Lithium-rich lithium iron phosphate materials and their preparation methods, positive electrode sheets and secondary batteries

Abstract

This application provides a lithium-rich lithium iron ferrite (LFO) material, its preparation method, a positive electrode sheet, and a secondary battery, belonging to the field of secondary battery technology. The preparation method includes: mixing a lithium source and an iron source to obtain a mixture; subjecting the mixture to preheating sintering, a first sintering, a first pulverization process, a second sintering, and a second pulverization process sequentially to obtain a sintered material; dispersing a carbon source and a phosphorus source in an organic solvent to obtain a coating solution; dispersing the sintered material in the coating solution and drying to obtain an intermediate product; sintering the intermediate product and pulverizing it to obtain the lithium-rich lithium iron ferrite material; wherein the lithium source is lithium hydroxide or a mixture of lithium hydroxide and lithium oxide; the temperature of the first sintering is controlled within the range of 410~550℃, and the temperature of the second sintering is controlled within the range of 600~780℃. This application aims to solve the technical problem of high production cost in existing LFO synthesis methods.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, and particularly relates to a lithium-rich lithium iron phosphate material and its preparation method, a positive electrode sheet and a secondary battery. Background Technology

[0002] During the first charge-discharge cycle, a solid electrolyte interphase (SEI) film forms on the surface of the negative electrode of a lithium-ion battery. This process irreversibly consumes a large amount of active lithium ions from the positive electrode, resulting in a significant "first coulombic efficiency loss," which severely restricts the overall energy density and effective service life of the battery. To compensate for this irreversible lithium loss, "pre-lithiation" technology has been widely recognized as a key strategy to overcome the energy density bottleneck. This involves adding a lithium replenishing agent to the positive or negative electrode material during the preparation of the positive or negative electrode.

[0003] Among numerous lithium replenishing agents, this one possesses a high theoretical lithium replenishing capacity (~700 mAh / g) and a suitable delithiation potential (~3.5 V vs. Li). + Lithium-rich lithium iron ore oxide (Li5FeO4, LFO) exhibits significant advantages. Its reaction products are mainly Li2O and Fe, which are generally electrochemically inert and have minimal impact on subsequent battery cycling.

[0004] However, despite the attractiveness of LFO in principle, its large-scale commercial application still faces significant challenges. Traditional LFO synthesis methods, such as the high-temperature solid-state method, involve mixing and sintering lithium and iron sources. Since lithium oxide (Li₂O) has a similar crystal structure to LFO, it is easier to prepare high-purity LFO using Li₂O. Therefore, Li₂O is typically used as the lithium source. However, the low yield of Li₂O and the complex preparation process result in its high price, leading to high production costs for LFO synthesis and hindering its industrial production and market promotion. Summary of the Invention

[0005] In view of the technical problems existing in the background art, this application provides a lithium-rich lithium iron phosphate material and its preparation method, positive electrode sheet and secondary battery, aiming to solve the technical problem of high production cost of existing LFO synthesis methods.

[0006] In a first aspect, embodiments of this application provide a method for preparing lithium iron phosphate material, comprising the following steps: The lithium source and the iron source are mixed to obtain a mixture; The mixture is subjected to preheating sintering, first sintering, first pulverization, second sintering and second pulverization in sequence to obtain sintered material; The carbon source and phosphorus source are dispersed in an organic solvent to obtain a coating solution; The sintered material is dispersed in the coating liquid and dried to obtain an intermediate product; The intermediate product is sintered and then pulverized to obtain lithium-rich lithium iron ferrite material; The lithium source is lithium hydroxide or a mixture of lithium hydroxide and lithium oxide; The temperature of the first sintering is controlled within the range of 410~550℃, and the temperature of the second sintering is controlled within the range of 600~780℃.

[0007] In the technical solution of this application embodiment, lithium hydroxide is used to partially or completely replace expensive lithium oxide as the lithium source, which significantly reduces the cost of raw materials. At the same time, by adopting a segmented sintering process, optimizing the sintering temperature curve, and pulverizing the material after each of the two sintering stages, the defects caused by the increased proportion of lithium hydroxide are effectively solved, ensuring that while the cost of raw materials is reduced, the product still has a low residual alkali content, high phase purity, and electrical performance. In addition, by contacting and sintering the sintered material with a coating liquid containing carbon and phosphorus sources, a uniform and complete composite coating layer is formed on the surface of the lithium iron ferrite (Li5FeO4, LFO) core. This composite coating layer contains highly conductive carbon and fast lithium-ion conductor Li3PO4, which not only further reduces the residual alkali content and the proportion of impurity phases, but also improves the stability of the material, reduces the powder internal resistance and charging voltage plateau, and improves the delithiation performance and reversible specific capacity of the material.

[0008] In some embodiments, when the lithium source is a mixture of lithium hydroxide and lithium oxide, the molar ratio of lithium oxide to lithium hydroxide is greater than 0 and less than or equal to 1.4:1.

[0009] In this embodiment, controlling the proportion of LiOH in the lithium source within the above-mentioned range helps to reduce raw material costs.

[0010] In some embodiments, the molar ratio of lithium in the lithium source to iron in the iron source is (5.03~5.15):1.

[0011] In this embodiment, controlling the Li / Fe ratio within the above range when adjusting the amount of lithium and iron sources helps to control the Li / Fe molar ratio in LFO, so that it can be controlled within the range of (5.0~5.12):1. In this way, the formation of solid solutions with excess or deficiency of lithium can be avoided, which helps to improve the delithiation performance and specific capacity of LFO.

[0012] In some embodiments, the lithium source has a water content of less than 1000 ppm, a magnetic impurity content of less than 10 ppm, and a purity of greater than or equal to 95%. The iron source has a water content of less than 5000 ppm, a magnetic impurity content of less than 100 ppm, and a purity of greater than or equal to 95%.

[0013] In this embodiment, controlling the moisture content of the lithium and iron sources helps improve the mixing effect. If the moisture content of both is too high, the materials are prone to agglomeration during the mixing process due to the high water absorption of lithium hydroxide and / or lithium oxide, resulting in uneven mixing and affecting the purity and Li / Fe ratio of the product. Controlling the magnetic impurities and purity of the lithium and iron sources helps reduce the content of magnetic impurities in the product and improve product purity.

[0014] In some embodiments, the average particle size of the iron source is less than 300 nm.

[0015] In this embodiment, controlling the size of the iron source helps promote ion migration, improve the reaction conversion rate, and ensure the phase purity of the product during the solid-phase reaction between the lithium source and the iron source.

[0016] In some embodiments, the iron source includes one or more of ferric oxide and ferric acetate.

[0017] In this embodiment, the iron source is inexpensive and readily available, and it is well-matched with the lithium source, without introducing other impurity elements such as S, N, and P.

[0018] In some embodiments, the preheating sintering temperature is 100~120℃, and the preheating sintering time is 2~4h.

[0019] In this embodiment, preheating sintering is carried out within the above-mentioned temperature and time range, which can fully remove residual moisture in the mixture and avoid severe agglomeration of the material during subsequent sintering, or reaction between the material and residual moisture, which is not conducive to the formation of the target product Li5FeO4.

[0020] In some embodiments, the first sintering time is 16 to 72 hours.

[0021] In this embodiment, sintering at a lower temperature for a longer period of time can promote the decomposition of LiOH to form Li2O, while avoiding excessive volatilization of LiOH, which would lead to lithium loss and uneven distribution of Li content between the upper and lower layers of the material.

[0022] In some embodiments, the first sintering includes a heat preservation section a, a heat preservation section b, and a heat preservation section c performed sequentially. The temperature of heat preservation section a is greater than or equal to 410°C and less than 430°C, and the heat preservation time of heat preservation section a is 1 to 5 hours. The temperature of heat preservation section b is greater than or equal to 430°C and less than 500°C, and the heat preservation time of heat preservation section b is 10 to 24 hours. The temperature of heat preservation section c is 500 to 550°C, and the heat preservation time of heat preservation section c is 5 to 24 hours.

[0023] In this embodiment, the first sintering process employs a gradient heating method, and the temperature curve of the first sintering is further optimized. First, the temperature is held at 410~430℃ for 1~5 hours to ensure uniform temperature inside and outside the material, promoting all LiOH to be in a near-molten state to facilitate uniform decomposition of LiOH in the subsequent process. This avoids the problem of increased impurities and residual alkali due to uneven internal and external temperatures during subsequent heating. Then, the temperature is held at 430~500℃ for 10~24 hours. Since LiOH decomposes rapidly within this temperature range, setting a holding section within this range helps promote the full and stable decomposition of lithium hydroxide and minimizes LiOH volatilization. This helps solve the problem of uneven Li content distribution between the upper and lower layers of the material, reducing impurities and residual alkali. Finally, the temperature is held at 500~550℃ for 5~24 hours to promote the formation of the target phase Li5FeO4, increasing the proportion of Li5FeO4 in the first material, thereby giving the lithium-rich lithium iron phosphate material a higher capacity.

[0024] In some embodiments, at least one of the insulation section a, the insulation section b, and the insulation section c is provided with a plurality of sub-insulation sections.

[0025] In this embodiment, the insulation sections a, b, and c are further refined, and one or more sub-insulation sections are set. This can further optimize the temperature curves of each insulation section, making its temperature change more gradual and adaptable to the reaction conditions. It avoids excessively rapid temperature increases and uneven temperature distribution in the sintering furnace, which can lead to excessively violent local reactions, thereby improving product performance and uniformity.

[0026] In some embodiments, during the first sintering, the temperature of two adjacent heat-insulating sections is increased at a rate of 2~5℃ / h.

[0027] In this embodiment, the heating rate is controlled. This range of heating rates is suitable for most sintering furnaces, avoiding excessively rapid heating that would increase the furnace preparation cost and consequently, the material production cost. Furthermore, slow heating combined with a longer first sintering time helps control the decomposition of LiOH at a lower rate, allowing the water generated during LiOH decomposition to be quickly expelled with the inert gas. This prevents Li₂O from absorbing water and transforming into lithium hydroxide, which is detrimental to the formation of the target phase, or from the iron source reacting with water at high temperatures to form ferric hydroxide. Additionally, controlling the heating rate at this level also avoids excessively long first sintering times due to slow heating, which would negatively impact production efficiency and increase production costs.

[0028] In some embodiments, the second sintering time is 12 to 50 hours.

[0029] In this embodiment, controlling the second sintering time can ensure that the solid-phase reaction is sufficient and complete, reduce impurities, thereby improving the first charge specific capacity of lithium iron ferrite material, reducing residual alkali, improving lithium replenishment effect, and improving gas generation problem. At the same time, it avoids the problem of low and uneven Li / Fe ratio in the product caused by Li volatilization due to excessive heat preservation time.

