Preparation method of manganese iron phosphate precursor, precursor and positive electrode material

Abstract

The application discloses a preparation method of a manganese iron phosphate precursor, the precursor and a positive electrode material. The preparation method comprises the following steps: dissolving a manganese source solid in an inorganic acid to obtain a first solution; reacting an iron source solid with a first phosphorus source solution, and then filtering and washing to obtain a phosphorus iron filter cake; mixing the filter cake with the first solution and a second phosphorus source solution to form a manganese iron phosphorus homogeneous phase mixed solution; adding an oxidizing agent and a pH regulator for co-precipitation reaction, and then drying and roasting to obtain the precursor. Through the step-by-step process of 'firstly precipitating iron, then mixing manganese, and then co-precipitation', the atomic-level uniform mixing of Mn and Fe elements is realized, and the hydrolysis of iron ions and element segregation are avoided. The obtained precursor is a flaky secondary particle, the amount-of-substance ratio of manganese elements to iron elements can be controlled within 0.55:1-1.85:1, and the compaction density is greater than or equal to 2.18 g / cm3. The precursor and a lithium source can be sintered to obtain a high-performance manganese iron lithium phosphate positive electrode material, and the process is compatible with the existing iron phosphate production line, and is suitable for industrial popularization.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery manufacturing technology, specifically to a method for preparing a manganese iron phosphate precursor, the precursor, and a cathode material. Background Technology

[0002] The large-scale use of traditional fossil fuels has not only led to the depletion of non-renewable resources but also exacerbated environmental pollution. Therefore, promoting energy structure transformation has become a crucial objective. Developing high-energy, high-safety, high-efficiency, and low-cost materials and processes is key to achieving this goal.

[0003] Lithium manganese iron phosphate (LMFP) is an important cathode material for lithium-ion batteries. Due to its environmental friendliness, excellent safety, high energy density, low self-discharge rate, high operating voltage, long cycle life, and low cost, it has become a research hotspot in the field of battery materials.

[0004] Ferric manganese phosphate is a key precursor in the synthesis of lithium manganese iron phosphate, and its physicochemical properties, such as morphology, particle size, and elemental distribution, directly determine the performance of the final cathode material. Conventional co-precipitation methods for preparing precursors generally suffer from difficulty in controlling the product morphology, which in turn affects the material's compaction density. While existing equipment can control particle size, it often disrupts the integrity of primary particles, leading to a decline in the electrochemical performance of the battery material.

[0005] Currently disclosed methods for preparing manganese iron phosphate precursors mainly include one-step precipitation and stepwise precipitation. Chinese Patent CN202310936279.3 discloses a one-step precipitation method, which involves mixing manganese, iron, and phosphorus sources in a single step and then adjusting the pH for co-precipitation. This method has the following inherent drawbacks: 1. Mn 3+ and Fe 3+ The precipitation rate and pH of phosphates differ, and the precipitation of the two elements is asynchronous during one-step precipitation, which can easily lead to uneven element distribution; 2. Fe 3+ Hydrolysis begins at pH ≥ 2 to generate Fe(OH)3 colloid, which easily encapsulates other precipitated particles and hinders the uniform incorporation of Mn element; 3. The local supersaturation fluctuates greatly during the one-step precipitation process, making it difficult to control the morphology of the product and resulting in a wide particle size distribution.

[0006] Chinese patent CN202311641580.8 discloses a stepwise precipitation method, but it uses a sequence of first precipitating manganese and then iron, forming a core-shell structure with manganese phosphate monohydrate as the core and iron phosphate dihydrate as the shell. The interior is rich in manganese and the exterior is rich in iron, failing to achieve atomic-level uniform mixing of Mn and Fe elements. During charge and discharge, this core-shell structure may lead to stress concentration and phase separation due to the difference in composition between the interior and exterior, affecting the long-term cycling stability of the material.

[0007] Therefore, there is an urgent need in this field for a method to prepare a manganese iron phosphate precursor that can achieve atomic-level uniform mixing of Mn and Fe elements and controllable morphology. Summary of the Invention

[0008] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a manganese iron phosphate precursor, comprising the following steps: S1. Add the manganese source to the inorganic acid solution and react to obtain the first solution; S2. Mix the iron source with water to prepare an iron source solution, mix the phosphorus source with water to prepare a first phosphorus source solution, add the first phosphorus source solution and pH adjuster to the iron source solution, and react to obtain the first slurry; S3. After filtering the first slurry, the filter residue is retained, and after washing, the first filter cake is obtained. S4. Mix the phosphorus source with water to prepare a second phosphorus source solution. Add the second phosphorus source solution and the first filter cake to the first solution and react to obtain a homogeneous mixed solution of manganese, iron and phosphorus. S5. Add pH adjuster and oxidant to the homogeneous mixed solution of manganese, iron and phosphorus, react to obtain the second slurry, filter the second slurry and retain the filter residue, wash and obtain the second filter cake. S6. The second filter cake is dried and then calcined to obtain the manganese iron phosphate precursor.

[0009] Furthermore, in the homogeneous manganese-iron-phosphorus mixed solution, the ratio of the total amount of manganese and iron to the total amount of phosphorus is 1:(1-2).

[0010] Furthermore, the amount of phosphorus in the second phosphorus source, n(P2), satisfies: n(P2) = (1-2) × [n(Mn) + n(Fe)] - n(P1), where n(Mn) is the amount of manganese in the manganese source in step S1, n(Fe) is the amount of iron in the iron source in step S2, and n(P1) is the amount of phosphorus in the first phosphorus source solution in step S2.

[0011] Furthermore, the ratio of the sum of the amounts of manganese in the oxidant and manganese source and iron in the iron source is 0.5:1 to 1.5:1.

[0012] Furthermore, the manganese source includes one of manganese sulfate, manganese carbonate, manganese dioxide, manganese flakes, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate; The iron source is one of ferrous sulfate, ferrous nitrate, ferrous chloride, and ferrous oxalate; The first phosphorus source and the second phosphorus source are each one of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; The oxidant is one of hydrogen peroxide, nitric acid, and potassium permanganate; The pH adjuster is one of ammonia, sodium hydroxide, or potassium hydroxide.

