Preparation method of manganese iron phosphate and application thereof

Abstract

The application discloses a preparation method of manganese iron phosphate and application thereof. Through controlling the ratio of manganese source, iron source and phosphorus source and reaction conditions in the preparation process, divalent manganese and divalent iron are first precipitated to generate ammonium manganese iron phosphate, then the pH is adjusted and the temperature is increased to remove ammonium to generate red phosphorus manganese mine type manganese iron phosphate, and finally, crystallization is carried out to transform into manganese iron phosphate. The whole preparation process does not need inert gas protection and ammonia gas waste gas emission, effectively reduces the production cost and is environment-friendly. In addition, the synthesis cycle of the application is short, the cost is low, and the application is suitable for large-scale popularization and application.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a method for preparing and applying manganese iron phosphate. Background Technology

[0002] Lithium iron manganese phosphate (LMP) cathode materials combine high energy density and high safety performance. They can be used in combination with ternary materials to reduce costs and increase efficiency, or they can replace lithium iron phosphate to achieve breakthroughs in energy density, making them a highly promising next-generation cathode material. The preparation methods of LMP can be divided into solid-phase and liquid-phase methods. The liquid-phase method produces higher quality products and has become a technology under research and development for major companies. The liquid-phase method requires the preparation of LMP precursors, but currently there are no standardized precursors in the industry. Various precursors have their own advantages, presenting significant research and development potential. LMP precursors can be classified into different types, such as iron manganese oxides, iron manganese phosphates, and iron manganese oxalates. Among these, iron manganese phosphates are the most researched, and they can be further subdivided into ferromanganese phosphate, ferrous manganese phosphate, ferromanganese ammonium phosphate, and ferromanganese pyrophosphate.

[0003] Most methods for preparing ferromanganese phosphate involve mixing iron and manganese sources, adding phosphorus and an oxidant, and then performing crystallization transformation under specific temperature and time conditions to obtain the target product, ferromanganese phosphate. For example, Chinese patent CN115535992A discloses a preparation method that uses iron powder, manganese carbonate, and phosphoric acid to mix, and then obtains a ferromanganese phosphate precursor through grinding, spraying, and calcination. This patent does not specify whether the obtained precursor is pure-phase ferromanganese phosphate. Although the ratio of manganese, iron, and phosphorus elements meets the design values, impurities may be present, affecting the compaction density and specific surface area of ​​the finished lithium manganese phosphate product. Chinese patent CN106981656B discloses a method for preparing battery-grade manganese iron phosphate. The method involves preparing a low-concentration iron-manganese mixed solution, then adding a low-concentration oxidant and phosphate salt mixed solution, heating the reaction, and then adding a high-concentration iron-manganese mixed solution. After stirring evenly, a high-concentration oxidant and phosphate salt mixed solution is added, and after heating the reaction, phosphoric acid is finally added for transformation and crystallization to obtain battery-grade manganese iron phosphate. However, this method has a complex preparation process that is difficult to control and is not suitable for industrialization.

[0004] Current methods for synthesizing manganese iron phosphate suffer from high levels of impurity elements, which leads to poor performance of lithium manganese iron phosphate when it is used to prepare lithium manganese iron phosphate. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing ferromanganese phosphate and its application. The method employs a multi-step reaction: first, ferromanganese ammonium phosphate is synthesized; then, ammonium ions are removed to form manganese iron phosphate of the red phosphorus mineral type; finally, it crystallizes to transform into ferromanganese phosphate. This multi-step reaction facilitates the removal of impurity elements and improves product purity. Preparing pure-phase ferromanganese phosphate as a precursor simplifies the subsequent production process for lithium iron manganese phosphate.