[0030] In some embodiments, the temperature of the second sintering is increased from the temperature of the first sintering to the temperature of the second sintering at a heating rate of 2 to 5 °C / h.

[0031] In this embodiment, controlling the heating rate within the above-mentioned range can, on the one hand, adapt to most sintering furnaces, improve the compatibility of the method with existing production lines, and reduce equipment costs; on the other hand, it can release the moisture generated in the reaction and reduce impurities while ensuring production efficiency.

[0032] In some embodiments, the step of sequentially performing preheating sintering, first sintering, first pulverization, second sintering, and second pulverization on the mixture to obtain sintered material includes: sequentially performing preheating sintering and first sintering on the mixture to obtain a first material; performing a first pulverization on the first material to obtain pulverized material with a D50 particle size less than or equal to 15 μm and a D100 particle size less than or equal to 60 μm; performing a second sintering on the pulverized material to obtain a second material; and performing a second pulverization on the second material to obtain sintered material with a D10 particle size greater than or equal to 0.1 μm, a D50 particle size less than or equal to 15 μm, and a D100 particle size less than or equal to 35 μm.

[0033] In this embodiment, the materials that have undergone the first and second sintering processes are pulverized to obtain pulverized materials with the target particle size distribution. This helps to improve phase purity and improve the size of the finished lithium iron ferrite material, thereby enhancing its processing performance and electrochemical performance.

[0034] In some embodiments, the carbon source includes one or more of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG-600, PEG-1000), polyacrylonitrile, phenolic resin, asphalt, coal tar, and ethylene tar. The organic solvent includes one or more of anhydrous ethanol, acetone, chloroform, and tetrahydrofuran.

[0035] In this embodiment, the carbon source compound exhibits good carbon coating effect, forming a highly conductive carbon layer. The organic solvent described above can effectively disperse the carbon source.

[0036] In some embodiments, the phosphorus source includes one or more of ammonium polyphosphate with a degree of polymerization of 1000 or 1500 and lithium pyrophosphate.

[0037] In this embodiment, high-polymerization-degree ammonium polyphosphate or lithium pyrophosphate is used as the phosphorus source. High-polymerization-degree ammonium polyphosphate or lithium pyrophosphate can prevent the carbon-coated intermediate from decomposing and generating water during the sintering process at a low temperature of 200~300℃, thereby avoiding the increase in residual alkali content and impurity phase due to the generation of lithium hydroxide.

[0038] In some embodiments, the mass ratio of the lithium source to the carbon source is (5.7~11.5):1.

[0039] In this embodiment, the addition amounts of lithium source and carbon source are controlled as described above, which can form a coating layer with moderate thickness and high coating rate on the LFO surface, and control the carbon content in the lithium iron ferrite material within the range of 1.5~3%.

[0040] In some embodiments, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (149.3~384.6):1.

[0041] In this embodiment, by adding iron and phosphorus sources in the above proportions, sufficient Li3PO4 can be formed on the LFO surface, so that the molar ratio of Li3PO4 to Li5FeO4 in the lithium iron phosphate material is in the range of (0.24~0.67):100, thereby ensuring the uniform distribution of Li3PO4 in the composite coating layer and the thickness of the composite coating layer, and avoiding the adverse effects of too much or too little Li3PO4 layer on battery performance.

[0042] In some embodiments, the sintering temperature is 600~700℃ and the sintering time is 2~8h.

[0043] In this embodiment, controlling the sintering temperature and time within the aforementioned range allows the carbon source to fully decompose, carbonize, and densify, forming dense, highly conductive carbon and reducing the formation of loose, porous amorphous carbon. Furthermore, it promotes the phosphorus source to fully react with the residual alkali on the surface of the sintered material, forming Li3PO4.

[0044] In some embodiments, the D50 particle size of the lithium iron phosphate material is 5.76~20μm, and the D100 particle size is less than or equal to 45μm.

[0045] In this embodiment, to improve the problem of slight agglomeration of the product after sintering, the sintered material can be pulverized and its particle size distribution can be controlled within the above-mentioned range. In this way, on the one hand, it can avoid the composite coating layer from falling off the LFO surface due to excessive pulverization intensity, which would lead to direct exposure of the LFO and affect the air stability of the product. On the other hand, it can also avoid the product size being too large due to insufficient pulverization intensity, which would lead to large particles in the lithium iron phosphate material during the slurry coating process, thus affecting the coating uniformity and helping to improve the processing performance of the product at the battery end.

[0046] Secondly, embodiments of this application provide a lithium-rich lithium iron ferrite material, the lithium-rich lithium iron ferrite material comprising a core and a composite coating layer covering the core, the core being made of Li5FeO4 and the composite coating layer being made of Li3PO4 and carbon.

[0047] In the technical solution of this application embodiment, a composite coating layer is formed on the surface of LFO. This coating layer not only isolates LFO from contact with external air and moisture, preventing LFO from decomposing and producing strong alkaline substances such as lithium oxide, lithium carbonate, and lithium hydroxide, thus improving the stability of the material and reducing residual alkali, but also helps to improve the electronic conductivity and ion diffusion rate of lithium-rich lithium iron ferrite material. This promotes the complete decomposition of lithium-rich lithium iron ferrite material during the first charge, thereby improving the material's delithiation performance and reversible specific capacity, and avoiding the continuous gas generation problem caused by material decomposition during subsequent charge and discharge processes. This helps to improve the compatibility of lithium-rich lithium iron ferrite material with other cathode material components in secondary batteries, and helps to broaden the application of lithium-rich lithium iron ferrite material in the battery end.

[0048] In some embodiments, the molar ratio of Li3PO4 to Li5FeO4 in the lithium-rich lithium iron ore material is (0.24~0.67):100.

[0049] In this embodiment, the molar ratio of Li3PO4 to Li5FeO4 is within a suitable range, which can ensure the uniformity of Li3PO4 distribution and the thickness of the composite coating layer, and avoid the adverse effects of too much or too little Li3PO4 layer on battery performance.

[0050] In some embodiments, the mass percentage of carbon in the lithium iron phosphate material is 1.5-3%.

[0051] In this embodiment, the carbon content is controlled within a suitable range, which enables the formation of a coating layer with moderate thickness and high coverage on the LFO surface.

[0052] In some embodiments, the D50 particle size of the lithium iron phosphate material is 5.76~20μm, and the D100 particle size is less than or equal to 45μm.

[0053] In this embodiment, the lithium iron phosphate material with the above-mentioned particle size distribution not only has better processing performance at the battery end, but also better electrochemical performance. It is more fully activated during the first charge and discharge process, has a high specific capacity during the first charge and discharge process, and can release more or even completely oxygen during the first charge and discharge process. This can avoid continuous gas generation during subsequent cycles when used with other cathode materials, which would lead to battery bulging.

[0054] In some embodiments, the Raman spectrum of the lithium-rich lithium iron ore material shows that the symmetric stretching vibration of the Fe-O octahedron of Li5FeO4 occurs at 650 cm⁻¹. -1 The ratio of the Raman intensity at point I to the D peak is I. Fe-O / I D I Fe-O / I D Less than or equal to 0.52.

[0055] In this embodiment, I Fe-O / I D A concentration ≤0.52 indicates a higher degree of integrity in the coating of the Li3PO4 and carbon layers on the surface of the LFO, which helps to isolate the LFO from external air, significantly slowing down its reaction with CO2 and H2O and improving the LFO's air stability. Simultaneously, during charging and discharging, it helps to prevent direct contact between the LFO and the electrolyte, avoiding interfacial side reactions and the generation of CO2 gas, thus further preventing the continuous gas production problem of LFO during battery use. Furthermore, Li3PO4, as a fast ion conductor, enhances the Li... + The diffusion rate on the LFO surface prevents the capacity of the LFO from decreasing due to the inorganic layer coating the surface.

[0056] In some embodiments, the specific surface area of ​​the lithium iron phosphate material is 0.8~3m². 2 / g.

[0057] In this embodiment, the lithium iron ferrite material has a suitable BET. If the BET is too high, the material is prone to water absorption and decomposition, resulting in a decrease in its processing performance; if the BET is too low, it will restrict lithium ion diffusion, resulting in a decrease in the electrochemical reactivity of the material.

[0058] In some embodiments, the residual alkali content in the lithium iron phosphate material is less than or equal to 1.95% by mass.

[0059] In this embodiment, the lithium iron phosphate material has a low residual alkali content, high purity, and a low impurity ratio, which helps to improve the specific capacity during the first charge, avoids the gelation problem during the homogenization process, and helps to improve the processing performance of the material.

[0060] In some embodiments, the internal resistance of the lithium iron ferrite material under a pressure of 8 MPa is less than or equal to 74 Ω·cm.

[0061] In this embodiment, the lithium-rich lithium iron phosphate material has low powder internal resistance and high electronic conductivity, which is beneficial to the material's capacity utilization and full decomposition during the first charging process.

[0062] In some embodiments, the residual alkali content in the core is less than or equal to 4.5% by mass; and the molar ratio of Li to Fe in the core is (5.0~5.12):1.

[0063] In this embodiment, the core has a low residual alkali content and high purity, with a low impurity ratio, which helps to improve the electrochemical performance of lithium iron ferrite material; at the same time, the Li / Fe ratio of the core is controlled within a suitable range, which can avoid the formation of solid solutions with excess or deficient lithium, and helps to improve the delithiation performance and specific capacity of LFO.

[0064] Thirdly, embodiments of this application provide a positive electrode sheet, which includes a current collector and a positive electrode material disposed on at least one side of the current collector along its thickness direction. The positive electrode material includes lithium iron ferrite material prepared by the preparation method described above or lithium iron ferrite material as described above.

[0065] In the technical solution of this application embodiment, the cathode material prepared by using the above-mentioned lithium iron ferrite material can improve the energy density, cycle life and safety of the cathode sheet.

[0066] Fourthly, embodiments of this application provide a secondary battery, which includes the aforementioned positive electrode plate.

[0067] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and therefore has high energy density, cycle life and safety.

[0068] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0069] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0070] Figure 1 This is a schematic flowchart of a method for preparing lithium iron ferrite material according to an embodiment of this application; Figure 2 This is the Raman spectrum of the lithium-rich lithium iron ore material prepared in Example 1; Figure 3 This is the first charge-discharge curve of the lithium-rich lithium iron ore material prepared in Example 1 at a rate of 0.05C; Figure 4 This is the XRD pattern of the lithium iron ferrite material prepared in Example 1; Figure 5 This is the XRD pattern of the lithium iron ferrite material prepared in Example 10; Figure 6 This is the XRD pattern of the lithium-rich lithium iron ferrite material prepared in Example 11. Detailed Implementation

[0071] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0072] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0073] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0074] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0075] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0076] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0077] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0078] In the description of the embodiments of this application, "ppm" means the mass of the tested element, molecule or ion in parts per million of the sample mass.

