Magnesium-doped anhydrous iron phosphate and preparation method thereof, positive electrode material, positive electrode plate and secondary battery

Abstract

The invention provides magnesium-doped anhydrous iron phosphate and a preparation method thereof, a positive electrode material, a positive electrode plate and a secondary battery, and belongs to the technical field of secondary batteries, the preparation method comprises the following steps: carrying out aluminum removal treatment on wet-process phosphoric acid to obtain refined phosphate; the refined phosphate is mixed with phosphorite flotation liquid and an oxidizing agent, and a first mixture is obtained; mixing ferrite, a complexing agent and the first mixture, and then reacting to obtain magnesium-doped iron phosphate dihydrate; sequentially carrying out drying treatment and sintering treatment on the magnesium-doped iron phosphate dihydrate to obtain magnesium-doped anhydrous iron phosphate; wherein the phosphorite flotation liquid contains a magnesium element. The invention aims to solve the technical problem of low comprehensive utilization rate of phosphorite in the existing iron phosphate preparation process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a magnesium-doped anhydrous iron phosphate, a preparation method thereof, a positive electrode material, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium iron phosphate materials are commonly used positive electrode materials for power batteries. With the rapid development of new energy vehicles, the demand for lithium iron phosphate materials is growing rapidly, and the demand for iron phosphate, which is one of the key precursors of lithium iron phosphate materials, is also increasing.

[0003] The mainstream preparation method of iron phosphate is oxidation, that is, using ferrous salt as an iron source and battery-grade phosphate or phosphoric acid as a phosphorus source to synthesize iron phosphate through an oxidation reaction. The battery-grade phosphate or phosphoric acid is derived from the purified product of wet-process phosphoric acid. Among them, wet-process phosphoric acid refers to a crude product of phosphoric acid obtained by decomposing phosphate rock with inorganic acid, which contains a large amount of impurity elements such as Al and Mg. In order to achieve battery-grade purity, the impurity elements need to be removed through a removal process, but not only the Mg element is removed, causing the loss of magnesium element, but also a large amount of phosphorus element will form phosphate precipitate with Mg ions and be filtered out, resulting in low comprehensive utilization rate of phosphate rock and waste. SUMMARY

[0004] In view of the technical problems in the background art, the application provides a magnesium-doped anhydrous iron phosphate, a preparation method thereof, a positive electrode material, a positive electrode sheet and a secondary battery, aiming to solve the technical problem of low comprehensive utilization rate of phosphate rock in the existing preparation process of iron phosphate.

[0005] In a first aspect, the application provides a preparation method of a magnesium-doped anhydrous iron phosphate, comprising the following steps: performing aluminum removal treatment on the wet-process phosphoric acid to obtain refined phosphate; mixing the refined phosphate with a phosphate rock flotation liquid and an oxidizing agent to obtain a first mixture; mixing ferrous salt, a complexing agent and the first mixture, and then performing a reaction to obtain a wet material containing magnesium-doped dihydrate iron phosphate; performing drying treatment and sintering treatment on the wet material in sequence to obtain the magnesium-doped anhydrous iron phosphate; The phosphate rock flotation liquid contains magnesium element.

[0006] The technical scheme of the embodiment of the present application uses wet-process phosphoric acid as a phosphorus source and part of a magnesium source, and uses phosphate rock flotation liquid as another part of the magnesium source to synthesize magnesium-doped anhydrous iron phosphate, which not only effectively develops and utilizes the wet-process phosphoric acid and the phosphate rock flotation liquid, fully utilizes the magnesium element and the phosphorus element in the phosphate rock, improves the utilization efficiency of the phosphate rock resources, reduces waste, and reduces production costs, but also makes the magnesium-doped anhydrous iron phosphate have a suitable Fe / P ratio, a better phase purity, a suitable specific surface area, and good electronic conductivity, and is suitable for use as a lithium iron phosphate precursor. In addition, after the wet-process phosphoric acid is subjected to aluminum removal, a complexing agent is added, so that the ferric ions can be complexed to form stable complexes, the purpose of masking the ferric ions is achieved, the magnesium ion precipitation efficiency is effectively improved, and the synthesis of the magnesium-doped anhydrous iron phosphate is achieved.

[0007] In some embodiments, the step of mixing the ferrous salt, the complexing agent, and the first mixture, and then performing a reaction to obtain a wet material containing magnesium-doped dihydrate iron phosphate includes: mixing the ferrous salt with water to obtain a ferrous solution; under stirring conditions, the first mixture is added dropwise to the ferrous solution within a time range of 10 min to 60 min, and then the complexing agent is added, and the first mixture is continuously stirred for a first time to obtain a second mixture; the second mixture is heated to 80-95 DEG C for a second time, and then is kept for a third time to obtain a third mixture; the third mixture is subjected to solid-liquid separation to obtain a wet material containing magnesium-doped dihydrate iron phosphate.

[0008] In this embodiment, the first mixture is added dropwise to the ferrous solution within a limited time, which helps to avoid local overheating or violent reaction caused by rapid reaction, ensures smooth reaction, and reduces byproduct impurities; the complexing agent is added after the first mixture is mixed with the ferrous solution, which not only ensures that the ferric ions are fully masked and promotes the magnesium ion precipitation, but also avoids the precipitation of part of the unoxidized ferrous ions due to early addition, which affects the product purity and causes waste of iron elements.

[0009] In some embodiments, the content of magnesium ions in the first mixture is 5000 ppm to 6000 ppm.

[0010] In this embodiment, the content of magnesium elements in the first mixture is controlled, which can more accurately control the doping amount of magnesium elements in the subsequent reaction process, coordinate the ratio of magnesium, iron, and phosphorus, and obtain magnesium-doped anhydrous iron phosphate with excellent performance.

[0011] In some embodiments, the concentration of phosphorus elements in the first mixture is 1 mol / L to 2.5 mol / L.

[0012] In this embodiment, the content of phosphorus element in the first mixture is controlled, so that the ratio of magnesium, iron and phosphorus can be more accurately controlled in the subsequent reaction process, and the magnesium-doped anhydrous iron phosphate with excellent performance is obtained.

[0013] In some embodiments, the complexing agent comprises at least one of oxalic acid, oxalate and fluoride.

[0014] In this embodiment, oxalic acid, oxalate or fluoride is used as a complexing agent to form a stable complex ([Fe(C2O4)3] 3- or [FeF6] 3- ) with ferric ions, thereby masking ferric ions.

[0015] In some embodiments, when the complexing agent is selected from oxalic acid or oxalate, the molar ratio of the complexing agent to the ferrous salt is (0.03-0.04):1, and when the complexing agent is selected from fluoride, the molar ratio of the complexing agent to the ferrous salt is (0.06-0.08):1.

