Preparation method and application of ferrous oxalate

Abstract

The invention provides a preparation method and application of ferrous oxalate, and belongs to the field of comprehensive utilization of solid waste resources, the preparation method of the ferrous oxalate comprises the steps that iron ore waste residues are subjected to acid leaching treatment, and a first iron-containing solution is obtained; performing resin impurity removal on the first iron-containing solution to obtain a second iron-containing solution; and mixing the second iron-containing solution with a compound containing oxalate, and carrying out precipitation reaction to obtain ferrous oxalate. Then, the ferrous oxalate can be used as an iron source to be mixed with a lithium source, a phosphorus source, a carbon source and a solvent to obtain a mixture; and grinding and drying the mixture, and sequentially carrying out primary sintering and secondary sintering to obtain the lithium iron phosphate material. According to the technical scheme provided by the invention, the iron element can be extracted from the iron ore waste residues, the high-purity ferrous oxalate is prepared, and then the lithium iron phosphate material is prepared by taking the ferrous oxalate as an iron source, so that the preparation cost of the lithium iron phosphate material is effectively reduced, and efficient utilization of the iron ore waste residues is realized.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization technology of solid waste resources, specifically to a method for preparing ferrous oxalate and its application. Background Technology

[0002] With the rapid development of new energy vehicles, rechargeable batteries, which determine their driving range, have attracted much attention. Lithium iron phosphate (LiFePO4) is widely used as an important cathode material in electric vehicles and energy storage due to its high safety, long cycle life, and environmental friendliness. However, the expensive raw materials and complex manufacturing process of lithium iron phosphate result in high production costs.

[0003] Iron ore slag is a solid waste generated during the iron and steel smelting process. Its traditional treatment methods mainly involve stockpiling or landfilling, which not only consumes land resources but also potentially pollutes the environment. Iron ore slag typically contains large amounts of iron and other metallic elements. Iron is an important component of lithium iron phosphate (LFP). If iron ore slag could be used in the production of LFP, it would not only achieve comprehensive utilization of waste resources but also reduce the production cost of LFP, which is of great significance. Summary of the Invention

[0004] In view of the technical problems existing in the background art, this application provides a method for preparing ferrous oxalate and its application, aiming to solve the technical problems of high preparation cost and low utilization rate of iron ore waste in existing lithium iron phosphate.

[0005] In a first aspect, embodiments of this application provide a method for preparing ferrous oxalate, comprising the following steps: The iron ore slag was subjected to acid leaching to obtain the first iron-containing solution; The first iron-containing solution is subjected to resin purification to obtain the second iron-containing solution; The second iron-containing solution was mixed with a compound containing oxalate to carry out a precipitation reaction, yielding ferrous oxalate.

[0006] In the technical solution of this application embodiment, iron can be extracted from iron ore waste by acid leaching. Then, impurities in the first iron-containing solution are effectively removed by resin purification, thereby significantly improving the purity of the second iron-containing solution. This allows the second iron-containing solution to react with oxalate to generate high-purity ferrous oxalate. The technical solution of this application embodiment not only achieves effective utilization of iron ore waste but also enables the preparation of high-purity ferrous oxalate at a lower cost, allowing it to be used as an iron source in the preparation of lithium iron phosphate materials, thus reducing the production cost of lithium iron phosphate materials.

[0007] In some embodiments, in the second iron-containing solution: Fe2+ The mass concentration is 2.5%~15%; and / or, the total mass concentration of sodium and potassium is <300ppm, the mass concentration of aluminum is <100ppm, the mass concentration of titanium is <100ppm, the mass concentration of magnesium is <100ppm, the mass concentration of manganese is <50ppm, the mass concentration of calcium is <100ppm, the mass concentration of silicon is <50ppm, and the mass concentration of phosphorus is <100ppm.

[0008] In this embodiment, by adjusting the Fe in the second iron-containing solution... 2+ Controlling the mass concentration of Fe is beneficial for promoting Fe 2+ The precipitation reaction with oxalate ions proceeds while preventing other impurities from being trapped inside the formed ferrous oxalate crystals. Controlling the mass concentration of impurity elements such as aluminum, titanium, magnesium, manganese, calcium, silicon, and phosphorus helps to improve the purity of ferrous oxalate.

[0009] In some embodiments, resin impurity removal includes: The first iron-containing solution is subjected to a first impurity removal process using an acidic adsorption resin to obtain a first impurity-removed solution. The first impurity removal solution is subjected to a second impurity removal process using an ion-selective adsorption resin to obtain a second iron-containing solution. In the first purification solution, the total mass concentration of sodium and potassium is <300ppm, and the mass concentration of aluminum is <100ppm.

[0010] In this embodiment, by first using an acidic adsorption resin for initial impurity removal, Na in the first iron-containing solution can be effectively removed. + K + And Al 3+ This reduces the concentration of impurities in the first iron-containing solution and avoids the impact of these impurities on the second impurity removal process. Then, an ion-selective adsorption resin is used for the second impurity removal, which can effectively remove Ca... 2+ Mg 2+ It selectively adsorbs other impurity ions, thereby effectively reducing the impurity concentration in the second iron-containing solution.

[0011] In some embodiments, when the second iron-containing solution is mixed with the compound containing oxalate, the molar ratio of iron in the second iron-containing solution to oxalate in the compound containing oxalate is (1.05~1.2):1; and / or, the precipitation reaction conditions include: reacting at 20~80°C for 0.5~3 h at a pH of 2~4; and / or, the purity of ferrous oxalate is ≥99%; and / or, the compound containing oxalate includes oxalic acid and / or ammonium oxalate.