[0013] Furthermore, the reaction temperature in step S1 is 20-90℃; The reaction temperature in step S2 is 20-70℃; The reaction temperature in step S4 is 20-70℃; In step S5, when adding pH adjuster and oxidant, the stirring speed is 350-600 r / min, the pH adjuster is added to adjust the pH value to 3-6, and the reaction temperature is 20-90℃. In step S6, the drying temperature is 40-200℃, the drying time is 0.5-6 hours, the calcination temperature is 300-800℃, and the calcination time is 0.5-6 hours.

[0014] This invention also provides a manganese iron phosphate precursor, obtained by the above-described preparation method, wherein the chemical formula of the manganese iron phosphate precursor is Mn. x Fe (1-x) PO4, where 0.2 ≤ x ≤ 0.8.

[0015] The D50 of the manganese iron phosphate precursor is 2-20 μm; The specific surface area of ​​the manganese iron phosphate precursor is 3-15 m². 2 / g; The molar ratio of Mn / Fe in the manganese iron phosphate precursor is 0.55:1-1.85:1; The compacted density of the manganese iron phosphate precursor is ≥2.18 g / cm³.

[0016] Preferably, the molar ratio of manganese to iron in the manganese ferric phosphate precursor is 1.45:1 to 1.55:1.

[0017] The manganese iron phosphate precursor described above, wherein the average diameter of the primary particles of the manganese iron phosphate precursor is 10-100 nm.

[0018] The present invention also provides a lithium manganese iron phosphate cathode material, which is prepared from the above-mentioned manganese iron phosphate precursor.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the manganese iron phosphate precursor prepared by this invention, Mn and Fe elements are uniformly mixed at the atomic level. The EDS surface distribution diagram intuitively proves that Mn and Fe elements are completely uniformly distributed on the particle cross section. The element surface distribution diagram of the core-shell structure precursor will show obvious gradient distribution characteristics of Mn-rich core and Fe-rich shell.

[0020] 2. In the manganese iron phosphate precursor prepared by this invention, the sample morphology is uniform and regular. The primary particles are nano-dots, and the secondary particles are formed by sintering multiple primary particles into a sheet-like structure. This sheet-like secondary particle structure has the following advantages: 1. The nanoscale thickness greatly shortens the lithium-ion transport path, improving the rate performance and cycle stability of the material; 2. The sheet-like particles are easy to arrange and densely pack during the subsequent electrode pressing process, improving the compaction density and volumetric energy density of the electrode; 3. The regular particle structure is conducive to the uniform coating of the subsequent carbon coating layer, further improving the electronic conductivity of the material.

[0021] 3. By precisely controlling the proportion of raw materials and the pH value of the reaction, the present invention stabilizes the precipitation reaction process, ensuring uniform nucleation of the product without phase separation and good product uniformity; moreover, the Mn / Fe ratio in the precursor is controllable and adjustable, making it easy to realize industrial production.

[0022] 4. Compared with the existing one-step coprecipitation method, the stepwise precipitation process of this invention, through the sequence of first precipitating iron—then mixing manganese—and then coprecipitating, ensures that Mn and Fe are atomically homogeneously mixed before coprecipitation, fundamentally guaranteeing the elemental homogeneity of the product and effectively avoiding Fe... 3+ Hydrolyzed colloids interfere with the precipitation process, resulting in products with regular morphology and narrow particle size distribution.

[0023] 5. Compared with the core-shell structure precursors prepared by the existing stepwise precipitation method, the manganese iron phosphate precursor prepared by the present invention is a single solid solution phase in which Mn and Fe are uniformly mixed at the atomic level, which avoids the problems of lattice mismatch and stress concentration at the core-shell interface and fundamentally ensures the structural stability of the material during long-term cycling.

[0024] 6. The process method described in this invention is highly compatible with existing iron phosphate production lines, and can achieve rapid product iteration and upgrading without the need to build a new dedicated production line. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall process of the present invention; Figure 2 Mn prepared in Example 1 of the present invention x Fe (1-x) SEM images of PO4·2H2O samples (after drying and before calcination); Figure 3 Mn prepared in Example 1 of the present invention x Fe (1-x) SEM image of PO4 sample (after calcination); Figure 4 Mn prepared in Example 1 of the present invention x Fe (1-x)EDS elemental distribution map of PO4 sample. Detailed Implementation

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0027] According to a first aspect of the present invention, a method for preparing a manganese iron phosphate precursor includes the following steps: S1. Add the manganese source to the inorganic acid solution and react to obtain the first solution; the inorganic acid is preferably phosphoric acid or sulfuric acid, more preferably phosphoric acid, in order to reduce the introduction of impurity anions; S2. Mix the iron source with water to prepare an iron source solution, mix the phosphorus source with water to prepare a first phosphorus source solution, add the first phosphorus source solution and pH adjuster to the iron source solution, and react to obtain the first slurry; S3. After filtering the first slurry, the filter residue is retained, and after washing, the first filter cake is obtained. S4. Mix the phosphorus source with water to prepare a second phosphorus source solution. Add the second phosphorus source solution and the first filter cake to the first solution and react to obtain a homogeneous mixed solution of manganese, iron and phosphorus. S5. Add pH adjuster and oxidant to the homogeneous mixed solution of manganese, iron and phosphorus, react to obtain the second slurry, filter the second slurry and retain the filter residue, wash and obtain the second filter cake. S6. The second filter cake is dried and then calcined to obtain the manganese iron phosphate precursor.

[0028] This invention employs a stepwise precipitation process involving first precipitating iron, then mixing manganese, and finally co-precipitating. This process differs fundamentally from existing one-step co-precipitation methods, specifically in the following ways: Part 1 (Steps S1-S2): Iron precipitation – Using the iron source solution as the base solution, a first phosphorus source solution and a pH adjuster are added to allow Fe²⁺ to preferentially react with PO₄³⁻ to form iron phosphate precipitate. The core functions of this step are: 1. To pre-fix the iron element as a phosphate precipitate, providing an anchoring point for the subsequent uniform incorporation of manganese; 2. By pre-precipitating iron, impurity ions that may be introduced from the iron source raw material can be effectively removed, improving the purity of the final product.

