Ammonium manganese ferrophosphate, lithium manganese ferrophosphate and their preparation methods and applications

Abstract

The present invention provides ammonium manganese ferrous phosphate, lithium manganese ferrous phosphate and their preparation method and application, belonging to the technical field of inorganic material preparation; the present invention uses the corrosion reaction of iron powder and manganese powder in water, the hydrolysis reaction of ammonium ions and the regulation of the pH of the reaction system by ammonia water to make the metal ions react with NH3 and PO4 in the solution. 3‑ By combining to form ammonium manganese ferrous phosphate, Fe and Mn can be completely converted into the product, achieving uniform mixing of Fe, Mn, and P at the atomic level, and ammonium ions in the reaction solution can be recycled, greatly improving atom economy; the ammonium manganese ferrous phosphate is calcined to obtain manganese hydrogen phosphate, and then calcined with a lithium source and a carbon source to finally obtain a manganese iron lithium phosphate material having spherical secondary particles with high purity and high tap density, and a nanometer-scale and uniformly distributed particle size, and having good discharge specific capacity and rate performance. When applied to lithium-ion batteries, it can improve the electrochemical performance of lithium-ion batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic material preparation, and in particular relates to ammonium manganese iron phosphate, lithium manganese iron phosphate and a preparation method and application thereof. Background Art

[0002] Against the backdrop of resource scarcity and the international trend of energy conservation and emission reduction, lithium-ion batteries have become the most popular chemical energy storage devices in current research due to their many advantages, including high specific energy, no memory effect, high safety, and long cycle life. As a core component that determines battery capacity and cost, the optimization of lithium-ion battery cathode materials is crucial to improving battery performance. Currently, there are many types of lithium-ion battery cathode materials on the market. Among them, lithium iron manganese phosphate cathode materials stand out among many cathode materials because they combine the excellent rate performance of lithium iron phosphate with the advantages of the high voltage platform of lithium manganese phosphate, as well as the advantages of low cost and high safety.

[0003] At present, the synthesis methods of lithium manganese iron phosphate mainly include solid-phase method, co-precipitation method and hydrothermal method. The solid-phase method mainly forms lithium manganese iron phosphate material by directly mixing raw materials such as iron source, manganese source, phosphorus source, lithium source and then sintering them at high temperature. However, this method has high energy consumption, the product particle size is difficult to control, and uniform mixing at the atomic level cannot be achieved; the hydrothermal method requires a high temperature and high pressure environment, and has high requirements for production equipment. There are problems such as cumbersome production operations, poor safety, high energy consumption and low production capacity; the co-precipitation method first uses a wet method to synthesize a manganese iron phosphate precursor, and then mixes it with a lithium source and sintered. The synthesis route has mild conditions, and the prepared product has uniform element distribution, precise and adjustable proportions, and stable quality. It is currently a popular route.

[0004] Ammonium manganese ferric phosphate contains three elements, Fe, Mn, and P, and has a suitable stoichiometric ratio and stable physical and chemical properties. It is an ideal precursor for the preparation of lithium manganese ferric phosphate. The current methods for preparing ammonium manganese ferric phosphate mainly include co-precipitation and mechanical activation. The co-precipitation method is to mix ferrous salts, manganese salts and phosphorus sources, and carry out co-precipitation reaction under appropriate pH and temperature conditions to produce ammonium manganese ferric phosphate; the mechanical activation method is to mix manganese salts, iron salts and phosphorus sources, and carry out mechanical activation in a ball mill to fully mix, crush and react the raw materials to produce ammonium manganese ferric phosphate; but both methods use metal salts as raw materials, so they will produce a large amount of unusable anions and have low atomic utilization. Summary of the Invention

[0005] The purpose of the present invention is to provide ammonium manganese iron phosphate, lithium manganese iron phosphate and their preparation methods and applications, so as to solve the problems raised in the background technology.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] The first aspect of the present invention provides a method for preparing ammonium iron manganese phosphate, comprising the following steps: Under an inert atmosphere and stirring conditions, iron powder and manganese powder with a molar ratio of 1.5 - 9:1 are added to an appropriate amount of ammonium salt solution. Subsequently, a solution containing a metal-free phosphorus source and an ammonium source is added, and ammonia water is added to keep the pH of the reaction system alkaline. Stirring is continued for the reaction. When the rate of bubble generation in the reaction system slows down, the amount of ammonia water added is adjusted to make the pH of the reaction system weakly acidic, and stirring is continued until the reaction ends. The solid product is filtered out, washed, and dried to obtain ammonium iron manganese phosphate.