[0016] In this embodiment, the amount of the complexing agent is controlled within the above range, so that the free ferric ions are fully complexed, and the settling rate of magnesium ions is improved.

[0017] In some embodiments, the first time is 10-30 min.

[0018] In this embodiment, the first time is controlled within the above range, so that the complexing agent fully contacts with the free ferric ions, and the ferric ions are fully complexed.

[0019] In some embodiments, the second time is 30-90 min, and the third time is 30-90 min.

[0020] In this embodiment, the second mixture is heated to 80-95℃, and then the reaction is controlled within the second time range, so that the amorphous iron phosphate is fully converted into ferriphosphate dihydrate, and the magnesium element is incorporated into the crystal structure to form magnesium-doped ferriphosphate dihydrate; further, the third time is controlled within the above range for aging, so that the crystal grains grow and the size gradually increases, and the product with appropriate specific surface area and particle size is obtained.

[0021] In some embodiments, the step of removing aluminum from the wet-process phosphoric acid to obtain a refined phosphate salt comprises: diluting the wet-process phosphoric acid to obtain a dilute phosphoric acid solution with a mass percentage of phosphorus of 5-10%; adjusting the pH of the dilute phosphoric acid solution to 4.0-4.5 to obtain a conditioning solution; heating the conditioning solution to 85-95℃ and reacting for 1-4 h, and then performing solid-liquid separation to obtain a refined phosphate salt.

[0022] In this embodiment, adjusting the pH of wet-process phosphoric acid can promote the precipitation of aluminum ions contained in the wet-process phosphoric acid, thereby removing impurities.

[0023] In some embodiments, the drying temperature is 90°C to 110°C, and the drying time is 8 hours to 12 hours.

[0024] In this embodiment, controlling the drying temperature and time within the above range allows the wet material to be fully dried, free water to be removed, and magnesium-doped iron phosphate dihydrate to be obtained.

[0025] In some embodiments, the sintering temperature is 550°C to 650°C, and the sintering time is 1 hour to 3 hours.

[0026] In this embodiment, controlling the sintering temperature and time within the above range allows for the complete removal of water of crystallization from the crystal, resulting in anhydrous products. Simultaneously, high temperature promotes crystal structure reconstruction, making its internal structure more ordered, regulating the crystal size and specific surface area, and improving the electrochemical performance of the material.

[0027] In some embodiments, the oxidant includes at least one selected from hydrogen peroxide, sodium peroxide, potassium peroxide, sodium hypochlorite, potassium hypochlorite, sodium persulfate, and potassium persulfate.

[0028] In this embodiment, the above-mentioned compound is inexpensive and readily available, and has good oxidizing properties, which can oxidize ferrous ions to ferric ions.

[0029] In some embodiments, the molar ratio of the oxidant to the ferrous salt is 1.1 to 1.5 times the stoichiometric ratio of the oxidant to the ferrous salt.

[0030] In this embodiment, controlling the amount of oxidant added within the above range can promote the full reaction between ferrous ions and oxidant. While promoting the full oxidation of ferrous ions and improving the purity and electrochemical performance of the product, it avoids side reactions caused by excessive addition of oxidant or waste of oxidant.

[0031] In some embodiments, the ferrous salt includes at least one of ferrous sulfate, ferrous chloride, or ferrous nitrate, wherein the ferrous sulfate is a byproduct of titanium dioxide production.

[0032] In this embodiment, a byproduct generated during the production of titanium dioxide is used. The ferrous sulfate product after refining and removing impurities from the byproduct is taken as a ferrous salt, which develops a new use for titanium dioxide byproducts and helps to save costs.

[0033] In some embodiments, the molar ratio of iron in the ferrous salt to phosphorus in the first mixture is 1:(1.05~1.20).

[0034] In this embodiment, controlling the addition amount of ferrous salt and the first mixture to be within the above-mentioned ratio range is beneficial to promoting the full reaction of ferrous salt to generate magnesium-doped ferric phosphate dihydrate.

[0035] Secondly, embodiments of this application provide a magnesium-doped anhydrous iron phosphate, prepared by the preparation method described above.

[0036] In the technical solution of this application embodiment, magnesium-doped anhydrous iron phosphate can be prepared by the above preparation method. This magnesium-doped anhydrous iron phosphate has a suitable Fe / P ratio, good phase purity, suitable specific surface area, and good electronic conductivity, making it suitable as a precursor for lithium iron phosphate materials. Furthermore, compared to undoped anhydrous iron phosphate, the magnesium-doped iron phosphate proposed in this application, through modification of the iron phosphate crystal structure by incorporating magnesium, further improves the conductivity of the material and enhances the ion diffusion rate. When using the magnesium-doped anhydrous iron phosphate proposed in this application as a precursor, it helps to improve the electrochemical performance of lithium iron phosphate materials, resulting in batteries with higher specific capacity and rate performance. Simultaneously, it also helps to suppress the volume expansion of the cathode material during charge and discharge, extending the battery's cycle performance.

[0037] In some embodiments, the magnesium doping concentration in the magnesium-doped anhydrous iron phosphate is 500 ppm to 3000 ppm.

[0038] In this embodiment, controlling the magnesium doping concentration within the above range can improve the material's conductivity and ion diffusion rate while avoiding the impact of excessive magnesium doping on the specific capacity.

[0039] In some embodiments, the D50 particle size of the magnesium-doped anhydrous iron phosphate is 2 μm to 25 μm.

[0040] In this embodiment, controlling the size of the material within the above-mentioned range helps to shorten the solid-phase diffusion path of lithium ions in the cathode material, thereby improving the rate performance, cycle life and other electrochemical performance of the cathode material.

[0041] In some embodiments, the specific surface area of ​​the magnesium-doped anhydrous iron phosphate is 5 m². 2 / g~10m 2 / g.

[0042] In this embodiment, controlling the BET specific surface area to meet the above range is beneficial to providing sufficient active sites, thereby further improving the electrochemical performance of the cathode material.

[0043] In some embodiments, the molar ratio of iron to phosphorus in the magnesium-doped anhydrous iron phosphate is 0.965 to 0.975.

[0044] In this embodiment, controlling the molar ratio of Fe to P in the material to meet the above-mentioned range helps to balance and improve the purity and stability of the material, which is beneficial for preparing cathode materials with excellent performance.

[0045] Thirdly, embodiments of this application provide a cathode material, which includes lithium iron phosphate or doped lithium iron phosphate, and the cathode material is prepared using magnesium-doped anhydrous iron phosphate as a precursor as described above.