[0006] To achieve the above objectives, this invention provides ferromanganese phosphate, whose general chemical formula can be expressed as Mn1-xFexPO4·yH2O, where 0 < x ≤ 0.4 and 1.4 ≤ y < 2.0. This invention also provides a method for preparing ferromanganese phosphate, specifically comprising the following steps: (1) Dissolve the soluble ferrous iron source, the soluble ferrous manganese source and the antioxidant in pure water and adjust the pH to ≤ 2.0 to form a mixed solution; A soluble phosphorus source, a soluble ammonia source, and a surfactant are dissolved in pure water to form a base solution, and the pH of the base solution is adjusted to 6.0~6.5. (2) Continue stirring the bottom liquid, slowly add the mixed solution to the bottom liquid, and add ammonia water at the same time to maintain the pH of the bottom liquid at 6.0~6.5 until the mixed solution is completely added. Continue stirring for a period of time and then perform multiple filtrations and washings to obtain manganese iron ammonium phosphate. (3) Mix manganese iron ammonium phosphate with pure water and keep stirring. Then adjust the pH to 3.0~5.0, then heat to 80~99℃, keep warm for 1~3h, and then filter, wash and dry multiple times to obtain red phosphorus manganese ore type manganese iron phosphate. (4) Mix red phosphorus manganese ore type manganese iron phosphate with pure water and keep stirring. At the same time, add phosphoric acid and oxidant, and heat to 80~99℃ and keep warm for 1~6h. Control the pH of the reaction endpoint to be 2.0~5.0. After multiple filtrations, washing and drying, pure phase manganese iron phosphate is obtained.

[0007] Preferably, in step (1), the soluble ferrous iron source is one or more of ferrous sulfate, ferrous nitrate, ferrous chloride and ferrous acetate, and more preferably, the soluble ferrous iron source is ferrous sulfate.

[0008] Preferably, in step (1), the soluble divalent manganese source is one or more of manganese sulfate, manganese nitrate, manganese chloride and manganese acetate, and more preferably, the soluble divalent manganese source is manganese sulfate.

[0009] Preferably, in step (1), the antioxidant is one or more of citric acid, vitamin C, vitamin E and tea polyphenols, and more preferably, the antioxidant is vitamin C.

[0010] Preferably, in step (1), the soluble phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, ammonium phosphate, and sodium phosphate. More preferably, the soluble phosphorus source is phosphoric acid, ammonium dihydrogen phosphate, or any mixture thereof in any proportion.

[0011] Preferably, in step (1), the soluble ammonia source is one or more of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and ammonium sulfate. More preferably, the soluble ammonia source is ammonium phosphate, ammonium sulfate or any mixture thereof in any proportion.

[0012] Preferably, in step (1), the surfactant is one or more of polyethylene glycol 200, polyethylene glycol 400, polyvinylpyrrolidone, calcium stearate and butyl stearate, and more preferably, the surfactant is polyethylene glycol 400 or polyvinylpyrrolidone.

[0013] Preferably, in step (1), the soluble divalent iron source (calculated as Fe), the soluble divalent manganese source (calculated as Mn), and the soluble phosphorus source (calculated as PO4) are... 3- (calculated) and soluble ammonia sources (calculated as NH4) + The molar ratio of (calculated) is x : (1-x) : (1.0~1.2) : (1.0~1.2), where 0<x≤0.4.

[0014] Preferably, in step (1), the concentration of metal ions (Mn+Fe) in the mixed solution is 0.5~2 mol / L.

[0015] Preferably, in step (1), the antioxidant accounts for 0.1 to 5 wt% of the total mass of the raw materials (excluding pure water).

[0016] Preferably, in step (1), the surfactant accounts for 0.1 to 2 wt% of the total mass of the raw materials (excluding pure water).

[0017] Preferably, in step (1), sulfuric acid or phosphoric acid is used to adjust the pH value of the base solution, and more preferably, phosphoric acid is used for adjustment.

[0018] Preferably, in step (2), the stirring speed is 1000~1500 rpm during the dripping process.

[0019] Preferably, in step (2), the mixed solution is added from below the liquid surface to the bottom liquid with a drop rate of 5~20 mL / min.

[0020] Preferably, in step (2), the stirring speed is reduced to 500 rpm after the dripping is completed, and the stirring time is continued for 30~60 min.

[0021] Preferably, in step (2), the termination condition for the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.5mS / cm.

[0022] Preferably, in step (3), the solid content of the mixture of manganese ferric ammonium phosphate and pure water is 10~20wt%.

[0023] Preferably, in step (3), the termination condition for the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.2mS / cm.

[0024] Preferably, in step (4), the solid content of the red phosphorus manganese ore type manganese iron phosphate mixed with pure water is 10~20wt%.

[0025] Preferably, in step (4), the termination condition for the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.2mS / cm.

