A method for preparing lithium iron phosphate materials with low iron phosphate content and its application

Abstract

The application provides a preparation method of a lithium iron phosphate material with low ferrophosphorus content, comprising the following steps: mixing and dissolving anhydrous iron phosphate, a lithium source, a carbon source, a dopant and deionized water to obtain a mixed solution; performing wet grinding and spray drying on the mixed solution to obtain a sintering precursor; and performing heat treatment and crushing on the sintering precursor to obtain the lithium iron phosphate material; wherein the heat treatment process comprises preheating, low-temperature sintering, high-temperature sintering and cooling, the temperature of the preheating is lower than that of the low-temperature sintering, the temperature of the low-temperature sintering is lower than that of the high-temperature sintering, and the temperature of the cooling is lower than that of the high-temperature sintering; the heat treatment process is performed in a rare gas atmosphere, and the content of the rare gas in the high-temperature sintering is higher than that in the low-temperature sintering. Compared with a conventional process, the lithium iron phosphate material prepared by the application has high purity, the ferrophosphorus content is significantly reduced, the high-pressure compact density is maintained, and the electrochemical performance is excellent. The application further provides the lithium iron phosphate material and a lithium ion battery using the lithium iron phosphate material.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a method for preparing a lithium iron phosphate material with low iron phosphate content, as well as the lithium iron phosphate material and a lithium-ion battery. Background Technology

[0002] As the best performing secondary battery currently available, lithium-ion batteries have been commercialized since the 1990s. After years of research, lithium iron phosphate has become one of the best performing technologies in the field of lithium-ion batteries.

[0003] Lithium iron phosphate (LFP), as a cathode material for lithium-ion batteries, possesses advantages such as high voltage, high specific capacity, long cycle life, small size, and low cost, making it a strong competitor for batteries used in electric vehicles. However, its poor electronic conductivity and low ionic conductivity limit the performance of LFP materials. Carbon coating, as one of the main modification methods for LFP materials, can effectively improve its conductivity. Carbon coating of LFP materials requires high temperature and a strong reducing environment (such as C, CO, or H2), which makes the reaction kinetics between the LFP material and the selected carbon interface extremely intense, easily leading to the formation of secondary phases at the interface, ultimately altering the ionic and electronic conductivity of the LFP material. LFP materials generate different secondary phases depending on the sintering temperature and atmosphere, such as magnetic iron phosphate compounds FeP, Fe2P, and Fe3P. The impurity phase Fe2P can also weaken the cycle performance of LFP batteries; therefore, the formation of iron phosphate must be avoided in actual production.

[0004] Chinese patent CN116161638A discloses a low-magnetic-impurity, high-compact lithium iron phosphate material, its preparation method, and its application. The method involves mixing lithium, iron, phosphorus, carbon sources, and dopants via liquid-phase ball milling, drying, and then performing a first high-temperature sintering to obtain a first-burned powder. After determining the iron phosphide content in the first-burned powder, the magnetic material is dissolved in acid. Then, based on the iron phosphide content, lithium, phosphorus, and carbon sources are added, and the mixture is ball-milled until the slurry particle size (D50) is 300–800 nm. After drying, a second low-temperature sintering is performed to obtain lithium iron phosphate material with a magnetic impurity content of less than 100 ppb. While this method can reduce the magnetic material content in lithium iron phosphate, it requires a second sintering process, resulting in a long cycle. Furthermore, after the first sintering, the amount of lithium, phosphorus, and carbon sources to be added needs to be determined based on the iron phosphide content generated each time, leading to poor feasibility, high production costs, and failure to meet the requirements of industrial production. The industry lacks more effective technical means. Summary of the Invention

[0005] In view of the above, the present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a method for preparing lithium iron phosphate materials with low iron phosphate content and its application. By separately controlling the temperature and gas content during the lithium iron phosphate sintering process, both efficient synthesis of lithium iron phosphate can be ensured, while preventing excessive reduction, thereby improving material purity, compaction density, and electrochemical performance. Furthermore, the preparation method of this lithium iron phosphate material has a simple process flow and is suitable for large-scale industrial production.

[0006] Therefore, in a first aspect, embodiments of the present invention provide a method for preparing lithium iron phosphate materials with low iron phosphate content, the preparation method comprising:

[0007] S10. Anhydrous iron phosphate is mixed and dissolved with lithium source, carbon source, dopant and deionized water in a certain proportion to obtain a mixed solution.

[0008] S20. The mixture is wet-milled and spray-dried to obtain a sintering precursor.

[0009] S30. The sintering precursor is heat-treated and pulverized to obtain lithium iron phosphate material; wherein the heat treatment process includes preheating, low-temperature sintering, high-temperature sintering and cooling, the preheating temperature is lower than the low-temperature sintering temperature, the low-temperature sintering temperature is lower than the high-temperature sintering temperature, and the cooling temperature is lower than the high-temperature sintering temperature; the heat treatment process is carried out in a rare atmosphere, and the rare gas content of the high-temperature sintering is greater than the rare gas content of the low-temperature sintering.