[0046] In the technical solution of this application embodiment, the positive electrode material is prepared by using the above-mentioned magnesium-doped anhydrous iron phosphate as a precursor, and thus has good charge specific capacity, discharge specific capacity, rate performance and cycle life.

[0047] Fourthly, embodiments of this application provide a positive electrode sheet, which includes the aforementioned positive electrode material.

[0048] In the technical solution of this application embodiment, the positive electrode sheet contains the above-mentioned positive electrode material, and thus has good charging specific capacity, discharging specific capacity, rate performance and cycle life.

[0049] Fifthly, embodiments of this application provide a secondary battery, which includes the aforementioned positive electrode plate.

[0050] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has good charging specific capacity, discharging specific capacity, rate performance and cycle life.

[0051] 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, specific embodiments of this application are given below. Attached Figure Description

[0052] 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.

[0053] Figure 1 This is a schematic flowchart of a method for preparing magnesium-doped anhydrous iron phosphate according to an embodiment of this application; Figure 2 This is a schematic flowchart of a method for preparing magnesium-doped anhydrous iron phosphate according to another embodiment of this application; Figure 3This is a process flow diagram of a method for preparing magnesium-doped anhydrous iron phosphate according to an embodiment of this application; Figure 4 This is the XRD pattern of magnesium-doped anhydrous iron phosphate provided in Example 1; Figure 5 The XRD pattern of anhydrous ferric phosphate provided in Comparative Example 1 is shown. Figure 6 This is a SEM image of magnesium-doped anhydrous iron phosphate provided in Example 1. Detailed Implementation

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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).

[0060] 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.

[0061] 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.

[0062] 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.

[0063] The mainstream method for preparing ferric phosphate is the oxidation method, which uses ferrous salts as the iron source and battery-grade phosphate or phosphoric acid as the phosphorus source to synthesize ferric phosphate through an oxidation reaction. Battery-grade phosphate or phosphoric acid is derived from the purified product of wet-process phosphoric acid. Wet-process phosphoric acid refers to the crude phosphoric acid product obtained by decomposing phosphate rock with inorganic acids. It contains a large amount of impurities such as Al and Mg. To achieve battery-grade purity, a purification process is required to remove these impurities. However, in this purification process, not only is Mg removed, resulting in magnesium loss, but a large amount of phosphorus also reacts with Mg ions to form phosphate precipitates that are filtered out, leading to low overall utilization of the phosphate rock and waste.

[0064] To address the technical problem of low comprehensive utilization rate of phosphate rock in existing iron phosphate preparation processes, this application proposes a magnesium-doped anhydrous iron phosphate, its preparation method, positive electrode material, positive electrode sheet, and secondary battery.

[0065] Firstly, this application provides a method for preparing magnesium-doped anhydrous iron phosphate. Please refer to [link to relevant documentation]. Figure 1 and Figure 3 The preparation method includes the following steps: S10 is used to remove aluminum from wet-process phosphoric acid to obtain refined phosphate. S20, the refined phosphate is mixed with phosphate rock flotation solution and an oxidant to obtain a first mixture. The phosphate rock flotation solution contains magnesium. S30, ferrous salt, complexing agent and the first mixture are mixed and then reacted to obtain a wet material containing magnesium-doped ferric phosphate dihydrate; S40, the wet material is sequentially dried and sintered to obtain magnesium-doped anhydrous iron phosphate.

[0066] Phosphate rock mainly exists in the form of phosphate ore. Processing phosphate rock with inorganic acids such as sulfuric acid, nitric acid, or hydrochloric acid yields crude phosphoric acid containing large amounts of Al and Mg. In this application, this crude phosphoric acid is termed wet-process phosphoric acid, in which the mass percentage of phosphorus is 10%–27%. Simultaneously, due to the presence of associated minerals, phosphate rock also contains high-magnesium minerals (such as dolomite CaMg(CO3)2). To improve the filtration efficiency and reduce the consumption of inorganic acids during the preparation of wet-process phosphoric acid, the phosphate rock is typically pretreated with grinding, milling, and flotation to obtain a phosphorus-enriched concentrate. Flotation involves using flotation reagents and gases to alter the hydrophilicity and hydrophobicity of phosphate minerals, causing them to float with the froth. The flotation process generates magnesium-rich wastewater, which is termed phosphate rock flotation solution in this paper, with a magnesium ion concentration of approximately 1%–2%.

[0067] In this application, wet-process phosphoric acid is used as the phosphorus source and part of the magnesium source, while phosphate rock flotation liquor is used as the other part of the magnesium source to synthesize magnesium-doped anhydrous iron phosphate. This not only effectively develops and utilizes wet-process phosphoric acid and phosphate rock flotation liquor, fully utilizing the magnesium and phosphorus elements in the phosphate rock, improving the utilization efficiency of phosphate rock resources, reducing waste, and lowering production costs, but also produces magnesium-doped anhydrous iron phosphate with a suitable Fe / P ratio, good phase purity, suitable specific surface area, and good electronic conductivity, making it suitable as a precursor for lithium iron phosphate materials. Furthermore, since the Ksp of iron phosphate FePO4·2H2O is 9.91 × 10⁻⁶, it is suitable for use as a precursor for lithium iron phosphate materials. -16 The Ksp of magnesium phosphate Mg3(PO4)2 is 1.04 × 10⁻⁶. -24 When ferric ions, phosphate ions, and magnesium ions are in the same system, ferric ions can inhibit the formation of magnesium phosphate precipitate, preventing magnesium ions from settling. Therefore, in this embodiment, after removing aluminum from wet-process phosphoric acid, a complexing agent is added to enable ferric ions to form stable complexes, thereby masking ferric ions and effectively improving the precipitation efficiency of magnesium ions, thus enabling the synthesis of magnesium-doped anhydrous ferric phosphate.

[0068] Furthermore, compared to undoped anhydrous iron phosphate, the magnesium-doped iron phosphate proposed in this application modifies the iron phosphate crystal structure by incorporating magnesium, thereby further improving the conductivity of the material and enhancing the ion diffusion rate. When using the magnesium-doped anhydrous iron phosphate proposed in this application as a precursor, it helps to improve the electrochemical performance of lithium iron phosphate materials, enabling the battery to have higher specific capacity and rate performance. At the same time, it also helps to suppress the volume expansion of the cathode material during charging and discharging, and extend the cycle performance of the battery.

[0069] For further details, please refer to Figure 2 In some embodiments, step S10 can be performed as follows: S11, dilute the wet-process phosphoric acid to obtain a dilute phosphoric acid solution with a phosphorus mass percentage of 5% to 10%; S12, adjust the pH of the dilute phosphoric acid solution to 4.0 to 4.5 to obtain a conditioning solution; S13, heat the conditioning solution to 85°C to 95°C and react for 1 to 4 hours, then perform solid-liquid separation to obtain refined phosphate.