[0026] Preferably, in step (4), the oxidant is one or more of hydrogen peroxide, sodium chlorate, sodium hypochlorite, high-purity oxygen, and nitric acid.

[0027] Preferably, in step (4), the oxidant accounts for 5 to 15 wt% of the total mass of the raw materials (red phosphogypsum type manganese iron phosphate and oxidant).

[0028] This invention also provides an application of manganese iron phosphate in the preparation of battery cathode materials.

[0029] Preferably, the positive electrode material of the battery is lithium manganese iron phosphate, and the preparation method of lithium manganese iron phosphate includes the following steps: (1) The lithium source, manganese iron phosphate, carbon source and pure water are mixed and then milled to obtain a slurry with a particle size D50≤300nm; (2) The slurry is spray-dried to obtain powder; (3) The powder is calcined, crushed and sieved under inert gas protection to obtain lithium manganese iron phosphate.

[0030] Preferably, in step (1), the molar ratio of lithium source (calculated as Li) and manganese iron phosphate (calculated as Mn+Fe) is 1.0-1.05:1.0, and the carbon source accounts for 2-8 wt% of the total mass of the raw materials (excluding pure water).

[0031] The beneficial effects of this invention are as follows: This invention first synthesizes ammonium manganese phosphate hydrate, then adjusts the pH and raises the temperature to convert it into manganese iron phosphate of the red phosphorus ore type, and then crystallizes it to obtain ferromanganese phosphate. Compared with the one-step direct synthesis of ferromanganese phosphate, this method has advantages such as lower impurity content, higher uniformity of manganese, iron, and phosphorus elements, and better controllability. Furthermore, this invention eliminates the need for a protective atmosphere and ammonia emissions during the synthesis of ferromanganese phosphate, reducing production costs and being environmentally friendly.

[0032] This invention proposes ferromanganese phosphate as a precursor. Compared to conventional single manganese and iron sources, ferromanganese phosphate provides a more uniform mixing of manganese and iron, effectively suppressing manganese dissolution caused by the Jan Taylor effect. Compared to ammonium ferromanganese phosphate, ferromanganese phosphate does not contain ammonium ions, thus avoiding the release of ammonia gas during the synthesis of lithium iron phosphate and preventing its impact on product quality. Compared to ferrous manganese phosphate, ferromanganese phosphate is compatible with mainstream lithium salts such as lithium carbonate and lithium hydroxide, demonstrating promising industrialization prospects. Based on these advantages, using ferromanganese phosphate as a precursor to prepare lithium iron phosphate cathode materials offers advantages in terms of low cost and high quality, making it a highly promising precursor material. Attached Figure Description

[0033] Figure 1 This is the XRD pattern of the ferric ammonium manganese phosphate prepared in Example 2.

[0034] Figure 2 This is the XRD pattern of the red phosphorus manganese ore type manganese iron phosphate prepared in Example 2.

[0035] Figure 3 This is the XRD pattern of the manganese iron phosphate prepared in Example 2.

[0036] Figure 4 This is the XRD pattern of lithium manganese iron phosphate prepared in Example 2. Detailed Implementation

[0037] The technical solution of the present invention will be further explained and described below with reference to specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0038] Example 1 (1) Preparation of manganese iron phosphate Mix 6.08g ferrous sulfate, 9.06g manganese sulfate, 0.04g ascorbic acid, and 200ml pure water evenly, then adjust the pH to 0.5 to form a mixed solution. Dissolve 9.8g phosphoric acid, 13.21g ammonium sulfate, and 0.04g polyvinylpyrrolidone in 500ml pure water to form a base solution. Stir the base solution continuously at 1000rpm. Pump the mixed solution into the base solution from below the liquid surface at a rate of 10ml / min, while simultaneously pumping in ammonia water to maintain the pH of the base solution at 6.0. After the mixed solution has been added, reduce the stirring speed to 500rpm and continue stirring for 60min. Then filter and wash repeatedly until the conductivity of the filtrate is ≤0.5mS / cm to obtain manganese ferric ammonium phosphate. A slurry with a solid content of 10 wt% was formed by mixing manganese ferric ammonium phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and nitric acid was added to adjust the pH to 5.0. The temperature was raised to 80℃ and held for 1 hour. After multiple filtrations and washings until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was then vacuum dried to obtain red phosphorus manganese phosphate. Alternatively, a slurry with a solid content of 10 wt% was formed by mixing red phosphorus manganese phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and phosphoric acid and 0.77 g of hydrogen peroxide were added simultaneously. The temperature was raised to 80℃ and held for 1 hour until the final pH reached 2.0. After multiple filtrations and washings until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was then dried to obtain pure phase manganese ferric phosphate.

