Carbon-coated lithium iron ferrite, its preparation methods, cathode lithium supplementation materials and applications

Abstract

This invention provides a carbon-coated lithium ferrite rich in lithium iron ferrite, its preparation method, a cathode lithium supplement material, and its applications. The preparation method includes: step S1, crystallizing an iron source and a structure inducer in an organic solvent to obtain a crystallized product; step S2, micro-reacting a lithium source and the crystallized product in a microwave reactor to obtain a precursor; and step S3, sintering the precursor to obtain carbon-coated lithium ferrite rich in lithium iron ferrite. The structure inducer includes organic amines and polyphenolic compounds. The lithium ferrite rich in lithium iron ferrite prepared by this invention exhibits higher ionic conductivity and structural stability, higher purity, lower residual alkali content, and reduced average particle size, resulting in superior overall performance.

Description

Technical Field

[0001] This invention relates to the field of lithium battery additives, and more specifically, to a carbon-coated lithium iron ferrite, its preparation method, positive electrode lithium replenishment material, and its application. Background Technology

[0002] In the field of lithium-ion batteries, cathode lithium replenishment technology has attracted increasing attention due to its simple operation and reliable safety. Typically, cathode lithium replenishment technology only requires adding a certain amount of cathode lithium replenishment additive during the preparation of the cathode slurry in the battery manufacturing process, without altering the lithium-ion battery production process, thus making it a promising technology for industrial application.

[0003] Lithium-rich lithium iron ferrite (Li5FeO4) material possesses extremely high lithium capacity, good compatibility with battery systems, and low production cost and non-toxicity, making it one of the most commercially promising cathode lithium supplementation additives. However, lithium-rich lithium iron ferrite material has low intrinsic conductivity, and the sintered particles of lithium-rich lithium iron ferrite material obtained by conventional solid-state preparation methods are relatively large, resulting in low lithium-ion and electron transport efficiency and poor rate performance, often failing to achieve ideal results when used as an additive. On the other hand, lithium-rich lithium iron ferrite material itself has poor structural stability and is extremely unstable in air, readily reacting rapidly with water in the air to form lithium hydroxide, resulting in excessively high residual alkali on the surface, thus leading to capacity reduction and increasing processing difficulty.

[0004] Existing lithium-rich lithium iron ferrite materials suffer from low ion transport efficiency or poor rate performance due to their excessively high residual alkali (lithium hydroxide) content on the surface and large particle size. Therefore, there is an urgent need to provide a new lithium-rich lithium iron ferrite material to improve these problems. Summary of the Invention

[0005] The main objective of this invention is to provide a carbon-coated lithium iron ferrite, its preparation method, positive electrode lithium replenishment material, and its application, in order to solve the problem that existing lithium iron ferrite has low ion transport efficiency or poor rate performance due to its high residual alkali (lithium hydroxide) content on the surface and large particle size.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing carbon-coated lithium iron ferrite is provided, the method comprising: step S1, taking an iron source and a structure inducer and performing a crystallization reaction in an organic solvent to obtain a crystallized product; step S2, taking a lithium source and the crystallized product and performing a microwave reaction in a microwave reactor to obtain a precursor; step S3, sintering the precursor to obtain carbon-coated lithium iron ferrite; wherein the structure inducer includes organic amines and polyphenolic compounds.

[0007] Further, the organic amine is a fatty amine compound, preferably selected from one or more of ethylenediamine, diethylamine or triethylamine; the polyphenol compound is preferably a flavanol compound, more preferably selected from one or more of catechin, epicatechin or flavanol; the weight ratio of iron source to polyphenol compound is preferably 1:(0.05-0.1); the weight ratio of organic amine to organic solvent is preferably 1:(1-25).

[0008] Further, the organic solvent is selected from one or more of methanol, ethanol, isopropanol, ethylene glycol or n-propanol; preferably, the iron source is selected from one or more of ferrous chloride, ferrous nitrate or ferrous acetate; preferably, the lithium source is selected from one or more of lithium hydroxide, lithium nitrate or lithium acetate; preferably, the molar ratio of the lithium source to the iron source is (5.5 to 6.0):1.