[0010] Preferably, the preheating temperature is 300℃-550℃, the low-temperature sintering temperature is 680℃-775℃, the high-temperature sintering temperature is 776℃-820℃, and the cooling temperature is 300℃-720℃; and / or,

[0011] The low-temperature sintering time is longer than the preheating time, the high-temperature sintering time, and the cooling time, respectively; preferably, the preheating time is 2h-3h, the low-temperature sintering time is 6h-10h, the high-temperature sintering time is 2h-6h, and the cooling time is 2h-3h.

[0012] Preferably, the rare atmosphere includes nitrogen, the nitrogen content in the high-temperature sintering atmosphere is greater than 96%, and the nitrogen content in the low-temperature sintering atmosphere is greater than 94.5%.

[0013] Preferably, the heat treatment process further includes detecting the content of other gases in the rare atmosphere, including at least one of oxygen, carbon monoxide, carbon dioxide, and hydrogen.

[0014] Preferably, the heat treatment is carried out in a nitrogen-sealed roller furnace, which includes a front gas replacement chamber, a preheating section, a low-temperature sintering section, a high-temperature sintering section, a cooling section, a rear gas replacement chamber, and heating and roller window sections. The nitrogen main inlet flow rate of the roller furnace is 290 Nm³. 3 / h-330Nm 3 / h, the nitrogen intake of the pre-gas replacement chamber accounts for 8%-10% of the total intake, the nitrogen intake of the preheating section accounts for 10%-20% of the total intake, the nitrogen intake of the low-temperature sintering section accounts for 15%-20% of the total intake, the nitrogen intake of the high-temperature sintering section accounts for 20%-25% of the total intake, the nitrogen intake of the cooling section accounts for 8%-12% of the total intake, the nitrogen intake of the post-gas replacement chamber accounts for 8%-15% of the total intake, and the nitrogen intake of the heating and roller window accounts for 8%-12% of the total intake.

[0015] Preferably, the temperature of the preheating section gradually increases with the feeding direction, the temperatures of the low-temperature sintering section and the high-temperature sintering section remain constant with the feeding direction, and the temperature of the cooling section gradually decreases with the feeding direction.

[0016] Preferably, the anhydrous ferric phosphate has an iron-to-phosphorus molar ratio (Fe / P) of 0.96-0.98; and / or,

[0017] The lithium source includes at least one of lithium carbonate, lithium phosphate, and lithium hydroxide, and the molar ratio of the lithium source to iron phosphate (Li / Fe) is between 1.01 and 1.07; and / or,

[0018] The carbon source includes at least one selected from glucose, sucrose, polyethylene glycol, and citric acid, and the amount of carbon source added satisfies the target carbon content mass ratio for preparing lithium iron phosphate materials being 1.0 wt%-1.8 wt%; and / or,

[0019] The dopant includes at least one of titanium dioxide and ammonium metavanadate; the amount of titanium dioxide and ammonium metavanadate added is 0.05wt%-0.20wt% of the mass of iron phosphate.

[0020] Preferably, the wet grinding is performed at 30℃-45℃, the grinding particle size D50 is less than 0.5μm, and the solid content is 30wt%-45wt%; and / or,

[0021] The spray drying process has the following characteristics: the inlet air temperature is 180℃-240℃, and the outlet air temperature is 90℃-110℃; the particle size D50 of the intermediate product obtained by spray drying is 20μm-45μm, and the moisture content is ≤1.5%.

[0022] Secondly, embodiments of the present invention also provide a lithium iron phosphate material, which is prepared by the preparation method described in the first aspect above.

[0023] Thirdly, embodiments of the present invention also provide a lithium-ion battery, the lithium-ion battery comprising: a battery positive electrode made of the lithium iron phosphate material described in the second aspect above.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The method of this invention controls the temperature and atmosphere of the primary sintering process of lithium iron phosphate according to the formation conditions of iron phosphide, effectively reducing the iron phosphide content in lithium iron phosphate. This sintering control method does not require the selection of different raw materials or the addition of complex demagnetization and secondary sintering processes to achieve the preparation of low-magnetic lithium iron phosphate. The process is simple, can be carried out using existing production lines, has low cost, and is highly feasible.

[0026] The method of this invention performs segmented sintering of lithium iron phosphate material by setting low-temperature and high-temperature sections with different durations. On the one hand, this avoids the lithium iron phosphate material being reduced to iron phosphide when sintering for a long time above the decomposition temperature. On the other hand, since there is a temperature gradient in different positions of the lithium iron phosphate precursor powder in the sagger, when the sintering temperature changes, the lithium iron phosphate material on the surface layer of the sagger experiences a more significant change in heat than the middle layer. This makes it easier for larger lithium iron phosphate material particles to form on the surface layer, while smaller lithium iron phosphate material particles are more likely to form in the middle layer. This achieves a wider range of particle size distribution, improving the compaction density and specific capacity of the lithium iron phosphate material.

