Manganese iron phosphate and method for preparing same, lithium manganese iron phosphate and method for preparing same

Abstract

The application discloses manganese iron phosphate and a preparation method thereof, lithium manganese iron phosphate and a preparation method thereof. The manganese iron phosphate is secondary particles formed by agglomeration of primary particles, and the manganese iron phosphate comprises a core and a shell layer located on the surface of the core, the porosity of the core is less than the porosity of the shell layer, wherein the radius of the manganese iron phosphate is R, the radius of the core is r1, the thickness of the shell layer is r2, r1 / R = 0.3-0.6, r2 / R = 0.4-0.7, and (r1+r2) / R = 1. Thus, the porosity of the core of the manganese iron phosphate is lower than that of the shell layer, a porosity gradient distribution is formed, the grinding efficiency of the manganese iron phosphate can be effectively improved, the gram capacity of the prepared lithium manganese iron phosphate is higher, the consistency of the deintercalation degree of lithium in the charging and discharging process is higher, and the gram capacity is better.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries, specifically to iron manganese phosphate and its preparation method, and lithium manganese iron phosphate and its preparation method. Background Technology

[0002] With the continuous development of the new energy industry, consumers are constantly raising new requirements for battery performance. Lithium iron phosphate (LFP) has become the preferred positive electrode active material for batteries due to its relatively good electrochemical performance and excellent safety performance. However, the actual capacity of LFP has approached its theoretical capacity, and further improvements in electrochemical performance are not possible. Introducing manganese into LFP to raise its voltage plateau can improve capacity. Furthermore, lithium manganese iron phosphate (LFP) and ternary cathodes have similar potential windows, and the two can be mixed in any proportion, further expanding its application scenarios. LFP has thus become a hot topic in development and research.

[0003] Lithium manganese iron phosphate (LMP) is typically produced by sintering a precursor containing manganese and iron with a lithium source. The characteristics of the manganese-iron precursor play a decisive role in the electrochemical properties of LMP. Common precursor materials include ferromanganese phosphate, ammonium manganese phosphate, and ferromanganese carbonate.

[0004] It should be noted that the above statements are only used to provide background information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0005] In a first aspect of this application, a ferromanganese phosphate is provided, wherein the ferromanganese phosphate is a secondary particle formed by the agglomeration of primary particles, the ferromanganese phosphate includes a core and a shell layer located on the surface of the core, the porosity of the core is less than the porosity of the shell layer, wherein the radius of the ferromanganese phosphate is R, the radius of the core is r1, the thickness of the shell layer is r2, r1 / R = 0.3-0.6, r2 / R = 0.4-0.7, and (r1+r2) / R = 1. Therefore, the core porosity of this manganese iron phosphate is lower than that of the shell, forming a porosity gradient distribution. The thickness ratio of the core to the shell is relatively moderate. The porous structure can effectively improve the grinding efficiency of manganese iron phosphate, which is beneficial to improving the subsequent preparation efficiency of lithium manganese iron phosphate. The dense core makes the material structurally stable, and the porous shell helps to expose abundant reactive sites, promoting a full and uniform reaction with the lithium source to generate lithium manganese iron phosphate with uniform elemental distribution. This effectively improves the elemental distribution uniformity and structural stability of the prepared lithium manganese iron phosphate. The lithium manganese iron phosphate has a high specific capacity and a high consistency in the degree of lithium insertion / extraction during charging and discharging, resulting in better specific capacity performance.

[0006] In some embodiments, the manganese and iron elements in the ferromanganese phosphate are uniformly distributed in the core and the shell. This helps to improve the elemental uniformity of the subsequently produced lithium manganese iron phosphate.

[0007] In some embodiments, the porosity of the core is 8%-20%, and / or the pore size of the core is 10nm-50nm. Therefore, the internal structure of manganese iron phosphate is relatively dense and has high structural stability.

[0008] In some embodiments, the porosity of the shell is 25%-50%, and / or the pore size of the shell is 10nm-60nm. Thus, the external structure of manganese iron phosphate is relatively loose, which helps to expose more reactive sites.

