Carbon-coated doped manganese iron pyrophosphate, preparation method and application thereof

Abstract

The application discloses carbon-coated doped manganese iron pyrophosphate, a preparation method and application thereof. The application controls the ratio of manganese source, iron source and phosphorus source and reaction conditions in the preparation process, so that divalent manganese and divalent iron are precipitated to form manganese iron ammonium phosphate, impurities are removed and crystallization is carried out, and finally, manganese iron pyrophosphate is obtained by calcination. The whole precipitation process does not need inert gas protection, effectively reducing the production cost; the removal of impurities and crystallization can improve the uniformity of the finished product and effectively avoid the existence of non-divalent manganese and divalent iron impurities in the finished product, improving the purity. Meanwhile, the carbon source and dopant are used for coating and doping manganese iron pyrophosphate, so that the carbon-coated doped manganese iron pyrophosphate can inhibit the elution of manganese ions when applied in lithium manganese iron phosphate, the secondary carbon coating makes the carbon layer more uniform and dense, and the performance of lithium manganese iron phosphate is significantly improved. In addition, the synthesis period 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 carbon-coated manganese pyrophosphate, its preparation method, and its application. Background Technology

[0002] Manganese ferric pyrophosphate (Mn) 1-x Fe x P₂O₇, 0 < x < 1, is a solid solution of manganese pyrophosphate and iron pyrophosphate, in which manganese and iron are miscible in any proportion. Therefore, it has been considered one of the excellent precursors for the preparation of lithium manganese iron phosphate in recent years. Compared with conventional single manganese and iron sources, manganese iron pyrophosphate produces a more uniform mixture of manganese and iron, which can effectively suppress manganese dissolution caused by the Jan Taylor effect; compared with manganese iron phosphate (Mn 1-x Fe x PO4, 0 < x < 1), manganese ferric pyrophosphate is easier to synthesize and has higher purity; compared to manganese ferrous phosphate ((Mn 1-x Fe x (3(PO4)2, 0<x<1), the Mn+Fe / P ratio in ferromanganese pyrophosphate is 1:1, making feeding more convenient. Due to the above advantages, the preparation of ferromanganese phosphate cathode materials using ferromanganese pyrophosphate as a precursor has advantages in terms of low cost and industrialization, and is a very promising precursor material.

[0003] There are several methods for preparing ferromanganese pyrophosphate. The general process involves mixing manganese, iron, and phosphorus sources, followed by calcination under a protective atmosphere to obtain the target product. For example, Chinese patent CN105190951B discloses a method for preparing a battery composite material and its precursor. This method involves reacting iron, manganese, and phosphoric acid in an aqueous system for 8 hours, followed by calcination at above 500°C under a protective atmosphere to obtain ferromanganese pyrophosphate. This method suffers from a long reaction cycle in the first step, which is prone to incomplete reaction and uneven mixing of manganese and iron, thus increasing production costs or affecting product quality. Chinese patent CN105355885A discloses a lithium-ion battery composite cathode material, LiMn. 1-x Fe xThe synthesis method of PO4 / C involves mixing manganese, iron, phosphorus, and carbon sources in a high-energy ball mill at a speed of 600-1500 rpm for 2-4 hours, followed by calcination at 500-700℃ in an inert atmosphere for 4-8 hours to obtain manganese ferrophosphate pyrophosphate. This method, using high-energy ball milling, suffers from uneven mixing of manganese and iron, and the high energy consumption of high-energy ball mills makes it unsuitable for industrialization. Chinese patent CN116750743A discloses a high-performance lithium manganese iron phosphate precursor, a method for preparing lithium manganese iron phosphate cathode material, and a battery using this cathode material. This method involves reacting a phosphorus source, a manganese iron metal salt solution, and ammonia in a reactor, adjusting the reaction temperature to 20-70℃, the stirring speed to 300-900 rpm, and the pH to 4-7 to obtain manganese iron ammonium monohydrate phosphate, which is then calcined in a sintering furnace at 300-800℃ for 4-15 hours to obtain the manganese ferrophosphate pyrophosphate product. This method requires the introduction of a protective atmosphere into the reactor when synthesizing manganese ferric ammonium hydrate, which increases the cost. Furthermore, the precipitation and crystallization processes are completed in one step in the reactor. Impurity ions in the solution system (such as sulfate ions, ammonium ions, etc.) can affect the crystallization effect and may be difficult to clean after crystallization by being encapsulated in manganese ferric ammonium hydrate species, ultimately resulting in a high content of impurity elements in manganese ferric pyrophosphate.