[0012] In this embodiment, by controlling the molar ratio of iron in the second iron-containing solution to oxalate in the oxalate-containing compound, an appropriate excess of iron is achieved. This ensures complete precipitation of oxalate without wasting a large amount of iron. Controlling the precipitation reaction conditions promotes efficient reaction between iron and oxalate, leading to the formation of ferrous oxalate precipitate. Optimizing the types of oxalate-containing compounds provides oxalate while avoiding the introduction of excessive impurities, thus improving the purity of ferrous oxalate. The ferrous oxalate prepared in this application has a purity ≥99%, making it a high-quality iron source for the preparation of lithium iron phosphate. This reduces the cost of lithium iron phosphate production while also improving its electrochemical performance.

[0013] In some embodiments, before acid leaching, the iron ore waste residue is further subjected to crushing and screening; and / or, the acid leaching process includes: mixing the iron ore waste residue with an acid leaching agent, reacting it fully at 70-90°C, and obtaining a first iron-containing solution after solid-liquid separation; the acid leaching agent includes sulfuric acid and / or hydrochloric acid.

[0014] In this embodiment, crushing and screening the iron ore waste slag before acid leaching facilitates full contact between the iron elements in the waste slag and the acid leaching agent, thereby improving reaction efficiency. By using sulfuric acid and / or hydrochloric acid as the acid leaching agent and controlling the reaction temperature, it is beneficial to promote the efficient reaction between the iron elements in the waste slag and the acid leaching agent, thereby increasing the leaching rate of iron elements in the waste slag and achieving efficient utilization of the waste slag.

[0015] Secondly, embodiments of this application provide a method for preparing lithium iron phosphate material, comprising the following steps: Ferrous oxalate prepared by the method provided in the first aspect is mixed with a lithium source, a phosphorus source, a carbon source, and a solvent to obtain a mixture; After grinding and drying, the mixture is subjected to a first sintering and a second sintering to obtain lithium iron phosphate material.

[0016] In the technical solution of this application embodiment, by using high-purity ferrous oxalate provided in the first aspect as an iron source, the impurity content in the raw materials can be effectively reduced, while the content of the active component lithium iron phosphate in the product can be increased, so that the prepared lithium iron phosphate material has a higher compaction density and better charge and discharge performance.

[0017] In some embodiments, the molar ratio of lithium, iron, and phosphorus in the mixture is (0.9~1.1):(0.9~1.1):(0.9~1.1); and / or, the lithium source includes at least one of lithium dihydrogen phosphate, lithium carbonate, and lithium acetate; and / or the phosphorus source includes at least one of lithium dihydrogen phosphate, ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate; and / or, the carbon source includes at least one of glucose, sucrose, and citric acid; and / or, in the lithium iron phosphate material, the mass fraction of carbon is 0.8%~3%.

[0018] In this embodiment, by controlling the molar ratio of lithium, iron, and phosphorus, impurity formation can be avoided, thereby increasing the content of active components in the lithium iron phosphate material and thus improving its charge-discharge performance. Controlling the mass fraction of carbon facilitates the formation of a uniform carbon coating layer on the surface of the lithium iron phosphate particles, improving the ionic conductivity of the material and enhancing its electrical performance. Furthermore, the lithium, phosphorus, and carbon sources used in this application have a wide range of selection options and low raw material costs, which is beneficial for industrial-scale production.

[0019] In some embodiments, the primary sintering method includes: holding at 320~550℃ for 2~5 hours in an inert atmosphere; the secondary sintering method includes: holding at 550~800℃ for 5~15 hours in an inert atmosphere; the inert atmosphere includes at least one of nitrogen, argon, and hydrogen.

[0020] In this embodiment, by performing a primary and secondary sintering under an inert atmosphere, the oxidation of ferrous iron can be avoided, thus preventing the formation of metallic compounds containing high-valence iron. Furthermore, by performing a primary sintering at a lower temperature, this embodiment facilitates the decomposition of ferrous oxalate into ferrous oxide and allows for the slow release of the generated gases, thereby increasing the compaction density of the product. Subsequent secondary sintering at a higher temperature effectively suppresses the formation of large particles, thereby improving the electrochemical performance of the lithium iron phosphate material.

[0021] Thirdly, embodiments of this application provide a positive electrode sheet comprising lithium iron phosphate material prepared by the preparation method provided in the second aspect.

[0022] In this embodiment, the positive electrode sheet contains the aforementioned lithium iron phosphate material, thus possessing the advantages of low cost and good electrochemical performance.

[0023] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode provided in the third aspect.

[0024] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing the advantages of low cost and good electrochemical performance.

[0025] 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

[0026] 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.

[0027] Figure 1 This is a process flow diagram of the preparation method of lithium iron phosphate material provided in the embodiments of this application; Figure 2 The image shows the XRD pattern of the lithium iron phosphate material prepared in Example 1 of this application. Figure 3 This is a SEM image of the lithium iron phosphate material prepared in Example 1 of this application. Detailed Implementation

[0028] 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.

[0029] 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, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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).

[0034] The high cost of raw materials and complex manufacturing process for lithium iron phosphate (LFP) results in the high production cost of existing LFP materials. Iron ore slag is a solid waste generated during steel smelting, typically containing large amounts of iron and other metallic elements. However, its traditional treatment methods often involve stockpiling or landfilling, making it difficult to effectively utilize iron ore slag. This not only leads to resource waste but also environmental pollution.

[0035] To address the technical challenges of high production costs and low utilization rates of iron ore waste in lithium iron phosphate (LFP) production, this application provides a method for preparing and applying ferrous oxalate. Iron is extracted from iron ore waste through acid leaching, and after impurity removal with resin, it reacts with oxalate to generate high-purity ferrous oxalate. This ferrous oxalate is then used as the iron source to prepare LFP materials. This method effectively reduces production costs while ensuring excellent electrochemical performance of LFP materials and achieves efficient utilization of iron ore waste, resulting in significant economic and social benefits.