[0027] The method of this invention does not require increasing the total nitrogen flow rate by increasing costs to control the sintering atmosphere. Instead, it rationally distributes the nitrogen flow rate in each area by adjusting the air intake and exhaust devices of the roller furnace, thereby increasing the proportion of nitrogen flow rate in the high-temperature sintering section. This reduces the content of reducing gases such as CO and H2 when the lithium iron phosphate material is in the high-temperature sintering section, and further prevents the lithium iron phosphate material from being reduced to iron phosphide during the formation of large particles in the high-temperature sintering process. Attached Figure Description

[0028] Figure 1 This is a flowchart of a method for preparing lithium iron phosphate material according to an embodiment of the present invention;

[0029] Figure 2 SEM image of the lithium iron phosphate material prepared in Example 1 of this invention;

[0030] Figure 3 The image shows a SEM image of the lithium iron phosphate material prepared in Comparative Example 1 of this invention.

[0031] Figure 4 The image shows a SEM image of the lithium iron phosphate material prepared in Comparative Example 2 of this invention.

[0032] Figure 5 The images show the XRD patterns of lithium iron phosphate materials prepared in Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0033] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0034] The following disclosure provides numerous different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples. Such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. Additionally, examples of various specific processes and materials are provided in this invention; however, those skilled in the art will recognize the applicability of other processes and / or the use of other materials.

[0035] Please refer to Figure 1 In a first aspect, embodiments of the present invention provide a method for preparing lithium iron phosphate materials with low iron phosphate content, the preparation method comprising:

[0036] S10. Anhydrous iron phosphate is mixed and dissolved with lithium source, carbon source, dopant and deionized water in a certain proportion to obtain a mixed solution.

[0037] S20. The mixture is wet-milled and spray-dried to obtain a sintering precursor.

[0038] S30. The sintering precursor is heat-treated and pulverized to obtain lithium iron phosphate material; wherein the heat treatment process includes preheating, low-temperature sintering, high-temperature sintering and cooling, the preheating temperature is lower than the low-temperature sintering temperature, the low-temperature sintering temperature is lower than the high-temperature sintering temperature, and the cooling temperature is lower than the high-temperature sintering temperature; the heat treatment process is carried out in a rare atmosphere, and the rare gas content of the high-temperature sintering is greater than the rare gas content of the low-temperature sintering.

[0039] In this embodiment, step S30 is performed in a roller furnace. The roller furnace is equipped with a preheating section, a low-temperature sintering section, a high-temperature sintering section, and a cooling section along the feeding direction, respectively implementing the preheating process, the low-temperature sintering process, the high-temperature sintering process, and the cooling process. It can be understood that the material sintering process can also be performed in other sintering furnaces; the temperature of the sintering furnace can be set according to the preheating, low-temperature sintering, high-temperature sintering, and cooling processes. Furthermore, setting different rare gas contents for high-temperature sintering and low-temperature sintering, combined with different sintering temperatures and different rare atmospheres, effectively improves the sintering effect, avoids the formation of iron phosphide, and maintains the high purity and high compaction of the material.

[0040] Furthermore, the preheating temperature is 300℃-550℃, the low-temperature sintering temperature is 680℃-775℃, the high-temperature sintering temperature is 776℃-820℃, and the cooling temperature is 300℃-720℃.

[0041] Furthermore, the low-temperature sintering time is longer than the preheating time, the high-temperature sintering time, and the cooling time, respectively; preferably, the preheating time is 2h-3h, the low-temperature sintering time is 6h-10h, the high-temperature sintering time is 2h-6h, and the cooling time is 2h-3h.

[0042] Furthermore, the rare atmosphere includes nitrogen, the nitrogen content in the high-temperature sintering atmosphere is greater than 96%, and the nitrogen content in the low-temperature sintering atmosphere is greater than 94.5%.

[0043] Furthermore, the heat treatment process also includes detecting the content of other gases in the rare atmosphere, including at least one of oxygen, carbon monoxide, carbon dioxide, and hydrogen. It is understood that the effectiveness of rare gas content control is confirmed by detecting the content of other gases. Specifically, in the low-temperature sintering section, the nitrogen content is ≥94.5%, carbon monoxide content ≤0.6%, hydrogen content ≤0.7%, and oxygen content ≤0.5%; in the high-temperature sintering section, the nitrogen content is ≥96%, carbon monoxide content ≤0.4%, hydrogen content ≤0.7%, and oxygen content ≤0.5%.

[0044] Furthermore, the heat treatment is carried out in a nitrogen-sealed roller furnace, which includes a front gas replacement chamber, a preheating section, a low-temperature sintering section, a high-temperature sintering section, a cooling section, a rear gas replacement chamber, and heating and roller window sections. The nitrogen main inlet flow rate of the roller furnace is 290 Nm³. 3 / h-330Nm 3 / h, the nitrogen intake of the pre-gas replacement chamber accounts for 8%-10% of the total intake, the nitrogen intake of the preheating section accounts for 10%-20% of the total intake, the nitrogen intake of the low-temperature sintering section accounts for 15%-20% of the total intake, the nitrogen intake of the high-temperature sintering section accounts for 20%-25% of the total intake, the nitrogen intake of the cooling section accounts for 8%-12% of the total intake, the nitrogen intake of the post-gas replacement chamber accounts for 8%-15% of the total intake, and the nitrogen intake of the heating and roller window accounts for 8%-12% of the total intake.

[0045] Furthermore, the temperature of the preheating section gradually increases with the feeding direction, the temperatures of the low-temperature sintering section and the high-temperature sintering section remain constant with the feeding direction, and the temperature of the cooling section gradually decreases with the feeding direction.

