A method for preparing lithium iron phosphate by using a seed crystal

Abstract

The application provides a method for preparing lithium iron phosphate by using a seed crystal, and comprises the following steps: adding a strip-shaped lithium iron phosphate seed crystal in a sintering process to prepare a lithium iron phosphate crystal. In the application, the strip-shaped lithium iron phosphate seed crystal is mixed with a lithium iron phosphate precursor, and a strip-shaped large-particle lithium iron phosphate crystal with a micron level is prepared through sintering, so that the compaction density and the energy density of the lithium iron phosphate are improved, and the lithium iron phosphate crystal prepared has a strip-shaped crystal morphology and has good rate performance.

Description

Technical Field

[0001] This invention relates to the field of battery materials, and more specifically, to a method for preparing lithium iron phosphate by seed crystals. Background Technology

[0002] Lithium iron phosphate (LFP) is widely used as a cathode material in lithium-ion batteries due to its stable material structure, high safety, and high energy density. The production processes of LFP can be mainly divided into solid-state and liquid-state methods. Currently, domestic industrial production of LFP primarily uses the high-temperature solid-state method and the hydrothermal method. The morphology and size of LFP particles prepared by the solid-state method are difficult to control, resulting in inconsistent material performance. The hydrothermal method for producing LFP is inefficient, consumes a lot of energy, and easily leads to environmental pollution problems from industrial wastewater.

[0003] CN110127646A discloses a method for preparing lithium iron phosphate cathode material. The method involves mixing lithium iron phosphate seed crystals with iron phosphate, a lithium source, a carbon source, and additives, dispersing the mixture in water, then coarsely grinding, finely grinding, spray drying, and sintering to obtain the lithium iron phosphate cathode material. However, because the seed crystals are added during the sand milling process, the size of the seed crystals is altered after sand milling. Furthermore, after prolonged ball milling, the crystals tend to become smooth spheres, which is not conducive to the seed crystals playing a role in the superposition and growth of unit cells during sintering. This patent does not report on controlling the morphology of lithium iron phosphate crystals through lithium iron phosphate seed crystals.

[0004] CN109336077A can prepare elongated lithium iron phosphate primary particles at the nanoscale, but the increased compaction density of lithium iron phosphate is mainly achieved through a combination of elongated and spherical primary particles. The patent does not mention a method for preparing elongated lithium iron phosphate particles at the micrometer scale.

[0005] Therefore, there is a need to develop a highly efficient and low-pollution process that can effectively control the morphology and size of lithium iron phosphate particles, improve the compaction density and rate performance of lithium iron phosphate, and is easy to industrialize. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for preparing lithium iron phosphate through seed crystals. By adding bar-shaped lithium iron phosphate seed crystals during the sintering process, micron-sized bar-shaped large-particle lithium iron phosphate crystals are prepared, thereby improving the compaction density and energy density of lithium iron phosphate. At the same time, the prepared lithium iron phosphate crystals have a bar-shaped morphology and good rate performance.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] In a first aspect, the present invention provides a method for preparing lithium iron phosphate, comprising the following steps: adding bar-shaped lithium iron phosphate seed crystals in a sintering process to prepare lithium iron phosphate crystals.

[0009] This invention introduces bar-shaped lithium iron phosphate seed crystals during the sintering process, rather than during grinding. This promotes the growth of large-particle crystals during sintering, which increases the compaction density of lithium iron phosphate and thus improves the energy density of the battery. Furthermore, the bar-shaped seed crystals induce the superposition and growth of unit cells in a specific orientation of the crystal during sintering, thereby better controlling the morphology of the sintered lithium iron phosphate crystals to be bar-shaped and improving the rate performance of the battery.

[0010] Preferably, the length of the bar-shaped lithium iron phosphate seed crystals is 20-400 nm, for example: 20 nm, 60 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm. The bar-shaped lithium iron phosphate seed crystals used in this invention are conventional products that can be obtained commercially.