[0036] In a first aspect, embodiments of this application provide a method for preparing ferrous oxalate, comprising the following steps: The iron ore slag was subjected to acid leaching to obtain the first iron-containing solution; The first iron-containing solution is subjected to resin purification to obtain the second iron-containing solution; The second iron-containing solution was mixed with a compound containing oxalate to carry out a precipitation reaction, yielding ferrous oxalate.

[0037] In this application, by acid leaching the iron ore slag, the iron element contained in the iron ore slag can be fully leached out to improve the utilization rate of resources. Since the iron ore slag also contains impurities such as silicon, aluminum, calcium, magnesium, sulfur, phosphorus, manganese, titanium, sodium, and potassium, this application uses resin to remove impurities from the first iron-containing solution. The resin can selectively adsorb the above-mentioned impurities, thereby greatly improving the purity of the second iron-containing solution. The second iron-containing solution reacts with oxalate to generate high-purity ferrous oxalate.

[0038] The technical solution provided in this application not only enables the effective utilization of iron ore waste but also allows for the preparation of high-purity ferrous oxalate at a lower cost. The purity of this ferrous oxalate can reach over 99.5%, making it suitable as an iron source in the preparation of lithium iron phosphate materials. This not only effectively reduces the preparation cost of lithium iron phosphate materials but also improves the electrochemical performance of the prepared materials due to its high purity.

[0039] Furthermore, in some embodiments, in the second iron-containing solution: Fe 2+ The mass concentration is 2.5%~15%; and / or, the total mass concentration of sodium and potassium is <300ppm, the mass concentration of aluminum is <100ppm, the mass concentration of titanium is <100ppm, the mass concentration of magnesium is <100ppm, the mass concentration of manganese is <50ppm, the mass concentration of calcium is <100ppm, the mass concentration of silicon is <50ppm, and the mass concentration of phosphorus is <100ppm.

[0040] In this application, Fe in the second iron-containing solution 2+ Controlling the mass concentration of Fe is beneficial for promoting Fe 2+ The precipitation reaction with oxalate ions proceeds while preventing other impurities from being trapped inside the formed ferrous oxalate crystals. This application further improves the purity of ferrous oxalate by controlling the mass concentrations of impurity elements such as aluminum, titanium, magnesium, manganese, calcium, silicon, and phosphorus within a low range.

[0041] In the second iron-containing solution, if Fe 2+ The mass concentration was too low. After mixing the second iron-containing solution with a compound containing oxalate, Fe... 2+ The reaction efficiency with oxalate is low, which is not conducive to the precipitation reaction; if Fe 2+ If the mass concentration is too high, the second iron-containing solution will also contain a high mass concentration of anions (such as SO42-). 2- Cl - (etc.), after mixing the second iron-containing solution with a compound containing oxalate, Fe 2+It can react with oxalate ions to form ferrous oxalate. During its growth, ferrous oxalate tends to trap these anions within its crystals, resulting in impurities and affecting its purity. Specifically, in the second iron-containing solution, Fe... 2+ The mass concentration can be 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 10%, 13%, 15%, or any value in the range of 2.5% to 15%.

[0042] Furthermore, in some embodiments, resin impurity removal includes: The first iron-containing solution is subjected to a first impurity removal process using an acidic adsorption resin to obtain a first impurity-removed solution. The first impurity removal solution is subjected to a second impurity removal process using an ion-selective adsorption resin to obtain a second iron-containing solution. In the first purification solution, the total mass concentration of sodium and potassium is <300ppm, and the mass concentration of aluminum is <100ppm.

[0043] In this application, by first using an acidic adsorption resin for the initial impurity removal, Na in the first iron-containing solution can be effectively removed. + K + And Al 3+ This process ensures that the total mass concentration of sodium and potassium in the first purification solution is <300 ppm, and the mass concentration of aluminum is <100 ppm. This reduces the impurity concentration in the first iron-containing solution and avoids the impact of these impurities on the second purification process. Subsequently, an ion-selective adsorption resin is used for the second purification, which can effectively remove Ca... 2+ Mg 2+ Other impurity ions are selectively adsorbed, thereby effectively reducing the impurity concentration in the second iron-containing solution. The ion-selective adsorption resin can be a single resin capable of simultaneously and selectively adsorbing various impurity ions, or it can be composed of multiple resins connected in series, with each resin selectively adsorbing one or more impurity ions. For example, Ca... 2+ Selective adsorption resin and Mg 2+ Selective adsorption resins in series are used for the adsorption of Ca 2+ and Mg 2+ Selective adsorption is performed.

[0044] Specifically, in some embodiments of this application, the acidic adsorption resin used is preferably an organic weakly acidic cation exchange resin; more specifically, in some embodiments of this application, the organic weakly acidic cation exchange resin uses a copolymer of methacrylic acid and divinylbenzene as its backbone, and the functional group is mainly a carboxyl group (-COOH). In some embodiments of this application, the preferred operating temperature of the acidic adsorption resin is 10~30℃, and the preferred pH value is 2~5; by adjusting the pH value of the first iron-containing solution to 2~5 and purifying it by passing it through the acidic adsorption resin at 10~30℃, the Na in the first iron-containing solution can be removed. + K + And Al 3+ Sufficient adsorption is achieved. The ion-selective adsorption resin is preferably a weakly acidic cation adsorption resin, with sulfonic acid groups being the preferred functional groups. It is mainly used to adsorb impurity ions such as Ca and Mg. Its operating temperature is preferably 10-30℃, and its operating pH value is preferably 3-5. The pH value of the first impurity removal solution is adjusted to 3-5, and it is purified by passing it through the ion-selective adsorption resin at 10-30℃. This ensures sufficient adsorption of impurity ions in the first impurity removal solution, thereby improving the purity of the second iron-containing solution. Furthermore, both the acidic adsorption resin and the ion-selective adsorption resin used in this application can be reused after regeneration, exhibiting a long service life and being more environmentally friendly.