[0046] Furthermore, the iron-to-phosphorus molar ratio (Fe / P) of the anhydrous iron phosphate is between 0.96 and 0.98.

[0047] Furthermore, the lithium source includes at least one of lithium carbonate, lithium phosphate, and lithium hydroxide, and the molar ratio of the lithium source to iron phosphate (Li / Fe) is between 1.01 and 1.07.

[0048] Furthermore, the carbon source includes at least one of glucose, sucrose, polyethylene glycol, and citric acid, and the amount of carbon source added satisfies the target carbon content mass ratio of the prepared lithium iron phosphate material being 1.0wt%-1.8wt%.

[0049] Furthermore, the dopant includes at least one of titanium dioxide and ammonium metavanadate; the amount of titanium dioxide and ammonium metavanadate added is 0.05wt%-0.20wt% of the mass of iron phosphate.

[0050] Furthermore, the wet grinding is carried out at 30℃-45℃, the grinding particle size D50 is less than 0.5μm, and the solid content is 30wt%-45wt%.

[0051] Furthermore, in the spray drying process: the inlet air temperature is 180℃-240℃, and the outlet air temperature is 90℃-110℃; the particle size D50 of the intermediate product obtained by spray drying is 20μm-45μm, and the moisture content is ≤1.5%.

[0052] Secondly, embodiments of the present invention also provide a lithium iron phosphate material, which is prepared by the preparation method described in the first aspect above.

[0053] Thirdly, embodiments of the present invention also provide a lithium-ion battery, the lithium-ion battery comprising: a battery positive electrode made of the lithium iron phosphate material described in the second aspect above.

[0054] This invention proposes a method for preparing lithium iron phosphate materials with low iron phosphate content and their applications. By separately controlling the temperature and gas content during the lithium iron phosphate sintering process, the method can ensure efficient synthesis of lithium iron phosphate while preventing excessive reduction, thereby improving material purity, compaction density, and electrochemical performance.

[0055] The following detailed description, in conjunction with some specific embodiments, further illustrates the specific process and effects of the preparation method of lithium iron phosphate material using the present invention, but does not limit the scope of protection of the present invention.

[0056] Example 1

[0057] This embodiment prepares a lithium iron phosphate material, specifically including the following steps:

[0058] 1. Mix 160 kg of ferric phosphate (Fe / P = 0.97), 41 kg of lithium carbonate, 13 kg of glucose, 7 kg of polyethylene glycol, 500 g of titanium dioxide, 150 g of ammonium metavanadate, and deionized water, and then perform sand milling at 30°C. Control the sand milling particle size D50 to be 0.33 μm to obtain slurry A with a solid content of 35 wt%.

[0059] 2. Spray dry the obtained slurry A, controlling the spray inlet temperature to be 220℃ and the outlet temperature to be 90℃, controlling the particle size D50 of the sprayed particles to be 35μm, and the moisture content to be ≤1.5%, to obtain the precursor powder B to be sintered.

[0060] 3. Precursor powder B is loaded into graphite saggers at a rate of 6 kg per sagger and placed in a nitrogen-sealed roller furnace for sintering. Before sintering begins, the roller furnace is set up sequentially according to the feeding direction: a preheating section with a heating time of 3 hours and a temperature gradually increasing from 300℃, 370℃, 440℃, to 500℃; a low-temperature sintering section with a heating time of 6 hours and a temperature of 740℃; a high-temperature sintering section with a heating time of 4 hours and a temperature of 810℃; and a cooling section with a heating time of 3 hours and a temperature gradually decreasing from 710℃, 640℃, 520℃, to 390℃.

[0061] 4. After sintering begins, maintain the nitrogen main flow rate at 321 Nm. 3With the nitrogen flow rate kept constant, the opening of the nitrogen inlet valves in the low-temperature sintering section and the high-temperature sintering section is increased, while the opening of the nitrogen inlet valves in other temperature zones is appropriately decreased. This ensures that the nitrogen flow rates in the front gas replacement chamber, preheating section, low-temperature sintering section, high-temperature sintering section, cooling section, rear gas replacement chamber, heating section, and roller window are 8%, 12%, 20%, 25%, 12%, 12%, and 11% of the total pipe flow rate, respectively. The position of the material is observed through the roller furnace observation window. When the material reaches the low-temperature sintering section and the high-temperature sintering section, the gas from each heating section is extracted into a 2L aluminum foil gas bag. The gas composition and content in the gas bag are analyzed using a GC-MS gas chromatography-mass spectrometry instrument. After adjusting the nitrogen flow rate in each temperature zone, the main gas contents in the low-temperature sintering section are: N2 95.78%, O2 0.35%, CO 0.52%, CO2 2.28%, and H2 0.51%; the main gas contents in the high-temperature sintering section are: N2 97.97%, O2 0.41%, CO 0.38%, CO2 0.26%, and H2 0.64%.

[0062] 5. The sintered material C obtained in step 4 is subjected to air jet milling, and the particle size D50 is controlled to be 1.10 μm to obtain a lithium iron phosphate product D with low iron phosphate content.

[0063] Figure 2 The image shows a SEM image of the high-density lithium iron phosphate material prepared in Example 1. Figure 2 As can be seen, the primary particles have good sphericity, and the particles of different sizes are closely connected, presenting a spherical structure similar to a "pomegranate".