[0029] Part Two (Steps S3-S4): Manganese Mixing – The phosphorus-iron filter cake obtained in Step One is mixed with the manganese source solution (the first solution), and a second phosphorus source solution is added to redissolve the phosphorus-iron filter cake under acidic conditions, forming a homogeneous mixed solution in which manganese, iron, and phosphorus coexist. The core functions of this step are: 1. To achieve pre-dispersion of iron at the molecular scale by precipitating and then dissolving iron, avoiding the uneven distribution caused by the easy hydrolysis of Fe³⁺ to form colloids in the one-step coprecipitation method; 2. To achieve uniform mixing of Mn²⁺ and Fe²⁺ at the molecular level, laying the foundation for subsequent atomic-level coprecipitation.

[0030] Part 3 (Steps S5-S6): Co-precipitation—An oxidant and pH adjuster are added to the homogeneous mixed solution of manganese, iron, and phosphorus, causing Mn²⁺ and Fe²⁺ to be oxidized to trivalent valence while co-precipitating with PO₄³⁻ to form Mn. x Fe (1-x) PO4 precipitation. The core functions of this step are: 1. To co-precipitate Mn and Fe after they have achieved a molecular-level homogeneous mixture, ensuring that the two elements are evenly distributed at the atomic level in the precipitate; 2. The addition of the oxidant causes the divalent metal ions to be oxidized in situ to trivalent ions, directly generating the target product, the precursor of manganese iron phosphate, Mn. x Fe (1-x) PO4 avoids the inhomogeneity introduced by oxidation alone.

[0031] In a homogeneous mixed solution of manganese, iron, and phosphorus, the molar ratio of manganese to iron is (1-2):1. The electrochemical performance of lithium manganese iron phosphate is closely related to the Mn / Fe ratio. 2+ / Mn 3+ The redox couple provides a high voltage platform of 4.1V, while Fe 2 + / Fe 3+ The electrode provides a 3.4V platform. Higher Mn content results in higher operating voltage and energy density, theoretically increasing energy density by approximately 15%-20% compared to lithium iron phosphate. However, excessively high Mn content can lead to... 3+ The Jahn-Teller effect is also exacerbated, leading to material structural distortion, accelerated capacity decay, and reduced cycle stability. Therefore, it is necessary to control the Mn / Fe ratio range. This invention limits the molar ratio of manganese to iron to (1-2):1, that is, the Mn / Fe molar ratio range is 0.55:1-1.85:1. Below the lower limit of this range (Mn / Fe<0.55), the Mn content is too low, the material voltage plateau is close to that of lithium iron phosphate, the energy density improvement is limited, and the high voltage advantage of lithium manganese iron phosphate cannot be reflected. Above the upper limit of this range (Mn / Fe>1.85), the Mn content is too high, the Jahn-Teller effect is significant, leading to decreased structural stability and rapid capacity decay during charge-discharge cycles, affecting the service life.

[0032] In a homogeneous mixed solution of manganese, iron, and phosphorus, the ratio of the total amount of manganese and iron to the total amount of phosphorus is 1:(1-2). The chemical formula of the manganese iron phosphate precursor is Mn. x Fe (1-x)Theoretically, the ratio of the total moles of metal elements to the moles of phosphorus in PO4 should be 1:1. However, in actual co-precipitation processes, some phosphorus sources remain in the liquid phase, are lost during washing, or volatilize as gas during subsequent calcination, resulting in a relatively insufficient phosphorus content in the final product, which in turn affects the phase purity and crystallinity of the product. This invention sets the total molar ratio of manganese and iron to the total molar ratio of all phosphorus sources to 1:(1-2). Below this lower limit ((Mn+Fe):P<1:1), the phosphorus source is relatively insufficient, and metal ions cannot be completely converted into phosphate precipitates, easily leading to the introduction of metal oxide impurities into the product and reducing material purity. Above this upper limit ((Mn+Fe):P>1:2), the phosphorus source is excessive, causing raw material waste and increased costs. Furthermore, the excessive phosphorus source is difficult to completely remove during washing, and the residual phosphate will affect crystal phase formation during calcination, resulting in uneven particle size distribution of the product. This invention controls the (Mn+Fe):P ratio within the range of 1:1 to 1:2, which ensures that the metal ions are fully precipitated and converted into phosphates, while avoiding the negative effects of excessive phosphorus sources, thereby guaranteeing the high purity and uniformity of the precursor.

[0033] The amount of phosphorus in the second phosphorus source, n(P2), satisfies: n(P2) = (1-2) × [n(Mn) + n(Fe)] - n(P1), where n(Mn) is the amount of manganese source in step S1, n(Fe) is the amount of iron source in step S2, and n(P1) is the amount of phosphorus in the first phosphorus source solution in step S2.

[0034] The ratio of the sum of the molar amounts of manganese and iron in the oxidant and manganese / iron sources is 0.5:1 to 1.5:1. In the preparation method of this invention, the role of the oxidant is to oxidize Mn²⁺ and Fe²⁺ to their trivalent state (Mn³⁺ / Fe³⁺) to form stable manganese iron phosphate (Mn...). x Fe (1-x) (PO4) precursor. Theoretically, only 0.5 mol of oxidant is needed to oxidize 1 mol of metal ions from divalent to trivalent. Taking hydrogen peroxide (H2O2) as an example, each mole of H2O2 can provide 2 electrons to oxidize 2 mol of divalent metal ions. The molar ratio of oxidant to the total amount of metal ions in the manganese and iron sources is limited to 0.5:1-1.5:1. Below this lower limit (oxidant:(Mn+Fe)<0.5:1), the metal ions are not completely oxidized to trivalent, and some metal ions exist in the form of divalent phosphate, resulting in an impure product phase and the presence of divalent metal phosphate impurities in the crystal structure, reducing the voltage plateau and specific capacity of the material. Above this upper limit (oxidant:(Mn+Fe)>1.5:1), excessive strong oxidant will over-oxidize Mn²⁺ to a higher valence state of Mn. 4 ⁺ or Mn 7⁺ These high-valence manganese ions cannot exist stably in the olivine structure and will form impurity phases such as MnO2; at the same time, excessive oxidant not only increases the cost of raw materials, but also introduces additional impurity ions into the reaction system, increasing the difficulty of washing and also affecting the purity of the product. This invention controls the molar ratio of oxidant to the total amount of manganese and iron sources within the range of (0.5-1.5):1, which ensures that the metal ions are fully and appropriately oxidized to the trivalent state to form a pure phase of manganese iron phosphate product, while avoiding the impurity problems caused by excessive oxidation, thus ensuring the phase purity and electrochemical performance of the precursor.