[0070] In this embodiment, adjusting the pH of wet-process phosphoric acid can promote the precipitation of aluminum ions, thus removing impurities. Specifically, in step S11, the phosphorus content in the wet-process phosphoric acid can be detected first, and then diluted with water until it reaches 5wt%~10wt%. In step S12, pH adjustment can be achieved by adding an acid-base adjuster to the dilute phosphoric acid solution. The acid-base adjuster can be any common alkali, such as ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, etc. In step S13, heating and aging helps promote the precipitation of aluminum ions and improves precipitation efficiency. It can be understood that solid-liquid separation can be achieved through filtration, vacuum filtration, pressure filtration, centrifugation, etc.

[0071] Furthermore, in some embodiments, in step S20, the magnesium ion content in the first mixture is 5000ppm to 6000ppm; for example, it can be 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm, 6000ppm, or any value between any two of the above.

[0072] In this embodiment, controlling the magnesium content in the first mixture allows for more precise control of the magnesium doping amount during subsequent reactions, coordinating the ratio of magnesium, iron, and phosphorus to obtain magnesium-doped anhydrous iron phosphate with excellent performance. Specifically, the magnesium content in the phosphate rock flotation solution can be detected first, and then the amount of phosphate rock flotation solution added can be controlled according to the magnesium ion content in the first mixture. It can be understood that in actual preparation, water can be added when mixing the refined phosphate with the phosphate rock flotation solution and oxidant to further precisely control the magnesium content in the phosphate rock flotation solution.

[0073] Furthermore, in some embodiments, in step S20, the concentration of phosphorus in the first mixture is 1 mol / L to 2.5 mol / L; for example, it can be 1 mol / L, 1.2 mol / L, 1.4 mol / L, 1.5 mol / L, 1.7 mol / L, 1.9 mol / L, 2 mol / L, 2.1 mol / L, 2.3 mol / L, 2.5 mol / L, or any value between any two of the above.

[0074] In this embodiment, controlling the phosphorus content in the first mixture allows for more precise control of the magnesium, iron, and phosphorus ratio during subsequent reactions, resulting in magnesium-doped anhydrous iron phosphate with excellent performance. It is understood that in actual preparation, the phosphorus content can be adjusted by regulating the ratio of refined phosphate to phosphate rock flotation solution. Furthermore, the phosphorus content can be further precisely controlled by adding water when mixing the refined phosphate with the phosphate rock flotation solution and oxidant.

[0075] Furthermore, in some embodiments, in step S20, the oxidant may include, but is not limited to, at least one of hydrogen peroxide, sodium peroxide, potassium peroxide, sodium hypochlorite, potassium hypochlorite, sodium persulfate, and potassium persulfate.

[0076] In this embodiment, the above-mentioned compound is inexpensive and readily available, and has good oxidizing properties, capable of oxidizing ferrous ions to ferric ions. Furthermore, hydrogen peroxide is preferably used as the oxidant, as it has strong oxidizing properties and does not easily introduce impurities.

[0077] Furthermore, in some embodiments, in step S20, the molar ratio of the oxidant to the ferrous salt is 1.1 to 1.5 times the stoichiometric ratio of the oxidant to the ferrous salt; for example, 1.1 times, 1.2 times, 1.3 times, 1.4 times, 1.5 times, and any value between any two of the above.

[0078] In this embodiment, controlling the amount of oxidant added within the aforementioned range can promote the full reaction between ferrous ions and the oxidant. This promotes the complete oxidation of ferrous ions, improves product purity and electrochemical performance, while avoiding side reactions or waste due to excessive oxidant addition. It can be understood that the stoichiometric ratio of the oxidant to the ferrous salt refers to the molar ratio of the two substances in the reaction equation. For example, when the oxidant is hydrogen peroxide and the ferrous salt is ferrous sulfate, the reaction equation for hydrogen peroxide and ferrous sulfate can be: H₂O₂ + 2Fe 2+ +2H + =2Fe 3+ +2H2O; The stoichiometric ratio of hydrogen peroxide to ferrous ions is 0.5. Therefore, in this embodiment, the molar ratio of hydrogen peroxide to ferrous ions is (0.55~0.75):1.

[0079] Furthermore, in some embodiments, in step S30, the molar ratio of iron in the ferrous salt to phosphorus in the first mixture is 1:(1.05~1.20); for example, it can be 1:1.05, 1:1.07, 1:1.09, 1:1.10, 1:1.13, 1:1.15, 1:1.18, 1:1.20, or any value between any two of the above.

[0080] In this embodiment, controlling the addition amount of ferrous salt and the first mixture to be within the above-mentioned ratio range is beneficial to promoting the full reaction of ferrous salt to generate magnesium-doped ferric phosphate dihydrate.

[0081] Furthermore, in some embodiments, in step S30, the complexing agent may include, but is not limited to, at least one of oxalic acid, oxalate, and fluoride; wherein, oxalate may be any soluble oxalate, including but not limited to at least one of ammonium oxalate, lithium oxalate, and magnesium oxalate, and fluoride may be any soluble fluoride, including but not limited to at least one of ammonium fluoride, lithium fluoride, and magnesium fluoride.

[0082] In this embodiment, oxalic acid, oxalate, or fluoride is used as a complexing agent to form a stable complex ([Fe(C2O4)3)) with ferric ions. 3- Or [FeF6] 3- ), concealing trivalent iron.

[0083] Furthermore, in some embodiments, in step S30, the molar ratio of the complexing agent to the ferrous ions in the ferrous salt is 0.01 to 0.014 times the stoichiometric ratio of the complexing agent to the ferric ions. Even further, in some embodiments, in step S30, when the complexing agent is selected from oxalic acid or oxalate, the molar ratio of the complexing agent to the ferrous salt is (0.03 to 0.04):1; when the complexing agent is selected from fluorides, the molar ratio of the complexing agent to the ferrous salt is (0.06 to 0.08):1.

[0084] In this embodiment, in the reaction system of step S30, when the ferrous salt and the oxidant are mixed, the ferrous ions are converted into ferric ions, and the ferric ions react with the complexing agent to form a complex ([Fe(C2O4)3)). 3- Or [FeF6] 3- The stoichiometric ratio of the complexing agent to ferric ions was 3 ([Fe(C2O4)3]). 3- ) or 6 ([FeF6] 3- Since the molar amount of ferric ions is equal to that of ferrous ions, the actual feeding amount can be controlled according to the following ratio: if the complexing agent is selected from oxalic acid or oxalate, the molar ratio of the complexing agent to the ferrous salt can be controlled at (0.03~0.04):1; if the complexing agent is selected from fluoride, the molar ratio of the complexing agent to the ferrous salt can be controlled at (0.06~0.08):1. Controlling the amount of complexing agent added within the above range can ensure that the free ferric ions are fully complexed, thereby improving the sedimentation rate of magnesium ions.