[0064] Figure 5 The image shows the XRD pattern of the high-density lithium iron phosphate material prepared in Example 1. Figure 5 As can be seen from the spectrum, the peak positions match those of the lithium iron phosphate standard PDF card, indicating the absence of impurities and high material purity.

[0065] Example 2

[0066] This embodiment prepares a lithium iron phosphate material, specifically including the following steps:

[0067] 1. Mix 180 kg of ferric phosphate with a molar ratio of Fe / P = 0.965, 45 kg of lithium carbonate, 10 kg of sucrose, 8 kg of citric acid, 600 g of ammonium metavanadate and deionized water, and then perform sand milling at 30℃. Control the sand milling particle size D50 to be 0.33 μm to obtain slurry A with a solid content of 35 wt%.

[0068] 2. Spray dry the obtained slurry A, controlling the spray inlet temperature to be 220℃ and the outlet temperature to be 90℃, controlling the particle size D50 of the sprayed particles to be 30μm, and the moisture content to be ≤1.5%, to obtain the precursor powder B to be sintered.

[0069] 3. Precursor powder B is loaded into graphite saggers at a rate of 6 kg per sagger and placed in a nitrogen-sealed roller furnace for sintering. Before sintering begins, the roller furnace is set up sequentially according to the feeding direction: a preheating section with a heating time of 3 hours and a temperature gradually increasing from 320℃, 400℃, 460℃, to 520℃; a low-temperature sintering section with a heating time of 6 hours and a temperature of 760℃; a high-temperature sintering section with a heating time of 5 hours and a temperature of 790℃; and a cooling section with a heating time of 3 hours and a temperature gradually decreasing from 700℃, 620℃, 500℃, to 380℃.

[0070] 4. After sintering begins, maintain the nitrogen main flow rate at 315 Nm. 3 With the nitrogen flow rate kept constant, the opening of the nitrogen inlet valves in the low-temperature sintering section and the high-temperature sintering section is increased, while the opening of the nitrogen inlet valves in other temperature zones is appropriately decreased. This ensures that the nitrogen flow rates in the front gas replacement chamber, preheating section, low-temperature sintering section, high-temperature sintering section, cooling section, rear gas replacement chamber, heating section, and roller window are 8%, 12%, 21%, 24%, 12%, 12%, and 11% of the total pipe flow rate, respectively. The position of the material is observed through the roller furnace observation window. When the material reaches the low-temperature sintering section and the high-temperature sintering section, the gas in that heating section is extracted into a 2L aluminum foil gas bag. The gas composition and content in the gas bag are analyzed using a GC-MS gas chromatography-mass spectrometry instrument. After adjusting the nitrogen flow rate in each temperature zone, the main gas contents in the low-temperature sintering section are: N2 95.82%, O2 0.29%, CO 0.3%, CO2 3.03%, and H2 0.27%. The main gas contents in the high-temperature sintering section are: N2 97.60%, O2 0.24%, CO 0.26%, CO2 1.38%, and H2 0.31%. Sintered material C is obtained after sintering.

[0071] 5. The sintered material C obtained in step 4 is subjected to air jet milling, and the particle size D50 is controlled to be 1.05μm to obtain a lithium iron phosphate product D with low iron phosphate content.

[0072] Example 3

[0073] This embodiment prepares a lithium iron phosphate material, specifically including the following steps:

[0074] 1. Mix 170 kg of ferric phosphate (Fe / P = 0.98), 43 kg of lithium carbonate, 14 kg of glucose, 5 kg of polyethylene glycol, 800 g of titanium dioxide and deionized water, and then mill them at 30°C. Control the milling particle size D50 to be 0.36 μm to obtain slurry A with a solid content of 33 wt%.

[0075] 2. Spray dry the obtained slurry A, control the spray inlet temperature to be 225℃ and the outlet temperature to be 95℃, control the particle size D50 of the sprayed particles to be 32μm, and the moisture content to be ≤1.5%, to obtain the precursor powder B to be sintered.

[0076] 3. Precursor powder B is loaded into graphite saggers at a rate of 6 kg per sagger and placed in a nitrogen-sealed roller furnace for sintering. Before sintering begins, the roller furnace is set up sequentially according to the feeding direction: a preheating section with a heating time of 3 hours and a temperature gradually increasing from 330℃, 390℃, 470℃, to 540℃; a low-temperature sintering section with a heating time of 8 hours and a temperature of 740℃; a high-temperature sintering section with a heating time of 4 hours and a temperature of 780℃; and a cooling section with a heating time of 3 hours and a temperature gradually decreasing from 700℃, 600℃, 480℃, to 350℃.