[0079] In the description of the embodiments of this application, unless otherwise specified, the solvent in the "solution" is selected from at least one of distilled water, deionized water, deionized water, pure water, and ultrapure water.

[0080] In traditional LFO synthesis methods, taking the high-temperature solid-state method as an example, lithium and iron sources are mixed and sintered. Since lithium oxide (Li₂O) and LFO have similar crystal structures, it is easier to prepare high-purity LFO using Li₂O. Therefore, Li₂O is usually used as the lithium source. However, the yield of Li₂O is low, and the preparation process is difficult, resulting in its high price. This makes the production cost of LFO synthesis high, hindering its industrial production and market promotion.

[0081] In view of this, this application proposes a lithium-rich lithium iron phosphate material and its preparation method, a positive electrode sheet, and a secondary battery. By using an inexpensive and readily available lithium source and in conjunction with an optimized sintering temperature profile, the cost of raw materials is significantly reduced while ensuring that the resulting material has excellent lithium replenishment performance (high purity, high lithium replenishment capacity, low residual alkali), good air stability, and physical properties suitable for large-scale battery production (such as suitable particle size distribution). When using this lithium-rich lithium iron phosphate material to prepare positive electrode sheets and secondary batteries, it helps to improve the energy density, cycle life, and safety of the positive electrode sheets and secondary batteries.

[0082] In a first aspect, embodiments of this application provide a method for preparing lithium-rich lithium iron phosphate materials. Please refer to... Figure 1 The preparation method includes the following steps: Step S10: Mix the lithium source and the iron source to obtain a mixture. The lithium source is lithium hydroxide or a mixture of lithium hydroxide and lithium oxide.

[0083] Step S20: The mixture is subjected to preheating sintering, first sintering, first pulverization, second sintering and second pulverization in sequence to obtain sintered material.

[0084] The temperature of the first sintering is controlled within the range of 410~550℃, for example, it can be 410℃, 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, or any value between any two of the above; the temperature of the second sintering is controlled within the range of 600~780℃, for example, it can be 600℃, 620℃, 650℃, 680℃, 700℃, 730℃, 750℃, 780℃, or any value between any two of the above.

[0085] Step S30: Disperse the carbon source and phosphorus source in an organic solvent to obtain a coating solution.

[0086] Step S40: Disperse the sintered material in the coating liquid and dry it to obtain an intermediate product.

[0087] Step S50: The intermediate product is sintered and then pulverized to obtain lithium iron ferrite material.

[0088] In the technical solution of this application embodiment, lithium hydroxide is used to partially or completely replace expensive lithium oxide as the lithium source, which significantly reduces the cost of raw materials. At the same time, by adopting a segmented sintering process, optimizing the sintering temperature curve, and pulverizing the material after each of the two sintering stages, the defects caused by the increased proportion of lithium hydroxide are effectively solved, ensuring that while the cost of raw materials is reduced, the product still has a low residual alkali content, high phase purity, and electrical performance. In addition, by contacting and sintering the sintered material with a coating liquid containing carbon and phosphorus sources, a uniform and complete composite coating layer is formed on the surface of the lithium iron ferrite (Li5FeO4, LFO) core. This composite coating layer contains highly conductive carbon and fast lithium-ion conductor Li3PO4, which not only further reduces the residual alkali content and the proportion of impurity phases, but also improves the stability of the material, reduces the powder internal resistance and charging voltage plateau, and improves the delithiation performance and reversible specific capacity of the material.

[0089] Specifically, in current mainstream processes, lithium oxide is generally used as the lithium source. However, lithium oxide is expensive, while lithium hydroxide is inexpensive and readily available. Replacing lithium oxide partially or completely with lithium hydroxide can significantly reduce raw material costs. However, if the lithium source contains too much lithium hydroxide or is entirely composed of lithium hydroxide, it can also lead to several problems: Because LiOH is volatile during sintering, the Li content in the LFO product is unstable, resulting in material inhomogeneity. The Li / Fe molar ratio and the proportion of the Li5FeO4 phase in the resulting product are difficult to control, leading to poor product stability during industrial production. Simultaneously, the limited conversion of LiOH to Li2O and its reaction with the iron source during high-temperature sintering increases the proportion of impurity phases (such as LiOH, LiFeO2, etc.) in the LFO product, resulting in lower purity of the LFO. In response, a segmented sintering process is adopted in step S20, and the sintering temperature and time of each segment are optimized to form a better sintering temperature curve: (1) During preheating sintering, the residual moisture in the mixture is removed to reduce the impurity phase caused by moisture. (2) The first sintering is carried out in the range of 410~550℃. The low temperature sintering promotes the decomposition of LiOH to form Li2O, while avoiding excessive volatilization of LiOH, which leads to lithium loss and uneven distribution of Li content in the upper and lower layers of the material. If the sintering temperature in this stage is too high, LiOH will volatilize rapidly, resulting in a low Li / Fe molar ratio in the material obtained by the first sintering, or uneven distribution of Li content in the upper and lower layers of the material, which in turn leads to a low proportion of LFO in the generated lithium iron ferrite material. At the same time, incomplete decomposition of LiOH will also form more residual alkali, which in turn leads to a decrease in the electrical performance of the lithium iron ferrite material. If the sintering temperature at this stage is too low, the conversion rate of LiOH to Li₂O will be low, resulting in a higher LiOH content in the material obtained from the first sintering. Consequently, during the second sintering at a higher temperature, the volatilization of LiOH increases, leading to a lower Li / Fe molar ratio in the second sintered material or increased uneven distribution of Li content between the upper and lower layers. This, in turn, results in an increase in alkaline impurities and a decrease in the electrical performance of the lithium iron ferrite material. Furthermore, pulverizing after the first sintering can improve the unevenness of the first sintered product and prevent the unevenness from being exacerbated during the second sintering. (3) The second sintering is carried out in the range of 600~780℃. Through the solid-phase reaction at a higher temperature, the reaction of multiple phases (LiOH, Li2O, LiFeO2, Li5FeO4) in the material obtained by the first sintering is promoted to generate the target phase of Li5FeO4. This can reduce the proportion of alkaline impurities LiOH and Li2O in the material obtained by the second sintering (LFO) and increase the proportion of Li5FeO4 phase in the material (in some embodiments, the proportion of Li5FeO4 phase in LFO is ≥95%, and the residual alkali is ≤3%).If the second sintering temperature is too low, the solid-phase reaction is prone to incompleteness, resulting in a higher content of inert impurities such as LiOH and LiFe5O8 in the LFO. This not only easily leads to a lower initial charge specific capacity, higher residual alkali, and poor lithium replenishment effect in lithium-rich lithium iron phosphate materials, but also causes negative effects such as continuous gas production due to the easy side reactions of these impurities with the electrolyte in the battery system. If the second sintering temperature is too high, it easily causes Li volatilization and uneven lithium content in the upper and lower layers of the material after sintering, resulting in a low and uneven Li / Fe molar ratio in the LFO, which in turn leads to a decrease in the initial charge specific capacity of lithium-rich lithium iron phosphate materials.

[0090] Furthermore, based on step S20, steps S30 to S50 are performed. On the one hand, the carbon source can decompose, carbonize, and densify to form highly conductive carbon. On the other hand, the phosphorus source can fully react with the residual alkali (LiOH, Li2O) on the LFO surface to form Li3PO4, further consuming the residual alkali and thus reducing the residual alkali content and impurity phase. At the same time, the composite coating layer formed on the LFO surface not only isolates the LFO from contact with the outside air and moisture, preventing the LFO from decomposing to produce strong alkaline substances such as lithium oxide, lithium carbonate, and lithium hydroxide, thus improving the stability of the material and reducing residual alkali, but also helps to improve the electronic conductivity and ion diffusion rate of lithium-rich lithium iron phosphate material, promotes the complete decomposition of lithium-rich lithium iron phosphate material during the first charge, thereby improving the delithiation performance and reversible specific capacity of the material, and avoiding the continuous gas generation problem caused by the material decomposition during subsequent charge and discharge processes. This helps to improve the compatibility of lithium-rich lithium iron phosphate material with other cathode material components in secondary batteries and helps to broaden the application of lithium-rich lithium iron phosphate material in batteries.

[0091] In some embodiments, when the lithium source is a mixture of lithium hydroxide (LiOH) and lithium oxide (Li2O), the molar ratio of lithium oxide to lithium hydroxide is greater than 0 and less than or equal to 1.4:1, for example, it can be 0.1:1, 0.3:1, 0.5:1, 0.8:1, 1.0:1, 1.2:1, 1.4:1, and any value between any two of the above.

[0092] In this embodiment, controlling the proportion of LiOH in the lithium source within the above-mentioned range helps to reduce raw material costs.

[0093] In some embodiments, the molar ratio of lithium in the lithium source to iron in the iron source is (5.03~5.15):1; for example, it can be 5.03:1, 5.04:1, 5.05:1, 5.06:1, 5.07:1, 5.08:1, 5.09:1, 5.10:1, 5.11:1, 5.12:1, 5.13:1, 5.14:1, 5.15:1, or any value between any two of the above.

[0094] In this embodiment, controlling the Li / Fe ratio within the aforementioned range when adjusting the amount of lithium and iron sources helps to regulate the Li / Fe molar ratio in the LFO, ensuring it remains within the range of (5.0~5.12):1. This avoids the formation of solid solutions with excess or deficient lithium, thus improving the delithiation performance and specific capacity of the LFO. Excessive lithium addition, resulting in a high Li / Fe ratio, can lead to a higher residual alkali content (lithium hydroxide, lithium oxide, and lithium carbonate formed upon further contact with air) in the LFO, resulting in a lower proportion of the Li5FeO4 phase and affecting the specific capacity and electrochemical stability of the product. Insufficient lithium addition, coupled with lithium volatilization during the two relatively long sintering processes, can result in vacancies at lithium sites in the structure, affecting the product's electrical performance. Therefore, controlling the Li / Fe molar ratio in the feed at (5.03~5.15):1, and fully considering Li volatilization during sintering, allows for a final Li / Fe molar ratio of (5.0~5.12):1 in the product.

[0095] In some embodiments, the lithium source has a water content of less than 1000 ppm, a magnetic impurity content of less than 10 ppm, and a purity of greater than or equal to 95%. The iron source has a water content of less than 5000 ppm, a magnetic impurity content of less than 100 ppm, and a purity of greater than or equal to 95%.