[0008] Preferably, the inert atmosphere is argon or nitrogen, the reaction stirring rate is 300 - 600 rpm, the reaction temperature is 25 - 80 °C, the reaction time after complete feeding is 1 - 4 h, the weak alkalinity is: 7.0 < pH ≤ 8.5, and the weak acidity is: 5.0 ≤ pH ≤ 6.5.

[0009] Preferably, the ammonium salt solution is one or two of ammonium sulfate solution and ammonium chloride solution, the solution containing a metal-free phosphorus source and an ammonium source is one or more of ammonium phosphate solution, ammonium dihydrogen phosphate solution, and diammonium hydrogen phosphate solution, and the molar amounts of the phosphorus source and the ammonium source in the solution containing a metal-free phosphorus source and an ammonium source are both 1 - 1.2 times the total molar amount of the manganese powder and the iron powder.

[0010] Preferably, the concentration of the ammonium salt solution is 0.1 - 1 mol / L, the concentration of the solution containing a metal-free phosphorus source and an ammonium source is 0.5 - 1 mol / L, and the concentration of the ammonia water is 1 - 2 mol / L.

[0011] The second aspect of the present invention provides an ammonium iron manganese phosphate prepared by the above preparation method, and its chemical formula is NH4Fe x Mn 1-x PO4, 0 < x ≤ 0.4.

[0012] The third aspect of the present invention provides a method for preparing lithium iron manganese phosphate using the above ammonium iron manganese phosphate, comprising the following steps:

[0013] 1) Place the ammonium iron manganese phosphate under argon or nitrogen and calcine it at a temperature of 200 °C - 400 °C for 3 - 8 h to obtain a manganese iron hydrogen phosphate powder material;

[0014] 2) Disperse the manganese iron hydrogen phosphate powder material, lithium source, and carbon source in step 1) in a solvent. After sand grinding and spray drying, calcine it at a temperature of 600 - 800 °C for 6 - 8 h under an inert atmosphere to obtain lithium iron manganese phosphate.

[0015] Preferably, in step 2), the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium acetate, the carbon source is one or more of glucose, sucrose, starch, and cellulose, the molar amount of the lithium source is 1-1.03 times the total molar amount of iron and manganese in the ferromanganese hydrogen phosphate powder, and the mass of the carbon source is 1%-20% of the mass of the ferromanganese hydrogen phosphate powder.

[0016] Preferably, in step 2), the solvent is at least one of water and ethanol, and the total mass of the ferromanganese hydrogen phosphate powder, lithium source and carbon source solids accounts for 20%-40% of the total mass of the solids and the solvent.

[0017] The fourth aspect of the present invention provides a lithium manganese iron phosphate prepared by the above preparation method, whose chemical formula is LiFe y Mn 1-y PO4,0 <y≤0.4。

[0018] A fifth aspect of the present invention provides the use of lithium manganese iron phosphate in a positive electrode material for lithium-ion batteries.

[0019] The beneficial effects of the present invention are:

[0020] 1. The present invention utilizes the corrosion reaction of iron powder and manganese powder in water to produce Fe 2+ 、Mn 2+ Through the hydrolysis reaction of ammonium ions and the regulation of pH of the reaction system by ammonia water, the metal ions in the solution react with NH4 + and PO4 3- Combined to form ammonium manganese ferrous phosphate, Fe and Mn can be completely converted into the product, achieving uniform mixing of Fe, Mn, and P at the atomic level and precise adjustment of the stoichiometric ratio of Fe, Mn, and P in ammonium manganese ferrous phosphate. The remaining ammonium ions in the reaction solution can be recycled, greatly improving the atom economy. The process is simple and easy to control, with low equipment requirements and mild reaction conditions.

[0021] 2. In the early and middle stages of the reaction, the pH of the reaction system is regulated to be alkaline by ammonia water, so as to give full play to the complexing effect of NH3 and make the metal ions generated by the corrosion reaction form complexes (M (NH3)2 2+ , M=Mn, Fe) exists in the solution in the form of hydroxide precipitates instead of forming hydroxide precipitates that coat the surface of the metal particles, thereby promoting sufficient corrosion of the metal and generating a large number of bubbles. As the reaction proceeds, the bubble generation rate gradually slows down, and the formation of ammonium manganese ferrous phosphate precipitates causes the pH value of the system to gradually decrease. In the later stage of the reaction, ammonia water is used to adjust the reaction system to a specific weak acid environment to ensure sufficient precipitation of ammonium manganese ferrous phosphate, thereby improving the product yield;

[0022] 3. The ammonium manganese ferric phosphate obtained by the preparation method of the present invention is high in purity, free of impurities, and has a nano-sheet structure. As a result, the lithium manganese iron phosphate material synthesized using it as a precursor has high purity, spherical secondary particles, high tap density, and nanometer-scale and uniformly distributed particle size. The fine particles shorten the lithium ion transmission path, effectively improving the discharge capacity and rate performance of the lithium manganese iron phosphate material.