[0011] In this invention, the longest diameter inside the rod-shaped lithium iron phosphate is called the length dimension, and the longest diameter perpendicular to the length dimension is called the radial dimension. The length dimension is greater than or equal to the radial dimension.

[0012] Preferably, the method for preparing lithium iron phosphate includes the following steps:

[0013] S1: Preparation of lithium iron phosphate precursor;

[0014] S2: The aforementioned bar-shaped lithium iron phosphate seed crystals, lithium iron phosphate precursors, and reducing agents are sintered to obtain bar-shaped large-particle lithium iron phosphate crystals.

[0015] The bar-shaped lithium iron phosphate prepared by this invention is a large-particle crystal with high compaction density. At the same time, the bar-shaped lithium iron phosphate can shorten the migration path of lithium ions in the crystal, making lithium ion insertion and extraction easier, reducing the polarization effect of lithium iron phosphate batteries and thus improving the rate performance of high-compact lithium iron phosphate materials lithium batteries.

[0016] Preferably, the length of the rod-shaped large-particle lithium iron phosphate crystal is 3-8 μm, for example: 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm.

[0017] The rod-shaped large-particle lithium iron phosphate crystals obtained by the preparation method of this invention are at the micron level. Compared with the nano-level, the micron-sized rod-shaped lithium iron phosphate has a relatively low specific surface area and a higher compaction density. Therefore, during electrode coating, the amount of adhesive used can be reduced, and the overall areal density of the electrode can be increased.

[0018] Preferably, in step S1, the preparation of the lithium iron phosphate precursor includes: mixing and grinding iron phosphate, lithium salt and water, followed by spray drying to obtain the lithium iron phosphate precursor.

[0019] Preferably, the molar ratio of iron to phosphorus in the ferric phosphate is (0.9-0.98):1, for example: 0.9:1, 0.91:1, 0.92:1, 0.93:1, 0.94:1, 0.95:1, 0.96:1, 0.97:1, 0.98:1; and the specific surface area is 8-12 g / cm³. 3 For example: 8g / cm 3 9g / cm 3 10g / cm 3 11g / cm 3 12g / cm 3 .

[0020] Preferably, the molar ratio of Li:Fe:P in the lithium iron phosphate precursor is (1-1.2):(0.9-1.1):1, for example: 1:0.9:1, 1:1:1, 1:1.1:1, 1.1:0.9:1, 1.1:1:1, 1.1:1.1:1, 1.2:0.9:1, 1.2:1:1, 1.2:1.1:1.

[0021] Preferably, the lithium salt is one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, lithium nitrate or lithium acetate, and more preferably lithium carbonate.

[0022] Preferably, step S1 further includes adding citric acid, PEG, and titanium dioxide for spray drying; the amount of citric acid is 0.1-1% of the total weight of the added substances, for example: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%; the amount of titanium dioxide is 0.1-0.4% of the total weight of the added substances, for example: 0.1%, 0.2%, 0.3%, 0.4%. The total weight of the added substances refers to the total mass of iron phosphate, lithium salt, citric acid, PEG, and titanium dioxide, excluding the mass of water.

[0023] More preferably, the method for preparing the lithium iron phosphate includes the following steps:

[0024] S1: Iron phosphate, lithium salt, citric acid, PEG, titanium dioxide and water are mixed, ground and then spray-dried to prepare lithium iron phosphate precursor;

[0025] S2: Add the rod-shaped lithium iron phosphate seed crystals, lithium iron phosphate precursor and reducing agent described in this invention to a sintering pot, and sinter in a sintering furnace to obtain rod-shaped large-particle lithium iron phosphate crystals.

[0026] Preferably, in step S2, the reducing agent is one or more of glucose, activated carbon, carbon nanotubes, carbon fibers, or graphite.