[0035] For example, the manganese source includes one of manganese sulfate, manganese carbonate, manganese dioxide, manganese flakes, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate.

[0036] For example, the iron source is one of ferrous sulfate, ferrous nitrate, ferrous chloride, and ferrous oxalate.

[0037] For example, the first phosphorus source is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate.

[0038] For example, the second phosphorus source is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate.

[0039] For example, the oxidant is one of hydrogen peroxide, nitric acid, or potassium permanganate.

[0040] For example, the pH adjuster is one of ammonia, sodium hydroxide, or potassium hydroxide.

[0041] For example, the reaction temperature in step S2 is 20-70°C.

[0042] For example, the reaction temperature in step S4 is 20-70°C.

[0043] For example, when adding the pH adjuster and oxidant in step S5, the stirring speed is 350-600 r / min, the pH value is 3-6, and the reaction temperature is 20-90℃. pH is the most critical process parameter in the co-precipitation process, directly affecting the nucleation rate, crystal growth, elemental distribution, and product purity of the precipitate. This invention controls the pH of the precipitation reaction at 3-6. Below this lower limit (pH<3), the solution is highly acidic, the combination rate of metal ions and phosphate is too fast, the precipitation rate is high, and large-diameter, irregularly shaped precursor particles are easily formed. Furthermore, the difference in precipitation rates between Mn and Fe is amplified, leading to uneven elemental distribution. Above this upper limit (pH>6), although precipitation tends to be complete, excessively high pH can intensify the hydrolysis of metal ions, generating hydroxide precipitates instead of phosphate precipitates, introducing impurity phases; severely affecting product purity and morphology. This invention controls the pH within a moderately acidic to weakly acidic range of 3-6. Within this pH range, the phosphate precipitation rates of Mn²⁺ and Fe³⁺ are similar, ensuring synchronous and uniform precipitation of the two elements and achieving atomic-level homogeneous mixing. Simultaneously, the suitable pH value is conducive to the formation of uniformly sized and regularly morphologically regular nanoscale primary particles, which can then be sintered to form a plate-like secondary particle structure, thus balancing elemental uniformity and morphological controllability.

[0044] The present invention preferably controls the stirring speed at 350-600 r / min. Below the lower limit of this range (<350 r / min), the liquid phase mixing is insufficient, a concentration gradient exists in the reaction system, and local supersaturation is uneven, resulting in a wide particle size distribution and irregular morphology of the precipitated product, making it difficult to achieve atomic-level uniform mixing of Mn and Fe elements. Above the upper limit of this range (>600 r / min), excessive shear force will destroy the integrity of the growing primary particles, causing nanoscale primary particles (10-100 nm) to fragment; at the same time, excessively high speed will entrain a large amount of air into the slurry, which may cause premature oxidation of Fe²⁺ and interfere with the normal precipitation reaction process.

[0045] This invention controls the stirring speed within the range of 350-600 r / min, achieving thorough mixing and uniform contact and reaction of liquid phase materials, while protecting the integrity of nanoscale primary particles. This allows the primary particles to be orderly assembled into sheet-like secondary particles during subsequent sintering, ultimately obtaining a precursor material with higher compaction density.

[0046] For example, in step S6, the drying temperature is 40-200℃, the drying time is 0.5-6 hours, and the calcination temperature is 300-800℃, with a calcination time of 0.5-6 hours. This invention controls the drying temperature within the range of 40-200℃. Below this lower limit (<40℃), the drying efficiency is extremely low, free water and some water of crystallization cannot be removed within a reasonable time, resulting in low production efficiency. Furthermore, prolonged low-temperature drying may lead to slow chemical changes on the precursor surface. Above this upper limit (>200℃), the drying rate is too fast, and the stress generated by the rapid evaporation of moisture inside the particles causes primary particle breakage, affecting the orderly transformation of the crystal phase during subsequent calcination.

[0047] This invention controls the drying temperature within the range of 40-200℃, which can efficiently remove free water and water of crystallization from the precursor, while protecting the integrity of the primary particles and the chemical composition of the precursor, laying the foundation for obtaining a pure phase product through subsequent high-temperature calcination.

[0048] This invention controls the calcination temperature between 300-800℃. Below the lower limit of this range (<300℃), the water of crystallization in the precursor cannot be completely removed, and Mn... x Fe (1-x) PO4·2H2O cannot be completely converted into anhydrous Mn. x Fe (1-x) PO4 has an incomplete crystal structure and low crystallinity. Above the upper limit of this range (>800℃), primary particles grow excessively, the specific surface area decreases sharply, and the lamellar secondary particle structure is destroyed.

[0049] This invention controls the calcination temperature at 300-800℃, ensuring complete removal of crystal water from the precursor and the formation of a complete olivine crystal structure while effectively suppressing excessive growth of primary particles and preserving the favorable morphology of nanoscale plate-like secondary particles. Preferably, at a suitable calcination temperature, such as 500-700℃, a precursor material with good crystallinity and excellent electrochemical performance can be obtained.

[0050] According to a second aspect of the present invention, a manganese iron phosphate precursor is provided, obtained by the above-described preparation method, wherein the chemical formula of the manganese iron phosphate precursor is Mn. x Fe (1-x) PO4, where 0.2 ≤ x ≤ 0.8.

[0051] In some implementations, the D50 of the manganese iron phosphate precursor is 2-20 μm.

[0052] In some embodiments, the specific surface area of ​​the manganese iron phosphate precursor is 3-15 m². 2 / g.

[0053] In some embodiments, the molar ratio of Mn / Fe in the manganese iron phosphate precursor is 0.55:1 to 1.85:1.