[0004] Currently, there is very little research on the synthesis of manganese iron pyrophosphate. Existing synthesis methods also have problems such as complex preparation processes, high costs, impurities, and uneven mixing of manganese and iron, which lead 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 carbon-coated doped ferromanganese pyrophosphate, its preparation method, and its application. The method involves co-precipitating divalent manganese and divalent iron in a liquid phase to form ferromanganese ammonium hydrate, followed by regeneration and heating to remove impurities. Finally, the hydrate is calcined together with a carbon source and dopant elements to obtain carbon-coated doped ferromanganese pyrophosphate. The absence of a protective atmosphere during the precipitation process reduces production costs, and the prepared ferromanganese pyrophosphate has high purity, simplifying the subsequent production process for lithium iron manganese phosphate and improving the uniformity of carbon coating.

[0006] To achieve the above objectives, the present invention provides a carbon-coated doped manganese pyrophosphate, wherein the carbon content in the carbon-coated doped manganese pyrophosphate is 0.1% to 1.0%, and the doping element content is 500 to 4000 ppm.

[0007] This invention also provides a method for preparing carbon-coated manganese pyrophosphate, specifically including the following steps:

[0008] (1) Dissolve soluble ferrous iron source, soluble ferrous manganese source, soluble phosphorus source, soluble ammonia source, antioxidant and surfactant in pure water, and adjust the pH to ≤ 2.0 to form a mixed solution;

[0009] (2) Prepare a solution containing an ammonia source as the base solution, and adjust the pH of the base solution to 6.0~7.0;

[0010] The present invention first achieves a pH less than 2, primarily to prevent the oxidation of ferrous manganese in solution, as it is not easily oxidized in an acidic environment. Experiments have shown that a pH less than 2 is suitable and also saves costs. Next, a pH of 6-7 is achieved, mainly to maintain the entire co-precipitation process at a stable pH of 6-7. Therefore, the pH of the base solution needs to be pre-adjusted to the range of 6-7. (If the ferrous manganese precipitates immediately upon contact with the base solution, the precipitate will not be oxidized within the pH range of 6-7).

[0011] (3) 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 6.0~7.0 until the mixed solution is completely added. Continue stirring for a period of time and then perform multiple filtrations and washings to obtain filter cake A.

[0012] (4) Mix filter cake A, phosphorus source, ammonium sulfate, reducing agent and pure water, stir continuously for a period of time, then adjust the pH to 6.0~7.0 and raise the temperature to 40~60℃, and continue stirring for a period of time. Then perform multiple filtrations and washes to obtain filter cake B.

[0013] (5) After mixing filter cake B, carbon source, dopant and pure water, the mixture is sand milled to make the slurry particle size D50≤300nm. Then, it is spray dried, calcined under an inert atmosphere and ball milled to obtain carbon-coated doped manganese pyrophosphate.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] Preferably, in step (1), the surfactant is one or more of polyethylene glycol 200, polyethylene glycol 400, polyvinylpyrrolidone, calcium stearate, and butyl stearate. More preferably, the surfactant is polyethylene glycol 400 or polyvinylpyrrolidone. The main purpose of the surfactant is to disperse particles and adjust the specific surface area of ​​the product.

[0019] Preferably, in step (1), the antioxidant is one or more of citric acid, vitamin C, vitamin E, and tea polyphenols; more preferably, the antioxidant is vitamin C. The main purpose of the antioxidant is to prevent ferrous manganese from being oxidized before precipitation.

[0020] 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... - (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.