[0039] (2) Preparation of lithium manganese iron phosphate 17.55g of manganese iron phosphate, 3.81g of lithium carbonate, 0.57g of sucrose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination at 700℃ for 10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0040] Example 2 (1) Preparation of manganese iron phosphate Mix 6.07g ferrous sulfate, 9.06g manganese sulfate, 0.39g ascorbic acid, and 100ml pure water evenly, then adjust the pH to 0.8 to form a mixed solution. Dissolve 10.29g phosphoric acid, 13.87g ammonium sulfate, and 0.39g polyvinylpyrrolidone in 500ml pure water to form a base solution. Stir the base solution continuously at 1200rpm. Pump the mixed solution into the base solution from below the liquid surface at a rate of 10ml / min, while simultaneously pumping in ammonia water to maintain the pH of the base solution at 6.2. After the mixed solution has been added, reduce the stirring speed to 500rpm and continue stirring for 60min. Then filter and wash repeatedly until the conductivity of the filtrate is ≤0.5mS / cm to obtain manganese ferric ammonium phosphate. A slurry with a solid content of 10 wt% was formed by mixing manganese ferric ammonium phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and nitric acid was added to adjust the pH to 3.0. The mixture was then heated to 90℃ and held for 2 hours. After repeated filtration and washing until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was vacuum dried to obtain manganese ferric phosphate of the red phosphorus ore type. Alternatively, a slurry with a solid content of 10 wt% was formed by mixing manganese ferric phosphate of the red phosphorus ore type with pure water. The mixture was continuously stirred at 1000 rpm, and phosphoric acid and 1.62 g of hydrogen peroxide were added simultaneously. The mixture was heated to 90℃ and held for 3 hours until the final pH reached 3.0. After repeated filtration and washing until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was dried to obtain pure phase manganese ferric phosphate.

[0041] (2) Preparation of lithium manganese iron phosphate 17.55g of manganese iron phosphate, 3.81g of lithium carbonate, 0.97g of glucose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination at 700℃ for 10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0042] Example 3 (1) Preparation of manganese iron phosphate Mix 5.40g ferrous nitrate, 12.53g manganese nitrate, 2.16g ascorbic acid, and 50ml pure water thoroughly, then adjust the pH to 1.0 to form a mixed solution. Dissolve 10.78g phosphoric acid, 14.54g ammonium sulfate, and 0.86g polyvinylpyrrolidone in 500ml pure water to form a base solution. Stir the base solution continuously at 1500rpm. Pump the mixed solution into the base solution from below the liquid surface at a rate of 10ml / min, while simultaneously pumping in ammonia water to maintain the pH of the base solution at 6.5. After the mixed solution has been added, reduce the stirring speed to 500rpm and continue stirring for 45min. Then filter and wash repeatedly until the conductivity of the filtrate is ≤0.5mS / cm to obtain manganese ferric ammonium phosphate. A slurry with a solid content of 10 wt% was formed by mixing manganese ferric ammonium phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and sulfuric acid was added to adjust the pH to 4.0. The temperature was raised to 99℃ and held for 3 hours. After multiple filtrations and washings until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was then vacuum dried to obtain red phosphorus manganese phosphate. Alternatively, a slurry with a solid content of 10 wt% was formed by mixing red phosphorus manganese phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and phosphoric acid and 2.58 g of hydrogen peroxide were added simultaneously. The temperature was raised to 99℃ and held for 6 hours until the final pH reached 4.0. After multiple filtrations and washings until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was then dried to obtain pure phase manganese ferric phosphate.