[0085] Furthermore, in some embodiments, in step S30, the ferrous salt may include, but is not limited to, at least one of ferrous sulfate, ferrous chloride, or ferrous nitrate. Preferably, the ferrous salt may be ferrous sulfate, and further, the ferrous sulfate may be a byproduct of titanium dioxide production.

[0086] In this embodiment, a byproduct generated during the production of titanium dioxide is used. The ferrous sulfate product after refining and removing impurities from the byproduct is taken as a ferrous salt, thus developing a new use for the titanium dioxide byproduct and helping to save costs. In some embodiments, the ferrous sulfate content in the titanium dioxide byproduct is approximately 80-90% by mass.

[0087] For further details, please refer to Figure 2In some embodiments, step S30 can be performed as follows: S31, ferrous salt is mixed with water to obtain a ferrous solution; S32, under stirring conditions, the first mixture is added dropwise to the ferrous solution within a time range of 10 min to 60 min, then a complexing agent is added, and stirring is continued for a first time to obtain a second mixture; S33, the second mixture is heated to 80 to 95°C and reacted for a second time, then kept at that temperature for a third time to obtain a third mixture; S34, the third mixture is subjected to solid-liquid separation to obtain a wet material containing magnesium-doped ferric phosphate dihydrate.

[0088] In this embodiment, adding the first mixture to the ferrous solution dropwise within a limited time helps to avoid local overheating or violent reactions caused by rapid reactions, ensuring a stable reaction and reducing byproduct impurities. Adding the complexing agent after mixing the first mixture with the ferrous solution not only ensures that ferric iron is fully masked and promotes magnesium ion precipitation, but also avoids premature addition that would cause some unoxidized ferrous ions to precipitate, affecting product purity and wasting iron.

[0089] Furthermore, in some embodiments, in step S31, the concentration of ferrous ions in the ferrous solution is 0.5 mol / L to 1.5 mol / L; for example, it can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any value between any two of the above.

[0090] Furthermore, in some embodiments, in step S32, the first time is 10 min to 30 min; for example, it can be 10 min, 15 min, 20 min, 25 min, 30 min, or any value between any two of the above.

[0091] In this embodiment, controlling the time within the above range can promote sufficient contact between the complexing agent and free ferric ions, ensuring that the ferric ions are fully complexed.

[0092] Furthermore, in some embodiments, in step S33, the second time is 30 min to 90 min; for example, it can be 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, or any value between any two of the above. The third time is 30 min to 90 min; for example, it can be 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, or any value between any two of the above.

[0093] In this embodiment, the second mixture is heated to 80-95°C and then the reaction is controlled within a second time range, which allows the amorphous iron phosphate to undergo sufficient crystal transformation to form iron phosphate dihydrate. At the same time, magnesium is incorporated into the crystal structure to form magnesium-doped iron phosphate dihydrate. Furthermore, after ripening within the above-mentioned third time range, the grains grow and the size gradually increases, resulting in a product with suitable specific surface area and particle size.

[0094] Furthermore, in some embodiments, in step S40, the drying temperature is 90℃~110℃, for example, it can be 90℃, 95℃, 100℃, 105℃, 110℃ and any two of the above values; the drying time is 8h~12h, for example, it can be 8h, 9h, 10h, 11h, 12h and any two of the above values.

[0095] In this embodiment, controlling the drying temperature and time within the above range allows the wet material to be fully dried, free water to be removed, and magnesium-doped iron phosphate dihydrate to be obtained.

[0096] Furthermore, in some embodiments, in step S40, the sintering temperature is 550℃~650℃, for example, it can be 550℃, 570℃, 580℃, 590℃, 600℃, 610℃, 630℃, 650℃, or any value between any two of the above; the sintering time is 1h~3h, for example, it can be 1h, 1.5h, 2h, 2.5h, 3h, or any value between any two of the above.

[0097] In this embodiment, controlling the sintering temperature and time within the above range allows for the complete removal of water of crystallization from the crystal, resulting in anhydrous products. Simultaneously, high temperature promotes crystal structure reconstruction, making its internal structure more ordered, regulating the crystal size and specific surface area, and improving the electrochemical performance of the material.

[0098] Secondly, this application provides a magnesium-doped anhydrous iron phosphate, which is prepared by the preparation method described above.

[0099] In the technical solution of this application embodiment, magnesium-doped anhydrous iron phosphate can be prepared by the above preparation method. This magnesium-doped anhydrous iron phosphate has a suitable Fe / P ratio, good phase purity, suitable specific surface area, and good electronic conductivity, making it suitable as a precursor for lithium iron phosphate materials. Furthermore, compared to undoped anhydrous iron phosphate, the magnesium-doped iron phosphate proposed in this application, through modification of the iron phosphate crystal structure by incorporating magnesium, further improves the conductivity of the material and enhances the ion diffusion rate. When using the magnesium-doped anhydrous iron phosphate proposed in this application as a precursor, it helps to improve the electrochemical performance of lithium iron phosphate materials, resulting in batteries with higher specific capacity and rate performance. Simultaneously, it also helps to suppress the volume expansion of the cathode material during charge and discharge, extending the battery's cycle performance.

[0100] Furthermore, in some embodiments, the magnesium doping concentration in the magnesium-doped anhydrous ferric phosphate is 500 ppm to 3000 ppm; for example, it can be 500 ppm, 600 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, or any value between two of the above. In other embodiments, the chemical formula of the magnesium-doped anhydrous ferric phosphate obtained in this application can be Fe. x Mg 1-x PO4, where 0.98 ≤ x < 1.

[0101] In this embodiment, controlling the magnesium doping concentration within the above range can improve the material's conductivity and ion diffusion rate while avoiding the impact of excessive magnesium doping on the specific capacity.

[0102] Furthermore, in some embodiments, the D50 particle size of the magnesium-doped anhydrous iron phosphate is 2μm to 25μm; for example, it can be 2μm, 3μm, 5μm, 8μm, 10μm, 13μm, 15μm, 18μm, 20μm, 23μm, 25μm, or any value between any two of the above.

[0103] In this embodiment, controlling the size of the material within the above-mentioned range helps to shorten the solid-phase diffusion path of lithium ions in the cathode material, thereby improving the rate performance, cycle life and other electrochemical performance of the cathode material.