[0045] Further, in some embodiments, when the second iron-containing solution is mixed with the compound containing oxalate, the molar ratio of iron in the second iron-containing solution to oxalate in the compound containing oxalate is (1.05~1.2):1; and / or, the precipitation reaction conditions include: reacting at 20~80°C for 0.5~3 h at a pH of 2~4; and / or, the purity of ferrous oxalate is ≥99%; and / or, the compound containing oxalate includes oxalic acid and / or ammonium oxalate.

[0046] In this application, by controlling the molar ratio of iron in the second iron-containing solution to oxalate in the oxalate-containing compound, an appropriate excess of iron is achieved, ensuring complete precipitation of oxalate without wasting a large amount of iron. Specifically, the molar ratio of iron in the second iron-containing solution to oxalate in the oxalate-containing compound can be any ratio within the range of 1.05:1, 1.1:1, 1.15:1, 1.2:1, or (1.05~1.2):1.

[0047] This application utilizes controlled pH, reaction temperature, and reaction time during the precipitation reaction to promote a complete reaction between iron and oxalate, resulting in the formation of high-purity ferrous oxalate precipitate. Specifically, the pH of the precipitation reaction can be any value within the range of 2, 2.5, 3, 3.5, 4, or 2–4; the temperature of the precipitation reaction can be any value within the range of 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, or 20–80°C; and the reaction time at this temperature can be any value within the range of 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, or 0.5–3 h.

[0048] This application optimizes the types of compounds containing oxalate, thereby providing oxalate while avoiding the introduction of excessive impurity elements, which is beneficial for improving the purity of ferrous oxalate. Simultaneously, the resin purification process performed on the first iron-containing solution in this application significantly reduces the impurity concentration, resulting in a purity of ≥99% for the ferrous oxalate prepared in this application. This ferrous oxalate can serve as a high-quality iron source for the preparation of lithium iron phosphate, reducing the production cost of lithium iron phosphate while also improving its electrochemical performance.

[0049] Furthermore, in some embodiments, before acid leaching, the iron ore waste residue is crushed and screened; and / or, the acid leaching process includes: mixing the iron ore waste residue with an acid leaching agent, reacting fully at 70~90°C, and obtaining a first iron-containing solution after solid-liquid separation; the acid leaching agent includes sulfuric acid and / or hydrochloric acid.

[0050] In this application, by crushing and screening the iron ore waste residue before acid leaching, it is beneficial to ensure sufficient contact between the iron element in the iron ore waste residue and the acid leaching agent, promote the full reaction, and shorten the reaction time, thereby improving the leaching efficiency of the acid leaching treatment and the leaching rate of iron element in the iron ore waste residue. This application also optimizes the reaction by using sulfuric acid and / or hydrochloric acid as the acid leaching agent and controlling the reaction temperature, which further promotes the efficient reaction between the iron element in the iron ore waste residue and the acid leaching agent, thereby increasing the leaching rate of iron element in the iron ore waste residue and achieving efficient utilization of the iron ore waste residue. Specifically, the reaction temperature between the iron ore waste residue and the acid leaching agent can be 70℃, 75℃, 80℃, 85℃, 90℃, or any value within the range of 70~90℃; the concentration of the acid leaching agent is preferably 1.5~2.5mol / L, and the mass-to-volume ratio of the iron ore waste residue to the acid leaching agent is preferably 1kg : (1~3)L.

[0051] Furthermore, in some embodiments, the iron content in the iron ore slag is 20-50 wt%, and the main impurities in the iron ore slag include: 1-25 wt% silicon, 0.5-10 wt% aluminum, 0.1-10 wt% calcium, 0.1-5 wt% magnesium, 0.01-2 wt% sulfur, 0.01-1.5 wt% phosphorus, 0.1-5 wt% manganese, 0.05-5 wt% titanium, sodium and potassium, each with a content of less than 1 wt%, and 1-10 wt% water. Acid leaching of the iron ore slag can initially remove some insoluble impurities, resulting in a first iron-containing solution with an iron concentration of 3-10%. The main impurities include: silicon (0.01-0.5%), aluminum (0.008-2%), calcium (0.002-0.01%), magnesium (0.01-1%), phosphorus (0.001-0.1%), manganese (0.01-1%), titanium (0.01-0.1%), and sodium and potassium (both <0.2%). Resin purification of the first iron-containing solution can effectively reduce the impurity content, facilitating the preparation of high-purity ferrous oxalate.

[0052] Secondly, embodiments of this application provide a method for preparing lithium iron phosphate material, comprising the following steps: Ferrous oxalate prepared by the method provided in the first aspect is mixed with a lithium source, a phosphorus source, a carbon source, and a solvent to obtain a mixture; After grinding and drying, the mixture is subjected to a first sintering and a second sintering to obtain lithium iron phosphate material.

[0053] In this application, by using high-purity ferrous oxalate as the iron source provided in the first aspect, the impurity content in the raw materials can be reduced to an extremely low level, while the content of the active component can be significantly increased, resulting in a lithium iron phosphate material with high compaction density and good charge-discharge performance. Specifically, ferrous oxalate prepared at low cost in the prior art usually contains a lot of impurities. If it is used to prepare lithium iron phosphate materials, the impurity elements will not only interfere with the crystal growth of lithium iron phosphate, resulting in a low compaction density, but also affect the charge-discharge performance of the lithium iron phosphate material. For example, the presence of impurity elements such as Mn, Ti, and Zn will cause lattice distortion in lithium iron phosphate and shift the growth direction of the lithium iron phosphate crystal, resulting in a less dense lithium iron phosphate material with a low compaction density; moreover, the increase in the content of impurity elements will lead to a decrease in the content of the active component lithium iron phosphate, thereby resulting in a deterioration in the charge-discharge performance of the lithium iron phosphate material.