[0021] Preferably, in step (1), the concentration of metal ions (Mn+Fe) in the mixed solution is 0.5~2mol / L.

[0022] Preferably, in step (1), the antioxidant accounts for 0.1 to 5 wt% of the total mass of the raw materials (excluding antioxidants, surfactants and pure water).

[0023] Preferably, in step (1), the surfactant accounts for 0.1 to 2 wt% of the total mass of the raw materials (excluding antioxidants, surfactants and pure water).

[0024] Preferably, in step (2), the bottom liquid ammonia source (in NH4) - The concentration of (calculated) is 0.1~1 mol / L.

[0025] Preferably, in step (2), 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.

[0026] Preferably, in step (3), the stirring speed is 1000~1500 rpm during the dripping process.

[0027] Preferably, in step (3), the mixed solution is added from below the liquid surface to the bottom liquid with a drop rate of 5~20 mL / min.

[0028] Preferably, in step (3), the stirring speed is reduced to 500 rpm after the dripping is completed, and the stirring time is continued for 30~60 min.

[0029] Preferably, in step (3), the termination condition for the multiple filtrations and washings is that the conductivity of the filtrate is ≤1.0mS / cm.

[0030] Preferably, in step (4), the solid content of the filter cake A and pure water mixture is 10~20wt%.

[0031] Preferably, in step (4), the stirring speed is 1000~1500 rpm.

[0032] Preferably, in step (4), the phosphorus source is phosphoric acid.

[0033] Preferably, in step (4), the ammonia source is ammonium sulfate.

[0034] Preferably, in step (4), the reducing agent is vitamin C.

[0035] Since no protective atmosphere was introduced during the experiment, ferrous manganese divalent may be oxidized. The addition of phosphorus source, ammonium source and reducing agent is to convert the oxidized ferrous manganese divalent into ferric manganese ammonium phosphate, thereby further improving the purity of the product.

[0036] Preferably, in step (4), the phosphorus source (in PO4) - (calculated) and ammonia source (in NH4) - The molar ratio of (calculated) is 1:1.

[0037] Preferably, in step (4), the continuous stirring time is 30~120 min.

[0038] Preferably, in step (4), the termination condition for the multiple filtrations and washings is that the conductivity of the filtrate is ≤0.2mS / cm.

[0039] Preferably, in step (5), the carbon source is one or more of starch, sucrose, glucose, polyethylene glycol and dextrin, and more preferably, the carbon source is sucrose, glucose or a mixture thereof.

[0040] Preferably, in step (5), the carbon source accounts for 2 to 8 wt% of the total mass of the raw materials (excluding pure water).

[0041] Preferably, in step (5), the dopant is a raw material for forming doping of any one or more elements such as Ti, Mg, Nb, Mo, and V. The dopant is one or more of titanium dioxide, magnesium oxide, niobium pentoxide, ammonium molybdate, and ammonium metavanadate. More preferably, the dopant is magnesium oxide, niobium pentoxide, metavanadate, or a mixture thereof.

[0042] Preferably, in step (5), the dopant accounts for 0.1 to 0.5 wt% of the total mass of the raw materials (excluding pure water).

[0043] Preferably, in step (5), the solid content of the slurry is 20~30wt%.

[0044] Preferably, in step (5), the inert atmosphere is nitrogen or argon.

[0045] Preferably, in step (5), the calcination temperature is 500~700℃ and the calcination time is 6~10h.

[0046] Preferably, in step (5), the particle size D50 after pulverization is ≤1.0μm.

[0047] This invention also provides an application of carbon-coated doped manganese iron pyrophosphate in the preparation of battery cathode materials.

[0048] 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:

[0049] (1) The lithium source, carbon-coated doped manganese iron pyrophosphate, carbon source and pure water are mixed and then milled to obtain a slurry with a particle size D50≤300nm;

[0050] (2) The slurry is spray-dried to obtain powder;

[0051] (3) The powder is calcined, crushed and sieved under inert gas protection to obtain lithium manganese iron phosphate.