[0009] In some embodiments, the porosity of the ferromanganese phosphate is 25%-40%, and / or the specific surface area of ​​the ferromanganese phosphate is 40 m². 2 / g-100m 2 / g. This helps expose more reactive sites, which facilitates a full reaction with the lithium source to form lithium manganese iron phosphate.

[0010] In some embodiments, the Dv50 particle size of the ferromanganese phosphate is 1.0 μm-2.5 μm, and / or the primary particle size of the ferromanganese phosphate is 20 nm-80 nm. Therefore, lithium iron phosphate has a smaller particle size, facilitating grinding into small particles.

[0011] In a second aspect of this application, a method for preparing the aforementioned ferromanganese phosphate is proposed, comprising: adding hydrogen peroxide to a metal ion solution to obtain a first solution, wherein the metal ion solution comprises divalent manganese ions and divalent iron ions, and the pH of the metal ion solution is ≤3; adding hydrogen peroxide to the first solution under alkaline conditions and performing a first heating treatment to obtain a second solution; adding a supplementary oxidant to the second solution to obtain a third solution; adding phosphoric acid to the third solution and performing a second heating treatment to obtain the ferromanganese phosphate. Thus, through stepwise oxidation, both manganese and iron ions can be oxidized to trivalent states while forming a loose ferromanganese precipitate with a small particle size, which is then subjected to a crystallization process to obtain ferromanganese phosphate with a uniform distribution of manganese and iron elements.

[0012] In some embodiments, the molar ratio of manganese ions to iron ions in the metal ion solution is (5-8):(2-5). Therefore, by adjusting the molar ratio of manganese ions to iron ions in the metal ion solution, the molar ratio of manganese ions to iron ions in ferric manganese phosphate can be controlled.

[0013] In some embodiments, the molar ratio of hydrogen peroxide added to the first solution to the molar ratio of hydrogen peroxide added to the second solution is 1-3. This helps to improve the utilization rate of hydrogen peroxide and achieve stepwise and complete oxidation of manganese and iron ions.

[0014] In some embodiments, the sum of the molar amounts of hydrogen peroxide added to the first solution and the second solution is 'a', and the sum of the molar amounts of divalent manganese ions and divalent iron ions in the metal ion solution is 'b', where a / b is 0.6-1.5. This facilitates the complete oxidation of manganese and iron ions.

[0015] In some embodiments, the supplementary oxidant includes one or more of ammonium persulfate, sodium persulfate, potassium permanganate, and potassium perchlorate, and the molar amount of the supplementary oxidant added to the third solution is c, where c / b ≤ 0.1. Thus, the addition of the supplementary oxidant facilitates the rapid and complete oxidation of manganese ions.

[0016] In some embodiments, the molar amount of phosphoric acid added is d, and the d / b ratio is 1-1.2. This facilitates the complete crystallization of ferromanganese precipitate to form ferromanganese phosphate.

[0017] In some embodiments, the pH of the alkaline conditions is 7.5-9.5, and / or the temperature of the first heat treatment is 40°C-70°C, and / or the temperature of the second heat treatment is 60°C-90°C. This helps to improve the yield of ferromanganese phosphate.

[0018] In a third aspect, this application proposes a method for preparing lithium manganese iron phosphate, comprising: mixing and sintering ferromanganese phosphate, a lithium source, and a carbon source at a high temperature to obtain the lithium manganese iron phosphate, wherein the ferromanganese phosphate includes ferromanganese phosphate prepared by the aforementioned method, or the aforementioned ferromanganese phosphate. Thus, lithium manganese iron phosphate with excellent electrochemical performance can be prepared by a simple process.

[0019] In a fourth aspect, this application proposes a lithium manganese iron phosphate, prepared using the aforementioned method. Therefore, this lithium manganese iron phosphate possesses all the features and advantages of the aforementioned method for preparing lithium manganese iron phosphate, which will not be repeated here. Attached Figure Description

[0020] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0021] Figure 1 This is a scanning electron microscope image of the manganese iron phosphate prepared in Example 1 of this application;

[0022] Figure 2A scanning electron microscope image of the cross section of the manganese iron phosphate prepared in Example 1 of this application;

[0023] Figure 3 This is a scanning electron microscope-energy scattering spectrum of lithium iron phosphate prepared in Example 1 of this application;

[0024] Figure 4 This is an X-ray diffraction pattern of lithium iron phosphate prepared in Example 1 of this application. Detailed Implementation

[0025] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit the application; unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0026] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are open-ended expressions, meaning they include what is specified in this application but do not exclude other aspects.