[0054] Further, in some embodiments, the molar ratio of lithium, iron, and phosphorus in the mixture is (0.9~1.1):(0.9~1.1):(0.9~1.1); and / or, the lithium source includes at least one of lithium dihydrogen phosphate, lithium carbonate, and lithium acetate; and / or the phosphorus source includes at least one of lithium dihydrogen phosphate, ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate; and / or, the carbon source includes at least one of glucose, sucrose, and citric acid; and / or, in the lithium iron phosphate material, the mass fraction of carbon is 0.8%~3%.

[0055] In this application, by controlling the molar ratio of lithium, iron, and phosphorus to meet the basic stoichiometric requirements, the generation of impurities can be effectively avoided, thereby increasing the content of the active component, lithium iron phosphate, in the lithium iron phosphate material and thus improving its charge-discharge performance. This application also controls the mass fraction of carbon in the prepared lithium iron phosphate material, which facilitates the formation of a uniform carbon coating layer on the surface of the lithium iron phosphate particles, improving the ionic conductivity and reducing the powder resistance, thereby enhancing the electrical performance of the lithium iron phosphate material. If the carbon content is too low, the carbon coating layer will be too thin or incomplete, affecting the electrical performance of the lithium iron phosphate material; if the carbon content is too high, the carbon coating layer will be too thick, hindering lithium-ion transport and reducing the content of the active component, lithium iron phosphate, thus lowering the charge-discharge performance of the lithium iron phosphate material. Furthermore, this application optimizes the types of lithium, phosphorus, and carbon sources, facilitating the preparation of lithium iron phosphate materials at a lower cost, and the wide range of raw material selection is beneficial for industrial-scale production.

[0056] Specifically, the molar ratio of lithium, iron, and phosphorus can be any ratio within the range of 0.9:1.0:1.0, 1.0:1.0:1.0, 1.1:1.0:1.0, 1.0:0.9:1.0, 1.0:1.1:1.0, 1.0:1.0:0.9, 1.0:1.0:1.1, 1.1:0.9:1.0, 1.1:1.0:0.9, or (0.9~1.1):(0.9~1.1):(0.9~1.1). The mass fraction of carbon in the lithium iron phosphate material can be any value within the range of 0.8%, 1%, 2%, 3%, or 0.8%~3%. To ensure that the mass fraction of carbon in the prepared lithium iron phosphate material is within the above range, the amount of carbon source added can be controlled. Preferably, the amount of carbon source added accounts for 2%~15% of the total mass of lithium source, phosphorus source, and carbon source.

[0057] Furthermore, in some embodiments, when ferrous oxalate is mixed with a lithium source, phosphorus source, carbon source, and solvent, additives may be added as needed to improve the electrochemical performance of the lithium iron phosphate material; the additive is preferably a metal oxide or metal salt of titanium, vanadium, nickel, aluminum, niobium, or manganese, and its addition amount is preferably 300~10000ppm. The solvent is preferably water and / or alcohol, and the mass fraction of the solvent in the mixture is preferably 50~70%.

[0058] Furthermore, in some embodiments, the grinding method of the mixture includes coarse grinding and fine grinding. The coarse grinding method is preferably a rod-mill, resulting in a particle size D100 < 20 μm. The fine grinding method is preferably a rod-mill (with grinding media particle size smaller than that of coarse grinding), resulting in a particle size (D50) ranging from 200 to 500 nm. By subjecting the mixture to coarse and fine grinding, the mixture can be mechanically and chemically activated to reduce the solid-phase reaction interface energy barrier, thereby improving the specific capacity and compaction density of the prepared lithium iron phosphate material.

[0059] Furthermore, in some embodiments, the primary sintering method includes: holding at 320~550℃ for 2~5 hours in an inert atmosphere; the secondary sintering method includes: holding at 550~800℃ for 5~15 hours in an inert atmosphere; the inert atmosphere includes at least one of nitrogen, argon, and hydrogen.

[0060] In this application, by performing primary and secondary sintering under an inert atmosphere, the oxidation of ferrous iron can be avoided, thus preventing the formation of metallic compounds containing high-valence iron. Furthermore, when hydrogen is present in the inert atmosphere, ferric iron can be reduced to ferrous iron, ensuring that ferrous iron dominates the subsequent crystallization process, which is beneficial for forming well-defined lithium iron phosphate crystals and improving its electrochemical performance. Moreover, by performing a primary sintering at a lower temperature, ferrous oxalate is decomposed into ferrous oxide. The preferred heating rate during the primary sintering is 1-8°C / min, allowing for the slow release of gases generated during the primary sintering process, preventing the formation of porous structures within the product, and thus increasing the product's compaction density. After the first sintering, a second sintering process is performed at a higher temperature. The temperature and holding time of the second sintering are controlled to ensure that the lithium iron phosphate crystals can grow fully and become dense during the second sintering process, while effectively suppressing the formation of large particles. During the second sintering process, the carbon source is pyrolyzed and carbonized, uniformly coating the surface of the lithium iron phosphate particles, thereby constructing a highly efficient conductive network and ensuring that the lithium iron phosphate particles are tightly packed, thus increasing the compaction density of the lithium iron phosphate material. The preferred heating rate during the second sintering is 3~13℃ / min, which helps to save energy and reduce carbon dioxide emissions.

[0061] Specifically, the temperature for the first sintering can be 320℃, 350℃, 400℃, 450℃, 500℃, 550℃, or any value within the range of 320~550℃, and the holding time can be 2h, 3h, 4h, 5h, or any value within the range of 2~5h. The temperature for the second sintering can be 550℃, 600℃, 700℃, 800℃, or any value within the range of 550~800℃, and the holding time can be 5h, 8h, 10h, 12h, 15h, or any value within the range of 5~15h.