[0043] (2) Preparation of lithium manganese iron phosphate 17.55g of manganese iron phosphate, 3.81g of lithium carbonate, 1.60g of glucose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination at 700℃ for 10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0044] Example 4 (1) Preparation of manganese iron phosphate 5.40g ferrous nitrate, 12.53g manganese nitrate, 1.37g ascorbic acid, and 67ml pure water were mixed evenly, and the pH was adjusted to 1.5 to form a mixed solution. 11.76g phosphoric acid, 15.86g ammonium sulfate, and 0.23g polyvinylpyrrolidone were dissolved in 500ml pure water to form a base solution. The base solution was continuously stirred at 1000rpm. The mixed solution was pumped into the base solution from below the liquid surface at a rate of 10ml / min, while ammonia water was pumped in simultaneously to maintain the pH of the base solution at 6.3. After the mixed solution was completely added, the stirring speed was reduced to 500rpm and stirring was continued for 60min. Then, the solution was filtered and washed multiple times until the conductivity of the filtrate was ≤0.5mS / cm to obtain manganese ferric ammonium phosphate. A slurry with a solid content of 10 wt% was formed by mixing manganese ferric ammonium phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and hydrochloric acid was added to adjust the pH to 3.5. The mixture was then heated to 95℃ and held for 1.5 h. After repeated filtration and washing until the conductivity of the filtrate was ≤0.2 mS / cm, it was vacuum dried to obtain manganese ferric phosphate of the red phosphorus ore type. Alternatively, a slurry with a solid content of 10 wt% was formed by mixing manganese ferric phosphate of the red phosphorus ore type with pure water. The mixture was continuously stirred at 1000 rpm, and phosphoric acid and 1.99 g of hydrogen peroxide were added simultaneously. The mixture was heated to 95℃ and held for 4 h until the final pH reached 5.0. After repeated filtration and washing until the conductivity of the filtrate was ≤0.2 mS / cm, it was dried to obtain pure phase manganese ferric phosphate.

[0045] (2) Preparation of lithium manganese iron phosphate 17.55g of manganese iron phosphate, 3.81g of lithium carbonate, 0.77g of polyethylene glycol and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination at 700℃ for 10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0046] Example 5 (1) Preparation of manganese iron phosphate 2.54g ferrous chloride, 10.07g manganese chloride, 1.33g ascorbic acid, and 67ml pure water were mixed evenly, and the pH was adjusted to 1.0 to form a mixed solution. 10.78g phosphoric acid, 14.54g ammonium sulfate, and 0.57g polyvinylpyrrolidone were dissolved in 500ml pure water to form a base solution. The base solution was continuously stirred at 1500rpm. The mixed solution was pumped into the base solution from below the liquid surface at a rate of 10ml / min, while ammonia water was pumped in simultaneously to maintain the pH of the base solution at 6.4. After the mixed solution was completely added, the stirring speed was reduced to 500rpm and stirring was continued for 60min. Then, the solution was filtered and washed multiple times until the conductivity of the filtrate was ≤0.5mS / cm to obtain manganese ferric ammonium phosphate. A slurry with a solid content of 10 wt% was formed by mixing manganese ferric ammonium phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and phosphoric acid was added to adjust the pH to 4.5. The mixture was then heated to 88℃ and held for 2.5 hours. After repeated filtration and washing until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was vacuum dried to obtain red phosphorus manganese phosphate. Alternatively, a slurry with a solid content of 10 wt% was formed by mixing red phosphorus manganese phosphate and pure water. The mixture was continuously stirred at 1000 rpm, and phosphoric acid and 1.27 g of hydrogen peroxide were added simultaneously. The mixture was heated to 88℃ and held for 5 hours until the final pH reached 4.5. After repeated filtration and washing until the conductivity of the filtrate was ≤0.2 mS / cm, the slurry was dried to obtain pure phase manganese ferric phosphate.