[0027] In the description of this application, all figures disclosed herein, whether or not the words "approximately" or "about" are used, are approximate values. Each figure may vary by less than 10% or by a difference that is considered reasonable by one of the art, such as 1%, 2%, 3%, 4%, or 5%.

[0028] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0029] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. "First feature" and "second feature" may include one or more of the indicated feature.

[0030] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0031] In this application, the order in which the steps are written does not imply a strict execution order and does not limit the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps in this application can be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0032] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0033] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0034] In a first aspect of this application, a ferromanganese phosphate is provided, wherein the ferromanganese phosphate is a secondary particle formed by the agglomeration of primary particles, the ferromanganese phosphate includes a core and a shell layer located on the surface of the core, the porosity of the core is less than the porosity of the shell layer, wherein the radius of the ferromanganese phosphate is R, the radius of the core is r1, the thickness of the shell layer is r2, r1 / R = 0.3-0.6, r2 / R = 0.4-0.7, and (r1+r2) / R = 1. Therefore, the core porosity of this manganese iron phosphate is lower than that of the shell, forming a porosity gradient distribution. The thickness ratio of the core to the shell is relatively moderate. The porous structure can effectively improve the grinding efficiency of manganese iron phosphate, which is beneficial to improving the subsequent preparation efficiency of lithium manganese iron phosphate. The dense core makes the material structurally stable, and the porous shell helps to expose abundant reactive sites, promoting a full and uniform reaction with the lithium source to generate lithium manganese iron phosphate with uniform elemental distribution. This effectively improves the elemental distribution uniformity and structural stability of the prepared lithium manganese iron phosphate. The prepared lithium manganese iron phosphate has a high specific capacity and a high consistency of lithium insertion / extraction during charging and discharging, resulting in better specific capacity performance.

[0035] It should be noted that the core and shell in this application are only used to distinguish the structural regions of lithium iron phosphate. The two can be formed in one step, rather than forming the core first and then coating the core with a surface to form the shell.

[0036] As an example, r1 / R = 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, or 0.6.

[0037] As an example, r² / R can be 0.4, 0.45, 0.5, 0.55, 0.6, or 0.7.

[0038] When r1 / R and r2 / R are within the aforementioned ranges, the prepared lithium iron phosphate exhibits both high structural stability and excellent elemental distribution consistency, with superior compaction density and specific capacity.

[0039] In some embodiments, the manganese and iron elements in the ferromanganese phosphate are uniformly distributed in the core and the shell. This helps to improve the elemental uniformity of the subsequently produced lithium manganese iron phosphate, enhances the synergistic effect of manganese and iron, and ultimately improves the electrochemical performance of lithium manganese iron phosphate.

[0040] In some embodiments, the porosity of the core is 8%-20%, and / or the pore size of the core is 10nm-50nm. Therefore, the internal structure of manganese iron phosphate is relatively dense and has high structural stability.

[0041] As an example, the porosity of the kernel can be 8%, 10%, 12%, 14%, 16%, 18%, or 20%.

[0042] As an example, the aperture of the core can be 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, or 50nm.

[0043] In some embodiments, the porosity of the shell is 25%-50%, and / or the pore size of the shell is 10nm-60nm. Thus, the external structure of manganese iron phosphate is relatively loose, which helps to expose more reactive sites.

[0044] As an example, the porosity of the kernel can be 25%, 30%, 35%, 40%, 45%, or 50%.

[0045] As an example, the aperture of the core can be 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm or 60nm.

[0046] In some embodiments, the porosity of the ferromanganese phosphate is 25%-40%, and / or the specific surface area of ​​the ferromanganese phosphate is 40 m². 2 / g-100m 2 / g. This helps expose more reactive sites, which facilitates a full reaction with the lithium source to form lithium manganese iron phosphate.