[0096] In this embodiment, controlling the moisture content of the lithium and iron sources helps improve the mixing effect. If the moisture content of both is too high, the materials are prone to agglomeration during the mixing process due to the high water absorption of lithium hydroxide and / or lithium oxide, resulting in uneven mixing and affecting the purity and Li / Fe ratio of the product. Controlling the magnetic impurities and purity of the lithium and iron sources helps reduce the content of magnetic impurities in the product and improve product purity. Specifically, the water content can be detected using the Karl Fischer coulometric method specified in GB / T45330-2025, "Determination of Moisture Content in Cathode Materials for Lithium-ion Batteries"; the magnetic impurity content can be detected using the method specified in GB / T 41704—2022, "Test Methods for Cathode Materials of Lithium-ion Batteries: Determination of Magnetic Impurity Content and Residual Alkali Content".

[0097] In some embodiments, the average particle size of the iron source is less than 300 nm.

[0098] In this embodiment, controlling the size of the iron source helps promote ion migration, improve the reaction conversion rate, and ensure the phase purity of the product during the solid-phase reaction between the lithium source and the iron source.

[0099] Furthermore, in some embodiments, step S10 is performed in a closed device, and the ambient humidity is controlled to be less than 10% during the feeding and discharging process.

[0100] In this embodiment, since both lithium hydroxide and lithium oxide react with water and absorb water to agglomerate, ensuring the sealing of the mixing equipment and controlling humidity can prevent the materials from absorbing water and agglomerating, thus avoiding uneven mixing.

[0101] In some embodiments, the iron source may include, but is not limited to, one or more of ferric oxide and ferric acetate.

[0102] In this embodiment, the iron source is inexpensive and readily available, and it is well-matched with the lithium source, without introducing other impurity elements such as S, N, and P.

[0103] In some embodiments, the preheating sintering temperature is 100~120℃, for example, it can be 100℃, 105℃, 110℃, 115℃, 120℃ and any two of the above values; the preheating sintering time is 2~4h, for example, it can be 2h, 2.5h, 3h, 3.5h, 4h and any two of the above values.

[0104] In this embodiment, preheating sintering is carried out within the above-mentioned temperature and time range, which can fully remove residual moisture in the mixture and avoid severe agglomeration of the material during subsequent sintering, or reaction between the material and residual moisture, which is not conducive to the formation of the target product Li5FeO4.

[0105] In some embodiments, the first sintering time is 16 to 72 hours; for example, it can be 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 28 hours, 30 hours, 32 hours, 35 hours, 36 hours, 40 hours, 45 hours, 50 hours, 55 hours, 60 hours, 65 hours, 70 hours, 72 hours, or any value between any two of the above.

[0106] In this embodiment, sintering at a lower temperature for a longer period promotes the decomposition of LiOH into Li2O while avoiding excessive LiOH volatilization, which would lead to lithium loss and uneven Li content distribution between the upper and lower layers of the material. If the sintering time in this stage is too short, the conversion rate of LiOH to Li2O will be low; if the sintering time is too long, more LiOH will volatilize, resulting in a lower Li / Fe ratio in LFO, lower melting point, and consequently, lower production efficiency and increased preparation costs.

[0107] In some embodiments, the first sintering includes a heat preservation section a, a heat preservation section b, and a heat preservation section c performed sequentially. The temperature of heat preservation section a is greater than or equal to 410°C and less than 430°C, for example, it can be 410°C, 415°C, 420°C, 425°C, 429°C, or any value between any two of the above values; the duration of heat preservation section a is 1 to 5 hours, for example, it can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or any value between any two of the above values. The temperature of heat preservation section b is greater than or equal to 430°C and less than 500°C, for example, it can be 430°C, 440°C, 450°C, 460°C, 480°C, 499°C, or any value between any two of the above values; the duration of heat preservation section b is 10 to 24 hours, for example, it can be 10 hours, 12 hours, 15 hours, 16 hours, 18 hours, 20 hours, 24 hours, or any value between any two of the above values. The temperature of the insulation section c is 500~550℃, for example, it can be 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, or any value between two of the above; the insulation time of the insulation section c is 5~24h, for example, it can be 5h, 10h, 12h, 15h, 16h, 18h, 20h, 24h, or any value between two of the above. It can be understood that the total insulation time of multiple insulation sections is controlled within the range of 16~72h.

[0108] In this embodiment, during the first sintering stage, if the internal and external temperatures of the material are not uniform, for example, if the internal temperature of the material is lower than the surface temperature, it is easy to accelerate the volatilization of LiOH to the surface of the material, thereby precipitating and accumulating white LiOH crystals on the surface. In this way, this part of LiOH will have difficulty contacting the iron source to form the target phase LFO. At the same time, the precipitation of LiOH on the surface will also lead to an excess of Li in the upper layer of the material and a deficiency of Li in the lower layer, making it difficult to convert to LFO or Li2O, thereby increasing the amount of impurities and residual alkali in the product. This embodiment further optimizes the temperature curve design for the first sintering. First, holding at 410~430℃ for 1~5h ensures uniform temperature throughout the material, promoting all LiOH to be in a near-molten state, thus facilitating uniform decomposition of LiOH and preventing the increase of impurities and residual alkali due to uneven internal and external temperatures during subsequent heating. Then, holding at 430~500℃ for 10~24h, where LiOH decomposes rapidly, helps promote the full and stable decomposition of lithium hydroxide and minimizes LiOH volatilization, thus addressing the uneven distribution of Li content between the upper and lower layers and reducing impurities and residual alkali. Finally, holding at 500~550℃ for 5~24h promotes the formation of the target phase Li5FeO4, increasing the proportion of Li5FeO4 in the first material, resulting in higher capacity for the lithium-rich lithium iron ferrite material. At lower temperatures, the conversion rate of Li5FeO4 is lower, leading to a higher proportion of impurities and reduced capacity in the resulting lithium-rich lithium iron ferrite material.

[0109] Furthermore, in some embodiments, at least one of the insulation section a, insulation section b, and insulation section c is provided with multiple sub-insulation sections. That is, insulation sections a, b, and c can also adopt a gradient heating method, taking one or more insulation temperatures within their respective temperature ranges for insulation. It is understood that when multiple sub-insulation sections are set, the time of each insulation section still needs to be controlled within an appropriate range: that is, the total time of insulation section a is 1~5h, the total time of insulation section b is 10~24h, the total time of insulation section c is 5~24h, and the total insulation time of the entire first sintering is controlled within the range of 16~72h.

[0110] In this embodiment, the insulation sections a, b, and c are further refined, and one or more sub-insulation sections are set. This can further optimize the temperature curves of each insulation section, making its temperature change more gradual and adaptable to the reaction conditions. It avoids excessively rapid temperature increases and uneven temperature distribution in the sintering furnace, which can lead to excessively violent local reactions, thereby improving product performance and uniformity.

[0111] In some embodiments, during the first sintering, the temperature is increased at a rate of 2 to 5 °C / h between two adjacent heat-insulating sections. The heating rate can be 2 °C / h, 3 °C / h, 4 °C / h, 5 °C / h, or any value between any two of the above. It is understood that when there are three or more heat-insulating sections, the multiple heating rates can be the same or different.

[0112] In this embodiment, the heating rate is controlled. This range of heating rates is suitable for most sintering furnaces, avoiding excessively rapid heating that would increase the furnace preparation cost and consequently, the material production cost. Furthermore, slow heating combined with a longer first sintering time helps control the decomposition of LiOH at a lower rate, allowing the water generated during LiOH decomposition to be quickly expelled with the inert gas. This prevents Li₂O from absorbing water and transforming into lithium hydroxide, which is detrimental to the formation of the target phase, or from the iron source reacting with water at high temperatures to form ferric hydroxide. Additionally, controlling the heating rate at this level also avoids excessively long first sintering times due to slow heating, which would negatively impact production efficiency and increase production costs.

[0113] In some embodiments, the second sintering time is 12 to 50 hours; for example, it can be 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 28 hours, 30 hours, 36 hours, 40 hours, 45 hours, 48 ​​hours, 50 hours, or any value between any two of the above.

[0114] In this embodiment, controlling the second sintering time can ensure that the solid-phase reaction is sufficient and complete, reduce impurities, thereby improving the first charge specific capacity of lithium iron ferrite material, reducing residual alkali, improving lithium replenishment effect, and improving gas generation problem. At the same time, it avoids the problem of low and uneven Li / Fe ratio in the product caused by Li volatilization due to excessive heat preservation time.

[0115] In addition, in some embodiments, when the temperature of the second sintering is 600~700°C, the holding time is preferably 24~60h, and when the temperature of the second sintering is 700~780°C, the holding time is preferably 10~24h.

[0116] In this embodiment, when the holding temperature range of the second sintering is set at a lower temperature (600~700℃), the solid-phase reaction can be maintained steadily and slowly. Setting a longer holding time of 24~60h can promote the complete decomposition of LiOH, allowing the various phases in the material obtained from the first sintering to completely react and generate Li5FeO4, and avoiding the problem of lithium source volatilization caused by long-term holding during high-temperature sintering. When the holding temperature range of the second sintering is set at a higher temperature (700~780℃), the solid-phase reaction rate is relatively fast, and the rate at which the various phases in the material obtained from the first sintering react to generate Li5FeO4 is faster. Setting a shorter holding time of 10~24h can avoid excessive volatilization of lithium source and help maintain the Li / Fe molar ratio of the lithium iron phosphate material at (5.01~5.5):1.

[0117] In some embodiments, the temperature of the second sintering is increased from the temperature of the first sintering to the temperature of the second sintering at a heating rate of 2 to 5 °C / h. The heating rate can be 2 °C / h, 3 °C / h, 4 °C / h, 5 °C / h, or any value between any two of the above.

[0118] In this embodiment, controlling the heating rate within the above-mentioned range can, on the one hand, adapt to most sintering furnaces, improve the compatibility of the method with existing production lines, and reduce equipment costs; on the other hand, it can release the moisture generated in the reaction and reduce impurities while ensuring production efficiency.

[0119] In some embodiments, step S20 may be implemented according to the following steps: S21, the mixture is subjected to preheating sintering and first sintering in sequence to obtain a first material; S22, the first material is subjected to a first pulverization process to obtain pulverized material with a D50 particle size less than or equal to 15 μm and a D100 particle size less than or equal to 60 μm; S23, the pulverized material is subjected to a second sintering process to obtain a second material; S24, the second material is subjected to a second pulverization process to obtain sintered material with a D10 particle size greater than or equal to 0.1 μm, a D50 particle size less than or equal to 15 μm and a D100 particle size less than or equal to 35 μm.