[0062] Thirdly, embodiments of this application provide a positive electrode sheet comprising lithium iron phosphate material prepared by the preparation method provided in the second aspect.

[0063] In this application, the positive electrode sheet contains the aforementioned lithium iron phosphate material, thus possessing the advantages of low cost and good electrochemical performance.

[0064] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode provided in the third aspect.

[0065] In this application, the secondary battery includes the above-mentioned positive electrode sheet, thus having the advantages of low cost and good electrochemical performance. Its initial discharge specific capacity at 0.1C rate is 157~162mAh / g, and its capacity retention rate exceeds 92% after 1000 charge-discharge cycles.

[0066] 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.

[0067] I. Preparation Method Example 1 like Figure 1 As shown, this embodiment provides a method for preparing lithium iron phosphate material, including the following steps: S1. Take 1500 kg of iron ore waste (iron content 25.82%), crush it and pass it through a 200 mesh sieve. Take 1000 kg of the sieve-passing material as iron ore powder. Add 2000 L of 2 mol / L sulfuric acid solution to the iron ore powder, stir and react at 80℃ for 3 h, and filter to obtain the first iron-containing solution (iron content 4.7%). The iron leaching rate is calculated to be 95.2%.

[0068] S2. Adjust the pH of the first iron-containing solution to 4, and perform a first purification at 20°C using an acidic adsorption resin (an organic weakly acidic cation exchange resin with a copolymer of methacrylic acid and divinylbenzene as the backbone and carboxyl-COOH as the functional group) to obtain a first purified solution; then adjust the pH of the first purified solution to 4, and perform a second purification at 20°C using an ion-selective adsorption resin (a weakly acidic cation exchange resin containing sulfonate groups) to obtain a second purified solution (Fe 2+ The mass concentration was 4.6%); the second impurity removal solution was concentrated to obtain a second iron-containing solution (Fe). 2+ The mass concentration was 14.7%.

[0069] S3. Mix the second iron-containing solution with a 0.5 mol / L oxalic acid solution to reduce the Fe content in the solution. 2+ A precipitation reaction was carried out with a molar ratio of 1.05:1 to oxalate ions. The precipitation process included adjusting the pH of a mixture of a second iron-containing solution and an oxalic acid solution to 3, and stirring the mixture at 20°C for 1 hour. The resulting precipitate was filtered, washed three times at a solid-liquid ratio of 1:3, and then vacuum dried to obtain ferrous oxalate (FeC₂O₄·2H₂O), with a purity of 99.2%.

[0070] S4. The ferrous oxalate obtained in step S3 is mixed with lithium dihydrogen phosphate, glucose, and water to obtain a mixture. In this mixture, the molar ratio of lithium, iron, and phosphorus is 1:1:1, the mass of glucose accounts for 2.5% of the total mass of ferrous oxalate, lithium dihydrogen phosphate, and glucose, and the mass of water accounts for 60% of the total mass of the mixture.

[0071] S5. The mixture obtained in step S4 is subjected to coarse grinding and fine grinding in sequence; wherein, both coarse grinding and fine grinding are carried out using a rod-type coarse grinding equipment, and the particle size (D50) obtained after fine grinding is 300nm; then the finely ground material is subjected to spray drying treatment, and the particle size (D50) of the dried powder obtained is 25.6μm.

[0072] S6. Spread the dried powder obtained in step S5 evenly on a ceramic boat and place it in an atmosphere furnace for primary sintering. The primary sintering method includes: introducing argon gas as a protective atmosphere (argon gas flow rate of 250 mL / min), heating to 450°C at a heating rate of 5°C / min, holding at that temperature for 3 hours, and obtaining the primary sintered product.

[0073] S7. After cooling the first-burned product obtained in step S6 to room temperature, agglomeration is eliminated by grinding and crushing. The product is then transferred to a high-temperature furnace for secondary sintering. The secondary sintering process includes: introducing argon gas as a protective atmosphere (argon flow rate of 250 mL / min), heating to 750℃ at a rate of 8℃ / min, and holding at this temperature for 10 hours to obtain the second-burned product. After the second-burned product cools naturally to room temperature, it is pulverized into fine powder with a D50 of 1.3 μm using an airflow milling device. Magnetic impurities are then removed by magnetic separation to obtain lithium iron phosphate material. The mass fraction of carbon in this lithium iron phosphate material is 1.3%.

[0074] In this embodiment, the contents of the main impurities in the iron ore waste residue in step S1, the first iron-containing solution, and the first impurity removal solution and the second iron-containing solution in step S2 are shown in Table 1.

[0075] Table 1 As can be seen from Table 1, although the iron ore slag contains a lot of impurities, the method provided in this embodiment can effectively remove impurities, so that the concentration of each impurity in the obtained second iron-containing solution is reduced to a low level, which is beneficial to improving the purity of ferrous oxalate.

[0076] Examples 2-3 and Comparative Examples 1-2 Examples 2-3 and Comparative Examples 1-2 each provide a method for preparing lithium iron phosphate materials. Compared with Example 1, the only difference is the change in the Fe content of the second iron-containing solution. 2+ The specific mass concentration is shown in Table 2. The remaining steps are the same as in Example 1 and will not be repeated here.

[0077] Table 2 Examples 4-5 and Comparative Examples 3-4 Examples 4-5 and Comparative Examples 3-4 respectively provide a method for preparing lithium iron phosphate material. Compared with Example 1, the only difference is that the pH value during the first and second purification in step S2 is changed, as shown in Table 3. The remaining steps are the same as in Example 1 and will not be repeated here.

[0078] Table 3 Examples 6-7 and Comparative Examples 5-6 Examples 6-7 and Comparative Examples 5-6 respectively provide a method for preparing lithium iron phosphate material. Compared with Example 1, the only difference is that the precipitation reaction conditions in step S3 are changed, as shown in Table 4. The remaining steps are the same as in Example 1 and will not be described again here.