[0027] Preferably, the weight ratio of the rod-shaped lithium iron phosphate seed crystals, the lithium iron phosphate precursor, and the reducing agent is (0.1-0.2):1:(0.01-0.08), for example: 0.1:1:0.01, 0.15:1:0.01, 0.2:1:0.01, 0.1:1:0.05, 0.15:1:0.05, 0.2:1:0.05, 0.1:1:0.08, 0.15:1:0.08, 0.2:1:0.08.

[0028] Preferably, in step S2, the sintering is performed first at a low temperature under an inert atmosphere, and then at a high temperature.

[0029] The present invention employs a segmented sintering process, which can better control the carbon coating of lithium iron phosphate, enhance the conductivity of lithium iron phosphate material, and provide an electron channel for lithium ions in lithium iron phosphate, facilitating the insertion and extraction of lithium ions.

[0030] Preferably, the inert atmosphere is one or more of nitrogen or argon.

[0031] Preferably, the sintering pressure is 0.1-0.5 MPa, for example: 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa.

[0032] Preferably, the low-temperature sintering temperature is 250-400℃, for example: 250℃, 300℃, 350℃, 400℃, and the sintering time is 4-8h, for example: 4h, 5h, 6h, 7h, 8h; the high-temperature sintering temperature is 600-850℃, for example: 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, and the sintering time is 8-15h, for example: 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h.

[0033] Secondly, the present invention provides a lithium iron phosphate, which is prepared according to the preparation method described in the present invention.

[0034] The lithium iron phosphate prepared by the method of this invention consists of rod-shaped, large-particle lithium iron phosphate crystals with a length of 3-8 μm and a compaction density greater than 2.6 g / cm³. 3 This invention prepares micron-sized rod-shaped large-particle lithium iron phosphate crystals, which improves the compaction density and energy density of lithium iron phosphate and exhibits better rate performance.

[0035] Thirdly, the present invention provides a positive electrode, comprising lithium iron phosphate as described in the present invention.

[0036] Fourthly, the present invention provides a battery including the positive electrode described in the present invention.

[0037] The beneficial effects of this invention are as follows:

[0038] (1) This invention develops a method for generating large-particle lithium iron phosphate by mixing bar-shaped lithium iron phosphate seed crystals with lithium iron phosphate precursors and then sintering and directionally growing them. This method solves the problem of difficult control of crystal particle morphology in solid-phase lithium iron phosphate preparation and avoids the problem of generating a large amount of wastewater in industrial production processes similar to liquid-phase methods.

[0039] (2) In this invention, bar-shaped lithium iron phosphate seed crystals are added after grinding and mixed with lithium iron phosphate precursors before sintering. On the one hand, this promotes the growth of large-particle crystals during the crystal sintering process. Large-particle crystals have higher density, thereby improving the compaction density and energy density of lithium iron phosphate. On the other hand, bar-shaped lithium iron phosphate seed crystals can induce the superposition and growth of unit cells in a specific orientation of the crystal during the sintering process, thereby better controlling the morphology of the lithium iron phosphate crystals obtained after sintering to be bar-shaped, thereby improving the rate performance of lithium iron phosphate.

[0040] (3) The bar-shaped large-particle lithium iron phosphate crystals obtained by the preparation method of the present invention are at the micron level. Compared with the nano level, the micron-sized bar-shaped lithium iron phosphate has a relatively low specific surface area and a higher compaction density.

[0041] Terminology Definition

[0042] Unless explicitly stated otherwise, all scopes referenced in this invention include end values.

[0043] The terms "an" or "a" are used in this invention to describe the elements and components described herein. This is done merely for convenience and to provide a general meaning regarding the scope of the invention. This description should be understood to include one or at least one, and the singular includes the plural unless clearly otherwise indicated. "Multiple" means two or more.

[0044] All figures in this invention are approximate values, regardless of whether words such as "approximately" or "about" are used. The numerical values ​​may vary by 1%, 2%, 5%, 7%, 8%, 10%, etc. Whenever a number with a value of N is disclosed, any number having a value of N+ / -1%, N+ / -2%, N+ / -3%, N+ / -5%, N+ / -7%, N+ / -8%, or N+ / -10% will be explicitly disclosed, where "+ / -" indicates addition or subtraction, and the range between N-10% and N+10% is also disclosed.