[0009] Further, in step S2, the microwave frequency of the microwave reactor is 2400-2500MHz, preferably 2435-2465MHz, the reaction time is 1-3h, and the reaction temperature is 140-160℃; preferably, in step S1, the crystallization reaction is carried out in a hydrothermal reactor; preferably, the crystallization reaction temperature is 160-200℃, and the reaction time is 5-12h.

[0010] Furthermore, before the sintering process, step S2 also includes sequentially separating and drying the precursor to obtain the precursor; preferably, the separation process is carried out in a centrifuge; preferably, the drying process is carried out in a vacuum dryer; more preferably, the drying temperature is 60-100°C and the processing time is 6-12 hours.

[0011] Further, in step S3, the sintering process is carried out under a protective gas atmosphere, preferably nitrogen and / or argon; preferably, the sintering involves heating the precursor from the temperature after the microwave reaction to the sintering temperature, with the sintering temperature being 600–900°C, the sintering time being 4–8 h, and the heating rate being 0.5–2.0°C / min; preferably, the sintering process is carried out in one or more of the following equipment: an atmosphere box furnace, a tube furnace, or a roller kiln.

[0012] According to another aspect of the present invention, a carbon-coated lithium ferrite rich in lithium is provided, which is obtained by the above-described method for preparing carbon-coated lithium ferrite rich in lithium.

[0013] Furthermore, the carbon-coated lithium iron ferrite is in particulate form, preferably with an average particle size of 1–4 μm.

[0014] According to another aspect of the present invention, an application of carbon-coated lithium iron ferrite as a positive electrode lithium replenishment material is provided.

[0015] According to another aspect of the present invention, a lithium-ion battery is provided, the lithium-ion battery comprising a positive electrode lithium replenishment material, the positive electrode lithium replenishment material being the aforementioned carbon-coated lithium iron ferrite.

[0016] The lithium iron ferrite prepared by applying this invention has higher ionic conductivity and structural stability, higher purity, lower residual alkali content, and lower average particle size, resulting in better overall performance. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0018] Figure 1 The image shows a SEM image (magnification 30,000x) of carbon-coated lithium iron ferrite obtained in Example 1 of this application.

[0019] Figure 2 The image shown is a SEM image (magnification 30,000x) of carbon-coated lithium iron ferrite obtained in Comparative Example 1 of this application. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] As described in the background section of this invention, existing lithium-rich lithium ferrite suffers from low ion transport efficiency or poor rate performance due to its high residual alkali (lithium hydroxide) content and large particle size. This invention provides a method for preparing carbon-coated lithium-rich lithium ferrite, comprising: step S1, reacting an iron source and a structure inducer in an organic solvent to obtain a crystallized product; step S2, reacting a lithium source and the crystallized product in a microwave reactor to obtain a precursor; and step S3, sintering the precursor to obtain carbon-coated lithium-rich lithium ferrite. The structure inducer includes organic amines and polyphenolic compounds.

[0022] Existing lithium-rich lithium iron ferrite structures are unstable and readily react with water in the air to form lithium hydroxide, resulting in excessively high residual alkali content on the surface and reduced lithium replenishment capacity during application. Furthermore, lithium-rich lithium iron ferrite prepared using conventional methods tends to aggregate and form large sintered particles during sintering, thus affecting lithium-ion transport efficiency. Therefore, this invention provides a method for preparing carbon-coated lithium-rich lithium iron ferrite. First, an iron source and a structure inducer are subjected to a crystallization reaction in an organic solvent to obtain a crystallized product. This invention achieves this by coordinating iron ions from the iron source with the structure inducer, thereby obtaining a crystallized product in which structural particles (molecules, atoms, or ions) are arranged regularly and periodically in three-dimensional space. Then, the crystallized product is subjected to a microwave reaction with a lithium source. On the one hand, the microwave driving force allows the lithium source to enter the intercrystalline gaps of the crystallized product, ensuring uniform mixing and resulting in precursor particles with high structural regularity. On the other hand, under the action of the microwave driving force, the precursor particles generated in the microwave reactor can slowly aggregate into a tetrahedral structure, thereby increasing the wetting of lithium-rich lithium ferrite particles by the electrolyte during application, increasing the contact area between lithium ions and electrons, shortening the transfer path between particles, and thus improving cycle rate performance. Finally, the obtained precursor is sintered, which further improves the structural stability of the precursor, promotes the orderly arrangement of the precursor structure, and forms a crystalline phase. Moreover, the structure inducer in the raw material can provide a carbon source, forming a carbon coating layer on the outer surface of lithium-rich lithium ferrite during sintering, which can improve the structural stability of lithium-rich lithium ferrite, reduce its residual alkali content, and improve the lithium replenishment capacity and ion and electron transfer efficiency of lithium-rich lithium ferrite.