[0120] In this embodiment, the materials after the first and second sintering processes are pulverized to obtain pulverized materials with the target particle size distribution. This helps to improve phase purity and the size of the finished lithium iron ferrite material, thereby improving its processing performance and electrochemical performance. Specifically, the first material is pulverized, and the particle size of the pulverized material is controlled to meet the requirements of D100≤60μm and D50≤15μm. On the one hand, the mixing effect of pulverization can pulverize the first material, which may have differences in Li content between the upper and lower layers, and uniformly mix the various phases (LiOH, Li2O, LiFeO2, Li5FeO4), thereby promoting ion migration during the second sintering process and promoting the fusion of multiple phases to form a high-purity Li5FeO4 phase. On the other hand, by reducing the particle size of the first material, the ion migration path is shortened, further promoting the transformation of the various mixed phases in the first material into the Li5FeO4 phase. Preferably, the particle size of the pulverized material meets the requirements of D100≤45μm and D50≤15μm. Meanwhile, research has found that smaller lithium-rich lithium iron phosphate materials are more fully activated during the first charge-discharge cycle, resulting in a higher specific capacity. This also helps to completely release oxygen during the first charge-discharge cycle, preventing continuous gas generation during subsequent cycles when used with other cathode materials, which could lead to battery bulging. Furthermore, excessively large material sizes can negatively impact processing performance in battery applications. Therefore, it is necessary to control the size of lithium-rich lithium iron phosphate materials within an appropriate range. Therefore, this application pulverizes the second material and controls the particle size of the pulverized sintered material to meet the following requirements: D100≤35μm, D50≤15μm, D10≥0.1μm. This ensures that the particle size is controlled within a suitable range before subsequent shell coating. This prevents material agglomeration after coating, thus avoiding excessively large particle sizes and eliminating the need for post-coating pulverization or reducing the degree of pulverization. It also prevents damage to the composite coating layer due to over-pulverization. Furthermore, screening avoids the presence of highly reactive fine powder on the sintered material, thereby reducing impurities caused by the reduction of fine powder to elemental iron during carbon coating. Preferably, the particle size of the sintered material meets the following requirements: D100≤30μm, D50≤5μm, D10≥0.1μm.

[0121] Furthermore, to avoid the materials reacting with water during the crushing process and producing impurities such as LiOH, lithium carbonate, and LiFeO2, the crushing of the first and second materials can be carried out in an environment with a dew point temperature of less than or equal to -40°C.

[0122] Furthermore, to avoid the materials being affected by the environment, the segmented sintering process in step S20 can be carried out in an inert gas atmosphere, wherein the inert gas may include, but is not limited to, nitrogen, argon, etc.

[0123] In some embodiments, the carbon source may include, but is not limited to, one or more of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG-600, PEG-1000), polyacrylonitrile, phenolic resin, asphalt, coal tar, and ethylene tar. The organic solvent may include, but is not limited to, one or more of anhydrous ethanol, acetone, chloroform, and tetrahydrofuran.

[0124] In this embodiment, the carbon source compound exhibits good carbon coating effect, forming a highly conductive carbon layer. The organic solvent described above can effectively disperse the carbon source.

[0125] In some embodiments, the phosphorus source may include, but is not limited to, one or more of ammonium polyphosphate (degree of polymerization n=1000), ammonium polyphosphate (degree of polymerization n=1500), and lithium pyrophosphate.

[0126] In traditional processes, ammonium dihydrogen phosphate (MDH) and lithium dihydrogen phosphate (LFO) are commonly used as phosphorus sources. However, in the method of this application, if these compounds are used, the phosphorus source easily decomposes to produce water during co-sintering with LFO. Consequently, LFO readily reacts with water to form lithium hydroxide. Since the low-temperature sintering of lithium hydroxide to form LFO requires a long time, while the sintering time after coating is short, the generated lithium hydroxide will exist as a residual alkali impurity, leading to an increase in the residual alkali content and impurity phases in the product. Therefore, in this embodiment, highly polymerized ammonium polyphosphate or lithium pyrophosphate is used as the phosphorus source. Highly polymerized ammonium polyphosphate or lithium pyrophosphate can prevent the carbon-coated intermediate from decomposing and producing water during the low-temperature sintering process (200-300°C), thus avoiding the increase in residual alkali content and impurity phases due to the formation of lithium hydroxide.

[0127] In some embodiments, the water content of the carbon source, phosphorus source, and organic solvent is less than or equal to 0.1 wt%.

[0128] In this embodiment, controlling the water content of the carbon source, phosphorus source, and organic solvent can prevent the water in the carbon source, phosphoric acid, and organic solvent from reacting with LFO, which would lead to the decomposition of LFO and the generation of electrochemically inactive impurity phases, resulting in a reduction in the capacity of lithium iron ferrite material.

[0129] In some embodiments, step S30 may specifically include: adding a carbon source to an organic solvent, dispersing it by high-speed ball milling, then adding a phosphorus source and dispersing it by low-speed ball milling to obtain a coating solution. The high-speed ball milling process parameters are: rotation speed 800~1200 rpm, for example, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, and any value between any two of the above; and ball milling time 2~4 h, for example, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, and any value between any two of the above. The process parameters for low-speed ball milling are: rotation speed 400~800 rpm, for example, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, and any value between any two of the above; and ball milling time 1~3h, for example, 1h, 1.5h, 2h, 2.5h, 3h, and any value between any two of the above.

[0130] In this embodiment, the carbon source is first fully dispersed in an organic solvent by high-speed ball milling, and then the carbon source and phosphorus source are fully mixed by low-speed ball milling to form a uniformly distributed coating solution.

[0131] In some embodiments, the mass ratio of the lithium source to the carbon source is (5.7~11.5):1; for example, it can be 5.7:1, 8:1, 9:1, 10:1, 11:1, 11.4:1, 11.5:1, or any value between any two of the above.

[0132] In this embodiment, the addition amounts of lithium source and carbon source are controlled as described above, enabling the formation of a coating layer with moderate thickness and high coverage on the LFO surface, and controlling the carbon content in the lithium iron phosphate material within the range of 1.5% to 3%. If the carbon content is too low, it will not only be difficult to form a complete coating layer, resulting in decreased air stability and increased residual alkali, but it will also lead to decreased conductivity, reduced capacity, and exacerbated gas generation problems. On the other hand, if the carbon content is too high, it will easily lead to increased adsorption of water and organic solvents by the material, making it difficult to dry the electrode during the preparation of the positive electrode sheet, thus increasing the processing difficulty at the battery end.

[0133] In some embodiments, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (149.3~384.6):1; for example, it can be 149.3:1, 150:1, 160:1, 200:1, 250:1, 300:1, 350:1, 370:1, 380:1, 384:1, 384.6:1, or any value between any two of the above.

[0134] In this embodiment, adding iron and phosphorus sources in the above proportions enables the formation of sufficient Li3PO4 on the LFO surface. This ensures that the molar ratio of Li3PO4 to Li5FeO4 in the lithium iron phosphate material is within the range of (0.24~0.67):100, thereby guaranteeing the uniform distribution of Li3PO4 in the composite coating layer and the thickness of the composite coating layer. This avoids the adverse effects of too much or too little Li3PO4 on battery performance. If there is too much Li3PO4, it will not provide sufficient capacity, leading to a reduction in the capacity of the final product. If there is too little Li3PO4, a complete coating layer cannot be formed on the LFO surface, resulting in poor air isolation and making the product unsuitable for high-humidity environments.

[0135] In some embodiments, in step S40, in order to better improve the dispersibility of the sintered material in the coating liquid, the sintered material can be ball-milled after being added to the coating liquid, and the ball milling speed can be controlled at 300~600 rpm and the ball milling time is 1~2 hours. In this way, the carbon source, phosphorus source and sintered material can be mixed as uniformly as possible.

[0136] In addition, in order to better disperse the sintering materials, the dispersion step can be carried out at a temperature of 25~50℃. This can avoid the coating liquid viscosity being too high due to the temperature being too low, which would affect the dispersion effect of the carbon source and phosphorus source in the organic solvent. It can also avoid the organic solvent evaporation or vaporization due to the temperature being too high, which would affect the safety of equipment use.

[0137] In some embodiments, drying can be carried out in any manner, such as spray drying, rotary drying, etc.

[0138] In some embodiments, the sintering temperature is 600~700℃, for example, it can be 600℃, 610℃, 630℃, 650℃, 670℃, 680℃, 700℃ and any two of the above values; the sintering time is 2~8h, for example, it can be 2h, 3h, 4h, 5h, 6h, 7h, 8h and any two of the above values.

[0139] In this embodiment, controlling the sintering temperature and time within the aforementioned range allows for the full decomposition, carbonization, and densification of the carbon source, forming dense, highly conductive carbon and reducing the formation of loose, porous amorphous carbon. Furthermore, it promotes the phosphorus source to fully react with the residual alkali on the surface of the sintered material, forming Li3PO4. If the sintering temperature is too low or the holding time too short, not only will the carbon source be difficult to fully decompose, resulting in more amorphous carbon formation, but the phosphorus source will also be incompletely converted, leaving a large amount of residual lithium source that forms an inert impurity phase in the lithium iron phosphate material, affecting the material's capacity. If the sintering temperature is too high or the holding time too long, the ferric iron in the LFO is easily reduced to ferrous iron or elemental iron, forming an electrochemically inactive impurity phase, reducing the electrochemical performance of the lithium iron phosphate material and jeopardizing battery safety.

[0140] Furthermore, to avoid environmental impact on the materials, the sintering process can be carried out in an inert gas atmosphere, which may include, but is not limited to, nitrogen or argon with a purity greater than 99.99%. In addition, considering that LFO readily reacts with water under high temperature and humidity conditions, the sintering step can be carried out in an environment with an oxygen content of less than or equal to 20 ppm. This allows for control of the moisture content in the reaction environment, preventing LFO from reacting with water, while appropriately relaxing the oxygen content requirement, thus preventing increased equipment maintenance costs due to excessively low oxygen levels.

[0141] In some embodiments, the D50 particle size of the lithium iron phosphate material is 5.76~20μm, and the D100 particle size is less than or equal to 45μm. As an example, the D50 particle size can be 5.76μm, 6μm, 7μm, 8μm, 10μm, 13μm, 15μm, 18μm, 20μm, or any value between two of the above; the D100 particle size can be 25μm, 30μm, 35μm, 40μm, 45μm, etc.

[0142] In this embodiment, to improve the problem of slight agglomeration of the product after sintering, the sintered material can be pulverized and its particle size distribution can be controlled within the above-mentioned range. In this way, on the one hand, it can avoid the composite coating layer from falling off the LFO surface due to excessive pulverization intensity, which would lead to direct exposure of the LFO and affect the air stability of the product. On the other hand, it can also avoid the product size being too large due to insufficient pulverization intensity, which would lead to large particles in the lithium iron phosphate material during the slurry coating process, thus affecting the coating uniformity and helping to improve the processing performance of the product at the battery end.

[0143] Furthermore, to achieve better pulverization, an air jet mill can be used. Its process parameters can be controlled as follows: classifier speed 700~1000 r / min, pulverizing air pressure 0.2~0.5 MPa. In actual operation, the pulverization intensity can be adjusted by controlling the speed and air pressure, thereby regulating the particle size distribution after pulverization.