[0054] Preferably, the molar ratio of Mn / Fe in the manganese iron phosphate precursor is 1.45:1-1.55:1. Within this preferred range, the material's crystal structure is most stable, and the Mn-O and Fe-O bond lengths in the olivine structure have the best matching degree. This ensures the full utilization of the 4.1V high voltage plateau while effectively suppressing structural distortion caused by the Jahn-Teller effect, achieving an optimal balance between voltage plateau and cycle life. Simultaneously, the Mn and Fe ion radii are similar within this ratio range (Mn²⁺ 0.083nm, Fe²⁺ 0.078nm), which is beneficial for achieving atomically uniform mixing during co-precipitation and avoiding elemental segregation. Studies have shown that a Mn / Fe ratio of 6:4 results in optimal crystal structure stability and comprehensive electrochemical performance. The preferred range of 1.45:1-1.55:1 in this invention balances structural stability and high voltage output while further optimizing energy density.

[0055] In some implementations, the compacted density of the manganese ferric phosphate precursor is ≥2.18 g / cm³.

[0056] In some embodiments, the primary particle average diameter of the manganese iron phosphate precursor is 10-100 nm.

[0057] The present invention also provides a lithium manganese iron phosphate cathode material, which is prepared by mixing any of the above-mentioned manganese iron phosphate precursors with a lithium source and a carbon coating agent and then sintering at high temperature. The specific preparation process is as follows: the manganese iron phosphate precursor, lithium source, and carbon source are uniformly ball-milled and mixed at a total molar ratio of lithium metal to manganese and iron elements of 1.02 to 1.05. The mixed powder is calcined at 700 to 820°C for 6 to 12 hours under a nitrogen / argon inert atmosphere. After natural cooling, crushing and sieving, lithium manganese iron phosphate cathode material is obtained. The lithium source is selected from lithium carbonate, lithium hydroxide, and lithium acetate; the carbon source is selected from sucrose, glucose, PEG, and citric acid.

[0058] The present invention will be further described below through specific embodiments.

[0059] Example 1 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 33g of manganese tablets and add them to 470.4g of 25% phosphoric acid solution. Dissolve the solution at 40℃ to obtain the first solution. (2) Take 200 mL of 2 mol / L ferrous sulfate solution, add 230 g of 20% ammonium dihydrogen phosphate solution, adjust pH=4.08, precipitate out, react at 45℃ for 0.5 hours, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution and add 92.6g of 85% phosphoric acid. Dissolve the filter cake at 40℃ to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 81.6g of hydrogen peroxide to the mixed solution, adjust the pH to 5.1 with ammonia, raise the temperature to 65℃, and react at a speed of 400r / min for 1 hour to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 1000 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 120°C for 2 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 550℃ for 2 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0060] Figure 2 Mn prepared in Example 1 x Fe (1-x) SEM images of PO4·2H2O (after drying and before calcination). From Figure 2 It can be seen that the primary particles of the dried precursor are nanoparticles with an average diameter of 30nm-50nm. The particles are uniform in size, well-dispersed, and without abnormal morphology. There are preliminary signs of sintering connection between the primary particles, but a complete secondary particle structure has not yet been formed.

[0061] Figure 3 Mn prepared in Example 1 x Fe (1-x) SEM image of PO4 (after calcination). From Figure 3 It can be seen that during the sintering process, the nanoparticles grow and connect along specific crystal planes under the drive of surface energy, ultimately forming a regular plate-like secondary particle structure. The secondary particles are in the form of thin plates, with a plate thickness at the scale of the primary particles and a plate planar orientation at the micrometer level.

[0062] Figure 4 The EDS elemental distribution diagrams for the manganese iron phosphate precursor prepared in Example 1 show the distribution of Mn (red), Fe (green), and P (blue) elements on the particle cross-section.

[0063] from Figure 4It can be clearly seen that the distribution of Mn and Fe on the particle cross-section is completely overlapping and uniform, with no enrichment or segregation of any element observed, proving that Mn and Fe are uniformly mixed at the atomic scale. This indicates that the manganese iron phosphate precursor sample prepared by this invention achieves uniform mixing of Mn and Fe elements at the atomic scale, forming a single solid solution phase, rather than a simple physical mixture, thus ensuring the overall consistency of the material.