[0104] Furthermore, in some embodiments, the specific surface area of ​​the magnesium-doped anhydrous iron phosphate is 5 m². 2 / g~10m 2 / g; for example, it can be 5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 2 / g and any value between any two of the above values.

[0105] In this embodiment, controlling the BET specific surface area to meet the above range is beneficial to providing sufficient active sites, thereby further improving the electrochemical performance of the cathode material.

[0106] Furthermore, in some embodiments, the molar ratio of iron to phosphorus in the magnesium-doped anhydrous iron phosphate is 0.965 to 0.975; for example, it can be 0.965, 0.966, 0.967, 0.968, 0.969, 0.970, 0.971, 0.972, 0.973, 0.974, 0.975, or any value between any two of the above.

[0107] In this embodiment, controlling the molar ratio of Fe to P in the material to meet the above-mentioned range helps to balance and improve the purity and stability of the material, which is beneficial for preparing cathode materials with excellent performance.

[0108] Thirdly, embodiments of this application provide a cathode material, which includes lithium iron phosphate or doped lithium iron phosphate, and the cathode material is prepared using magnesium-doped anhydrous iron phosphate as a precursor as described above.

[0109] In the technical solution of this application embodiment, the positive electrode material is prepared by using the above-mentioned magnesium-doped anhydrous iron phosphate as a precursor, and thus has good charge specific capacity, discharge specific capacity, rate performance and cycle life.

[0110] Fourthly, embodiments of this application provide a positive electrode sheet, which includes the aforementioned positive electrode material.

[0111] In the technical solution of this application embodiment, the positive electrode sheet contains the above-mentioned positive electrode material, and thus has good charging specific capacity, discharging specific capacity, rate performance and cycle life.

[0112] Fifthly, embodiments of this application provide a secondary battery, which includes the aforementioned positive electrode plate.

[0113] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and thus has good charging specific capacity, discharging specific capacity, rate performance and cycle life.

[0114] Sixthly, 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. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; spacecraft can include airplanes, rockets, space shuttles, and spacecraft.

[0115] 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.

[0116] I. Preparation Method In the following examples, the composition of the wet-process phosphoric acid is shown in Table 1-1, the composition of the phosphate rock flotation solution is shown in Table 1-2, and the composition of the ferrous sulfate product after purification and impurity removal from titanium dioxide by-products is shown in Table 1-3.

[0117] Table 1-1 Composition of wet-process phosphoric acid

[0118] Table 1-2 Composition of Phosphate Rock Flotation Solution (Unit: ppm)

[0119] Table 1-3 Composition of ferrous sulfate solution prepared from titanium dioxide by-products after purification and impurity removal.

[0120] Example 1 (1) Take the refined and impurity-removed ferrous sulfate by-product of titanium dioxide, add pure water to prepare a ferrous sulfate solution with a ferrous divalent molar concentration of 1.0 mol / L, and set it aside.

[0121] (2) Take wet phosphoric acid and dilute it with pure water to form a dilute phosphoric acid solution with a phosphorus mass fraction of 8%. Under stirring, add ammonia water dropwise to the dilute phosphoric acid solution to adjust the pH value of the system to 4.2 to obtain the conditioning solution. Heat the conditioning solution to 90°C and react for 2 hours. Then filter to obtain the refined phosphate after aluminum removal.

[0122] (3) Take refined phosphate, add pure water, phosphate rock flotation solution and hydrogen peroxide to prepare a first mixture with a phosphorus molar concentration of 2.0 mol / L and a magnesium ion content of 5500 ppm, for later use. The amount of hydrogen peroxide and refined phosphate added is based on the ferrous ion content in the ferrous sulfate solution, satisfying the following: the molar ratio of hydrogen peroxide to ferrous ions is 0.6:1 (equivalent to 1.2 times the stoichiometric ratio), and the molar ratio of phosphorus to ferrous ions in the first mixture is 1.1:1.

[0123] (4) Add the ferrous sulfate solution to the reactor, turn on the reactor and stir. Then, use a peristaltic pump to add the first mixture dropwise into the reactor over 45 minutes. Then, add an aqueous solution of oxalic acid (wherein the molar ratio of oxalic acid to ferrous sulfate is 0.035:1) to the reactor. After the addition is complete, continue stirring for 20 minutes to obtain the second mixture. Subsequently, heat the second mixture to 90°C and react for 40 minutes. The reaction slurry changes from light white to light pink. Continue to keep it at this temperature for 45 minutes, and then filter to obtain a wet material containing magnesium-doped ferric phosphate dihydrate.

[0124] (5) The wet material was dried at 100°C for 10 hours, and then sintered at 600°C for 2 hours to obtain magnesium-doped anhydrous iron phosphate.

[0125] Example 2 (1) Take the refined and impurity-removed ferrous sulfate by-product of titanium dioxide, add pure water to prepare a ferrous sulfate solution with a ferrous divalent molar concentration of 1.0 mol / L, and set it aside.

[0126] (2) Take wet phosphoric acid and dilute it with pure water to form a dilute phosphoric acid solution with a phosphorus mass fraction of 5%. Under stirring, add ammonia water dropwise to the dilute phosphoric acid solution to adjust the pH value of the system to 4.5 to obtain the conditioning solution. Heat the conditioning solution to 85°C and react for 4 hours. Then filter to obtain the refined phosphate after aluminum removal.

[0127] (3) Take refined phosphate, add pure water, phosphate rock flotation solution and hydrogen peroxide to prepare a first mixture with a phosphorus molar concentration of 1.0 mol / L and a magnesium ion content of 5000 ppm, for later use. The amount of hydrogen peroxide and refined phosphate added is based on the ferrous ion content in the ferrous sulfate solution, satisfying the following: the molar ratio of hydrogen peroxide to ferrous ions is 0.55:1, and the molar ratio of phosphorus to ferrous ions in the first mixture is 1.05:1.

[0128] (4) Add ferrous sulfate solution to the reactor, start stirring, and then use a peristaltic pump to add the first mixture dropwise into the reactor over 45 minutes. Then add an aqueous solution of oxalic acid (wherein the molar ratio of oxalic acid to ferrous sulfate is 0.035:1) to the reactor. After the addition is complete, continue stirring for 10 minutes to obtain the second mixture. Subsequently, heat the second mixture to 80°C and react for 90 minutes. The reaction slurry changes from light white to light pink. Continue to keep it at this temperature for another 90 minutes, and then filter to obtain a wet material containing magnesium-doped ferric phosphate dihydrate.

[0129] (5) The wet material was dried at 90°C for 12 hours, and then sintered at 550°C for 3 hours to obtain magnesium-doped anhydrous iron phosphate.