[0079] Table 4 Examples 8-9 Examples 8 and 9 respectively provide a method for preparing lithium iron phosphate material. Compared with Example 1, the only difference is that the mass fraction of glucose in step S4 is changed so that the carbon content in the prepared lithium iron phosphate material changes, as shown in Table 5. The remaining steps are the same as in Example 1 and will not be repeated here.

[0080] Table 5 Examples 10-13 and Comparative Examples 7-9 Examples 10-13 and Comparative Examples 7-9 respectively provide a method for preparing lithium iron phosphate material. Compared with Example 1, the only difference is that the conditions for the first sintering and the second sintering in steps S6 and S7 are changed, as shown in Table 6. The remaining steps are the same as in Example 1 and will not be described again here.

[0081] Table 6 Among them, Comparative Example 7 refers to the sintering process performed directly according to the conditions of secondary sintering without first sintering.

[0082] Example 14 This embodiment provides a method for preparing lithium iron phosphate material. Compared with Example 1, the only difference is that manganese carbonate is added in step S4. The mass of this additive accounts for 0.03% of the total mass of the mixture. The remaining steps are the same as in Example 1 and will not be repeated here.

[0083] Comparative Example 10 This comparative example provides a method for preparing lithium iron phosphate material. Compared with Example 1, the only difference is the change in the impurity removal method in step S2. The remaining steps are the same as in Example 1 and will not be repeated here. In this comparative example, impurity removal is carried out by flocculation and precipitation. The specific steps are as follows: First, adjust the pH to 7 to facilitate the combination of flocculant and impurity ions and improve the flocculation effect; add flocculant (such as polyacrylamide) and auxiliary flocculant, and stir rapidly to facilitate the combination and flocculation of impurity ions; after flocculation, allow the flocculant groups to settle freely by gravity, and then perform mechanical separation to achieve the purpose of removing impurity ions.

[0084] II. Testing Methods 1. Property testing of lithium iron phosphate materials (1) Phase analysis: The lithium iron phosphate material prepared in Example 1 was characterized by X-ray powder diffraction (XRD), and the results are as follows: Figure 2 As shown.

[0085] (2) Morphology test: The morphology of the lithium iron phosphate material prepared in Example 1 was examined using a scanning electron microscope. The results are as follows: Figure 3 As shown.

[0086] (3) Compaction density test: The compaction density of the lithium iron phosphate materials prepared in the examples and comparative examples was tested using a compaction density meter in accordance with GB / T 24533-2019.

[0087] 2. Properties of secondary batteries Referring to standard GB / T 33822-2017, lithium iron phosphate material was mixed with conductive agent Super-P, binder PVDF and NMP to form a slurry. The slurry was uniformly coated on an aluminum foil and dried in an oven at 125℃ to form an electrode sheet. The negative electrode sheet was made of lithium metal. A simulated battery was assembled using 1 mol / L LiPF6 / (EC+DEC) as the electrolyte (the mass ratio of EC to DEC was 1:1) and electrochemical performance was tested.

[0088] III. Analysis of Test Results for Each Embodiment and Comparative Example Figure 2 The XRD pattern of the lithium iron phosphate material prepared in Example 1 is shown below. Figure 2 As can be seen, the method provided in Example 1 successfully prepared high-purity lithium iron phosphate using iron ore waste as one of the raw materials, which effectively reduced the preparation cost of lithium iron phosphate while improving the utilization rate of iron ore waste. Figure 3 Here is a SEM image of the lithium iron phosphate material prepared in Example 1. Figure 3 It can be seen that the dense packing of lithium iron phosphate particles is beneficial to improving the compaction density of lithium iron phosphate materials.

[0089] The purity of ferrous oxalate, the compaction density of lithium iron phosphate materials, and the initial discharge specific capacity and retention rate after 1000 charge-discharge cycles of secondary batteries prepared from the corresponding lithium iron phosphate materials at a 0.1C rate are shown in Table 7.

[0090] Table 7 As can be seen from Table 7, the methods provided in each embodiment of this application can extract iron from iron ore slag and prepare ferrous oxalate with a purity of not less than 99%. The ferrous oxalate is then used as an iron source to prepare lithium iron phosphate materials. This not only effectively reduces the preparation cost of lithium iron phosphate materials, but also enables the prepared lithium iron phosphate materials to have high compaction density, discharge specific capacity and capacity retention rate by controlling the preparation process.

[0091] Specifically, by comparing Examples 1-3 and Comparative Examples 1-2, it can be seen that when Fe in the second iron-containing solution...2+ When the mass concentration of Fe decreases, the purity of the prepared ferrous oxalate is higher, but due to Fe 2+ The lower the mass concentration of Fe in the second iron-containing solution, the larger the volume occupied, leading to greater resource consumption, resource waste, and increased costs; as the Fe in the second iron-containing solution... 2+ As the mass concentration increases, the purity of the prepared ferrous oxalate decreases. Ferrous oxalate with lower purity contains more impurity ions during its formation, which leads to a decrease in the initial discharge specific capacity when lithium iron phosphate materials are prepared based on this ferrous oxalate.

[0092] Comparative studies of Examples 1, 4-5, and Comparative Examples 3-4 show that appropriately lowering the pH value during the first and second purification processes within a certain range is more beneficial for improving the purity of ferrous oxalate. However, maintaining a lower pH requires acid-resistant treatment of the container surface to prevent corrosion, and maintaining acidity requires consuming more acid, leading to increased costs. Furthermore, when the pH value during the first and second purification processes is too low (as in Comparative Example 3), not only is higher cost required, but the purity of ferrous oxalate also decreases, thereby affecting the performance of the prepared lithium iron phosphate material.