[0064] Example 2 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 26.10g of anhydrous manganese sulfate, add it to 73.5g of 20% phosphoric acid solution, and dissolve it at 40℃ to obtain the first solution; (2) Measure 100 mL of 1 mol / L ferrous sulfate solution, add 76.6 g of 15% ammonium dihydrogen phosphate solution, adjust pH=5.0, precipitate out, react at 30℃ for 1 hour, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution and add about 27.5g of 85% phosphoric acid. Dissolve the filter cake at room temperature to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 30.91g of hydrogen peroxide to the mixed solution, adjust the pH to 5.5 with ammonia, heat to 75℃, and react at 450r / min for 1 hour to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 500 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 120°C for 2 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 550℃ for 2 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0065] Example 3 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 13.20g of manganese flakes and add them to 147g of 20% sulfuric acid solution. Dissolve the solution at 20℃ to obtain the first solution. (2) Measure 100 mL of 1 mol / L ferrous chloride solution, add 52.8 g of 25% diammonium hydrogen phosphate solution, adjust pH=4.5, precipitate out, react at 50℃ for 0.5 hours, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution and add 142.5g of ammonium dihydrogen phosphate solution with a mass fraction of 25%. Dissolve the filter cake at 40°C to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 30.91g of hydrogen peroxide to the mixed solution, adjust the pH to 5.0 with ammonia, heat to 90℃, and react at 450r / min for 1 hour to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 850 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 200°C for 1 hour to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 500℃ for 2.5 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0066] Example 4 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 78.5g of manganese carbonate, add it to 455.7g of 20% phosphoric acid solution, and dissolve it at 30℃ to obtain the first solution; (2) Take 380 mL of 1 mol / L ferrous oxalate solution, add 186.2 g of 15% phosphoric acid solution, adjust pH=4.0, precipitate out, react at 65℃ for 0.5 hours, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution and add 246g of diammonium hydrogen phosphate solution with a mass fraction of 20%. Dissolve the filter cake at 70°C to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 123.64g of hydrogen peroxide to the mixed solution, adjust the pH to 6.0 with potassium hydroxide solution, heat to 80℃, and react at 350r / min for 2 hours to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 400 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 150°C for 1.5 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 600℃ for 2 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0067] Example 5 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 78.5g of manganese carbonate, add it to 378g of 15% nitric acid solution, and dissolve it at 90℃ to obtain the first solution; (2) Take 400 mL of 1 mol / L ferrous nitrate solution, add 260 g of 20% sodium dihydrogen phosphate solution, adjust pH=3.8, precipitate out, react at 70℃ for 0.5 hours, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution, add 562g of 20% sodium hydrogen phosphate solution, and dissolve the filter cake at 60℃ to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 123.64g of hydrogen peroxide to the mixed solution, adjust the pH to 5.5 with potassium hydroxide solution, heat to 60℃, and react at 350r / min for 2 hours to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 400 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 150°C for 1.5 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 600℃ for 2 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0068] Example 6 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 48.4g of manganese oxalate and add it to 196g of 15% sulfuric acid solution. Dissolve the solution at 30℃ to obtain the first solution. (2) Measure 200 mL of 1 mol / L ferrous oxalate, add 142 g of 20% disodium hydrogen phosphate solution, adjust pH=5.0, precipitate out, react at 40℃ for 1 hour, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution, add 268g of sodium dihydrogen phosphate solution with a mass fraction of 20%, and dissolve the filter cake at 60℃ to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 48.61g of 65% nitric acid solution to the mixed solution, adjust the pH to 5.5 with sodium hydroxide solution, heat to 60℃, and react at 550r / min for 1 hour to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 600 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 150°C for 1.5 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 600℃ for 2 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0069] Example 7 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 66.2g of manganese dioxide, add 408g of 20% hydrochloric acid solution, and dissolve at 90℃ to obtain the first solution; (2) Measure 400 mL of 1 mol / L ferrous sulfate, add 328 g of 20% sodium phosphate solution, adjust pH to 4.2, precipitate out, react at 50℃ for 0.5 hours, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution, add 635g of sodium phosphate solution with a mass fraction of 20%, and dissolve the filter cake at 70℃ to obtain a homogeneous mixed solution of manganese, iron and phosphorus. (4) Add 126.4g of potassium permanganate solid (dissolved in an appropriate amount of deionized water before adding) to the mixed solution, adjust the pH to 3.0 with ammonia, heat to 80℃, and react at 600r / min for 2 hours to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 1000 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 180°C for 0.5 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 650℃ for 1 hour to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0070] Example 8 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 92.8g of manganese chloride, add 350g of 15% hydrochloric acid solution, and dissolve at 55℃ to obtain the first solution; (2) Take 200 mL of 2 mol / L ferrous chloride solution, add 298.2 g of 20% ammonium phosphate solution, adjust pH=5.2, precipitate out, react at 45℃ for 1 hour, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution, add 418g of phosphoric acid solution with a mass fraction of 30%, and dissolve the filter cake at 30℃ to obtain a homogeneous mixed solution of manganese, iron and phosphorus; (4) Add 86g of hydrogen peroxide to the mixed solution, adjust the pH to 4.5 with ammonia, heat to 75℃, and react at 450r / min for 1.5 hours to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 420 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 90°C for 4 hours to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 700℃ for 1 hour to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0071] Example 9 A method for preparing a manganese iron phosphate precursor, the specific steps of which are as follows: (1) Weigh 115.6g of manganese acetate, add 261g of 30% phosphoric acid solution, and dissolve at 30℃ to obtain the first solution; (2) Take 200 mL of 2 mol / L ferrous sulfate solution, add 196 g of 20% phosphoric acid solution, adjust pH=4.08, precipitate out, react at 30℃ for 1.5 hours, filter, wash and dry to obtain the first filter cake; (3) Add the first filter cake to the first solution, add 146g of phosphoric acid solution with a mass fraction of 30%, and dissolve the filter cake at 30℃ to obtain a homogeneous mixed solution of manganese, iron and phosphorus; (4) Add 81.6g of hydrogen peroxide to the mixed solution, adjust the pH to 5.2 with ammonia, heat to 45℃, and react at 500r / min for 0.5 hours to obtain the second slurry; (5) Filter the second slurry and wash the filter cake with deionized water until the conductivity of the wash water is 1080 μs / cm to obtain the second filter cake; (6) The second filter cake was dried in a forced-air drying oven at 150°C for 1 hour to obtain Mn. x Fe (1-x) PO4·2H2O; then calcined in a muffle furnace at 500℃ for 2 hours to obtain the manganese iron phosphate precursor Mn. x Fe (1-x) PO4 sample.

[0072] Example 10 The difference between Example 10 and Example 1 is that in step S5, the pH is adjusted to 3.0 and the temperature is raised to 90°C for the reaction.

[0073] Example 11 The difference between Example 11 and Example 1 is that in step S5, the pH is adjusted to 6.0 and the reaction is carried out at 20°C.

[0074] Example 12 The manganese iron phosphate precursor prepared by the method in Example 1 was used to prepare lithium manganese iron phosphate cathode material according to the lithiation sintering process.

[0075] Comparative Example 1 (Conventional One-Step Coprecipitation Method) Manganese flakes, ferrous sulfate, and phosphoric acid were mixed in the same proportions as in Example 1, and the pH was adjusted to 5.1 simultaneously. The same amount of hydrogen peroxide was added, and the mixture was reacted at 65°C and 400 r / min for 1 h. Other preparation conditions were the same as in Example 1.

[0076] Comparative Example 2 (non-step iron sinking) Instead of preparing a phosphorus-iron filter cake, manganese flakes and ferrous sulfate were directly mixed in the same proportion as in Example 1, and phosphoric acid and hydrogen peroxide were added as in Example 1. The pH was adjusted to 5.1, and the mixture was reacted at 65°C and 400 r / min for 1 h. Other conditions were the same as in Example 1.

[0077] Table 1. Mn synthesized in each embodiment and comparative example. x Fe (1-x) PO4 sample test data As shown in Table 1, the manganese iron phosphate precursor samples prepared in each embodiment of the present invention meet the various standard requirements of the battery industry for precursor materials in terms of chemical composition, particle size distribution, specific surface area, primary particle size, compaction density, and purity. Furthermore, all key indicators are significantly superior to those of Comparative Example 1 and Comparative Example 2. Specific analysis is as follows: 1. Particle Size Distribution (D50). The precursors prepared in each embodiment of this invention have a particle size of 4-8 μm, all falling within the 2-20 μm range defined by this invention. Furthermore, the variation between embodiments is small and the distribution is concentrated, indicating that the stepwise precipitation process of this invention has excellent control over the product particle size and can meet the differentiated requirements of different battery systems for precursor particle size. Comparative Examples 1 and 2 have D50 values ​​as high as 19.27 μm and 24.51 μm, respectively, with significantly larger particle sizes and uncontrollable distribution, proving that neither the one-step co-precipitation method nor the non-stepwise iron deposition method can effectively control the particle size.