[0045] Unless otherwise stated, the following definitions as used in this invention shall apply. For the purposes of this invention, chemical elements are consistent with the CAS version of the periodic table and the 75th edition of the *Handbook of Chemistry and Physics*, 1994. Furthermore, general principles of organic chemistry can be found in *Organic Chemistry*, Thomas Sorrell, University Science Books, Sausalito: 1999, and *March's Advanced Organic Chemistry* by Michael B. Smith and Jerry March, John Wiley & Sons, New York: 2007, the entire contents of which are incorporated herein by reference.

[0046] Unless otherwise defined, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although similar or equivalent methods and materials may be used in the implementation or testing of embodiments of this invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned in this invention are incorporated herein by reference in their entirety, except where specific paragraphs are cited. In case of any conflict, this specification and its included definitions shall prevail. Furthermore, materials, methods, and examples are illustrative only and are not intended to be limiting. Attached Figure Description

[0047] Figure 1 This is a scanning electron microscope image of sample 4 in Example 4 of the present invention. Detailed Implementation

[0048] The following description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0049] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0050] Unless otherwise stated, all temperatures described in the examples below are in degrees Celsius. All reagents used are commercially available or can be prepared by the methods described in this invention.

[0051] Example 1

[0052] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.966:1, specific surface area of ​​10g / cm³). 3 300g lithium carbonate, 6g citric acid, 3g PEG, 2g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0053] 20g of rod-shaped lithium iron phosphate seed crystals with a length of 400nm, 200g of precursor powder, and 10g of glucose were added to a sintering mortar and ball-milled until uniformly mixed. The mixture was then placed in a sintering furnace and sintered at 300℃ for 6 hours under an inert atmosphere with a pressure adjusted to 0.2MPa. The temperature was then increased to 700℃ and sintered for 9 hours. After cooling to room temperature, sample 1 was obtained.

[0054] Example 2

[0055] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.98:1, specific surface area of ​​11g / cm³). 3 300g lithium nitrate, 1.3g citric acid, 3g PEG, 5.2g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray-dried at 250℃ to obtain the precursor powder.

[0056] 30g of rod-shaped lithium iron phosphate seed crystals with a length of 360nm, 200g of precursor powder, and 15g of activated carbon were added to a sintering mortar and ball-milled until uniformly mixed. The mixture was then placed in a sintering furnace and sintered at 400℃ for 5h under an inert atmosphere with a pressure adjusted to 0.4MPa. The temperature was then increased to 680℃ and sintered for 8h. After cooling to room temperature, sample 2 was obtained.

[0057] Example 3

[0058] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.923:1, specific surface area of ​​12g / cm³). 3 100g lithium phosphate, 8g citric acid, 3g PEG, 4.6g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0059] 20g of 80nm long rod-shaped lithium iron phosphate seed crystals, 200g of precursor powder, and 2g of graphite were added to a sintering mortar and ball-milled until uniformly mixed. The mixture was then placed in a sintering furnace and sintered at 250℃ for 4 hours under an inert atmosphere with a pressure adjusted to 0.5MPa. The temperature was then increased to 650℃ and sintered for 15 hours. After cooling to room temperature, sample 3 was obtained.

[0060] Example 4

[0061] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.90:1, specific surface area of ​​9g / cm³). 3 300g lithium acetate, 13g citric acid, 3g PEG, 1.3g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0062] 40g of rod-shaped lithium iron phosphate seed crystals with a length of 280nm, 200g of precursor powder, and 16g of carbon nanotubes were added to a sintering mortar and ball-milled until uniformly mixed. The mixture was then placed in a sintering furnace and sintered at 360℃ for 8 hours under an inert atmosphere with a pressure adjusted to 0.1MPa. The temperature was then increased to 850℃ and sintered for 15 hours. After cooling to room temperature, sample 4 was obtained.