[0077] 4. After sintering begins, maintain the nitrogen main flow rate at 310 Nm. 3 With the nitrogen flow rate remaining constant, the opening of the nitrogen inlet valves in the low-temperature sintering section and the high-temperature sintering section is increased, while the opening of the nitrogen inlet valves in other temperature zones is appropriately decreased. This ensures that the nitrogen flow rates in the front gas replacement chamber, preheating section, low-temperature sintering section, high-temperature sintering section, cooling section, rear gas replacement chamber, heating section, and roller window are 10%, 12%, 20%, 23%, 12%, 12%, and 11% of the total pipe flow rate, respectively. The position of the material is observed through the roller furnace observation window. When the material reaches the low-temperature sintering section and the high-temperature sintering section, the gas in that heating section is extracted into a 2L aluminum foil gas bag. The gas composition and content in the gas bag are analyzed using a GC-MS gas chromatography-mass spectrometry instrument. After adjusting the nitrogen flow rate in each temperature zone, the main gas contents in the low-temperature sintering section are: N2 94.63%, O2 0.36%, CO 0.44%, CO2 3.13%, and H2 0.56%; the main gas contents in the high-temperature sintering section are: N2 96.27%, O2 0.28%, CO 0.29%, CO2 2.65%, and H2 0.31%. Sintered material C is obtained after sintering.

[0078] 5. The sintered material C obtained in step 4 is subjected to air jet milling, and the particle size D50 is controlled to be 1.05μm to obtain a lithium iron phosphate product D with low iron phosphate content.

[0079] Comparative Example 1

[0080] This embodiment prepares a lithium iron phosphate material, specifically including the following steps:

[0081] 1. Mix 160 kg of ferric phosphate (Fe / P = 0.97), 41 kg of lithium carbonate, 13 kg of glucose, 7 kg of polyethylene glycol, 500 g of titanium dioxide, 150 g of ammonium metavanadate, and deionized water, and then perform sand milling at 30°C. Control the sand milling particle size D50 to be 0.33 μm to obtain slurry A with a solid content of 35 wt%.

[0082] 2. Spray dry the obtained slurry A, controlling the spray inlet temperature to be 220℃ and the outlet temperature to be 90℃, controlling the particle size D50 of the sprayed particles to be 35μm, and the moisture content to be ≤1.5%, to obtain the precursor powder B to be sintered.

[0083] 3. Precursor powder B is loaded into graphite saggers at a rate of 6 kg per sagger and placed in a nitrogen-sealed roller furnace for sintering. Before sintering begins, the roller furnace is set up sequentially according to the feeding direction: a preheating section with a heating time of 3 hours and a temperature gradually increasing from 300℃, 370℃, 440℃, to 500℃; a sintering section with a heating time of 10 hours and a temperature gradually increasing from 800℃; and a cooling section with a heating time of 3 hours and a temperature gradually decreasing from 710℃, 640℃, 520℃, to 390℃. After sintering, sintered material C is obtained.

[0084] 4. The sintered material C obtained in step 3 is subjected to air jet milling to obtain lithium iron phosphate product D with a particle size D50 of 1.10 μm.

[0085] Figure 3 The image shows a SEM image of the lithium iron phosphate material prepared in Comparative Example 1. As can be seen from the image, there are many gaps between the primary particles, and the surface morphology of the particles is also poor.

[0086] Figure 5 In the XRD spectrum of the lithium iron phosphate material prepared in Comparative Example 1, it can be seen that there is an Fe2P impurity phase.

[0087] Comparative Example 2

[0088] This embodiment prepares a lithium iron phosphate material, specifically including the following steps:

[0089] 1. Mix 180 kg of ferric phosphate with a molar ratio of Fe / P = 0.965, 45 kg of lithium carbonate, 10 kg of sucrose, 8 kg of citric acid, 600 g of ammonium metavanadate and deionized water, and then perform sand milling at 30℃. Control the sand milling particle size D50 to be 0.33 μm to obtain slurry A with a solid content of 35 wt%.

[0090] 2. Spray dry the obtained slurry A, controlling the spray inlet temperature to be 220℃ and the outlet temperature to be 90℃, controlling the particle size D50 of the sprayed particles to be 30μm, and the moisture content to be ≤1.5%, to obtain the precursor powder B to be sintered.

[0091] 3. Precursor powder B is loaded into graphite saggers at a rate of 6 kg per sagger and placed in a nitrogen-sealed roller furnace for sintering. Before sintering begins, the roller furnace is set up sequentially according to the feeding direction: a preheating section with a heating time of 3 hours and a temperature gradually increasing from 300℃, 370℃, 440℃, to 500℃; a sintering section with a heating time of 14 hours and a temperature of 760℃; and a cooling section with a heating time of 3 hours and a temperature gradually decreasing from 710℃, 640℃, 520℃, to 390℃. After sintering, sintered material C is obtained.

[0092] 4. The sintered material C obtained in step 3 is subjected to air jet milling to obtain lithium iron phosphate product D with a particle size D50 of 1.10 μm.

[0093] Figure 4 The image shows a SEM image of the lithium iron phosphate material prepared in Comparative Example 2. As can be seen from the image, the primary particles are small in size and there are many gaps between the particles.

[0094] Comparative Example 3

[0095] This embodiment prepares a lithium iron phosphate material, specifically including the following steps:

[0096] 1. Mix 170 kg of ferric phosphate (Fe / P = 0.98), 43 kg of lithium carbonate, 14 kg of glucose, 5 kg of polyethylene glycol, 800 g of titanium dioxide and deionized water, and then mill them at 30°C. Control the milling particle size D50 to be 0.36 μm to obtain slurry A with a solid content of 33 wt%.