[0078] 2. Specific surface area. The specific surface area of ​​the precursors prepared in each embodiment of the present invention is 8-12 m² / g, all falling within the range of 3-15 m² / g defined by the present invention. A suitable specific surface area ensures both sintering activity and reactivity, while avoiding processing difficulties and increased side reactions caused by excessively high specific surface areas. The specific surface areas of Comparative Example 1 and Comparative Example 2 are 21.16 m² / g and 22.40 m² / g, respectively, more than twice that of the embodiments. Excessively high specific surface areas mean more surface defects and lower crystallinity, which will affect the electrochemical performance of the subsequent sintered products.

[0079] 3. Chemical Composition and Mn / Fe Ratio. The Mn% in each embodiment of this invention is 21.81%-22.61%, the Fe% is 14.80%-15.25%, and the P% is 20.36%-22.05%, which highly matches the theoretical chemical formula Mn0.6Fe0.4PO4, indicating that the stoichiometry of the precursor prepared by this invention is precise and controllable. The Mn / Fe molar ratio in each embodiment is 1.45-1.52, all falling within the preferred range of 1.45:1-1.55:1 of this invention, and the relative deviation between embodiments is less than 2%, fully demonstrating that the process of this invention has good controllability of the ratio and batch reproducibility. The Mn / Fe molar ratios of Comparative Example 1 and Comparative Example 2 are 1.31 and 1.29, respectively, significantly deviating from the target value and exhibiting elemental segregation, indicating that the one-step method cannot achieve simultaneous and uniform precipitation of Mn and Fe.

[0080] 4. Primary Particle Size. The average diameter of the primary precursor particles prepared in each embodiment of this invention is 20-90 nm, all falling within the 10-100 nm range defined in this invention. Nanoscale primary particles are beneficial for shortening the diffusion path of lithium ions in the solid phase, thereby effectively improving the rate performance and cycle stability of the material. Among them, the primary particles in Example 9 are the smallest (20-40 nm), and the primary particles in Example 11 are relatively large (50-90 nm), indicating that the primary particle size can be finely controlled within a certain range by adjusting the reaction pH and temperature. The primary particle size distribution of Comparative Example 1 is extremely wide (20-200 nm), and the morphology of the product in Comparative Example 2 is irregular, proving that the one-step method cannot achieve uniform and controllable growth of primary particles. Various indicators of the precursors prepared in each embodiment of this invention (e.g., specific surface area, D) are as follows. 50 The Mn / Fe ratio and compaction density of the present invention are significantly better than those of Comparative Example 1 and Comparative Example 2, demonstrating the significant superiority of the process of the present invention.

[0081] 5. Compacted density. The compacted density of each embodiment was 2.18-2.33 g / cm³, significantly better than Comparative Example 1 (1.85 g / cm³) and Comparative Example 2 (1.72 g / cm³). Combined with SEM morphology analysis ( Figure 2.2), this result directly proves the advantages of the sheet-like secondary particle structure of the present invention—the nano-dot primary particles are orderly assembled into sheet-like secondary particles during the calcination process. The sheet-like structure is easy to be arranged in parallel and tightly packed during the electrode pressing process, thereby obtaining a higher compaction density, which is beneficial to improving the volumetric energy density of the battery.

[0082] 6. Impurity Elements. The contents of key impurity elements in each embodiment are at low levels, with K content ranging from 17-64 ppm, Al content from 2-53 ppm, Ca content from 9-50 ppm, and Na content from 11-64 ppm, all meeting the purity requirements for battery-grade precursor materials. In Comparative Example 1, the K content is as high as 104 ppm, approximately twice the average of all embodiments, indicating that K⁺ introduced by the pH adjuster in the one-step co-precipitation method is easily adsorbed and entrained by the product and is difficult to remove by washing. In Comparative Example 2, the Al content (87 ppm) and Ca content (42 ppm) are significantly higher, indicating that the co-precipitation phenomenon of impurities in the raw materials is more severe when iron precipitation is not performed stepwise. Low impurity content is beneficial for obtaining high-purity lithium manganese iron phosphate cathode material during subsequent lithiation sintering, thereby ensuring the long-term cycle stability and safety of the battery.

[0083] The existing one-step coprecipitation method, which mixes manganese, iron, and phosphorus sources in one step and then adjusts the pH for coprecipitation, has the following inherent drawbacks: 1. Mn 3+ and Fe 3+ The precipitation rate and optimal precipitation pH of phosphates differ, and the precipitation of the two elements is asynchronous during one-step precipitation, which can easily lead to uneven element distribution; 2. Fe 3+ Hydrolysis begins at pH > 2 to form Fe(OH)3 colloid, which encapsulates other precipitated particles and hinders the uniform incorporation of Mn; 3. During the one-step precipitation process, the local supersaturation fluctuates greatly, the morphology of the product is difficult to control, and the particle size distribution is wide.

[0084] In comparison, the stepwise precipitation process of this invention has the following outstanding advantages: 1. By first precipitating iron—then mixing manganese—and then co-precipitating, it ensures that Mn and Fe are uniformly mixed at the molecular level before co-precipitation, fundamentally guaranteeing the elemental uniformity of the product; 2. The stepwise operation allows for independent optimization of the reaction conditions at each step, resulting in higher process control precision and better product consistency; 3. It effectively avoids Fe... 3+ Hydrolyzed colloids interfere with the precipitation process, resulting in products with regular morphology and narrow particle size distribution.

[0085] In this invention, a first phosphorus source solution is used in step S2, and a second phosphorus source solution is added in step S4. A strategy of adding two phosphorus sources in steps is employed, with the first and second phosphorus source solutions selected independently. These solutions can be prepared using the same type of phosphorus source or different types. The core functions of this design are: 1. Adding the first phosphorus source during the first step of iron precipitation ensures sufficient precipitation of iron; 2. Adding the second phosphorus source during the second step of manganese mixing replenishes phosphorus lost during iron precipitation and washing, and ensures that the final molar ratio of manganese, iron, and phosphorus precisely matches the target stoichiometric ratio ((Mn+Fe):P=1:1-1:2); 3. Stepwise phosphorus addition avoids the uncontrolled precipitation rate and morphological deterioration caused by excessively high local concentrations when adding a large amount of phosphorus source at once.