[0063] Example 5

[0064] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.947:1, specific surface area of ​​8g / cm³). 3 300g lithium dihydrogen phosphate, 10g citric acid, 3g PEG, 1.8g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0065] 35g of 200nm long rod-shaped lithium iron phosphate seed crystals, 200g of precursor powder, and 8g of carbon fiber were added to a sintering mortar and ball-milled until uniformly mixed. The mixture was then placed in a sintering furnace and sintered at 250℃ for 6 hours under an inert atmosphere with a pressure adjusted to 0.3 MPa. The temperature was then increased to 850℃ and sintered for 8 hours. After cooling to room temperature, sample 5 was obtained.

[0066] Example 6

[0067] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.971, specific surface area of ​​10g / cm³). 3300g lithium dihydrogen phosphate, 5.4g citric acid, 3g PEG, 3.8g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0068] 28g of 20nm long rod-shaped lithium iron phosphate seed crystals, 200g of precursor powder, and 11g of glucose were added to a sintering mortar and ball-milled until homogeneous. The mixture was then placed in a sintering furnace and sintered at 320℃ for 7 hours under an inert atmosphere with a pressure adjusted to 0.1 MPa. The temperature was then increased to 780℃ and sintered for 12 hours. After cooling to room temperature, sample 6 was obtained.

[0069] Comparative Example 1

[0070] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.966:1, specific surface area of ​​10g / cm³). 3 300g lithium carbonate, 117g glucose, 6g citric acid, 3g PEG, 2g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.35μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0071] The precursor was added to a sintering pot and placed in a sintering furnace, where it was calcined at 300°C for 6 hours under an inert atmosphere. Finally, the temperature was raised to 700°C for high-temperature sintering for 9 hours to obtain control sample 1.

[0072] Comparative Example 2

[0073] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.971:1, specific surface area of ​​10g / cm³). 3 300g lithium dihydrogen phosphate, 5.4g citric acid, 3g PEG, 3.8g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0074] 28g of non-strip-shaped lithium iron phosphate seed crystals with a particle diameter of 360nm, 200g of precursor powder, and 11g of glucose were added to a sintering mortar, ball-milled and mixed evenly, and then placed in a sintering furnace. Under an inert atmosphere, the pressure was adjusted to 0.1MPa, and sintering was carried out at 320℃ for 7h. Then the temperature was raised to 780℃ and sintered for 12h. After cooling to room temperature, the sample was removed to obtain control sample 2.

[0075] Comparative Example 3

[0076] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.971:1, specific surface area of ​​10g / cm³). 3 35g of rod-shaped lithium iron phosphate seed crystals with a length of 400nm, 300g of lithium dihydrogen phosphate, 5.4g of citric acid, 3g of PEG, 3.8g of titanium dioxide, and 3.5L of water were mixed and then added to a sand mill for coarse grinding for 30min. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was spray-dried at 250℃ to obtain precursor powder.

[0077] 200g of precursor powder and 15g of glucose were added to a sintering mortar, ball-milled and mixed evenly, and then placed in a sintering furnace. Under an inert atmosphere, the pressure was adjusted to 0.2 MPa, and sintering was carried out at 320℃ for 5 hours. Then the temperature was raised to 780℃ and sintered for 12 hours. After cooling to room temperature, the sample was taken out to obtain control sample 3.

[0078] Comparative Example 4

[0079] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.966:1, specific surface area of ​​10g / cm³). 3 300g lithium carbonate, 6g citric acid, 3g PEG, 2g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0080] 220g of precursor and 10g of glucose were added to a sintering mortar, ball-milled and mixed evenly, and then placed in a sintering furnace. Under an inert atmosphere, the mixture was calcined at 300℃ for 6 hours at normal pressure. Finally, the temperature was raised to 700℃ for high-temperature sintering for 9 hours to obtain control sample 4.