[0144] Secondly, embodiments of this application provide a lithium-rich lithium iron ferrite material, which can be prepared by the preparation method described above. The lithium-rich lithium iron ferrite material includes a core and a composite coating layer covering the core. The core is made of Li5FeO4, and the composite coating layer is made of Li3PO4 and carbon.

[0145] In the technical solution of this application embodiment, a composite coating layer is formed on the surface of LFO. This coating layer not only isolates LFO from contact with external air and moisture, preventing LFO from decomposing and producing strong alkaline substances such as lithium oxide, lithium carbonate, and lithium hydroxide, thus improving the stability of the material and reducing residual alkali, but also helps to improve the electronic conductivity and ion diffusion rate of lithium-rich lithium iron ferrite material. This promotes the complete decomposition of lithium-rich lithium iron ferrite material during the first charge, thereby improving the material's delithiation performance and reversible specific capacity, and avoiding the continuous gas generation problem caused by material decomposition during subsequent charge and discharge processes. This helps to improve the compatibility of lithium-rich lithium iron ferrite material with other cathode material components in secondary batteries, and helps to broaden the application of lithium-rich lithium iron ferrite material in the battery end.

[0146] In some embodiments, the residual alkali content in the core is less than or equal to 4.5%, preferably less than or equal to 3.0% by mass; and the molar ratio of Li to Fe in the core is (5.0~5.12):1.

[0147] In this embodiment, the core has a low residual alkali content and high purity, with a low impurity ratio, which helps to improve the electrochemical performance of lithium iron ferrite material; at the same time, the Li / Fe ratio of the core is controlled within a suitable range, which can avoid the formation of solid solutions with excess or deficient lithium, and helps to improve the delithiation performance and specific capacity of LFO.

[0148] In some embodiments, the molar ratio of Li3PO4 to Li5FeO4 in the lithium-rich lithium iron phosphate material is (0.24~0.67):100; for example, it can be 0.24:100, 0.26:100, 0.28:100, 0.3:100, 0.35:100, 0.4:100, 0.45:100, 0.5:100, 0.55:100, 0.6:100, 0.65:100, 0.67:100, or any value between any two of the above.

[0149] In this embodiment, the molar ratio of Li3PO4 to Li5FeO4 is within a suitable range, ensuring the uniform distribution of Li3PO4 and the thickness of the composite coating layer. This avoids the adverse effects of too much or too little Li3PO4 on battery performance. Too much Li3PO4 will not provide sufficient capacity, leading to a reduction in the final product's capacity. Too little Li3PO4 will prevent the formation of a complete coating layer on the LFO surface, resulting in poor air isolation and making the product unsuitable for high-humidity environments.

[0150] In some embodiments, the carbon mass percentage in the lithium iron phosphate material is 1.5 to 3 wt%; for example, it can be 1.5%, 2%, 2.5%, 3%, or any value between the above two.

[0151] In this embodiment, the carbon content is controlled within a suitable range to form a coating layer of moderate thickness and high coverage on the LFO surface. If the carbon content is too low, it will be difficult to form a complete coating layer, resulting in decreased air stability of the material, increased residual alkali, decreased conductivity, reduced capacity, and exacerbated gas generation problems. On the other hand, if the carbon content is too high, it will easily lead to increased adsorption of water and organic solvents by the material, making it difficult to dry the electrode during the preparation of the positive electrode and increasing the processing difficulty of the battery end.

[0152] In some embodiments, in the lithium iron phosphate material, the composite coating layer has a coating rate of 95% or greater on the core surface.

[0153] In this embodiment, the composite coating layer in the lithium-rich lithium iron phosphate material has a high coating rate, which can completely encapsulate the LFO and isolate the LFO from external contact. The coating rate refers to the percentage of the composite coating layer area to the core surface area. In actual testing, the material morphology can be observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to obtain the area S1 of the primary particle outline in the morphology image and the area S2 of the composite coating layer shown within that primary particle outline. The coating rate can then be obtained by calculating the percentage of S2 to S1.

[0154] In some embodiments, the D50 particle size of the lithium iron phosphate material is 5.76~20μm, and the D100 particle size is less than or equal to 45μm. For example, the D50 particle size can be 5.76μm, 6μm, 7μm, 8μm, 10μm, 13μm, 15μm, 18μm, 20μm, or any value between any two of the above; the D100 particle size can be 25μm, 30μm, 35μm, 40μm, 45μm, etc.

[0155] In this embodiment, the lithium iron phosphate material with the above-mentioned particle size distribution not only has better processing performance at the battery end, but also better electrochemical performance. It is more fully activated during the first charge and discharge process, has a high specific capacity during the first charge and discharge process, and can release more or even completely oxygen during the first charge and discharge process. This can avoid continuous gas generation during subsequent cycles when used with other cathode materials, which would lead to battery bulging.

[0156] In some embodiments, the Raman spectrum of the lithium-rich lithium iron ore material shows that the symmetric stretching vibration of the Fe-O octahedron of Li5FeO4 occurs at 650 cm⁻¹. -1 The ratio of the Raman intensity at point I to the D peak is I. Fe-O / I D I Fe-O / I D Less than or equal to 0.52, preferably less than or equal to 0.38.

[0157] In this embodiment, I Fe-O / I D A concentration ≤0.52 indicates a higher degree of integrity in the coating of the Li3PO4 and carbon layers on the surface of the LFO, which helps to isolate the LFO from external air, significantly slowing down its reaction with CO2 and H2O and improving the LFO's air stability. Simultaneously, during charging and discharging, it helps to prevent direct contact between the LFO and the electrolyte, avoiding interfacial side reactions and the generation of CO2 gas, thus further preventing the continuous gas production problem of LFO during battery use. Furthermore, Li3PO4, as a fast ion conductor, enhances the Li... + The diffusion rate on the LFO surface prevents the capacity of the LFO from decreasing due to the inorganic layer coating the surface.

[0158] In some embodiments, the specific surface area (BET) of the lithium iron phosphate material is 0.8~3 m². 2 / g; for example, it can be 0.8m 2 / g、1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g and any value between any two of the above values.

[0159] In this embodiment, the lithium iron ferrite material has a suitable BET. If the BET is too high, the material is prone to water absorption and decomposition, resulting in a decrease in its processing performance; if the BET is too low, it will restrict lithium ion diffusion, resulting in a decrease in the electrochemical reactivity of the material.

[0160] In some embodiments, the residual alkali content in the lithium iron phosphate material is less than or equal to 1.95%, preferably less than or equal to 0.56%.

[0161] In this embodiment, the lithium iron phosphate material has a low residual alkali content, high purity, and a low impurity ratio, which helps to improve the specific capacity during the first charge, avoids the gelation problem during the homogenization process, and helps to improve the processing performance of the material.

[0162] In some embodiments, the internal resistance of the lithium iron ferrite material under a pressure of 8 MPa is less than or equal to 74 Ω·cm, preferably less than or equal to 10 Ω·cm.

[0163] In this embodiment, the lithium-rich lithium iron phosphate material has low powder internal resistance and high electronic conductivity, which is beneficial to the material's capacity utilization and full decomposition during the first charging process.

[0164] Thirdly, embodiments of this application provide a positive electrode sheet, which includes a current collector and a positive electrode material disposed on at least one side of the current collector along its thickness direction. The positive electrode material includes lithium iron ferrite material prepared by the preparation method described above or lithium iron ferrite material as described above.

[0165] In the technical solution of this application embodiment, the cathode material prepared by using the above-mentioned lithium iron ferrite material can improve the energy density, cycle life and safety of the cathode sheet.

[0166] Fourthly, embodiments of this application provide a secondary battery, which includes the aforementioned positive electrode plate.

[0167] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and therefore has high energy density, cycle life and safety.

[0168] Fifthly, embodiments of this application provide an electrical device, including a secondary battery as described in the above embodiments. The electrical device provided in these embodiments can be, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0169] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0170] I. Preparation Method Example 1 (1) Mixing: 1.1 Lithium oxide and lithium hydroxide were used as lithium sources, and ferric oxide was used as iron source. The water content of lithium oxide was 550 ppm, the magnetic foreign matter content was 6 ppm, and the purity was 99.0%; the water content of lithium hydroxide was 670 ppm, the magnetic foreign matter content was 4 ppm, and the purity was 99.7%; the water content of ferric oxide was 800 ppm, the magnetic foreign matter content was 78 ppm, the purity was 97.7%, and the average particle size was 200 nm.

[0171] 1.2 Take 149.41 g of lithium oxide, 1032.53 g of lithium hydroxide and 851.24 g of ferric oxide (i.e., the molar ratio of Li2O to LiOH is 0.2326:1 and the molar ratio of Li / Fe is 5.09), put them into a sealed high-speed mixer for mixing, and control the humidity of the mixing environment to <10% to obtain a mixture.

[0172] (2) Segmented sintering: 2.1 The mixture is heated from room temperature to 120℃ at a rate of 5℃ / min and held at that temperature for 3 hours to obtain preheated material.

[0173] 2.2 The preheated material is subjected to the first sintering process as follows: the temperature is increased to 415℃ at 5℃ / min and held for 4h, then increased to 430℃ at 5℃ / min and held for 10h, then increased to 500℃ at 5℃ / min and held for 10h, then increased to 525℃ at 5℃ / min and held for 10h, and then cooled to room temperature to obtain the first material.

[0174] 2.3 The first material was subjected to a first crushing process to obtain crushed material with D100=44μm and D50=14μm.

[0175] 2.4 The pulverized material was subjected to a second sintering process as follows: the temperature was increased from room temperature to 650℃ at a rate of 5℃ / min and held for 36 hours, and then cooled to room temperature to obtain the second material.

[0176] 2.5 The second material is subjected to a second crushing process to obtain sintered material LFO with D100=28μm, D50=8μm, and D10=1μm.

[0177] (3) Preparation of composite coating layer: 3.1 Add 154.50g of PEG-1000 (equivalent to a lithium source to carbon source mass ratio of 7.65:1) to anhydrous ethanol and grind and disperse it in a high-speed sealed ball mill (control the ball mill speed at 1000 rpm, grind for 3 hours, and control the equipment temperature at 25~30℃). Then add 4.87g of ammonium polyphosphate and grind and disperse it at a low speed (ball mill speed at 500 rpm, grind for 2 hours) to obtain the coating solution.

[0178] 3.2 The above sintered materials were dispersed in a coating solution and ball-milled at a speed of 400 rpm for 1 hour to obtain a mixed slurry. The mixed slurry was then spray-dried to obtain an intermediate product.