[0023] 4. The metal ions in the ammonium manganese ferric phosphate obtained by the preparation method of the present invention are all +2 valence, so when it is used as a precursor for sintering, no additional carbon thermal reduction is required, which reduces the amount of carbon source used, and the NH4 + The prepared lithium manganese iron phosphate material has a porous structure, which is not only conducive to the infiltration of the electrolyte, but also improves the Li + The accessibility inside the microspheres can give full play to the performance of the material, increase the specific surface area of ​​the material, increase the surface active sites, and improve the specific energy of the lithium manganese iron phosphate material;

[0024] 5. The present invention pre-calcines ammonium ferromanganese phosphate at a low temperature to remove adsorbed water, crystallization water and ammonia to obtain ferromanganese hydrogen phosphate powder, and then uses sand milling, spray drying and solid phase sintering to synthesize ferromanganese phosphate material. This process is mature, has no harsh equipment and synthesis conditions, has a high safety factor, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 XRD patterns of ammonium manganese ferric phosphate samples A1, A2, A3, and B1 prepared in Example 1, Example 2, Example 3, and Comparative Example;

[0026] Figure 2 is a SEM image of ammonium manganese ferric phosphate sample A1 prepared in Example 1;

[0027] Figure 3 is a SEM image of the ammonium manganese ferric phosphate sample B1 prepared in the comparative example;

[0028] Figure 4 1 is the XRD pattern of lithium manganese iron phosphate samples C1 and D1 prepared in Example 1 and the comparative example;

[0029] Figure 5 This is a SEM image of the lithium manganese iron phosphate sample C1 prepared in Example 1;

[0030] Figure 6 This is an SEM image of the lithium iron manganese phosphate sample D1 prepared in the comparative example;

[0031] Figure 7 The charge and discharge curves of test battery 1 and test battery 2 at a rate of 0.2C are shown;

[0032] Figure 8 Figure 2 is the rate performance diagram of test battery 1 and test battery 2. DETAILED DESCRIPTION

[0033] The present invention is described in detail below through examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above-mentioned content of the present invention. In the following examples, reagents and instruments not specifically described are commercially available, and experimental operations not specifically described are carried out according to the manufacturer's instructions or conventional techniques in the field. Unless otherwise defined, all professional and scientific terms used in the text have the same meanings as those familiar to those skilled in the art. In addition, any methods and materials similar to or equivalent to those described can be applied to the present invention; the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein;

[0034] The first aspect of the present invention provides a method for preparing ammonium manganese ferric phosphate, comprising the following steps:

[0035] Under an inert atmosphere and a temperature of 25-80°C, at a stirring rate of 300-600 rpm, iron powder and manganese powder at a molar ratio of 1.5-9:1 are added to a 0.1-1 mol / L ammonium salt solution, followed by addition of a solution containing a metal ion-free phosphorus source and an ammonium source at a concentration of 0.5-1 mol / L, wherein the molar amounts of the phosphorus source and the ammonium source are 1-1.2 times the total molar amount of the manganese powder and the iron powder. 1-2 mol / L ammonia water is added to maintain the pH of the reaction system at a weak alkaline level. The reaction is continuously stirred and carried out. After the rate of bubble generation in the reaction system slows down, the amount of ammonia water added is adjusted to make the pH of the reaction system weakly acidic. After continuing the stirring reaction for 1-4 hours, the solid product is filtered out, washed, and dried to obtain ammonium manganese ferrous phosphate.

[0036] In some preferred embodiments of the present invention, the inert atmosphere is argon or nitrogen. The role of the inert atmosphere is to avoid Fe 2+ the reaction stirring rate may also be selected as 350rpm, 400rpm, 450rpm, 500rpm, 550rpm; the reaction temperature may also be selected as 30°C, 40°C, 50°C, 60°C, 70°C, and the reaction time may also be selected as 1.5h, 2h, 2.5h, 3h, 3.5h.