[0081] Comparative Example 5

[0082] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.971:1, specific surface area of ​​10g / cm³). 3 300g lithium dihydrogen phosphate, 5.4g citric acid, 3g PEG, 3.8g titanium dioxide, and 3.5L water were mixed and then added to a sand mill for coarse grinding for 30 minutes. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was introduced into a spray dryer and spray dried at 250℃ to obtain the precursor powder.

[0083] 28g of spherical lithium iron phosphate seed crystals with a particle diameter of 20nm, 200g of precursor powder, and 11g of glucose were added to a sintering mortar and ball-milled until uniformly mixed. The mixture was then placed in a sintering furnace and sintered at 320℃ for 7h under an inert atmosphere with a pressure adjusted to 0.1MPa. The temperature was then increased to 780℃ and sintered for 12h. After cooling to room temperature, the sample was removed to obtain control sample 5.

[0084] Comparative Example 6

[0085] Weigh 1200g of ferric phosphate (iron-phosphorus molar ratio of 0.971:1, specific surface area of ​​10g / cm³). 3 28g of rod-shaped lithium iron phosphate seed crystals with a length of 20nm, 300g of lithium dihydrogen phosphate, 5.4g of citric acid, 3g of PEG, 3.8g of titanium dioxide, and 3.5L of water were mixed and then added to a sand mill for coarse grinding for 30min. The mixture was then transferred to a fine sand mill for grinding to a particle size of 0.2μm. Finally, the mixture was spray-dried at 250℃ to obtain precursor powder.

[0086] 200g of precursor powder and 11g of glucose were added to a sintering mortar, ball-milled and mixed evenly, and then placed in a sintering furnace. Under an inert atmosphere, the pressure was adjusted to 0.1 MPa, and sintering was carried out at 320℃ for 7 hours. Then the temperature was raised to 780℃ and sintered for 12 hours. After cooling to room temperature, the sample was taken out to obtain control sample 6.

[0087] The samples 1-6 and control samples 1-6 were observed, analyzed, and statistically analyzed under a scanning electron microscope (SEM), and the compaction density of the powder was calculated. The results are shown in Table 1. The SEM image of sample 4 is shown below. Figure 1 As shown.

[0088] Table 1. SEM results and powder compaction density of the samples.

[0089] sample SEM lower shape Particle size / μm Powder compaction density Sample 1 bar-shaped 6.330 2.670 Sample 2 bar-shaped 5.782 2.661 Sample 3 bar-shaped 3.739 2.668 Sample 4 bar-shaped 4.986 2.656 Sample 5 bar-shaped 5.993 2.663 Sample 6 bar-shaped 7.865 2.706 Comparison Samples Non-bar-shaped 5.268 2.530 Comparison Samples Non-strip rod 6.243 2.547 Comparison Samples Non-bar-shaped 5.362 2.594 Comparison Samples Non-bar-shaped 2.964 2.431 Comparison Samples Non-bar-shaped 3.685 2.489 Comparison Samples Non-strip rod 2.964 2.431

[0090] Note: The particle size of bar-shaped lithium iron phosphate particles refers to the length dimension of the particle; the particle size of non-bar-shaped lithium iron phosphate particles refers to the length dimension of the particle (here, the length dimension refers to the longest diameter inside the non-bar-shaped lithium iron phosphate particle, which corresponds to the length dimension of the bar-shaped lithium iron phosphate particle).