[0047] (2) Preparation of lithium manganese iron phosphate 17.55g of manganese iron phosphate, 3.81g of lithium carbonate, 0.38g of dextrin and pure water were mixed and then milled in a sand mill until the slurry D50 ≤ 300nm. After spray drying, calcination at 700℃ for 10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0048] Example 6 (1) Preparation of manganese iron phosphate 2.54g ferrous chloride, 10.07g manganese chloride, 1.52g ascorbic acid, and 67ml pure water were mixed evenly, and the pH was adjusted to 0.9 to form a mixed solution. 10.78g phosphoric acid, 14.54g ammonium sulfate, and 0.57g polyvinylpyrrolidone were dissolved in 500ml pure water to form a base solution. The base solution was continuously stirred at 1250rpm. The mixed solution was pumped into the base solution from below the liquid surface at a rate of 10ml / min, while ammonia water was pumped in simultaneously to maintain the pH of the base solution at 6.1. After the mixed solution was completely added, the stirring speed was reduced to 500rpm and stirring was continued for 60min. Then, the solution was filtered and washed multiple times until the conductivity of the filtrate was ≤0.5mS / cm to obtain manganese ferric ammonium phosphate. A slurry with a solid content of 10 wt% was formed by mixing manganese ferric ammonium phosphate and pure water. The slurry was continuously stirred at 1000 rpm, and phosphoric acid was added to adjust the pH to 3.8. The mixture was then heated to 92℃ and held for 2.0 h. After multiple filtrations and washings until the conductivity of the filtrate was ≤0.2 mS / cm, it was finally dried under vacuum to obtain manganese ferric phosphate of the red phosphorus ore type. Alternatively, a slurry with a solid content of 10 wt% was formed by mixing manganese ferric phosphate of the red phosphorus ore type and pure water. The mixture was continuously stirred at 1000 rpm, and phosphoric acid and 1.62 g of hydrogen peroxide were added simultaneously. The mixture was heated to 92℃ and held for 5 h until the final pH reached 3.5. After multiple filtrations and washings until the conductivity of the filtrate was ≤0.2 mS / cm, it was finally dried to obtain pure phase manganese ferric phosphate.

[0049] (2) Preparation of lithium manganese iron phosphate 17.54g of manganese iron phosphate, 3.81g of lithium carbonate, 1.17g of glucose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination at 700℃ for 10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0050] Comparative Example 1 3.81g lithium carbonate, 6.03g iron phosphate, 4.58g manganese tetroxide, 6.90g ammonium dihydrogen phosphate, 1.85g glucose and 54g pure water were mixed and then added to a sand mill and milled until the slurry D50 ≤ 300nm. Then, the mixture was spray dried, calcined at 700℃ for 10h under a nitrogen atmosphere, pulverized and sieved to obtain lithium manganese iron phosphate cathode material.

[0051] Comparative Example 2 3.81g lithium carbonate, 4.52g iron phosphate, 5.26g manganese tetroxide, 8.05g ammonium dihydrogen phosphate, 2.14g sucrose and 55g pure water were mixed and then added to a sand mill and milled until the slurry D50 ≤ 300nm. Then, the mixture was spray dried, calcined at 700℃ for 10h under a nitrogen atmosphere, pulverized and sieved to obtain lithium manganese iron phosphate cathode material.

[0052] Comparative Example 3 3.81g lithium carbonate, 3.02g iron phosphate, 6.09g manganese tetroxide, 9.20g ammonium dihydrogen phosphate, 2.46g polyethylene glycol and 57g pure water were mixed and then added to a sand mill and milled until the slurry D50 ≤ 300nm. Then, the mixture was spray dried, calcined at 700℃ for 10h under a nitrogen atmosphere, pulverized and sieved to obtain lithium manganese iron phosphate cathode material.

[0053] The aforementioned lithium manganese iron phosphate cathode material, polyvinylidene fluoride (PVDF), Super-p conductive carbon black, and N-methylpyrrolidone were homogenized into a slurry, with the mass ratio of cathode material, PVDF, and Super-p being 90:5:5, and the solid content of the slurry being 30%. The slurry was then coated, dried, and punched to obtain circular electrode sheets. Finally, the circular electrode sheets, separator, and lithium sheet were assembled into coin cells for testing in a glove box. The coin cells were charged and discharged within a voltage range of 2.5–4.5 V.

[0054] The prepared ferromanganese phosphate and lithium manganese iron phosphate were subjected to performance tests, and the results are shown in Tables 1 and 2. The data in the tables show that when lithium manganese iron phosphate material is prepared using the ferromanganese phosphate prepared in this invention, its compaction density and electrical properties are higher than those of the comparative example, while its specific surface area is lower. This indicates that using high-purity ferromanganese phosphate as a precursor for lithium manganese iron phosphate can improve its physicochemical and electrical properties.