[0037] In some preferred embodiments of the present invention, the weak alkalinity is defined as: 7.0 < pH ≤ 8.5, and the weak acidity is defined as: 5.0 ≤ pH ≤ 6.5; the weak alkalinity can also be selected as: 7.3, 7.5, 7.8, 8.0, 8.2, 8.4; the weak acidity can also be selected as 5.2, 5.4, 5.6, 5.8, 6.0, 6.2, 6.4;

[0038] In some preferred embodiments of the present invention, the ammonium salt solution is one or both of ammonium sulfate solution and ammonium chloride solution, and the solution containing a metal - free ion phosphorus source and an ammonium source is one or more of ammonium phosphate solution, ammonium dihydrogen phosphate solution, and diammonium hydrogen phosphate solution.

[0039] In some preferred embodiments of the present invention, the ammonium salt solution can also be selected as 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, the concentration of the solution containing a metal - free ion phosphorus source and an ammonium source can also be selected as 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, and the concentration of the ammonia water can also be selected as 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L.

[0040] The technical principle of this preparation method is:

[0041] NH₄⁺ in the ammonium salt solution + undergoes a hydrolysis reaction to generate NH₃ (Reaction formula (1)), and then the subsequently added metal单质 M (M = Mn, Fe) undergoes a corrosion reaction with water, simultaneously generating three kinds of particles, M 2+ , H·, and OH⁻ on the surface of the metal particles (Reaction formula (2)). At this time, due to the strong complexing effect of NH₃ in an alkaline environment, the M 2+ generated by Reaction formula (2) will preferentially undergo a complexing reaction with NH₃ to generate the M(NH₃)₂ 2+ complex (Reaction formula (3)); the M(NH₃)₂ 2+ complex is transferred to the deep part of the solution with stirring and combines with PO₄ 3- and NH₄ + in the solution to form a precipitate (Reaction formula (4)), and this process releases NH₃ into the solution to combine with M 2+ again, forming a complete cycle; in addition, the H· generated in this process combines in pairs to generate hydrogen gas (Reaction formula (5)). The entire reaction process is as follows:

[0042] NH₄⁺ ++OH - →NH3 + H2O (1)

[0043] M + H2O → M 2+ + H· + OH - (2)

[0044] M 2+ + 2NH3 → M(NH3)2 2+ (3)

[0045] M(NH3)2 2+ + PO4 3- + NH4 + → NH4MPO4↓ + 2NH3 (4)

[0046] 2H· → H2↑ (5)

[0047] It should be noted that the role of the ammonium salt solution is to provide an "ammonium / ammonia" environment to promote the corrosion reaction of metal powder, which can exist in the solution for recycling. The solution containing a metal-free ion phosphorus source and an ammonium source is used to provide the NH4 + and PO4 3- .

[0048] The present invention also provides ammonium iron manganese phosphate prepared by using the above preparation method of ammonium iron manganese phosphate, and its chemical formula is NH4Fe x Mn 1-x PO4, 0 < x ≤ 0.4, with high purity, no impurity phase and having a nano-sheet structure.

[0049] The present invention also provides lithium iron manganese phosphate prepared by using the above-mentioned ammonium iron manganese phosphate. The chemical formula of the lithium iron manganese phosphate is LiFe y Mn 1-y PO4, 0 < y ≤ 0.4. This material has spherical secondary particles, high tap density, particle size in the nano-scale and uniform distribution, and has excellent discharge specific capacity and rate performance.

[0050] The present invention also provides a preparation method of lithium iron manganese phosphate as described above, including the following steps:

[0051] 1) Place the above-mentioned ammonium iron manganese phosphate under argon or nitrogen and calcine it at a temperature of 200°C - 400°C for 3 - 8 h to obtain manganese iron phosphate hydrogen powder material;

[0052] 2) Disperse the manganese iron phosphate hydrogen powder material, lithium source, and carbon source in step 1) in a solvent, and after sand grinding and spray drying, calcine it at a temperature of 600 - 800°C for 6 - 8 h in an inert atmosphere to obtain lithium iron manganese phosphate.

[0053] In some preferred embodiments of the present invention, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium acetate, the carbon source is one or more of glucose, sucrose, starch, and cellulose, the molar amount of the lithium source is 1-1.03 times the total molar amount of iron and manganese in ferromanganese hydrogen phosphate, and the mass of the carbon source is 1%-20% of the mass of the ferromanganese hydrogen phosphate powder; the molar amount of the lithium source can also be selected as 1.01 times or 1.02 times the total molar amount of iron and manganese in ferromanganese hydrogen phosphate; the mass of the carbon source can also be selected as 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, or 18% of the mass of the ferromanganese hydrogen phosphate powder.

[0054] In some preferred embodiments of the present invention, the solvent is at least one of water and ethanol, and the total solid mass of the manganese hydrogen phosphate ferrophosphate powder, lithium source and carbon source accounts for 20%-40% of the total mass of the solid and the solvent; the total solid mass can also account for 25%, 30%, or 35% of the total mass of the solid and the solvent.

