the temperature at 650°C-750°C for 5-20 hours, and then cooling down. An application of lithium iron phosphate material as described in any one of claims 1-3 as a cathode material in lithium-ion batteries.