[0055] Table 1. Test results of ferric manganese phosphate

[0056] Table 2 Test results of lithium manganese iron phosphate

Claims

1. A method for preparing ferric manganese phosphate, characterized in that, Includes the following steps: (1) Dissolve a soluble ferrous iron source, a soluble ferrous manganese source, and an antioxidant in pure water to form a mixed solution; dissolve a soluble phosphorus source, a soluble ammonia source, and a surfactant in pure water to form a base solution; (2) Continue stirring the bottom liquid, add the mixed solution to the bottom liquid until the mixed solution is completely added, continue stirring for a period of time, and then perform multiple filtrations and washings to obtain manganese iron ammonium phosphate. (3) Mix manganese iron ammonium phosphate with pure water and keep stirring. Then adjust the pH to 3.0~5.0, then heat to 80~99℃, keep warm for 1~3h, and then filter, wash and dry multiple times to obtain red phosphorus manganese ore type manganese iron phosphate. (4) Mix red phosphorus manganese ore type manganese iron phosphate with pure water and keep stirring. At the same time, add phosphoric acid and oxidant, and heat to 80~99℃ and keep warm for 1~6h. Control the pH of the reaction endpoint to be 2.0~5.

0. After multiple filtrations, washing and drying, pure phase manganese iron phosphate is obtained.

2. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (1), the pH of the mixed solution is ≤2.0; the pH of the base solution is 6.0~6.5; sulfuric acid or phosphoric acid is used to adjust the pH of the base solution.

3. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (1), the soluble ferrous iron source is one or more of ferrous sulfate, ferrous nitrate, ferrous chloride, and ferrous acetate; The soluble divalent manganese source is one or more of manganese sulfate, manganese nitrate, manganese chloride, and manganese acetate. The antioxidant is one or more of citric acid, vitamin C, vitamin E, and tea polyphenols; The soluble phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, diammonium hydrogen phosphate, disodium hydrogen phosphate, ammonium phosphate, and sodium phosphate. The soluble ammonia source is one or more of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium sulfate; the surfactant is one or more of polyethylene glycol 200, polyethylene glycol 400, polyvinylpyrrolidone, calcium stearate, and butyl stearate.

4. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (1), the soluble divalent iron source is calculated as Fe, the soluble divalent manganese source as Mn, and the soluble phosphorus source as PO4. 3- Calculation, soluble ammonia source according to NH4 + The calculated molar ratio is x : (1-x) : (1.0~1.2) : (1.0~1.2), where 0 < x ≤ 0.4; The concentration of metal ions Mn+Fe in the mixed solution is 0.5~2 mol / L, the antioxidant accounts for 0.1~5 wt% of the total mass of the raw materials, and the surfactant accounts for 0.1~2 wt% of the total mass of the raw materials.

5. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (2), the stirring speed is 1000~1500 rpm during the dropping process, the mixed solution is added from below the liquid surface with a dropping rate of 5~20 mL / min, the stirring speed is reduced to 300-500 rpm after the dropping is completed, the stirring time is 30~60 min, and the condition for the end of the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.5 mS / cm.

6. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (2), while adding the mixed solution, ammonia water is added at the same time to maintain the pH of the bottom solution at 6.0~6.

5.

7. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (3), the solid content of the mixture of manganese iron ammonium phosphate and pure water is 10~20wt%, the end condition of the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.2mS / cm, and the drying method is vacuum drying.

8. The method for preparing ferric manganese phosphate according to claim 1, characterized in that: In step (4), the solid content of the red phosphorus manganese ore type manganese iron phosphate mixed with pure water is 10~20wt%, and the end condition for the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.2mS / cm; The oxidant is one or more of hydrogen peroxide, perchloric acid, sodium hypochlorite, oxygen, ozone, and nitric acid, and the oxidant accounts for 5 to 15 wt% of the total mass of the raw materials.

9. A type of manganese iron phosphate, characterized in that, A manganese iron phosphate prepared by the preparation method according to any one of claims 1-8 has the general chemical formula Mn. 1-x Fe x PO4·yH2O, 0<x≤0.4, 1.4≤y<2.

0.

10. A positive electrode material, characterized in that, Includes the manganese iron phosphate as described in claim 9.

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