[0052] Preferably, in step (1), the molar ratio of lithium source (calculated as Li) to carbon-coated doped manganese iron pyrophosphate (calculated as Mn+Fe) is 1.0-1.05:1.0, and the carbon source accounts for 1-5 wt% of the total mass of the raw materials (excluding pure water).

[0053] The beneficial effects of this invention are as follows:

[0054] (1) The present invention improves the crystallinity of manganese ferric ammonium hydrate generated by co-precipitation by crystallizing and removing impurities, while redissolving the non-divalent iron and manganese precipitates contained therein and converting them into manganese ferric ammonium hydrate, thereby improving the purity of the subsequently calcined manganese ferric pyrophosphate.

[0055] (2) In the process of synthesizing manganese ferric ammonium hydrate, the present invention does not require the introduction of a protective atmosphere, which reduces the overall production cost of manganese ferric pyrophosphate and is suitable for industrial promotion.

[0056] (3) The present invention performs carbon coating and doping on manganese iron pyrophosphate. When preparing lithium manganese iron phosphate using carbon-coated doped manganese iron pyrophosphate, the process can be simplified, the carbon coating uniformity can be improved, and manganese dissolution can be suppressed (co-precipitation makes manganese iron uniformly distributed in the precursor, which can suppress manganese dissolution), thereby improving the electrical performance. Attached Figure Description

[0057] Figure 1 The image shows the XRD pattern of filter cake B (ammonium manganese phosphate hydrate) prepared in Example 4.

[0058] Figure 2 The image shows the XRD pattern of the carbon-coated doped manganese iron pyrophosphate prepared in Example 4.

[0059] Figure 3 This is the XRD pattern of lithium manganese iron phosphate prepared in Example 4. Detailed Implementation

[0060] 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.

[0061] Example 1

[0062] (1) Preparation of manganese ferric pyrophosphate

[0063] Mix 6.08g ferrous sulfate, 9.06g manganese sulfate, 9.80g phosphoric acid, 13.21g ammonium sulfate, 0.04g polyvinylpyrrolidone, 0.04g vitamin C, and 200ml pure water thoroughly, then adjust the pH to 0.5 to form a mixed solution. Dissolve 16.52g ammonium sulfate in 500ml pure water and adjust the pH to 7.0 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 7.0. 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 ≤1.0mS / cm to obtain filter cake A. Filter cake A, 0.98g phosphoric acid, 1.321g ammonium sulfate, 0.004g ascorbic acid (phosphoric acid, ammonium sulfate, and vitamin C were 1 / 10 of the raw materials), and pure water were mixed to form a slurry with a solid content of 10wt%. The mixture was stirred continuously for 30min, then the pH was adjusted to 6.0 and the temperature was raised to 60℃. The mixture was stirred for another 60min, and then filtered and washed multiple times until the conductivity of the filtrate was ≤0.2mS / cm, yielding filter cake B. Filter cake B, 0.95g sucrose, 0.02g titanium dioxide, 0.03g magnesium oxide, 0.01g niobium pentoxide, and 53ml pure water were mixed and then milled until the slurry particle size D50 was ≤300nm. The mixture was then vacuum dried, and the dried material was placed in a tube furnace and calcined at 600℃ for 6h under nitrogen protection. Finally, the material was pulverized until the particle size D50 was ≤1μm, yielding carbon-coated doped manganese ferrophosphate material.

[0064] (2) Preparation of lithium manganese iron phosphate

[0065] 14.23g of carbon-coated doped manganese iron pyrophosphate, 3.81g of lithium carbonate, 0.56g of sucrose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination at 600-800℃ for 6-10h under nitrogen atmosphere, pulverization and sieving were performed to obtain lithium manganese iron phosphate cathode material.