[0023] In particular, the structure-inducing agents used in this invention include organic amines and polyphenolic compounds. The use of organic amines and polyphenolic compounds is based on the fact that metal ions (iron ions) can coordinate with the amino groups in organic amines and the hydroxyl groups in polyphenolic compounds, thereby generating crystallized products in which structural particles are regularly arranged and periodically repeating in three-dimensional space. Compared to conventional methods, the operation of preparing lithium-rich lithium ferrite by sintering a mixture of iron and lithium sources results in a crystallized product with relatively high regularity. Lithium-rich lithium ferrite prepared using this raw material exhibits higher ionic conductivity and structural stability, resulting in superior performance.

[0024] In a preferred embodiment, the organic amine is an aliphatic amine compound, and more preferably, the aliphatic amine compound is selected from one or more of ethylenediamine, diethylamine, or triethylamine, which enables the metal ions (iron ions) to better coordinate with the amine groups in the organic amine, thereby generating a crystallized product in which the structural particles are arranged regularly and periodically in three-dimensional space, thereby improving the structural stability and lithium-ion transport efficiency of the product lithium iron ferrite.

[0025] In a preferred embodiment, the polyphenolic compound is a flavanol compound, more preferably selected from one or more of catechin, epicatechin, or flavanols. This allows the metal ions (iron ions) to better coordinate with the hydroxyl groups in the flavanol compound, thereby generating a crystalline product with regularly arranged and periodically repeating structural particles in three-dimensional space. Compared to conventional methods that involve sintering a mixture of iron and lithium sources to prepare lithium-rich lithium ferrite, the lithium-rich lithium ferrite prepared by the method of this invention exhibits higher ionic conductivity and structural stability, resulting in superior performance.

[0026] To further improve the reaction rate of the crystallization reaction and the yield of the crystallization product, the weight ratio of iron source to polyphenol compound is preferably 1:(0.05-0.1), and the weight ratio of organic amine to organic solvent is even more preferably 1:(4-25).

[0027] To further promote the crystallization reaction and allow the iron source and structure inducer to dissolve better, the organic solvent is preferably selected from one or more of methanol, ethanol, isopropanol, ethylene glycol, or n-propanol. To ensure sufficient reaction between the lithium source and the iron source reactants and improve the reaction rate and product yield, the iron source is preferably selected from one or more of ferrous chloride, ferrous nitrate, or ferrous acetate; the lithium source is preferably selected from one or more of lithium hydroxide, lithium nitrate, or lithium acetate. More preferably, the molar ratio of lithium source to iron source is (5.5-6.0):1.

[0028] In a preferred embodiment, the iron source is better promoted to enter the intercrystalline gaps of the crystallization product under the action of microwave driving force, so that the lithium source and the crystallization product are mixed evenly, thereby promoting the precursor particles generated in the microwave reactor to slowly aggregate into a tetrahedral structure, thereby increasing the lithium-ion transport efficiency of lithium iron ferrite in the application process. Preferably, the microwave frequency of the microwave reactor is 2400-2500MHz, more preferably 2435-2465MHz, the reaction time is 1-3h, and the reaction temperature is 140-160℃.

[0029] In a preferred embodiment, the crystallization reaction is further promoted so that the iron ions in the iron source coordinate with the structure inducer, thereby obtaining a crystallized product in which the structural particles (molecules, atoms or ions) are arranged more regularly and periodically in three-dimensional space. Preferably, in step S1, the crystallization reaction is carried out in a hydrothermal reactor; more preferably, the crystallization reaction temperature is 160-200°C and the reaction time is 5-12 hours.