[0097] 2. Spray dry the obtained slurry A, control the spray inlet temperature to be 225℃ and the outlet temperature to be 95℃, control the particle size D50 of the sprayed particles to be 32μm, and the moisture content to be ≤1.5%, to obtain the precursor powder B to be sintered.

[0098] 3. Precursor powder B is loaded into graphite saggers at a rate of 6 kg per sagger and placed in a nitrogen-sealed roller furnace for sintering. Before sintering begins, the roller furnace is set up sequentially according to the feeding direction: a preheating section with a heating time of 3 hours and a temperature gradually increasing from 330℃, 390℃, 470℃, to 540℃; a low-temperature sintering section with a heating time of 8 hours and a temperature of 740℃; a high-temperature sintering section with a heating time of 4 hours and a temperature of 780℃; and a cooling section with a heating time of 3 hours and a temperature gradually decreasing from 700℃, 600℃, 480℃, to 350℃.

[0099] 4. After sintering begins, the nitrogen flow rate in each zone remains unchanged; the total nitrogen flow rate is 310 Nm³. 3The nitrogen flow rates in the preheating section, preheating section, low-temperature sintering section, high-temperature sintering section, cooling section, post-gas replacement chamber, heating section, and roller window were 20%, 15%, 15%, 15%, 20%, and 15% of the total pipe flow rate, respectively. The material position was observed through the roller furnace observation window. When the material reached the low-temperature and high-temperature sintering sections, the gas from those sections was extracted into 2L aluminum foil gas bags. The gas composition and content in the gas bags were analyzed using GC-MS. The main gas contents in the low-temperature sintering section were: N2 93.63%, CO 1.44%, CO2 2.13%, and H2 0.56%; the main gas contents in the high-temperature sintering section were: N2 94.27%, CO 1.29%, CO2 2.65%, and H2 0.31%. Sintered material C was obtained after sintering.

[0100] 5. The sintered material C obtained in step 4 is subjected to air jet milling to obtain lithium iron phosphate product D with a particle size D50 of 1.05 μm.

[0101] Furthermore, the performance characterization methods for the lithium iron phosphate materials prepared in the examples and comparative examples are as follows:

[0102] Compacted density test method: The compacted density meter was used for testing. The mass of the test sample was 1.000±0.005g, and the test pressure was 3 tons.

[0103] The test method for magnetic iron is as follows: A magnetic rod with a magnetic field strength of 8000-12000GS is used to adsorb magnetic foreign objects. The adsorbed lithium iron phosphate on the magnetic rod is washed off with water. The remaining magnetic foreign objects are weighed and dissolved in acid to form a solution. The Fe and P content in the remaining magnetic foreign objects is tested by ICP. The content of magnetic iron can be calculated by the following formula: W_magnetic iron = (W_Fe - 1.8 * WP) * V / m, where W_Fe and WP are the Fe and P contents obtained from the ICP test, respectively, m is the weight of the magnetic foreign object, and V is the volume of the solution after acid dissolution.

[0104] Coin cell specific capacity testing method: The lithium iron phosphate cathode materials prepared in the examples and comparative examples were dispersed in NMP with Super-P and PVDF at a mass ratio of 80:10:10. After ball milling and uniform dispersion, the dispersion was coated on aluminum foil and vacuum dried to obtain the cathode electrode. The electrolyte was 1 mol / L LiPF6, with a solvent volume ratio of EC:DMC:EMC = 1:1:1 (volume ratio). The separator was Celigard polypropylene membrane, and the lithium metal sheet was used as the anode. All components were assembled into a coin cell. The test voltage range was 2.0V to 3.75V. The cells were charged to 3.75V using a constant current and constant voltage charging method and discharged to 2.0V using a constant current discharging method. The charge / discharge current was 0.1C. The test results are shown in Table 1 below.

[0105] Table 1 Test Results

[0106]

[0107] Examples 1-3 show lithium iron phosphate with low iron phosphide content prepared according to the present invention. Comparative Examples 1-2 show lithium iron phosphate prepared by conventional isothermal sintering, and Comparative Example 3 shows lithium iron phosphate prepared by segmented sintering without controlling the sintering atmosphere. The data shows that the lithium iron phosphate prepared according to the present invention has higher compaction density, 0.1C charge / discharge specific capacity, and initial coulombic efficiency than the lithium iron phosphate prepared by the comparative examples, and the iron phosphide content is less than 1 ppm. XRD analysis shows that, compared to Comparative Example 1, the lithium iron phosphate prepared by this process has higher purity and no Fe2P impurity phase is formed. Furthermore, scanning electron microscopy analysis of the products from the examples and the comparative examples reveals… Figure 3 Lithium iron phosphate is obtained by sintering at a relatively high constant temperature. Its particles are large and can also form large strip-shaped iron phosphate particles, which leads to a decrease in product performance. Figure 4 Lithium iron phosphate obtained by sintering at a relatively low constant temperature does not easily form iron phosphide, but due to the low sintering temperature, the particles are generally small and prone to agglomeration, leading to a decrease in compaction density; while... Figure 2 As shown, since the method of the present invention controls the formation of iron phosphide by setting a shorter high-temperature sintering period and increasing the nitrogen flow rate in the sintering period, no obvious strip-shaped iron phosphide particles appear in the figure. At the same time, the lithium iron phosphate particles prepared by the method of the present invention have a graded distribution of particle size, and the microscopic spherical structure similar to "pomegranate" helps to enhance the conductivity between particles and inside particles, thereby achieving lithium iron phosphate with low iron phosphide content while taking into account high compaction density and specific capacity.