[0066] Example 2

[0067] (1) Preparation of manganese ferric pyrophosphate

[0068] Mix 6.08g ferrous sulfate, 9.06g manganese sulfate, 10.29g phosphoric acid, 13.87g ammonium sulfate, 0.39g polyvinylpyrrolidone, 0.39g vitamin C, and 100ml pure water thoroughly, then adjust the pH to 0.8 to form a mixed solution. Dissolve 3.30g ammonium sulfate in 500ml pure water and adjust the pH to 6.0 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 6.0. 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 ≤1.0mS / cm to obtain filter cake A. Filter cake A, 1.029g phosphoric acid, 1.387g ammonium sulfate, 0.03g vitamin C, and pure water were mixed to form a slurry with a solid content of 10wt%. The mixture was stirred continuously for 40min, then the pH was adjusted to 6.5 and the temperature was raised to 60℃. The mixture was stirred for another 90min. After repeated filtration and washing, the conductivity of the filtrate was ≤0.2mS / cm, yielding filter cake B. Filter cake B, 1.62g glucose, 0.01g titanium dioxide, 0.02g magnesium oxide, 0.01g ammonium metavanadate, and 53ml pure water were mixed and then milled until the slurry particle size D50 ≤300nm. The mixture was then vacuum dried. The dried material was placed in a tube furnace and calcined at 560℃ for 10h under nitrogen protection. Finally, it was pulverized until the particle size D50 ≤1μm, yielding carbon-coated doped manganese ferropyrophosphate material.

[0069] (2) Preparation of lithium manganese iron phosphate

[0070] 14.23g of carbon-coated doped manganese iron pyrophosphate, 3.81g of lithium carbonate, 0.18g of glucose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization and sieving, lithium manganese iron phosphate cathode material was obtained.

[0071] Example 3

[0072] (1) Preparation of manganese ferric pyrophosphate

[0073] Mix 5.40g ferrous nitrate, 12.53g manganese nitrate, 10.78g phosphoric acid, 14.54g ammonium sulfate, 0.86g polyvinylpyrrolidone, 2.16g vitamin C, and 50ml pure water thoroughly, then adjust the pH to 1.0 to form a mixed solution. Dissolve 3.3g ammonium sulfate in 500ml pure water and adjust the pH to 6.7 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 6.7. 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 ≤1.0mS / cm to obtain filter cake A. Filter cake A, 1.078g phosphoric acid, 1.454g ammonium sulfate, 0.216g vitamin C, and pure water were mixed to form a slurry with a solid content of 10wt%. The mixture was stirred continuously for 60min, then the pH was adjusted to 6.5 and the temperature was raised to 40℃. The mixture was stirred for another 120min, and then filtered and washed multiple times until the conductivity of the filtrate was ≤0.2mS / cm, yielding filter cake B. Filter cake B, 0.38g starch, 0.005g magnesium oxide, 0.003g niobium pentoxide, 0.01g ammonium metavanadate, and 53ml pure water were mixed and then milled until the slurry particle size D50 was ≤300nm. The mixture was then vacuum dried, and the dried material was placed in a tube furnace and calcined at 650℃ for 8h under nitrogen protection. Finally, the material was pulverized until the particle size D50 was ≤1μm, yielding carbon-coated doped manganese ferropyrophosphate material.

[0074] (2) Preparation of lithium manganese iron phosphate

[0075] 14.23g of carbon-coated doped manganese iron pyrophosphate, 3.81g of lithium carbonate, 0.95g of starch and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization and sieving, lithium manganese iron phosphate cathode material was obtained.

[0076] Example 4

[0077] (1) Preparation of manganese ferric pyrophosphate

[0078] Mix 5.40g ferrous nitrate, 12.53g manganese nitrate, 11.76g phosphoric acid, 15.86g ammonium sulfate, 0.23g polyvinylpyrrolidone, 1.37g vitamin C, and 67ml pure water thoroughly, then adjust the pH to 1.5 to form a mixed solution. Dissolve 9.91g ammonium sulfate in 500ml pure water and adjust the pH to 6.5 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 6.5. 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 ≤1.0mS / cm to obtain filter cake A. Filter cake A, 1.176g phosphoric acid, 1.586g ammonium sulfate, 0.137g vitamin C, and pure water were mixed to form a slurry with a solid content of 20wt%. The mixture was stirred continuously for 30min, then the pH was adjusted to 7.0 and the temperature was raised to 45℃. The mixture was stirred for another 90min, and then filtered and washed multiple times until the conductivity of the filtrate was ≤0.2mS / cm, yielding filter cake B. Filter cake B, 0.60g polyethylene glycol, 0.02g titanium dioxide, 0.01g magnesium oxide, 0.01g niobium pentoxide, 0.01g ammonium molybdate, and 53ml pure water were mixed and then milled until the slurry particle size D50 was ≤300nm. The mixture was then vacuum dried, and the dried material was placed in a tube furnace and calcined at 700℃ for 6h under nitrogen protection. Finally, the material was pulverized until the particle size D50 was ≤1μm, yielding carbon-coated doped manganese ferropyrophosphate material.