[0047] As an example, the porosity of ferromanganese phosphate can be 25%, 30%, 35%, or 40%.

[0048] As an example, the specific surface area of ​​ferromanganese phosphate can be 40 m². 2 / g, 50m 2 / g、60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g or 100m 2 / g.

[0049] As an example, the porosity of ferromanganese phosphate can be obtained by the following method: porosity α of ferromanganese phosphate α = a1 / a2 × 100%; where a1 is the projected area of ​​the pores of the secondary particles of ferromanganese phosphate obtained by CP (ion milling), and a2 is the projected area of ​​the secondary particles of ferromanganese phosphate obtained by CP.

[0050] It can be understood that the porosity of the core region and the shell region can be measured using the same testing method as the overall porosity of the material, which will not be elaborated here.

[0051] As an example, the specific surface area of ​​manganese iron phosphate can be obtained by the following method: (1) Sample pretreatment: the sample is degassed under vacuum high temperature to remove impurities and water vapor adsorbed on the sample surface; (2) the sample is placed in a low temperature environment (liquid nitrogen temperature 77K), nitrogen is introduced to gradually increase the pressure, and the adsorption amount is recorded; (3) the adsorption amount is calculated according to the BET equation, the corresponding volume is obtained by linear fitting, and then the specific surface area is calculated.

[0052] In some embodiments, the Dv50 particle size of the manganese iron phosphate is 1.0 μm-2.5 μm.

[0053] In some embodiments, the primary particle size of the ferromanganese phosphate is 20 nm-80 nm. Therefore, the lithium iron phosphate has a smaller particle size, making it easier to grind into small particles.

[0054] As an example, the Dv50 particle size of ferromanganese phosphate can be tested using the following method: A Malvern 3000 laser particle size analyzer is used. The specific method is as follows: Deionized water is injected into the sample cell, ensuring the liquid surface submerges the measurement window; then background subtraction is performed to ensure a stable background signal; next, the dispersed sample is added to the sample cell, and the occlusion is adjusted to 5%-15%; finally, the "Start Test" button is clicked. After the test is completed, the instrument automatically generates a particle size distribution report, and the Dv50 value is directly displayed on the results interface.

[0055] As an example, the primary particle size of ferromanganese phosphate can be obtained by testing it as follows: Primary particle size: Statistically calculate the projected area of ​​each single crystal particle in the electron microscope, and then calculate its particle size. The specific method is as follows: Statistically calculate the projected area of ​​300 random primary particles in the scanning electron microscope image, regard the projected area as the area of ​​a standard circle with equal area, and then calculate the average diameter of the standard circle with equal area using the formula for the area of ​​a circle. This is the particle size of the primary particles of ferromanganese phosphate.

[0056] In some embodiments, the molecular formula of the manganese iron phosphate satisfies Fe X Mn 1-X PO4·xH2O, where 0<x≤0.8, and “xH2O” indicates that each mole of lithium manganese iron phosphate contains x mol of bound water, which is connected to lithium manganese iron phosphate through hydrogen bonds or coordinate bonds.

[0057] The preparation of lithium manganese iron phosphate (LFP) mainly includes two routes: solid-phase method and liquid-phase method. The solid-phase method typically requires two sintering processes, resulting in higher processing costs, and it is difficult to achieve atomic-level homogeneity in the mixing of manganese and iron. The liquid-phase method only requires one sintering process, saving processing costs. However, the poor uniformity of manganese and iron atom mixing in the liquid-phase precursor leads to uneven elemental distribution in the final LFP, hindering the full realization of the material's electrical properties. Therefore, the uniformity of manganese and iron mixing in the precursor has a significant impact on the electrochemical performance of LFP.

[0058] In this application, by controlling the distribution and use of hydrogen peroxide and the pH of the reaction process, the stepwise and complete oxidation of manganese and iron ions is achieved, thereby oxidizing both manganese and iron ions to the trivalent state and forming a loose, small-particle-size manganese-iron precipitate. Then, by adding an oxidant and phosphoric acid, manganese-iron crystallization is carried out, ultimately forming a manganese-iron phosphate precursor. This precursor has a uniform distribution of manganese and iron and a porous internal structure, which can significantly improve grinding efficiency and expose reactive sites, thereby reducing the processing cost of the positive electrode active material and improving electrochemical performance.