[0055] The lithium manganese iron phosphate of the present invention is used as a positive electrode material in a lithium ion battery, which can make the lithium ion battery have excellent specific capacity and cycle performance.

[0056] Hereinafter, the ammonium manganese iron phosphate, lithium manganese iron phosphate, and their preparation methods and applications will be further described through the following specific examples.

[0057] Example 1

[0058] 1) Under nitrogen atmosphere, stirring rate of 400 rpm and temperature of 60 ° C, weigh 14.76g of metal manganese powder and 10g of metal iron powder and add them to 1000mL of 0.5 mol / L ammonium sulfate solution, then add 985mL of 0.5 mol / L ammonium dihydrogen phosphate solution, and then add 1 mol / L ammonia solution and control its feeding rate to maintain the pH value of the reaction system at 7.5. Continue stirring to react. When the bubble generation rate slows down, adjust the ammonia feeding rate to reduce the pH value of the reaction system to 6.0. After the feeding is complete, continue stirring and react for 2 hours. Filter the solid product, wash and dry it, and obtain NH4Fe 0.4 Mn 0.6 PO4·H2O ammonium manganese ferrous phosphate sample A1;

[0059] 2) calcining the ammonium manganese ferrous phosphate sample A1 prepared in step 1) at 300° C. for 4 h under an argon atmosphere to obtain manganese ferrous hydrogen phosphate powder;

[0060] 3) The ferromanganese hydrogen phosphate powder, lithium carbonate, and glucose prepared in step 2) are dispersed in water at a molar ratio of (Fe+Mn):Li=1:1.02 and a glucose mass of 10% of the mass of the ferromanganese hydrogen phosphate powder, so that the total solid mass of the ferromanganese hydrogen phosphate powder, lithium carbonate, and glucose accounts for 30% of the total mass of the solid and solvent. After sand milling and spray drying, the mixture is calcined at 600°C for 8h under an argon atmosphere to obtain LiFe 0.4 Mn 0.6 PO4 lithium manganese iron phosphate sample C1.

[0061] Example 2

[0062] Under nitrogen atmosphere, stirring rate of 600 rpm and temperature of 40 ° C, 17.22g of metal manganese powder and 7.5g of metal iron powder were weighed and added to 1667mL of 0.3 mol / L ammonium sulfate solution, followed by adding 470mL of 1.0 mol / L ammonium dihydrogen phosphate solution, and then adding 2 mol / L ammonia solution and controlling the feeding rate to maintain the pH value of the reaction system at 7.5. The reaction was stirred continuously and the reaction was carried out. When the bubble generation rate slowed down, the ammonia feeding rate was adjusted to reduce the pH value of the reaction system to 6.5. After the feeding was completed, the reaction was stirred for 4h, and the solid product was filtered out for washing and drying to obtain NH4Fe 0.3 Mn 0.7 PO4·H2O ammonium manganese ferrous phosphate sample A2;

[0063] 2) calcining the ammonium manganese ferrous phosphate sample A2 prepared in step 1) at 350° C. for 3 h under an argon atmosphere to obtain manganese ferrous hydrogen phosphate powder;

[0064] 3) The ferromanganese hydrogen phosphate powder, lithium hydroxide, and sucrose prepared in step 2) were dispersed in ethanol at a molar ratio of (Fe+Mn):Li=1:1.03 and a sucrose mass ratio of 5% of the ferromanganese hydrogen phosphate powder mass, so that the total solid mass of the ferromanganese hydrogen phosphate powder, lithium hydroxide, and sucrose accounted for 30% of the total solid mass and solvent mass. After sand milling and spray drying, the mixture was calcined at 700°C for 6h under an argon atmosphere to obtain LiFe 0.3 Mn 0.7 PO4 lithium manganese iron phosphate sample C2.

[0065] Example 3

[0066] Under nitrogen atmosphere, stirring rate of 500 rpm and temperature of 30 ° C, 19.67g of metal manganese powder and 5g of metal iron powder were weighed and added to 1667mL of 0.3 mol / L ammonium sulfate solution, followed by 985mL of 0.5mol / L ammonium dihydrogen phosphate solution, and then 2 mol / L ammonia solution was added and the feeding rate was controlled to maintain the pH value of the reaction system at 7.5. The reaction was stirred continuously and the reaction was carried out. When the bubble generation rate slowed down, the ammonia feeding rate was adjusted to reduce the pH value of the reaction system to 5.0. After the feeding was completed, the reaction was stirred for 2 h, and the solid product was filtered out for washing and drying to obtain NH4Fe 0.2 Mn 0.8 PO4·H2O ammonium manganese ferric phosphate sample A3;