[0079] (2) Preparation of lithium manganese iron phosphate

[0080] 14.23g of carbon-coated doped manganese iron pyrophosphate, 3.81g of lithium carbonate, 0.75g of polyethylene glycol and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization and sieving, lithium manganese iron phosphate cathode material was obtained.

[0081] Example 5

[0082] (1) Preparation of manganese ferric pyrophosphate

[0083] 2.54g ferrous chloride, 10.07g manganese chloride, 10.78g phosphoric acid, 14.54g ammonium sulfate, 0.57g polyvinylpyrrolidone, 1.33g vitamin C, and 67ml pure water were mixed evenly, and the pH was adjusted to 1.0 to form a mixed solution. 23.12g ammonium sulfate was dissolved in 500ml pure water and the pH was adjusted to 6.7 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 6.7. After the mixed solution was completely added, the stirring speed was reduced to 500rpm and stirring was continued for 60min. Then, the mixture was filtered and washed multiple times until the conductivity of the filtrate was ≤1.0mS / cm, yielding filter cake A. Filter cake A, 1.078g phosphoric acid, 1.454g ammonium sulfate, 0.133g vitamin C, and pure water were mixed to form a slurry with a solid content of 15wt%. The mixture was stirred continuously for 30min, then the pH was adjusted to 6.5 and the temperature was raised to 50℃. The mixture was stirred for another 120min, and then filtered and washed multiple times until the conductivity of the filtrate was ≤0.2mS / cm, yielding filter cake B. Filter cake B, 1.3g dextrin, 0.02g titanium dioxide, 0.04g magnesium oxide, 0.04g ammonium molybdate, and 53ml pure water were mixed and then milled until the slurry particle size D50 was ≤300nm. The mixture was then vacuum dried, and the dried material was placed in a tube furnace and calcined at 600℃ for 8h under nitrogen protection. Finally, the material was pulverized until the particle size D50 was ≤1μm, yielding carbon-coated doped manganese ferropyrophosphate material.

[0084] (2) Preparation of lithium manganese iron phosphate

[0085] 14.23g of carbon-coated doped manganese iron pyrophosphate, 3.81g of lithium carbonate, 0.37g of dextrin and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization and sieving, lithium manganese iron phosphate cathode material was obtained.

[0086] Example 6

[0087] (1) Preparation of manganese ferric pyrophosphate

[0088] 2.54g ferrous chloride, 10.07g manganese chloride, 10.78g phosphoric acid, 14.54g ammonium sulfate, 0.57g polyvinylpyrrolidone, 1.52g vitamin C, and 67ml pure water were mixed evenly, and the pH was adjusted to 0.9 to form a mixed solution. 6.61g ammonium sulfate was dissolved in 500ml pure water and the pH was adjusted to 7.0 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 7.0. After the mixed solution was completely added, the stirring speed was reduced to 500rpm and stirring was continued for 60min. Then, the mixture was filtered and washed multiple times until the conductivity of the filtrate was ≤1.0mS / cm, yielding filter cake A. Filter cake A, 1.078g phosphoric acid, 1.454g ammonium sulfate, 0.152g vitamin C, and pure water were mixed to form a slurry with a solid content of 10wt%. The mixture was stirred continuously for 50min, then the pH was adjusted to 6.0 and the temperature was raised to 55℃. The mixture was stirred for another 80min, and then filtered and washed multiple times until the conductivity of the filtrate was ≤0.2mS / cm, yielding filter cake B. Filter cake B, 0.88g glucose, 0.02g magnesium oxide, 0.02g niobium pentoxide, 0.04g ammonium metavanadate, and 53ml pure water were mixed and then milled until the slurry particle size D50 was ≤300nm. The mixture was then vacuum dried, and the dried material was placed in a tube furnace and calcined at 600℃ for 7h under nitrogen protection. Finally, the material was pulverized until the particle size D50 was ≤1μm, yielding carbon-coated doped manganese ferropyrophosphate material.