[0130] Example 3 (1) Take the refined and impurity-removed ferrous sulfate by-product of titanium dioxide, add pure water to prepare a ferrous sulfate solution with a ferrous divalent molar concentration of 1.0 mol / L, and set it aside.

[0131] (2) Take wet phosphoric acid and dilute it with pure water to form a dilute phosphoric acid solution with a phosphorus mass fraction of 10%. Under stirring, add ammonia water dropwise to the dilute phosphoric acid solution to adjust the pH value of the system to 4.0 and obtain the conditioning solution. Heat the conditioning solution to 95°C and react for 1 hour. Then filter to obtain the refined phosphate after aluminum removal.

[0132] (3) Take refined phosphate, add pure water, phosphate rock flotation solution and hydrogen peroxide to prepare a first mixture with a phosphorus molar concentration of 2.5 mol / L and a magnesium ion content of 6000 ppm, for later use. The amount of hydrogen peroxide and refined phosphate added is based on the ferrous ion content in the ferrous sulfate solution, satisfying the following: the molar ratio of hydrogen peroxide to ferrous ions is 0.75:1, and the molar ratio of phosphorus to ferrous ions in the first mixture is 1.2:1.

[0133] (4) Add ferrous sulfate solution to the reactor, start stirring, and then use a peristaltic pump to add the first mixture dropwise into the reactor over 45 minutes. Then add an aqueous solution of oxalic acid (wherein the molar ratio of oxalic acid to ferrous sulfate is 0.035:1) to the reactor. After the addition is complete, continue stirring for 30 minutes to obtain the second mixture. Subsequently, heat the second mixture to 95°C and react for 30 minutes. The reaction slurry changes from light white to light pink. Continue to keep it at this temperature for another 30 minutes, and then filter to obtain a wet material containing magnesium-doped ferric phosphate dihydrate.

[0134] (5) The wet material was dried at 110°C for 8 hours, and then sintered at 650°C for 1 hour to obtain magnesium-doped anhydrous iron phosphate.

[0135] Example 4 The scheme in this embodiment is basically the same as that in embodiment 1, except that the complexing agent oxalic acid is replaced with ammonium fluoride, and the molar ratio of ammonium fluoride to ferrous sulfate is 0.07:1.

[0136] Example 5 The scheme in this embodiment is basically the same as that in embodiment 1, except that the molar ratio of oxalic acid to ferrous sulfate is 0.02:1 in this embodiment.

[0137] Example 6 The scheme in this embodiment is basically the same as that in embodiment 1, except that the molar ratio of oxalic acid to ferrous sulfate is 0.03:1.

[0138] Example 7 The scheme in this embodiment is basically the same as that in embodiment 1, except that the molar ratio of oxalic acid to ferrous sulfate is 0.04:1 in this embodiment.

[0139] Example 8 The scheme in this embodiment is basically the same as that in embodiment 1, except that the molar ratio of oxalic acid to ferrous sulfate is 0.05:1 in this embodiment.

[0140] Example 9 The scheme of this embodiment is basically the same as that of embodiment 1. The only difference is that in this embodiment (4), the dripping time of the first mixture is changed to 5 minutes when the peristaltic pump is used to add the mixture.

[0141] Example 10 The scheme of this embodiment is basically the same as that of embodiment 1. The only difference is that in this embodiment (4), the dripping time of the first mixture is changed to 10 min when the peristaltic pump is used.

[0142] Example 11 The scheme of this embodiment is basically the same as that of embodiment 1. The only difference is that in this embodiment (4), the dripping time of the first mixture is changed to 60 min when the peristaltic pump is used.

[0143] Example 12 The scheme of this embodiment is basically the same as that of embodiment 1. The only difference is that in this embodiment (4), the dripping time of the first mixture is changed to 65 min when the peristaltic pump is used.

[0144] Comparative Example 1 This comparative example is basically the same as Example 1, except that the complexing agent oxalic acid is not added in this comparative example.

[0145] Comparative Example 2 This comparative example is basically the same as Example 1, except that no phosphate rock flotation solution is added in this comparative example.

[0146] Comparative Example 3 The comparative example scheme is basically the same as that of Example 1. The only difference is that in this comparative example (4), the complexing agent is first added to the reaction vessel and mixed with the ferrous sulfate solution, and then the first mixture is added dropwise.

[0147] II. Testing Methods 1. XRD test: The crystal form of the products obtained in Example 1 and Comparative Example 1 was characterized using an X-ray diffractometer (SmartLab SE) manufactured by Rigaku Corporation, Japan. The results are shown in the figure. Figure 4 and Figure 5 In the figure, the horizontal axis Two-Theta represents the diffraction angle 2θ, and the vertical axis intensity represents the diffraction intensity.

[0148] 2. SEM testing: The morphology of the magnesium-doped anhydrous iron phosphate prepared in Example 1 was characterized using a field emission scanning electron microscope (Sigma 500) manufactured by Zeiss AG, Germany. The results are shown in the figure. Figure 6 .

[0149] 3. Elemental content: Inductively coupled plasma optical emission spectrometer (ICP-OES) was used. The content of each element in the iron phosphate material was tested by OES, and the results are shown in Tables 2 and 3.

[0150] 4. BET specific surface area: The BET method was used for gas adsorption, and the results are shown in Table 3.

[0151] 5. D50 particle size: The particle size was measured using a laser particle size analyzer, and the results are shown in Table 3.

[0152] III. Analysis of Test Results for Each Embodiment and Comparative Example Table 2. Impurity element content (unit: ppm)

[0153] Table 3 Product Characteristic Testing Table

[0154] Please see Figure 4 and Figure 5 As can be seen, the products obtained in Example 1 and Comparative Example 1 are both pure-phase structures with pure phases and no impurity peaks, indicating that the products obtained by the method of this application have few impurities and good magnesium doping effect. Please refer to Figure 6 ,pass Figure 6It can be seen that the magnesium-doped anhydrous iron phosphate prepared in the embodiments of this application has a plate-like structure, and no heterostructure was found. Figure 4 The data proves that magnesium ions enter the iron phosphate lattice, resulting in a good doping effect.

[0155] As shown in Tables 2 and 3, the impurity element content in the products obtained in each embodiment is relatively low. Furthermore, in the magnesium-doped anhydrous iron phosphate obtained in each embodiment, the Fe / P ratio is controlled within the range of 0.965 to 0.975, the magnesium content is controlled within the range of 500 ppm to 3000 ppm, the D50 particle size is controlled within the range of 2 μm to 25 μm, and the BET particle size is controlled within the range of 5 μm. 2 / g~10m 2 / g, which meets the requirements of HG / T4701-2021 standard for iron phosphate materials for batteries, and is suitable for use as a precursor for lithium iron phosphate cathode materials.