[0067] 2) calcining the ammonium manganese ferrous phosphate sample A3 prepared in step 1) at 400° C. for 4 h under an argon atmosphere to obtain manganese ferrous hydrogen phosphate powder;

[0068] 3) According to the molar ratio (Fe+Mn):Li=1:1.03, the mass of glucose is 5% of the mass of the ferromanganese hydrogen phosphate powder, and the ferromanganese hydrogen phosphate powder, lithium hydroxide, and glucose are dispersed in water so that the total mass of the solids of the ferromanganese hydrogen phosphate powder, lithium hydroxide, and glucose accounts for 30% of the total mass of the solid and solvent. After sand milling and spray drying, the mixture is calcined at 800°C for 6 h under an argon atmosphere to obtain LiFe 0.2 Mn 0.8 PO4 lithium manganese iron phosphate sample C3.

[0069] Comparative Example

[0070] 1) Manganese sulfate and ferrous sulfate metal salt are dissolved in water at a molar ratio of 6:4 to obtain a base liquid. Under a nitrogen atmosphere, 0.5 mol / L ammonium dihydrogen phosphate solution is added to the base liquid at a molar ratio of (Fe+Mn):P=1:1.1. The stirring speed is controlled at 400 rpm and the reaction temperature is 60°C. Then, 1 mol / L ammonia aqueous solution is added and the feeding rate is controlled so that the pH value of the reaction system is 6.0. After the feeding is complete, the reaction is stirred for 2 hours. After the reaction is completed, solid-liquid separation, washing, and drying are performed to obtain NH4Fe 0.4 Mn 0.6 PO4·H2O ammonium manganese ferric phosphate sample B1;

[0071] 2) The ammonium manganese ferrous phosphate sample B1 prepared in step 1) was calcined at 300°C for 4 h under an argon atmosphere to obtain manganese ferrous hydrogen phosphate powder;

[0072] 3) According to the molar ratio (Fe+Mn):Li=1:1.02, the mass of glucose is 10% of the mass of ferromanganese hydrogen phosphate powder, and ferromanganese hydrogen phosphate powder, lithium carbonate, and glucose are dispersed in water so that the total mass of ferromanganese hydrogen phosphate powder, lithium carbonate and glucose solids accounts for 30% of the total mass of solids and solvent. After sand grinding and spray drying, it is calcined at 600℃ for 8h under Ar atmosphere to obtain LiFe 0.4 Mn 0.6 PO4 lithium manganese iron phosphate sample D1.

[0073] Experimental Example 1

[0074] The ammonium manganese ferric phosphate samples A1, A2, A3 prepared in Example 1, Example 2, and Example 3 and the ammonium manganese ferric phosphate sample B1 prepared in Comparative Example 1 were subjected to X-ray diffraction (XRD) characterization analysis. The results are as follows: Figure 1 As shown by Figure 1 It can be seen that the characteristic diffraction peaks of the ammonium manganese ferrous phosphate samples A1, A2, A3, and B1 prepared in Example 1, Example 2, Example 3, and the comparative example all correspond to standard ammonium manganese ferrous phosphate, indicating that the ammonium manganese ferrous phosphate was successfully prepared and no impurities were present. Although both the present invention and the comparative example can successfully prepare ammonium manganese ferrous phosphate, by comparison, the preparation process of the present invention has extremely high atom economy. Starting from the metal powder, the Fe and Mn elements can be fully utilized and converted into the product, and the filtrate can be recycled, while the comparative example will inevitably produce a large amount of NH4 + 、SO4 2- wastewater, causing waste of resources.

[0075] Experimental Example 2

[0076] The ammonium manganese ferric phosphate sample A1 prepared in Example 1 and the ammonium manganese ferric phosphate sample B1 prepared in Comparative Example 1 were observed by scanning electron microscopy (SEM). Figure 2 and Figure 3 As shown by Figure 2 It can be seen that the ammonium manganese ferric phosphate sample A1 prepared in Example 1 has a nano-sheet structure with a sheet thickness of about 50nm. Figure 3 It can be seen that the ammonium manganese ferric phosphate sample B1 prepared in the comparative example does not have a nano-sheet structure. It is composed of blocks stacked on each other, and the thickness of a single block is about 2-3 μm. The morphology of the precursor has an important influence on the morphology and performance of the positive electrode material. The nano-sheet structure ammonium manganese ferric phosphate prepared by the present invention can be broken into particles with relatively small three-dimensional dimensions after the sand milling process, thereby shortening the Li +The transmission path is beneficial to improving the electrochemical performance of lithium manganese iron phosphate materials, while the three scales of ammonium manganese iron phosphate prepared in the comparative example are all micron-level, and it is difficult to obtain particles with fine particle size through the same sand milling process. Therefore, the ammonium manganese iron phosphate precursor prepared by the present invention has better morphology.