[0030] To further reduce the impact of impurities in the solution on the structure of lithium iron ferrite, step S2 before sintering further includes separating and drying the precursor sequentially to obtain the precursor. Preferably, the separation is carried out in a centrifuge and the drying is carried out in a vacuum dryer, which can further remove the solvent and other impurities, thereby reducing their impact on the formation of the regular structure of lithium iron ferrite during subsequent sintering and improving its electrochemical performance in application. More preferably, the drying temperature is 60-100℃ and the treatment time is 6-12h.

[0031] To reduce side reactions between oxygen and other components in the air and the reactants during sintering, which could affect the performance of lithium-rich lithium iron ferrite, step S3 is preferably performed under a protective gas atmosphere, more preferably nitrogen and / or argon. To promote a more ordered structural arrangement of lithium-rich lithium iron ferrite and to generate a surface carbon coating layer from the organic carbon chains in organic amines and polyphenolic compounds, thereby further inhibiting particle agglomeration, reducing particle size, stabilizing the product structure, and ultimately improving the rate performance of lithium-rich lithium iron ferrite, sintering is preferably performed by heating the precursor from the post-microwave reaction temperature to the sintering temperature of 600–900°C for 4–8 hours at a heating rate of 0.5–2.0°C / min. More preferably, the sintering process is carried out in one or more of the following equipment: an atmosphere-controlled box furnace, a tube furnace, or a roller kiln.

[0032] Another aspect of the present invention provides a carbon-coated lithium ferrite rich in lithium iron ferrite, which is obtained by the above-described method for preparing carbon-coated lithium ferrite rich in lithium iron ferrite. The carbon-coated lithium ferrite rich in lithium iron ferrite has a low residual alkali content on its surface, strong structural stability, and high ion transport efficiency.

[0033] In a preferred embodiment, the carbon-coated lithium iron ferrite is in particulate form, preferably with an average particle size of 1-4 μm. This reduces the agglomeration of the carbon-coated lithium iron ferrite particles, decreases their size, and results in a regular internal lattice structure, thus achieving higher ion transfer efficiency.

[0034] Another aspect of the present invention provides an application of carbon-coated lithium-rich lithium ferrite as a cathode lithium replenishment material. When the carbon-coated lithium-rich lithium ferrite of the present invention is used as a cathode lithium replenishment material, it has a high lithium replenishment capacity.

[0035] Another aspect of the present invention provides a lithium-ion battery comprising the above-mentioned positive electrode lithium replenishment material, which has high conductivity, excellent rate cycling performance and stability.

[0036] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0037] Example 1

[0038] 10 g of ferrous chloride (0.0789 mol), 20 mL of ethylenediamine, and 1 g of catechin were added to 200 mL of ethanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 200 °C for 12 h to obtain a crystallized product. Then, 10.6 g of lithium hydroxide (0.442 mol) was added to the crystallized product, with a Li to Fe molar ratio of 5.60. The mixture was then poured into a microwave reactor at a microwave frequency of 2450 MHz, a reaction temperature of 150 °C, and a reaction time of 3 h. The precipitate was then centrifuged and dried at 60 °C for 12 h under a vacuum of 0.02 MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900℃ for 5 hours with a heating rate of 1℃ / min to obtain carbon-coated lithium ferrite. The SEM image of this carbon-coated lithium ferrite is shown below. Figure 1 As shown.

[0039] Example 2

[0040] 10g of ferrous nitrate (0.0556mol), 20mL of ethylenediamine, and 0.6g of epicatechin were added to 400mL of methanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 180℃ for 6 hours to obtain a crystallized product. Then, 22.24g of lithium nitrate (0.323mol) was added to the crystallized product, with a Li to Fe molar ratio of 5.8. The mixture was then poured into a microwave reactor at a microwave frequency of 2460MHz, a reaction temperature of 140℃, and a reaction time of 1 hour. The precipitate was then centrifuged and dried at 100℃ for 6 hours under a vacuum of 0.01MPa to obtain precursor particles. Finally, the precursor particles were sintered in a tube furnace at 800℃ for 4 hours under a nitrogen atmosphere at a heating rate of 0.5℃ / min to obtain carbon-coated lithium iron ferrite.