[0156] Furthermore, comparing the data of Example 1 and Comparative Examples 1 and 2 in Table 3, it can be seen that the product of Example 1 has an extremely high magnesium content, which is much higher than that of Comparative Example 1 (91.95 ppm) which did not use a complexing agent to mask ferric ions. At the same time, although the magnesium content in Comparative Example 2 (120.69 ppm) which did not have magnesium added to the phosphate rock flotation solution is higher than that of Comparative Example 1, it is much lower than that of Example 1. This indicates that the scheme of the present invention can effectively utilize the magnesium element in wet phosphoric acid, promote the sedimentation of magnesium ions, and effectively utilize the magnesium element in the phosphate rock flotation solution to supplement magnesium, significantly improving the magnesium doping efficiency and effectively improving the utilization efficiency of phosphate rock resources.

[0157] Furthermore, comparing Examples 1 and 5 to 8, it can be seen that as the amount of complexing agent gradually increases, the sedimentation rate of magnesium ions gradually increases. However, when the molar ratio exceeds 0.04, the increase in magnesium ion content in the product is no longer significant. This indicates that controlling the molar amount of complexing agent at 0.01 to 0.014 times the stoichiometric ratio (that is, controlling the molar amount of oxalic acid at 0.03 to 0.04 times that of ferrous salt) helps to ensure that free ferric ions are fully complexed, thereby increasing the sedimentation rate of magnesium ions while minimizing the amount of complexing agent used and improving economic efficiency.

[0158] Comparing Example 1 and Comparative Example 3, it can be seen that the iron content of Comparative Example 3 is too high, with the Fe / P ratio exceeding 0.975, and the Mg content is extremely low. This indicates that adding the complexing agent after mixing the first mixture with the ferrous solution not only ensures that the ferric iron is fully masked and promotes the precipitation of magnesium ions, but also avoids the precipitation of some unoxidized ferrous ions due to premature addition, thus avoiding the waste of iron. Conversely, when the complexing agent is added in advance, the complexing agent (oxalate) will react with the ferrous iron to form ferrous oxalate precipitate, causing the iron content in the product to increase, while affecting the complexing effect and causing the magnesium content in the product to decrease.

[0159] 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 magnesium-doped anhydrous iron phosphate, characterized in that, Includes the following steps: Aluminum removal treatment is performed on wet-process phosphoric acid to obtain refined phosphate; The refined phosphate is mixed with phosphate rock flotation solution and oxidant to obtain a first mixture; The ferrous salt, complexing agent, and the first mixture are mixed and then reacted to obtain a wet material containing magnesium-doped ferric phosphate dihydrate. The wet material was sequentially dried and sintered to obtain magnesium-doped anhydrous iron phosphate. The phosphate rock flotation solution contains magnesium.

2. The preparation method according to claim 1, characterized in that, The steps of mixing ferrous salt, complexing agent, and the first mixture, and then reacting them to obtain a wet material containing magnesium-doped ferric phosphate dihydrate include: mixing ferrous salt with water to obtain a ferrous solution; adding the first mixture dropwise to the ferrous solution over a time range of 10 min to 60 min under stirring conditions, then adding the complexing agent, and continuing stirring for a first time to obtain a second mixture; heating the second mixture to 80-95°C and reacting for a second time, then maintaining the temperature for a third time to obtain a third mixture; performing solid-liquid separation on the third mixture to obtain a wet material containing magnesium-doped ferric phosphate dihydrate; and / or, The magnesium ion content in the first mixture is 5000 ppm to 6000 ppm; and / or, In the first mixture, the concentration of phosphorus is 1 mol / L to 2.5 mol / L; and / or, The complexing agent includes at least one of oxalic acid, oxalate, and fluoride.

3. The preparation method according to claim 2, characterized in that, When the complexing agent is selected from oxalic acid or oxalate, the molar ratio of the complexing agent to the ferrous salt is (0.03~0.04):1; when the complexing agent is selected from fluoride, the molar ratio of the complexing agent to the ferrous salt is (0.06~0.08):1; and / or, The first time is 10 min to 30 min; and / or, The second time is 30 min to 90 min; and / or, The third time is 30 min to 90 min.

4. The preparation method according to claim 1, characterized in that, The steps for removing aluminum from wet-process phosphoric acid to obtain refined phosphate include: diluting the wet-process phosphoric acid to obtain a dilute phosphoric acid solution with a phosphorus content of 5% to 10% by mass; adjusting the pH of the dilute phosphoric acid solution to 4.0 to 4.5 to obtain a conditioning solution; heating the conditioning solution to 85℃ to 95℃ and reacting for 1 to 4 hours, followed by solid-liquid separation to obtain refined phosphate; and / or, The drying temperature is 90℃~110℃, and the drying time is 8h~12h; and / or, The sintering temperature is 550℃~650℃, and the sintering time is 1h~3h.

5. The preparation method according to claim 1, characterized in that, The oxidant includes at least one selected from hydrogen peroxide, sodium peroxide, potassium peroxide, sodium hypochlorite, potassium hypochlorite, sodium persulfate, and potassium persulfate; and / or, The molar ratio of the oxidant to the ferrous salt is 1.1 to 1.5 times the stoichiometric ratio of the oxidant to the ferrous salt; and / or, The ferrous salt includes at least one of ferrous sulfate, ferrous chloride, or ferrous nitrate, wherein the ferrous sulfate is a byproduct of titanium dioxide production; and / or, The molar ratio of iron in the ferrous salt to phosphorus in the first mixture is 1:(1.05~1.20).

6. A magnesium-doped anhydrous iron phosphate, characterized in that, The magnesium-doped anhydrous iron phosphate is prepared by the preparation method according to any one of claims 1 to 5.

7. The magnesium-doped anhydrous iron phosphate according to claim 6, characterized in that, In the magnesium-doped anhydrous iron phosphate, the magnesium doping concentration is 500 ppm to 3000 ppm; and / or, The D50 particle size of the magnesium-doped anhydrous iron phosphate is 2 μm to 25 μm; and / or, The specific surface area of ​​the magnesium-doped anhydrous iron phosphate is 5 m². 2 / g~10m 2 / g; and / or, In the magnesium-doped anhydrous iron phosphate, the molar ratio of iron to phosphorus is 0.965 to 0.

975.

8. A positive electrode material, characterized in that, The cathode material includes lithium iron phosphate or doped lithium iron phosphate, and the cathode material is prepared using magnesium-doped anhydrous iron phosphate as a precursor as described in claim 6 or 7.

9. A positive electrode sheet, characterized in that, The positive electrode sheet includes the positive electrode material as described in claim 8.

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

Images