[0179] 3.3 The intermediate product was fed into a vacuum sintering furnace and sintered in an inert atmosphere (nitrogen with a purity greater than 99.99%) at 600℃ for 6 hours, while controlling the oxygen content in the furnace to ≤20ppm throughout the sintering process. The sintered material was then pulverized to obtain lithium iron ferrite material LFO@Li3PO4&C.

[0180] Example 2 The scheme in this embodiment is basically the same as that in embodiment 1, except that the lithium source and iron source are changed to 1344.70 g of lithium hydroxide and 888.89 g of ferric oxide, that is, the molar ratio of Li2O to LiOH is 0 and the molar ratio of Li / Fe is 5.15.

[0181] Example 3 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 1.2 of this embodiment, the lithium source and iron source are changed to: 433.30 g lithium oxide, 480.25 g lithium hydroxide and 758.76 g ferric oxide, that is, the molar ratio of Li2O to LiOH is 1.4:1 and the molar ratio of Li / Fe is 5.05.

[0182] Example 4 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 1.2 of this embodiment, the amount of ferric oxide added is changed to 862.38 g, that is, the Li / Fe molar ratio is 5.03.

[0183] Example 5 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 1.2 of this embodiment, the lithium source and iron source are changed to: 149.41 g lithium oxide, 1200.62 g lithium hydroxide and 941.28 g ferric oxide, that is, the molar ratio of Li2O to LiOH is 0.2:1 and the molar ratio of Li / Fe is 5.2.

[0184] Example 6 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 1.2 of this embodiment, the lithium source and iron source are changed to: 149.41 g lithium oxide, 960.50 g lithium hydroxide and 818.50 g ferric oxide, that is, the molar ratio of Li2O to LiOH is 0.2:1 and the molar ratio of Li / Fe is 5.

[0185] Example 7 The scheme in this embodiment is basically the same as that in embodiment 1, except that step 2.2 in this embodiment is changed to: The temperature was increased to 410℃ at 5℃ / min and held for 1 hour, then increased to 450℃ at 5℃ / min and held for 9 hours, and finally increased to 550℃ at 5℃ / min and held for 6 hours before being cooled to room temperature.

[0186] Example 8 The scheme in this embodiment is basically the same as that in embodiment 1, except that step 2.2 in this embodiment is changed to: The temperature was increased to 410℃ at 1℃ / min and held for 1 hour, then increased to 420℃ at 1℃ / min and held for 4 hours, then increased to 430℃ at 1℃ / min and held for 24 hours, then increased to 500℃ at 1℃ / min and held for 12 hours, then increased to 550℃ at 1℃ / min and held for 12 hours before being cooled to room temperature.

[0187] Example 9 The scheme in this embodiment is basically the same as that in embodiment 1, except that step 2.2 in this embodiment is changed to: Heat to 500℃ at a rate of 5℃ / min and hold for 34 hours.

[0188] Example 10 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 2.2 of this embodiment, the first sintering process is as follows: The temperature was increased to 430℃ at 5℃ / min and held for 5 hours, then increased to 500℃ at 5℃ / min and held for 4 hours before being cooled to room temperature.

[0189] Example 11 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 2.2 of this embodiment, during the first sintering, all heating rates are adjusted to 8℃ / min.

[0190] Example 12 The scheme in this embodiment is basically the same as that in embodiment 1, except that the second sintering process in step 2.4 of this embodiment is changed to: heating from room temperature to 600℃ at 2℃ / min and holding for 50h.

[0191] Example 13 The scheme in this embodiment is basically the same as that in embodiment 1, except that the second sintering process in step 2.4 of this embodiment is changed to: heating from room temperature to 780℃ at 3℃ / min and holding for 12h.

[0192] Example 14 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 2.5 of this embodiment, the particle size of the sintered material is: D100=35μm, D50=15μm, D10=2μm.

[0193] Example 15 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 2.5 of this embodiment, the particle size of the sintered material is: D100=27.32μm, D50=2.03μm, D10=0.1μm.

[0194] Example 16 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 2.3 of this embodiment, the particle size of the crushed material is controlled as follows: D100=98μm, D50=20μm.

[0195] Example 17 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 3.1 of this embodiment, the amount of ammonium polyphosphate added is 2.53g, which is equivalent to a molar ratio of iron in the iron source to phosphorus in the phosphorus source of 384.6:1.

[0196] Example 18 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 3.1 of this embodiment, the amount of ammonium polyphosphate added is 6.53 g, which is equivalent to a molar ratio of iron in the iron source to phosphorus in the phosphorus source of 149.3:1.

[0197] Example 19 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 3.1 of this embodiment: the added carbon source is PEG1000, the added amount is 103.03 g, and the added phosphorus source is lithium pyrophosphate, the added mass is 5.20 g. This is equivalent to a lithium source to carbon source mass ratio of 11.5, and a molar ratio of iron in the iron source to phosphorus in the phosphorus source of 200.

[0198] Example 20 The scheme in this embodiment is basically the same as that in embodiment 1, except that in step 3.1 of this embodiment: the added carbon source is PEG1000, the added amount is 206.06 g, and the added phosphorus source is lithium pyrophosphate, the added mass is 5.20 g. This is equivalent to a lithium source to carbon source mass ratio of 5.7, and a molar ratio of iron in the iron source to phosphorus in the phosphorus source of 200.

[0199] Example 21 (1) Mixing: 1.1 Lithium oxide and lithium hydroxide used in Example 1 were used as lithium sources, and ferric acetate was used as an iron source. The water content of ferric acetate was 790 ppm, the magnetic foreign matter content was 60 ppm, the purity was 96.0%, and the average particle size was 150 nm.

[0200] 1.2 With a molar ratio of Li2O to LiOH of 0.2:1 and a Li / Fe molar ratio of 5.09, lithium and iron sources were fed into a sealed high-speed mixer for mixing, and the humidity of the mixing environment was controlled to be <10% to obtain a mixture.

[0201] (2) Segmented sintering: 2.1 The mixture is heated from room temperature to 100℃ at a rate of 2℃ / min and held at that temperature for 4 hours to obtain preheated material.

[0202] 2.2 The preheated material is subjected to the first sintering process as follows: the temperature is increased to 415℃ at 5℃ / min and held for 4h, then increased to 430℃ at 5℃ / min and held for 10h, then increased to 500℃ at 5℃ / min and held for 10h, then increased to 525℃ at 5℃ / min and held for 10h, and then cooled to room temperature to obtain the first material.

[0203] 2.3 The first material was subjected to a first crushing process to obtain crushed material with D100=48μm and D50=12μm.

[0204] 2.4 The pulverized material was subjected to a second sintering process as follows: the temperature was increased from room temperature to 650℃ at a rate of 5℃ / min and held for 36 hours, and then cooled to room temperature to obtain the second material.

[0205] 2.5 The second material is subjected to a second crushing process to obtain sintered material LFO with D100=30μm, D50=10μm, and D10=0.8μm.

[0206] (3) Preparation of composite coating layer: 3.1 Add 92.73 g of polyvinylpyrrolidone to acetone and grind and disperse it in a high-speed sealed ball mill (control the ball mill speed at 1000 rpm, grind for 3 hours, and control the equipment temperature at 25~30℃). Then add 4.87 g of ammonium polyphosphate and grind and disperse it at a low speed (ball mill speed at 500 rpm, grind for 2 hours) to obtain the coating liquid.

[0207] 3.2 The above sintered materials were dispersed in a coating solution and ball-milled at a speed of 400 rpm for 1 hour to obtain a mixed slurry. The mixed slurry was then spray-dried to obtain an intermediate product.

[0208] 3.3 The intermediate product was fed into a vacuum sintering furnace and sintered in an inert atmosphere (nitrogen with a purity greater than 99.99%) at 700℃ for 2 hours, while controlling the oxygen content in the furnace to ≤20ppm throughout the sintering process. The sintered material was then pulverized to obtain lithium iron ferrite material LFO@Li3PO4&C.

[0209] Example 22 (1) Mixing: Take the mixture prepared in Example 1.

[0210] (2) Segmented sintering: 2.1 The mixture is heated from room temperature to 110℃ at a rate of 4℃ / min and held at that temperature for 2 hours to obtain preheated material.

[0211] 2.2 The preheated material is subjected to the first sintering process as follows: the temperature is increased to 415℃ at 5℃ / min and held for 4h, then increased to 430℃ at 5℃ / min and held for 10h, then increased to 500℃ at 5℃ / min and held for 10h, then increased to 525℃ at 5℃ / min and held for 10h, and then cooled to room temperature to obtain the first material.

[0212] 2.3 The first material was subjected to a first crushing process to obtain crushed material with D100=41μm and D50=14μm.

[0213] 2.4 The pulverized material was subjected to a second sintering process as follows: the temperature was increased from room temperature to 650℃ at a rate of 5℃ / min and held for 36 hours, and then cooled to room temperature to obtain the second material.

[0214] 2.5 The second material was subjected to a second crushing process to obtain sintered material LFO with D100=34μm, D50=13μm, and D10=0.6μm.

[0215] (3) Preparation of composite coating layer: 3.1 Add 92.73 g of polyacrylonitrile to tetrahydrofuran and grind and disperse it in a high-speed sealed ball mill (control the ball mill speed at 1000 rpm, grind for 3 h, and control the equipment temperature at 25~30℃). Then add 4.87 g of ammonium polyphosphate and grind and disperse it at a low speed (ball mill speed at 500 rpm, grind for 2 h) to obtain the coating solution.

[0216] 3.2 The above sintered materials were dispersed in a coating solution and ball-milled at a speed of 400 rpm for 1 hour to obtain a mixed slurry. The mixed slurry was then spray-dried to obtain an intermediate product.

[0217] 3.3 The intermediate product was fed into a vacuum sintering furnace and sintered in an inert atmosphere (nitrogen with a purity greater than 99.99%) at 650℃ for 8 hours, while controlling the oxygen content in the furnace to ≤20ppm throughout the sintering process. The sintered material was then pulverized to obtain lithium iron ferrite material LFO@Li3PO4&C.

[0218] Comparative Example 1 This comparative example is basically the same as Example 1, except that this comparative example uses a one-step sintering process, that is, steps 2.2 to 2.4 are changed to: The preheated material was sintered. The first sintering process was as follows: the temperature was increased to 415℃ at 5℃ / min and held for 4 hours; then increased to 430℃ at 5℃ / min and held for 10 hours; then increased to 500℃ at 5℃ / min and held for 10 hours; then increased to 525℃ at 5℃ / min and held for 10 hours; finally, the temperature was increased to 650℃ at 5℃ / min and held for 36 hours, and then cooled to room temperature. The sintered material was used for crushing in step 2.5.

[0219] Comparative Example 2 This comparative example is basically the same as Example 1, except that in step 2.2 of this comparative example, the first sintering process is as follows: the temperature is increased to 600℃ at 5℃ / min and held for 34 hours, and then cooled to room temperature.