[0041] Example 3

[0042] 10 g of ferrous nitrate (0.0556 mol), 50 mL of triethylamine, and 0.5 g of catechin were added to 300 mL of isopropanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 160 °C for 6 h to obtain a crystallized product. Then, 22.00 g of lithium acetate (0.333 mol) was added to the crystallized product, with a Li to Fe molar ratio of 5.99. The mixture was then poured into a microwave reactor at 2450 MHz, a reaction temperature of 140 °C, and a reaction time of 1 h. The precipitate was then centrifuged and dried at 80 °C for 8 h under a vacuum of 0.01 MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 800 °C for 4 h at a heating rate of 0.5 °C / min to obtain carbon-coated lithium iron ferrite.

[0043] Example 4

[0044] 10 g of ferrous acetate (0.0575 mol), 200 mL of ethylenediamine, and 1 g of flavanol were added to 200 mL of ethanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 200 °C for 12 h to obtain a crystallized product. Then, 7.60 g of lithium hydroxide (0.317 mol) was added to the crystallized product, with a Li to Fe molar ratio of 5.51. The mixture was then poured into a microwave reactor at 2450 MHz, a reaction temperature of 150 °C, and a reaction time of 3 h. The precipitate was then centrifuged and dried at 80 °C for 8 h under a vacuum of 0.02 MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900 °C for 5 h at a heating rate of 1 °C / min to obtain carbon-coated lithium iron ferrite.

[0045] Example 5

[0046] 10 g of ferrous chloride (0.0789 mol), 20 mL of ethylenediamine, and 0.8 g of catechin were added to 200 mL of ethanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 200 °C for 12 h to obtain a crystallized product. Then, 10.7 g of lithium hydroxide (0.446 mol) was added to the crystallized product, with a Li to Fe molar ratio of 5.65. The mixture was then poured into a microwave reactor at 2450 MHz, a reaction temperature of 150 °C, and a reaction time of 3 h. The precipitate was then centrifuged and dried at 100 °C for 8 h under a vacuum of 0.02 MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900 °C for 5 h at a heating rate of 1 °C / min to obtain carbon-coated lithium iron ferrite.

[0047] Comparative Example 1

[0048] 10g of ferrous chloride was added to 200mL of ethanol. After thorough mixing, 10.6g of lithium hydroxide was added, and the mixture was reacted at 150℃ for 3 hours. The precipitate was then centrifuged and dried at 60℃ for 12 hours under a vacuum of 0.02MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900℃ for 5 hours at a heating rate of 1℃ / min to obtain carbon-coated lithium ferrite. The SEM image of this carbon-coated lithium ferrite is shown below. Figure 2 As shown.

[0049] Comparative Example 2

[0050] 10g of ferrous chloride, 20mL of ethylenediamine, and 2g of catechin were added to 200mL of ethanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 200℃ for 12 hours, yielding a crystallized product. Then, 10.6g of lithium hydroxide was added to the crystallized product. The precipitate was then centrifuged and dried at 60℃ for 12 hours under a vacuum of 0.02MPa, yielding precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900℃ for 5 hours at a heating rate of 1℃ / min, yielding carbon-coated lithium iron ferrite.

[0051] Comparative Example 3

[0052] 10g of ferrous chloride and 20mL of ethylenediamine were added to 200mL of ethanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 200℃ for 12 hours to obtain a crystallized product. Then, 10.6g of lithium hydroxide was added to the crystallized product, and the mixture was poured into a microwave reactor. The microwave frequency was 2500MHz, the reaction temperature was 150℃, and the reaction time was 3 hours. The precipitate was then centrifuged and dried at 60℃ for 12 hours under a vacuum of 0.02MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900℃ for 5 hours at a heating rate of 1℃ / min to obtain carbon-coated lithium iron ferrite.

[0053] Comparative Example 4

[0054] 10g of ferrous chloride and 1g of catechin were added to 200mL of ethanol. After thorough mixing, the mixture was poured into a hydrothermal reactor for crystallization at 200℃ for 12 hours to obtain a crystallized product. Then, 10.6g of lithium hydroxide was added to the crystallized product, and the mixture was poured into a microwave reactor at 2500MHz, 150℃, and 3 hours. The precipitate was then centrifuged and dried at 60℃ for 12 hours under a vacuum of 0.02MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900℃ for 5 hours at a heating rate of 1℃ / min to obtain carbon-coated lithium iron ferrite.