[0093] By comparing Examples 1, 6-7 and Comparative Examples 5-6, it can be seen that, within a certain range, decreasing the pH value, increasing the reaction temperature, or extending the reaction time during the precipitation reaction are all beneficial to improving the purity of ferrous oxalate. However, decreasing the pH value and increasing the reaction temperature will lead to increased costs, while extending the reaction time will lead to decreased efficiency.

[0094] By comparing Examples 1 and 8-9, it can be seen that the mass fraction of carbon in lithium iron phosphate material is positively correlated with the mass fraction of glucose; and the mass fraction of carbon in lithium iron phosphate material is negatively correlated with the compaction density of lithium iron phosphate material, and positively correlated with the first discharge specific capacity and capacity retention rate of lithium iron phosphate material.

[0095] Comparing Examples 1, 10-13, and Comparative Examples 7-9, it can be seen that when sintering is performed twice, the compaction density of the prepared lithium iron phosphate material gradually increases with the increase of sintering temperature and the extension of holding time during the first or second sintering process. However, excessively high compaction density leads to a decrease in the initial discharge specific capacity and capacity retention of the lithium iron phosphate material. If the first sintering is omitted and only the sintering treatment is performed according to the conditions of the second sintering, both the compaction density and capacity retention of the prepared lithium iron phosphate material will decrease.

[0096] Comparing Examples 1 and 14, it can be seen that when ferrous oxalate is mixed with lithium source, phosphorus source, carbon source and solvent, adding manganese carbonate as an additive is beneficial to improving the compaction density and initial discharge specific capacity of the prepared lithium iron phosphate material.

[0097] Comparing Example 1 and Comparative Example 10, it can be seen that the ferrous oxalate obtained by flocculation and precipitation in Comparative Example 10 has significantly lower purity. When used in the preparation of lithium iron phosphate materials, the compaction density, first discharge specific capacity, and capacity retention of the prepared lithium iron phosphate materials are all low. In contrast, the resin purification method in Example 1 not only obtains ferrous oxalate with higher purity, but also enables the preparation of lithium iron phosphate materials with higher compaction density, first discharge specific capacity, and capacity retention.

[0098] 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 ferrous oxalate, characterized in that, Includes the following steps: The iron ore slag was subjected to acid leaching to obtain the first iron-containing solution; The first iron-containing solution is subjected to resin purification to obtain a second iron-containing solution; The second iron-containing solution was mixed with a compound containing oxalate to carry out a precipitation reaction, yielding ferrous oxalate.

2. The method for preparing ferrous oxalate according to claim 1, characterized in that, In the second iron-containing solution: Fe 2+ The mass concentration is 2.5%~15%; and / or, The total mass concentration of sodium and potassium is <300 ppm, the mass concentration of aluminum is <100 ppm, the mass concentration of titanium is <100 ppm, the mass concentration of magnesium is <100 ppm, the mass concentration of manganese is <50 ppm, the mass concentration of calcium is <100 ppm, the mass concentration of silicon is <50 ppm, and the mass concentration of phosphorus is <100 ppm.

3. The method for preparing ferrous oxalate according to claim 1, characterized in that, The resin impurity removal includes: The first iron-containing solution is subjected to a first impurity removal process using an acidic adsorption resin to obtain a first impurity-removed solution. The first impurity-removing solution is subjected to a second impurity removal process using an ion-selective adsorption resin to obtain the second iron-containing solution. In the first impurity removal solution, the total mass concentration of sodium and potassium is <300ppm, and the mass concentration of aluminum is <100ppm.

4. The method for preparing ferrous oxalate according to claim 1, characterized in that, When the second iron-containing solution is mixed with the compound containing oxalate, the molar ratio of iron in the second iron-containing solution to oxalate in the compound containing oxalate is (1.05~1.2):

1. And / or, The precipitation reaction conditions include: reacting at 20-80°C for 0.5-3 hours at a pH of 2-4; and / or, The purity of the ferrous oxalate is ≥99%; and / or, The compounds containing oxalate include oxalic acid and / or ammonium oxalate.

5. The method for preparing ferrous oxalate according to claim 1, characterized in that, Before the acid leaching treatment, the iron ore waste residue is also crushed and screened. And / or, The acid leaching treatment includes: mixing iron ore waste with an acid leaching agent, reacting fully at 70~90℃, and obtaining the first iron-containing solution after solid-liquid separation; the acid leaching agent includes sulfuric acid and / or hydrochloric acid.

6. A method for preparing lithium iron phosphate material, characterized in that, Includes the following steps: Ferrous oxalate prepared by any one of claims 1-5 is mixed with a lithium source, a phosphorus source, a carbon source, and a solvent to obtain a mixture; After grinding and drying, the mixture is subjected to a first sintering and a second sintering to obtain lithium iron phosphate material.

7. The method for preparing lithium iron phosphate material according to claim 6, characterized in that, In the mixture, the molar ratio of lithium, iron, and phosphorus is (0.9~1.1):(0.9~1.1):(0.9~1.1); and / or, The lithium source includes at least one of lithium dihydrogen phosphate, lithium carbonate, and lithium acetate; and / or, The phosphorus source includes at least one of lithium dihydrogen phosphate, ammonium phosphate, monoammonium phosphate, and ammonium dihydrogen phosphate; and / or, The carbon source includes at least one of glucose, sucrose, and citric acid; and / or, In the lithium iron phosphate material, the mass fraction of carbon element is 0.8% to 3%.

8. The method for preparing lithium iron phosphate material according to claim 6, characterized in that, The primary sintering method includes: holding at 320~550℃ for 2~5 hours under an inert atmosphere; The secondary sintering method includes: holding at 550~800℃ for 5~15h under an inert atmosphere; The inert atmosphere includes at least one of nitrogen, argon, and hydrogen.

9. A positive electrode sheet, characterized in that, This includes lithium iron phosphate materials prepared by the preparation method according to any one of claims 6-8.

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

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