[0220] Comparative Example 3 This comparative example is basically the same as Example 1, except that step 2.1 is removed in this comparative example, and the first sintering process in step 2.2 is: heating from room temperature to 600℃ at 5℃ / min and holding for 10h, then cooling to room temperature.

[0221] Comparative Example 4 This comparative example is basically the same as Example 1, except that in step 2.4 of this comparative example, the second sintering process is as follows: the temperature is increased from room temperature to 800℃ at 5℃ / min and held for 10 hours, and then cooled to room temperature.

[0222] II. Testing Methods 1. Residual alkali content: The test shall be conducted in accordance with the method specified in GB / T 41704—2022 Test Methods for Cathode Materials of Lithium-ion Batteries: Determination of Magnetic Foreign Matter Content and Residual Alkali Content.

[0223] 2. Phase purity: The purity was tested using an X-ray diffractometer (model SmartLab SE) manufactured by Rigaku Corporation of Japan.

[0224] 3. Chemical composition (Li / Fe molar ratio, C content), crystal structure (phase purity), BET, particle size distribution (D50 particle size, D100 particle size), and specific capacity: are tested according to the methods specified in GB / T 45327-2025 Lithium-rich Lithium Ferrite.

[0225] 4. Powder internal resistance: Tested using the four-probe method at a pressure of 8 MPa.

[0226] 5. Li3PO4 / LFO molar ratio: The peak intensity ratio of the Li3PO4 phase and the LFO phase was obtained by using an X-ray diffractometer (model SmartLabSE) manufactured by Rigaku Corporation of Japan. This ratio was used as the mass ratio of Li3PO4 to Li5FeO4, and then the Li3PO4 / LFO molar ratio was calculated.

[0227] 6. I Fe-O / I D The material was analyzed using a Raman spectrometer manufactured by Thermo Fisher Scientific, and the symmetric stretching vibrations of the Fe-O octahedron in Li5FeO4 at 650 cm⁻¹ were obtained. -1 The ratio of the Raman intensity at point I to the D peak. Fe-O / I D In the figure, the horizontal axis represents Raman shift, and the vertical axis represents peak intensity.

[0228] III. Analysis of Test Results for Each Embodiment and Comparative Example Table 1 Performance test data of LFO

[0229] Table 2 Performance test data of lithium iron phosphate materials

[0230] Table 3 Performance test data of lithium iron phosphate materials

[0231] Please see Figures 4 to 6 As can be seen, the product obtained in Example 1 is a lithium-rich lithium iron phosphate material with high phase purity. In contrast, the phase purity of the products in Examples 10 and 11 is relatively low. This may be because, in Example 10, the holding time during the first sintering was too short, resulting in a lower LFO content in the final product and the generation of more impurity phases, such as lithium oxide, lithium hydroxide, and ferric oxide, leading to an increase in residual alkali. In Example 11, the heating rate during the first sintering was too fast, which was not conducive to the decomposition of LiOH, and ferric oxide reacted with water at high temperatures to form ferric hydroxide hydrate, resulting in a low LFO content in the target phase.

[0232] See Figure 2 , Figure 3 As shown in Tables 1 to 3, compared with Comparative Example 1, the LFO prepared in each embodiment has higher phase purity, lower residual alkali content, and a suitable Li / Fe ratio. The prepared lithium iron ferrite material exhibits suitable specific surface area, particle size distribution, carbon content, and Li3PO4 / LFO, and shows lower residual alkali content, powder internal resistance, and higher specific capacity. This indicates that by reducing the proportion of lithium oxide, optimizing the sintering process, adopting a segmented sintering method, and using an optimized temperature curve, followed by pulverization after each of the two sintering stages, the defects caused by the increased proportion of lithium hydroxide can be solved. This ensures that while reducing raw material costs, the product still has a low residual alkali content, high phase purity, and good electrical performance. Furthermore, comparing Example 1 and Comparative Examples 2 to 4, the LFO and lithium iron ferrite materials prepared in Example 1 both exhibit significantly lower residual alkali content, and the LFO shows significantly higher phase purity. The lithium iron ferrite materials show lower powder internal resistance and higher specific capacity, further corroborating the above viewpoints. This indicates that sequentially performing preheating sintering, first sintering, first pulverization, second sintering, and second pulverization on the mixture, and controlling the temperature of the first sintering within the range of 410~550℃ and the temperature of the second sintering within the range of 600~780℃, helps to reduce residual alkali, improve phase purity, and solve problems such as unstable Li content in LFO, material inhomogeneity, and difficulty in controlling the Li / Fe molar ratio caused by excessively high LiOH content.

[0233] Furthermore, comparing Example 1 and Example 16, Example 1 has a higher phase purity. This may be because in Example 16, the size of the crushed material was controlled to be too large when crushing the first material, which failed to effectively regulate the size and improve the uniformity of the material. As a result, the residual LiOH, Li2O and other elements in the crushed material could not be uniformly mixed with the iron source, resulting in uneven distribution of Li and Fe elements, which in turn led to a lower phase purity of LFO.

[0234] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing lithium-rich lithium iron ferrite material, characterized in that, Includes the following steps: The lithium source and the iron source are mixed to obtain a mixture; The mixture is subjected to preheating sintering, first sintering, first pulverization, second sintering and second pulverization in sequence to obtain sintered material; The carbon source and phosphorus source are dispersed in an organic solvent to obtain a coating solution; The sintered material is dispersed in the coating liquid and dried to obtain an intermediate product; The intermediate product is sintered and then pulverized to obtain lithium-rich lithium iron ferrite material; The lithium source is lithium hydroxide or a mixture of lithium hydroxide and lithium oxide; The temperature of the first sintering is controlled within the range of 410~550℃, and the temperature of the second sintering is controlled within the range of 600~780℃.

2. The preparation method according to claim 1, characterized in that, When the lithium source is a mixture of lithium hydroxide and lithium oxide, the molar ratio of lithium oxide to lithium hydroxide is greater than 0 and less than or equal to 1.4:1; and / or, The molar ratio of lithium in the lithium source to iron in the iron source is (5.03~5.15):1; and / or, The preheating sintering temperature is 100~120℃, and the preheating sintering time is 2~4h; and / or, The first sintering time is 16~72h; and / or, The second sintering time is 12~50h; and / or, The temperature of the second sintering is increased from the temperature of the first sintering at a heating rate of 2~5℃ / h.

3. The preparation method according to claim 1, characterized in that, The first sintering process includes sequentially performed holding sections a, b, and c. The temperature of holding section a is greater than or equal to 410°C and less than 430°C, and the holding time is 1-5 hours. The temperature of holding section b is greater than or equal to 430°C and less than 500°C, and the holding time is 10-24 hours. The temperature of holding section c is 500-550°C, and the holding time of holding section c is 5-24 hours; and / or, The steps of sequentially subjecting the mixture to preheating sintering, first sintering, first pulverization, second sintering, and second pulverization to obtain sintered material include: sequentially subjecting the mixture to preheating sintering and first sintering to obtain a first material; subjecting the first material to first pulverization to obtain pulverized material with a D50 particle size less than or equal to 15 μm and a D100 particle size less than or equal to 60 μm; subjecting the pulverized material to second sintering to obtain a second material; and subjecting the second material to second pulverization to obtain sintered material with a D10 particle size greater than or equal to 0.1 μm, a D50 particle size less than or equal to 15 μm, and a D100 particle size less than or equal to 35 μm.

4. The preparation method according to claim 3, characterized in that, During the first sintering, the temperature is increased at a rate of 2~5℃ / h between two adjacent heat-insulating sections; and / or, At least one of the insulation section a, the insulation section b, and the insulation section c is provided with multiple sub-insulation sections.

5. The preparation method according to claim 1, characterized in that, The lithium source has a water content of less than 1000 ppm, a magnetic impurity content of less than 10 ppm, and a purity of greater than or equal to 95%; and / or, The iron source has a water content of less than 5000 ppm, a magnetic impurity content of less than 100 ppm, and a purity of greater than or equal to 95%; and / or, The iron source has an average particle size of less than 300 nm; and / or, The iron source includes one or more of ferric oxide and ferric acetate.

6. The preparation method according to claim 1, characterized in that, The carbon source includes one or more of the following: polyvinylpyrrolidone, polyethylene glycol, polyacrylonitrile, phenolic resin, asphalt, coal tar, and ethylene tar; and / or, The phosphorus source includes one or more of ammonium polyphosphate with a degree of polymerization of 1000 or 1500 and lithium pyrophosphate; and / or, The organic solvent includes one or more of anhydrous ethanol, acetone, chloroform, and tetrahydrofuran; and / or, The mass ratio of the lithium source to the carbon source is (5.7~11.5):1; and / or, The molar ratio of iron in the iron source to phosphorus in the phosphorus source is (149.3~384.6):1; and / or, The sintering temperature is 600~700℃, and the sintering time is 2~8h; and / or, The D50 particle size of the lithium iron ferrite material is 5.76~20μm, and the D100 particle size is less than or equal to 45μm.

7. A lithium-rich lithium iron phosphate material, characterized in that, The lithium-rich lithium iron phosphate material includes a core and a composite coating layer covering the core. The core is made of Li5FeO4, and the composite coating layer is made of Li3PO4 and carbon.

8. The lithium iron ferrite material according to claim 7, characterized in that, In the lithium iron ferrite material, the molar ratio of Li3PO4 to Li5FeO4 is (0.24~0.67):100; and / or, In the lithium iron ferrite material, the carbon mass percentage is 1.5% to 3%; and / or, The lithium iron ferrite material has a D50 particle size of 5.76~20μm and a D100 particle size of less than or equal to 46μm; and / or, In the Raman spectrum of the lithium iron ferrite-rich material, the symmetric stretching vibration of the Fe-O octahedron of Li5FeO4 is observed at 650 cm⁻¹. -1 The ratio of the Raman intensity at point I to the D peak is I. Fe-O / I D I Fe-O / I D Less than or equal to 0.52; and / or, The specific surface area of ​​the lithium iron ferrite material is 0.8~3m². 2 / g; and / or, In the lithium iron ferrite material, the residual alkali content by mass is less than or equal to 1.95%; and / or, The lithium iron ferrite material has a powder internal resistance of less than or equal to 74 Ω·cm under a pressure of 8 MPa; and / or, In the core, the residual alkali content by mass is less than or equal to 4.5%; and / or, In the core, the molar ratio of Li to Fe is (5.0~5.12):

1.

9. A positive electrode sheet, characterized in that, It includes a current collector and a positive electrode material disposed on at least one side of the current collector along its thickness direction, the positive electrode material including lithium iron ferrite material prepared by the preparation method according to any one of claims 1-6 or lithium iron ferrite material according to claim 7 or 8.

10. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet as described in claim 9.

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