[0055] Comparative Example 5

[0056] 10 g of ferrous chloride (0.0789 mol), 20 mL of ethylenediamine, and 2 g of catechin were added to 200 mL of ethanol, followed by 12.31 g of lithium hydroxide (0.513 mol), with a Li to Fe molar ratio of 6.5. The mixture was poured into a microwave reactor at a microwave frequency of 2500 MHz, a reaction temperature of 150 °C, and a reaction time of 3 h. The precipitate was then centrifuged and dried at 60 °C for 12 h under a vacuum of 0.02 MPa to obtain precursor particles. Finally, the precursor particles were sintered in a nitrogen atmosphere at 900 °C for 5 h at a heating rate of 1 °C / min to obtain carbon-coated lithium iron ferrite.

[0057] Performance testing:

[0058] (1) Initial charge capacity

[0059] The carbon-coated lithium iron ferrite prepared above was mixed and ground with carbon black and PVDF at a mass ratio of 8:1:1, and then coated onto aluminum foil as the positive electrode of a lithium-ion battery. CR2032 coin cells were assembled in an argon-filled glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L LiPF6 solution with a solvent ratio of EC:DMC:EMC = 1:1:1 (v / v / v). The coin cell measured voltage range was 2–4.5V, and the charge / discharge currents were 0.05C, 0.1C, and 0.2C.

[0060] (2) Residual alkali content

[0061] Accurately weigh 0.3 g of carbon-coated lithium iron ferrite into a clean, dry 100 mL gas washing bottle. Add 100 mL of ethanol to the gas washing bottle and stir for 5 min. After stirring, vacuum filter the solution and collect the filtrate in a 100 mL volumetric flask. Transfer 40 mL of the filtrate to a titration cup; titrate with 0.01 M HCl standard solution to obtain a titration curve, and record the volume of hydrochloric acid standard solution consumed, V1. Simultaneously, an analytical blank is recorded, the endpoint hydrochloric acid consumption volume is V0, and the LiOH content is 23.94 × 0.01 × (V1 - V0) / 120.

[0062] The lithium-ion batteries prepared in the above examples and comparative examples were tested, and the results are shown in Table 1 below.

[0063] Table 1

[0064]

[0065]

[0066] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0067] The test data from Examples 1, 2, 4, and Comparative Example 3 show that when the preparation method of the present invention is used, especially with the addition of the structure inducers of the present invention: polyphenolic compounds (such as catechin in Example 1, epicatechin in Example 2, and flavanol in Example 4), the carbon-coated lithium iron ferrite material prepared exhibits excellent lithium replenishment capacity, small average particle size, low residual alkali content, high ion transport efficiency, and excellent rate performance. However, when no polyphenolic compounds are added, as in Comparative Example 3, the lithium iron ferrite material prepared has low purity, high residual alkali content, large average particle size, and a significantly reduced initial lithium replenishment capacity when used as a lithium replenishment material.

[0068] The test data from Examples 1, 3, 4, and Comparative Example 4 show that when the preparation method of the present invention is used, especially with the addition of the structure inducer of the present invention: organic amines (e.g., ethylenediamine in Example 1, triethylamine in Example 3, and diethylamine in Example 4), the carbon-coated lithium iron ferrite material prepared exhibits excellent lithium replenishment capacity, small average particle size, low residual alkali content, high ion transport efficiency, and excellent rate performance. However, without the addition of organic amines, the lithium iron ferrite material prepared in Comparative Example 4 has low purity, high residual alkali content, large average particle size, and a significantly reduced initial lithium replenishment capacity when used as a lithium replenishment material.

[0069] The test data from Examples 1 and 5 and Comparative Examples 1, 2, and 5 show that when the preparation method of the present invention is used, especially when the raw materials are subjected to crystallization and microwave reaction treatments sequentially, the structural stability of lithium-rich lithium iron ferrite is improved, the residual alkali content on its surface is reduced, and the lithium replenishment capacity and ion and electron transfer efficiency of lithium-rich lithium iron ferrite are improved. In contrast, when conventional preparation methods are used without crystallization or microwave reaction, the resulting lithium-rich lithium iron ferrite material has low purity, large average particle size, and poor initial lithium replenishment capacity.