[0077] Experimental Example 3

[0078] The lithium manganese iron phosphate sample C1 prepared in Example 1 and the lithium manganese iron phosphate sample D1 prepared in Comparative Example 1 were subjected to X-ray diffraction (XRD) characterization analysis. The results are as follows: Figure 4 As shown by Figure 4 It can be seen that the characteristic diffraction peaks of the lithium iron manganese phosphate samples C1 and D1 prepared in Example 1 and the comparative example all correspond to standard lithium iron manganese phosphate, indicating that the lithium iron manganese phosphate was successfully prepared and no impurity phases were present.

[0079] Experimental Example 4

[0080] The lithium manganese iron phosphate sample C1 prepared in Example 1 and the lithium manganese iron phosphate sample D1 prepared in Comparative Example 1 were observed by scanning electron microscopy (SEM). Figure 5 and Figure 6 As shown by Figure 5 It can be seen that the lithium manganese iron phosphate sample C1 prepared in Example 1 is composed of secondary spherical particles formed by primary particles, the primary particles are small and the particle size distribution is uniform; Figure 6 It can be seen that the lithium manganese iron phosphate sample D1 prepared in the comparative example is composed of secondary spherical particles formed by primary particles, and the primary particles are large in size and unevenly distributed. This further explains that the use of ammonium manganese iron phosphate precursor with a nanosheet morphology to prepare lithium manganese iron phosphate can effectively reduce the particle size of the product. The fine particles shorten the path of lithium ion transmission and improve the discharge capacity and rate performance of the lithium manganese iron phosphate material.

[0081] Experimental Example 5

[0082] The lithium manganese iron phosphate material sample C1 prepared in Example 1 of the present invention and the lithium manganese iron phosphate material sample D1 prepared in the comparative example were used as positive electrode materials for lithium-ion batteries and assembled into CR2032 button batteries. The battery charge and discharge performance was tested using the Xinwei battery testing system:

[0083] Preparation of button cells:

[0084] The lithium iron manganese phosphate material and the conductive agent Super P were ground in a mortar for 20 minutes to mix them evenly, and then the binder polyvinylidene fluoride (PVDF) solution and an appropriate amount of nitrogen methyl pyrrolidone (NMP) solvent were added. After another 10 minutes of grinding, the resulting slurry was evenly coated on a carbon-coated aluminum foil, and then dried at 120°C in a vacuum oven for 8 hours. It was then punched out using a sheet puncher to obtain a circular positive electrode sheet with a diameter of 12 mm, wherein the lithium iron manganese phosphate material, the conductive agent and the binder were in a mass fraction ratio of 8:1:1; the above-mentioned positive electrode sheet was used as the positive electrode, the metal lithium sheet was used as the negative electrode, the polypropylene porous membrane was used as the separator, and an equal amount of 1 mol / L LiPF6, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) were selected as the electrolyte to form a 2032 button battery in an argon-filled glove box.

[0085] Among them, the battery using the lithium manganese iron phosphate material sample prepared in Example 1 as the battery positive electrode material is test battery 1, and the battery using the lithium manganese iron phosphate material sample prepared in the comparative example as the battery positive electrode material is test battery 2.

[0086] 2. Performance test and results:

[0087] (1) Test battery 1 and test battery 2 were respectively subjected to constant current charge and discharge tests at a rate of 0.2C, with a voltage test range of 2.0-4.5V. The results are as follows: Figure 7 shown by Figure 7 As shown, the two are at 3.5V, 4.1V (vs. Li + / Li) have two pairs of typical charge and discharge platforms, corresponding to Fe 2+ / Fe 3+ 、Mn 2+ / Mn 3+ The first discharge capacities of test cell 1 and test cell 2 at 0.2C were 148.24 mAh g -1 and 144.95mAh·g -1 ,In addition, the voltage difference between the charge and discharge platforms of test cell 1 is slightly smaller than that of test cell 2, ,indicating that test cell 1 has smaller electrochemical polarization.