[0086] In step S4 of this invention, the amount of the second phosphorus source solution added is such that the molar ratio of (Mn+Fe):P in the final reaction system reaches 1:1-1:2. Specifically, let the amount of manganese source in step S1 be n(Mn), the amount of iron source in step S2 be n(Fe), and the amount of phosphorus in the first phosphorus source solution in step S2 be n(P1). Then, the amount of phosphorus in the added second phosphorus source, n(P2), should satisfy: n(P2) = (1-2)×[n(Mn)+n(Fe)]-n(P1), that is, the amount of phosphorus added = (1-2)×(n(Mn)+n(Fe))-n(P1). In actual operation, the volume or mass of the second phosphorus source solution added can be determined by calculation based on the volume of the reaction system, the concentration and amount of each raw material solution. The specific amount of phosphorus added in each embodiment is detailed in the detailed implementation section.

[0087] In summary, the manganese iron phosphate precursors prepared in the various embodiments of the present invention are significantly superior to Comparative Example 1 and Comparative Example 2 in terms of key indicators such as the accuracy of chemical composition, the controllability of particle size distribution, the uniformity of primary particle size, compaction density, and purity. This fully verifies the technical advancement and industrial practical value of the stepwise precipitation process of the present invention, which involves "precipitating iron first, then mixing manganese, and then co-precipitating".

[0088] The method for preparing manganese iron phosphate precursor cathode material provided by this invention achieves atomic-level uniform mixing of Mn and Fe elements through precise control of raw material ratios and process parameters, resulting in a well-formed lamellar secondary particle structure, which significantly improves the compaction density and electrochemical performance of the precursor. Furthermore, the process method of this invention is highly compatible with existing iron phosphate production lines, eliminating the need for constructing new dedicated production lines and enabling rapid product iteration and upgrades, thus possessing broad prospects for industrial application.

Claims

1. A method for preparing a manganese iron phosphate precursor, characterized in that, Includes the following steps: S1. Add the manganese source to the inorganic acid solution and react to obtain the first solution; S2. Mix the iron source with water to prepare an iron source solution, mix the phosphorus source with water to prepare a first phosphorus source solution, add the first phosphorus source solution and pH adjuster to the iron source solution, and react to obtain the first slurry; S3. After filtering the first slurry, the filter residue is retained, and after washing, the first filter cake is obtained. S4. Mix the phosphorus source with water to prepare a second phosphorus source solution. Add the second phosphorus source solution and the first filter cake to the first solution and react to obtain a homogeneous mixed solution of manganese, iron and phosphorus. S5. Add pH adjuster and oxidant to the homogeneous mixed solution of manganese, iron and phosphorus, react to obtain the second slurry, filter the second slurry and retain the filter residue, wash and obtain the second filter cake. S6. The second filter cake is dried and then calcined to obtain the manganese iron phosphate precursor.

2. The method for preparing a manganese iron phosphate precursor according to claim 1, characterized in that, In the homogeneous manganese-iron-phosphorus mixed solution, the ratio of the total amount of manganese and iron to the total amount of phosphorus is 1:(1-2).

3. The method for preparing a manganese iron phosphate precursor according to claim 2, characterized in that, The amount of phosphorus in the second phosphorus source, n(P2), satisfies: n(P2) = (1-2) × [n(Mn) + n(Fe)] - n(P1), where n(Mn) is the amount of manganese in the manganese source in step S1, n(Fe) is the amount of iron in the iron source in step S2, and n(P1) is the amount of phosphorus in the first phosphorus source solution in step S2.

4. The method for preparing a manganese iron phosphate precursor according to claim 3, characterized in that, The ratio of the sum of the amounts of the oxidant and the manganese element in the manganese source, and the iron element in the iron source, is 0.5:1 to 1.5:

1.

5. The method for preparing a manganese iron phosphate precursor according to claim 4, characterized in that, The manganese source includes one of manganese sulfate, manganese carbonate, manganese dioxide, manganese flakes, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate. The iron source is one of ferrous sulfate, ferrous nitrate, ferrous chloride, and ferrous oxalate; The first phosphorus source and the second phosphorus source are each one of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; The oxidant is one of hydrogen peroxide, nitric acid, and potassium permanganate; The pH adjuster is one of ammonia, sodium hydroxide, or potassium hydroxide.

6. The method for preparing a manganese iron phosphate precursor according to claim 5, characterized in that, The reaction temperature in step S1 is 20-90℃; The reaction temperature in step S2 is 20-70℃; The reaction temperature in step S4 is 20-70℃; In step S5, when adding pH adjuster and oxidant, the stirring speed is 350-600 r / min, the pH adjuster is added to adjust the pH value to 3-6, and the reaction temperature is 20-90℃. In step S6, the drying temperature is 40-200℃, the drying time is 0.5-6 hours, the calcination temperature is 300-800℃, and the calcination time is 0.5-6 hours.

7. A manganese iron phosphate precursor, characterized in that, The precursor of manganese iron phosphate is obtained by the preparation method according to any one of claims 1-6, wherein the chemical formula of the precursor is Mn. x Fe (1-x) PO4, where 0.2 ≤ x ≤ 0.

8.

8. The manganese iron phosphate precursor according to claim 7, characterized in that, The D50 of the manganese iron phosphate precursor is 2-20 μm; The specific surface area of ​​the manganese iron phosphate precursor is 3-15 m². 2 / g; The molar ratio of Mn / Fe in the manganese iron phosphate precursor is 0.55:1-1.85:1; The compacted density of the manganese iron phosphate precursor is ≥2.18 g / cm³.

9. The manganese iron phosphate precursor according to claim 8, characterized in that, The molar ratio of manganese to iron in the manganese phosphate precursor is 1.45:1 to 1.55:

1.

10. A lithium manganese iron phosphate cathode material, characterized in that, It is prepared from the manganese iron phosphate precursor according to any one of claims 7.

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