[0108] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0109] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for preparing a low iron phosphite content lithium iron phosphate material, characterized in that, The preparation method comprises: S10, according to a certain proportion, anhydrous iron phosphate is mixed and dissolved with a lithium source, a carbon source, a dopant and deionized water to obtain a mixed solution; S20, the mixed solution is subjected to wet grinding and spray drying to obtain a sintering precursor; S30, the sintering precursor is subjected to heat treatment and crushing to obtain a lithium iron phosphate material; wherein the heat treatment process comprises preheating, low-temperature sintering, high-temperature sintering and cooling, the temperature of the preheating is lower than the temperature of the low-temperature sintering, the temperature of the low-temperature sintering is lower than the temperature of the high-temperature sintering, and the temperature of the cooling is lower than the temperature of the high-temperature sintering; the heat treatment process is carried out in an atmosphere of a rare gas, and the content of the rare gas in the high-temperature sintering is higher than the content of the rare gas in the low-temperature sintering; wherein the temperature of the preheating is 300-550 DEG C, the temperature of the low-temperature sintering is 680-775 DEG C, the temperature of the high-temperature sintering is 776-820 DEG C, and the temperature of the cooling is 300-720 DEG C; the rare gas comprises nitrogen, the content of nitrogen in the atmosphere of the high-temperature sintering is greater than 96%, and the content of nitrogen in the atmosphere of the low-temperature sintering is greater than 94.5%; The heat treatment is carried out in a nitrogen sealing roller furnace, which comprises a front gas displacement chamber, a preheating section, a low-temperature sintering section, a high-temperature sintering section, a cooling section, a rear gas displacement chamber, a heating and roller window, the total nitrogen inlet amount of the roller furnace is 290 Nm 3 / h-330 Nm 3 / h, the nitrogen inlet amount of the front gas displacement chamber accounts for 8%-10% of the total inlet amount, the nitrogen inlet amount of the preheating section accounts for 10%-20% of the total inlet amount, the nitrogen inlet amount of the low-temperature sintering section accounts for 15%-20% of the total inlet amount, the nitrogen inlet amount of the high-temperature sintering section accounts for 20%-25% of the total inlet amount, the nitrogen inlet amount of the cooling section accounts for 8%-12% of the total inlet amount, the nitrogen inlet amount of the rear gas displacement chamber accounts for 8%-15% of the total inlet amount, and the nitrogen inlet amount of the heating and roller window accounts for 8%-12% of the total inlet amount.

2. The production method according to claim 1, characterized by, the time of the low-temperature sintering is greater than the time of the preheating, the time of the high-temperature sintering and the time of the cooling respectively; the preheating time is 2-3 h, the low-temperature sintering time is 6-10 h, the high-temperature sintering time is 2-6 h, and the cooling time is 2-3 h.

3. The production method according to claim 1, characterized by, The heat treatment process further comprises detecting the content of other gases in the atmosphere, and the other gases comprise at least one of oxygen, carbon monoxide, carbon dioxide and hydrogen.

4. The method of claim 1, wherein, The temperature of the preheating section is gradually increased along the feeding direction, the temperature of the low-temperature sintering section and the high-temperature sintering section remains constant along the feeding direction, and the temperature of the cooling section is gradually decreased along the feeding direction.

5. The preparation method according to claim 1, characterized in that, The molar ratio of iron to phosphorus (Fe / P) of the anhydrous iron phosphate is 0.96-0.98; and / or, the lithium source comprises at least one of lithium carbonate, lithium phosphate and lithium hydroxide, and the molar ratio of the lithium source to iron (Li / Fe) is 1.01-1.07; and / or, the carbon source comprises at least one of glucose, sucrose, polyethylene glycol and citric acid, and the addition amount of the carbon source satisfies the target carbon content of the lithium iron phosphate material, which is 1.0wt%-1.8wt%; and / or, the dopant comprises at least one of titanium dioxide and ammonium metavanadate, and the addition amount of the titanium dioxide and the ammonium metavanadate satisfies 0.05wt%-0.20wt% of the mass of the iron phosphate.

6. The method of claim 1, wherein the lithium iron phosphate material is prepared by the steps of: The wet grinding is carried out at 30-45 DEG C, the grinding particle size D50 is less than 0.5 μm, and the solid content is 30wt%-45wt%; and / or, ​ the spray drying process: the inlet air temperature is 180-240 DEG C, and the outlet air temperature is 90-110 DEG C; the particle size D50 of the intermediate product obtained by the spray drying is 20-45 μm, and the moisture content is ≤1.5%.

7. A lithium iron phosphate material, characterized in that, The lithium iron phosphate material is prepared by the preparation method of any one of claims 1-6.

8. A lithium-ion battery, characterized by The lithium ion battery comprises: a battery anode prepared from the lithium iron phosphate material of claim 7.

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