[0088] (2) Test battery 1 and test battery 2 were tested for specific capacity at rates of 0.2C, 0.5C, 1C, 2C, 5C, and 10C, respectively. Figure 8 As shown by Figure 8 It can be seen that the discharge specific capacity of test battery 1 at the rates of 0.2C, 0.5C, 1C, 2C, 5C, and 10C is 148.24 mAh g -1 、146.65mAh·g -1、144.90mAh·g -1 、141.28mAh·g -1 、136.08mAh·g -1 、130.30mAh·g -1 The discharge capacity of test battery 2 at the rates of 0.2C, 0.5C, 1C, 2C, 5C, and 10C were 144.95 mAh g -1 、139.94mAh·g -1 , 132.99mAh·g -1 、127.16mAh·g -1 、112.62mAh·g -1 , 92.07mAh·g -1； From the above test results, it can be seen that even at low rates, the two have comparable discharge specific capacities, but as the rate increases, the difference in discharge specific capacities between the two increases. Compared with test battery 2, test battery 1 has better rate performance, which further indicates that the lithium manganese iron phosphate prepared by the present invention has better discharge specific capacity and rate performance. This is attributed to the fact that the lithium manganese iron phosphate material has finer primary particles, which makes Li + Has a shorter diffusion path.

[0089] In summary, it should be noted that the above is only a preferred embodiment of the present invention and does not overly limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still make some simple deductions, replacements, or equivalent replacements of some of the technical features of the technical solutions described in the aforementioned embodiments without departing from the concept of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing ammonium manganese ferric phosphate, characterized in that: It includes the following steps: Under an inert atmosphere and stirring conditions, iron powder and manganese powder with a molar ratio of 1.5 - 9:1 are added to an appropriate amount of ammonium salt solution. Subsequently, a solution containing a metal-free phosphorus source and an ammonium source is added, and ammonia water is added to keep the pH of the reaction system weakly alkaline. Stirring is continued for the reaction. When the rate of bubble generation in the reaction system slows down, the amount of ammonia water added is adjusted to make the pH of the reaction system weakly acidic, and stirring is continued until the reaction ends. The solid product is filtered out, washed, and dried to obtain ammonium iron manganese phosphate.

2. The method for preparing ammonium manganese ferric phosphate according to claim 1, wherein: The inert atmosphere is argon or nitrogen. The reaction stirring rate is 300 - 600 rpm, the reaction temperature is 25 - 80 °C, the reaction time after complete feeding is 1 - 4 h. The weakly alkaline condition is: 7.0 < pH ≤ 8.5, and the weakly acidic condition is: 5.0 ≤ pH ≤ 6.

5.

3. The method for preparing ammonium manganese ferric phosphate according to claim 1, wherein: The ammonium salt solution is one or both of ammonium sulfate solution and ammonium chloride solution. The solution containing a metal-free phosphorus source and an ammonium source is one or more of ammonium phosphate solution, ammonium dihydrogen phosphate solution, and diammonium hydrogen phosphate solution. The molar amounts of the phosphorus source and the ammonium source in the solution containing a metal-free phosphorus source and an ammonium source are both 1 - 1.2 times the total molar amount of the manganese powder and the iron powder.

4. The method for preparing ammonium manganese ferric phosphate according to claim 1, wherein: The concentration of the ammonium salt solution is 0.1 - 1 mol / L, the concentration of the solution containing a metal-free phosphorus source and an ammonium source is 0.5 - 1 mol / L, and the concentration of the ammonia water is 1 - 2 mol / L.

5. A method for preparing lithium manganese iron phosphate, characterized in that: It includes the following steps: 1) Prepare ammonium iron manganese phosphate according to the preparation method of ammonium iron manganese phosphate described in any one of claims 1 - 4. Place the ammonium iron manganese phosphate under argon or nitrogen and calcine it at a temperature of 200 °C - 400 °C for 3 - 8 h to obtain a manganese iron hydrogen phosphate powder material. 2) Disperse the manganese iron hydrogen phosphate powder material, lithium source, and carbon source in step 1) in a solvent. After sand grinding and spray drying, calcine it at a temperature of 600 - 800 °C for 6 - 8 h under an inert atmosphere to obtain lithium iron manganese phosphate.

6. The method for preparing lithium manganese iron phosphate according to claim 5, wherein: In step 2), the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium acetate. The carbon source is one or more of glucose, sucrose, starch, and cellulose. The molar amount of the lithium source is 1 - 1.03 times the total molar amount of iron and manganese in the manganese iron hydrogen phosphate powder material. The mass of the carbon source is 1% - 20% of the mass of the manganese iron hydrogen phosphate powder material.

7. The method for preparing lithium manganese iron phosphate according to claim 5, wherein: In step 2), the solvent is at least one of water and ethanol. The total mass of the solid of the manganese iron hydrogen phosphate powder material, lithium source, and carbon source accounts for 20% - 40% of the total mass of the solid and the solvent.

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