[0089] (2) Preparation of lithium manganese iron phosphate

[0090] 14.23g of carbon-coated doped manganese iron pyrophosphate, 3.81g of lithium carbonate, 0.56g of glucose and pure water were mixed and then milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, crushing and sieving, lithium manganese iron phosphate cathode material was obtained.

[0091] Comparative Example 1

[0092] 3.81g lithium carbonate, 6.03g iron phosphate, 4.58g manganese tetroxide, 6.90g ammonium dihydrogen phosphate, 1.85g glucose, 0.012g titanium dioxide, 0.024g magnesium oxide, 0.012g ammonium metavanadate, and 54g pure water were mixed and then added to a sand mill and milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization, and sieving, lithium manganese iron phosphate cathode material was obtained.

[0093] Comparative Example 2

[0094] 3.81g lithium carbonate, 4.52g iron phosphate, 5.26g manganese tetroxide, 8.05g ammonium dihydrogen phosphate, 2.14g sucrose, 0.012g titanium dioxide, 0.012g niobium pentoxide, 0.035g ammonium metavanadate, and 55g pure water were mixed and then added to a sand mill and milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization, and sieving, lithium manganese iron phosphate cathode material was obtained.

[0095] Comparative Example 3

[0096] 3.81g lithium carbonate, 3.02g iron phosphate, 6.09g manganese tetroxide, 9.20g ammonium dihydrogen phosphate, 2.46g polyethylene glycol, 0.03g magnesium oxide, 0.02g niobium pentoxide, 0.024g ammonium metavanadate, and 57g pure water were mixed and then added to a sand mill and milled until the slurry D50 ≤ 300nm. After spray drying, calcination under nitrogen atmosphere, pulverization, and sieving, lithium manganese iron phosphate cathode material was obtained.

[0097] 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.

[0098] The prepared ferromanganese pyrophosphate and lithium manganese iron phosphate were subjected to performance tests, and the results are shown in Table 1. The data in the table show that the lithium manganese iron phosphate material prepared using the carbon-coated doped ferromanganese pyrophosphate obtained in this invention has higher compaction density and electrical properties than the comparative example, while its specific surface area is lower. This indicates that using high-purity ferromanganese pyrophosphate as a precursor for lithium manganese iron phosphate can improve its physicochemical and electrical properties.

[0099] Table 1. Test results of ferromanganese pyrophosphate

[0100]

[0101] Table 2 Test results of lithium manganese iron phosphate

[0102]

Claims

1. A method for preparing carbon-coated doped manganese ferropyrophosphate, characterized in that, Includes the following steps: (1) Dissolve soluble ferrous iron source, soluble ferrous manganese source, soluble phosphorus source, soluble ammonia source, antioxidant and surfactant in pure water, and adjust the pH to ≤ 2.0 to form a mixed solution; (2) Prepare a solution containing an ammonia source as the base solution, and adjust the pH of the base solution to 6.0~7.0; (3) 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 6.0~7.0 until the mixed solution is completely added. Continue stirring for a period of time and then perform multiple filtrations and washings to obtain filter cake A. Filter cake A is manganese iron ammonium hydrate. (4) Mix filter cake A, phosphorus source, ammonium sulfate and reducing agent in pure water, stir continuously for a period of time, then adjust the pH to 6.0~7.0 and raise the temperature to 40~60℃, continue stirring for a period of time, and then perform multiple filtrations and washings to obtain filter cake B; this step crystallizes and removes impurities from the manganese iron ammonium hydrate generated by the co-precipitation method, improves its crystallinity and at the same time dissolves the non-divalent iron and manganese precipitates contained therein and transforms them into manganese iron ammonium hydrate; (5) After mixing filter cake B, carbon source, dopant and pure water, the mixture is sand milled to make the slurry particle size D50≤300nm. Then, it is spray dried, calcined under an inert atmosphere and ball milled to obtain carbon-coated doped manganese pyrophosphate.