[0091] The comparative samples 1-3 and 1-6 were mixed with SP carbon powder, PVDF, and NMP at a ratio of 92:4:0.1:3.9 and stirred until homogeneous. The resulting slurry was then coated onto a 20μm aluminum foil. After drying at 100℃ and vacuum drying at 120℃, proto-electrode sheets with a diameter of 150mm were cut out for button cell assembly. Finally, the cells were placed in a button cell performance testing cabinet for testing at different rates, and the following results were obtained (see Table 2):

[0092] Table 2 Performance of button batteries obtained from sample preparation

[0093]

[0094] As can be seen from the examples, the micron-sized lithium iron phosphate particles prepared according to the method of the present invention have a rod-like morphology, which improves the compaction density of lithium iron phosphate. The resulting lithium iron phosphate not only has a good specific capacity at low rates but also a high specific capacity at high rates, exhibiting good overall rate performance. Comparative Example 1, without adding seed crystals and using a conventional sintering method, yielded lithium iron phosphate with an irregular morphology, moderate compaction density, and low specific capacity at high rates. Comparative Example 2, with the addition of non-rod-like lithium iron phosphate seed crystals during sintering, resulted in lithium iron phosphate with improved compaction density and specific capacity compared to Comparative Example 1, but the specific capacity at high rates was limited. Comparative Example 3, with the addition of rod-like lithium iron phosphate seed crystals during sand milling, ultimately yielded lithium iron phosphate with a non-rod-like morphology, and lower compaction density and specific capacity compared to the examples. Comparative Example 4, without adding seed crystals and using an atmospheric pressure sintering method, yielded lithium iron phosphate with an irregular morphology, moderate compaction density, and lower specific capacity at high rates compared to the examples. Comparative Example 5 added spherical seed crystals to the precursor and sintered it, but did not obtain rod-shaped lithium iron phosphate. Its compaction density and high-rate 5C specific capacity were lower than those of Example 6. Comparative Example 6 added rod-shaped lithium iron phosphate seed crystals during the precursor preparation process and blended and milled it, but did not obtain rod-shaped lithium iron phosphate. Its compaction density and 5C specific capacity were also lower.

Claims

1. A method for producing lithium iron phosphate, characterized by, Includes the following steps: S1: Preparation of lithium iron phosphate precursor; S2: Sinter the rod-shaped lithium iron phosphate seed crystals, the lithium iron phosphate precursor obtained in step S1 and the reducing agent to obtain rod-shaped large-particle lithium iron phosphate crystals. The length of the bar-shaped lithium iron phosphate seed crystals is 20-400 nm; In step S1, the preparation of the lithium iron phosphate precursor includes: mixing and grinding iron phosphate, lithium salt and water, followed by spray drying to obtain the lithium iron phosphate precursor; The S1 step also includes adding citric acid, PEG and titanium dioxide for spray drying; In step S2, the sintering is performed first at a low temperature under an inert atmosphere, followed by a high temperature sintering. The inert atmosphere is one or more of nitrogen or argon. The sintering pressure is 0.1-0.5 MPa. The low-temperature sintering temperature is 250-400℃, and the sintering time is 4-8 h. The high-temperature sintering temperature is 600-850℃, and the sintering time is 8-15 h.

2. The production method according to claim 1, characterized by, The length of the rod-shaped large-particle lithium iron phosphate crystals is 3-8 μm.

3. The preparation method according to claim 1, characterized in that, The iron and phosphorus element molar ratio in the iron phosphate is (0.9-0.98):1, and the specific surface area is 8-12 g / cm 3 The Li:Fe:P element molar ratio in the lithium iron phosphate precursor is (1-1.2):(0.9-1.1):1; and the lithium salt is one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, lithium nitrate or lithium acetate.

4. The production method according to claim 3, characterized by, The lithium salt is lithium carbonate.

5. The method of claim 1, wherein, The amount of citric acid used is 0.1-1% of the total weight of the added substances; the amount of titanium dioxide used is 0.1-0.4% of the total weight of the added substances; wherein, the total weight of the added substances refers to the total mass of iron phosphate, lithium salt, citric acid, PEG, and titanium dioxide, excluding the mass of water.

6. The method of claim 1, wherein, In step S2, the reducing agent is one or more of glucose, activated carbon, carbon nanotubes, carbon fibers or graphite; the weight ratio of the rod-shaped lithium iron phosphate seed crystals, lithium iron phosphate precursor and reducing agent is (0.1-0.2):1:(0.01-0.08).

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

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