[0070] In summary, compared with the conventional method of preparing lithium-rich lithium ferrite by mixing and sintering iron and lithium sources, the iron source crystallization product prepared by the present invention has relatively high regularity. The lithium-rich lithium ferrite prepared by this raw material has high ionic conductivity and structural stability, high purity, significantly reduced residual alkali content and reduced average particle size, and better overall performance.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing carbon-coated lithium iron ferrite, characterized in that, The preparation method includes: Step S1: Take an iron source and a structure inducer and carry out a crystallization reaction in an organic solvent to obtain a crystallized product; Step S2: Take the lithium source and the crystallized product and carry out microwave reaction in a microwave reactor to obtain the precursor; Step S3: The precursor is sintered to obtain the carbon-coated lithium iron ferrite. The structure inducers include organic amines and polyphenolic compounds; Wherein, the organic amine is a fatty amine compound; the fatty amine compound is selected from one or more of ethylenediamine, diethylamine, or triethylamine; the polyphenol compound is a flavanol compound, the flavanol compound is selected from one or more of catechin and epicatechin; The weight ratio of the iron source to the polyphenol compound is 1:(0.05-0.1). The weight ratio of the organic amine to the organic solvent is 1:(1~25). The crystallization reaction temperature is 160~200℃, and the reaction time is 5~12h.

2. The preparation method according to claim 1, characterized in that, The organic solvent is selected from one or more of methanol, ethanol, isopropanol, ethylene glycol, or n-propanol.

3. The preparation method according to claim 1, characterized in that, The iron source is selected from one or more of ferrous chloride, ferrous nitrate, or ferrous acetate.

4. The preparation method according to claim 1, characterized in that, The lithium source is selected from one or more of lithium hydroxide, lithium nitrate, or lithium acetate.

5. The preparation method according to claim 1, characterized in that, The molar ratio of the lithium source to the iron source is (5.5~6.0):

1.

6. The preparation method according to claim 1, characterized in that, In step S2, the microwave frequency of the microwave reactor is 2400~2500MHz, the reaction time is 1~3h, and the reaction temperature is 140~160℃.

7. The preparation method according to claim 6, characterized in that, In step S2, the microwave frequency of the microwave reactor is 2435~2465MHz.

8. The preparation method according to claim 1, characterized in that, In step S1, the crystallization reaction is carried out in a hydrothermal reactor.

9. The preparation method according to any one of claims 1 to 8, characterized in that, Before the sintering process, step S2 further includes sequentially separating and drying the precursor to obtain the precursor.

10. The preparation method according to claim 9, characterized in that, The separation process is carried out in a centrifuge.

11. The preparation method according to claim 9, characterized in that, The drying process is carried out in a vacuum dryer.

12. The preparation method according to claim 9, characterized in that, The drying process is carried out at a temperature of 60-100℃ for 6-12 hours.

13. The preparation method according to claim 1, characterized in that, In step S3, the sintering process is carried out under a protective gas atmosphere.

14. The preparation method according to claim 13, characterized in that, The protective gas is nitrogen and / or argon.

15. The preparation method according to claim 1, characterized in that, The sintering process involves heating the precursor from the temperature after the microwave reaction to the sintering temperature, which is 600~900℃, for 4~8h, and at a heating rate of 0.5~2.0℃ / min.

16. The preparation method according to claim 1, characterized in that, The sintering process is carried out in one or more of the following devices: atmosphere box furnace, tube furnace, or roller kiln.

17. A carbon-coated lithium iron ferrite, characterized in that, The carbon-coated lithium ferrite is obtained by the method for preparing carbon-coated lithium ferrite according to any one of claims 1 to 16.

18. The carbon-coated lithium iron ferrite according to claim 17, characterized in that, The carbon-coated lithium iron ferrite is in granular form.

19. The carbon-coated lithium iron ferrite according to claim 18, characterized in that, The carbon-coated lithium iron ferrite particles have an average particle size of 1~4μm.

20. The application of carbon-coated lithium iron ferrite according to any one of claims 17 to 19 as a positive electrode lithium replenishment material.

21. A lithium-ion battery, comprising a positive electrode lithium replenishment material, characterized in that, The positive electrode lithium replenishment material is carbon-coated lithium iron ferrite as described in any one of claims 17 to 19.

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