2. The method for preparing carbon-coated doped manganese ferropyrophosphate 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 ferrous manganese source is one or more of manganese sulfate, manganese nitrate, manganese chloride, and manganese acetate; 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; and the antioxidant is one or more of citric acid, vitamin C, vitamin E, and tea polyphenols.

3. The method for preparing carbon-coated doped manganese ferropyrophosphate according to claim 1, characterized in that: In step (1), the molar ratio of the soluble ferrous iron source, the soluble ferrous manganese source, the soluble phosphorus source, and the soluble ammonia source is x :(1-x) : (1.0~1.2) : (1.0~1.2), where 0<x≤0.4, wherein the soluble ferrous iron source is calculated as Fe, the soluble ferrous manganese source is calculated as Mn, and the soluble phosphorus source is calculated as PO4. 3- Calculated, soluble ammonia source as NH4 + count; The concentration of metal ions 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.

4. The method for preparing carbon-coated doped manganese ferropyrophosphate according to claim 1, characterized in that: In step (2), the concentration of the ammonia source in the bottom liquid is 0.1~1 mol / L, and the pH value of the bottom liquid is adjusted by sulfuric acid or phosphoric acid. The ammonia source in the bottom liquid is NH4 + count.

5. The method for preparing carbon-coated doped manganese ferropyrophosphate according to claim 1, characterized in that: In step (3), the stirring speed is 1000~1500 rpm during the dropping process, the mixed solution is added from below the liquid surface with a dropping speed of 5~20 mL / min, the stirring speed is reduced to 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 ≤1.0 mS / cm.

6. The method for preparing carbon-coated doped manganese ferropyrophosphate according to claim 1, characterized in that: In step (4), the phosphorus source is phosphoric acid, the reducing agent is vitamin C, and the molar ratio of the phosphorus source to the ammonia source is 1:

1. The phosphorus source is prepared according to PO4. 3- Calculated, soluble ammonia source as NH4 + count; The filter cake A, after being mixed with pure water, has a solid content of 10-20 wt% and a stirring time of 30-60 min. After heating, the stirring time continues for 60-120 min. The conditions for ending the multiple filtrations and washings are that the conductivity of the filtrate is ≤0.2 mS / cm.

7. The method for preparing carbon-coated doped manganese ferropyrophosphate according to claim 1, characterized in that: In step (5), the carbon source is one or more of starch, sucrose, glucose, polyethylene glycol and dextrin, and the carbon source accounts for 2 to 8 wt% of the total mass of the raw materials. The dopant is a raw material that forms doping of any one or more elements such as Ti, Mg, Nb, Mo and V, and the dopant accounts for 0.1 to 0.5 wt% of the total mass of the raw materials. Except for pure water, the solid content of the slurry is 20 to 30 wt%.

8. The method for preparing carbon-coated doped manganese ferropyrophosphate according to claim 1, characterized in that: In step (5), the inert atmosphere is nitrogen or argon, the calcination temperature is 500~700℃, the calcination time is 6~10h, and finally the particles are crushed to a particle size D50≤1.0μm.

9. The application of carbon-coated manganese iron pyrophosphate prepared by any one of claims 1-8 in the preparation of battery cathode materials.

10. The application according to claim 9, characterized in that: The positive electrode material of the battery is lithium manganese iron phosphate, wherein the molar ratio of lithium source and carbon-coated doped manganese iron pyrophosphate is 1.0-1.05:1.0, the lithium source is calculated as Li, and the carbon-coated doped manganese iron pyrophosphate is calculated as Fe+Mn.

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