[0059] Compared to conventional methods of adding oxidants to metal ion solutions and introducing gas, the introduced gas often fails to achieve rapid and uniform distribution within the reaction system, leading to inconsistent reaction states and hindering the formation of a porous structure while achieving uniform and complete oxidation of metal ions, as described in this application. In this application, some hydrogen peroxide molecules added to the reaction system react with the active metal particles through oxidation, while the remainder decomposes to produce gas. This results in a uniform gas distribution within the reaction system, significantly improving the uniformity of elemental distribution and contributing to the formation of a more uniform porous structure.

[0060] In a second aspect of this application, a method for preparing the aforementioned ferromanganese phosphate is proposed. Through stepwise oxidation, both manganese and iron ions can be oxidized to the trivalent state while simultaneously forming a loose ferromanganese precipitate with small particle size. Subsequently, a crystallization treatment is performed to obtain ferromanganese phosphate with a uniform distribution of manganese and iron elements. Specifically, the method includes:

[0061] S1: Add hydrogen peroxide to the metal ion solution

[0062] In some embodiments, in this step, hydrogen peroxide is added to an acidic metal ion solution to obtain a first solution. Specifically, the metal ion solution includes divalent manganese ions and divalent iron ions, and the pH of the metal ion solution is ≤3.

[0063] Hydrogen peroxide undergoes a slow decomposition reaction in acidic metal ion solutions, oxidizing ferrous ions to ferric ions.

[0064] In some embodiments, this step can be performed at room temperature, for example, at 25°C.

[0065] In some embodiments, the molar ratio of manganese ions to iron ions in the metal ion solution is (5-8):(2-5). Therefore, by adjusting the molar ratio of manganese ions to iron ions in the metal ion solution, the molar ratio of manganese ions to iron ions in ferric manganese phosphate can be controlled.

[0066] As an example, the molar ratio of manganese ions to iron ions in a metal ion solution can be 5:5, 6:4, 7:3, or 8:2.

[0067] As an example, deionized water can be used as a solvent for metal ion solutions.

[0068] As an example, the sum of the molar concentrations of manganese ions and iron ions in a metal ion solution can be 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, or 2 mol / L.

[0069] In some embodiments, the amount of hydrogen peroxide added in step S1 is related to the iron ion content in the metal ion solution, so as to ensure that the iron ions in the metal ion solution are fully oxidized.

[0070] In some embodiments, the solute in the metal ion solution includes one or more of manganese sulfate, manganese nitrate, and manganese chloride, as well as one or more of ferrous sulfate, ferrous nitrate, and ferrous chloride, thereby introducing relatively stable ferrous ions and manganese ions into the metal ion solution.

[0071] S2: Under alkaline conditions, hydrogen peroxide is added to the first solution, and a first heat treatment is performed.

[0072] In some embodiments, in this step, hydrogen peroxide is added again under alkaline conditions and the mixture is heated to obtain a second solution.

[0073] Divalent manganese ions cannot be oxidized under acidic conditions. Adjusting the reaction system to an alkaline environment facilitates the complete oxidation of manganese ions, forming trivalent manganese ions and a loose, small-particle-size manganese-iron precipitate. Using high-temperature conditions accelerates the decomposition of hydrogen peroxide and the rate of the oxidation reaction, further promoting the complete oxidation of manganese ions.

[0074] In some embodiments, the molar ratio of hydrogen peroxide added to the first solution to the molar ratio of hydrogen peroxide added to the second solution is 1-3. This helps to improve the utilization rate of hydrogen peroxide and achieve stepwise and complete oxidation of manganese and iron ions.

[0075] As an example, the ratio of the molar amount of hydrogen peroxide added in the first solution to the molar amount of hydrogen peroxide added in the second solution can be 1, 1.5, 2, 2.5 or 3.

[0076] In some embodiments, the sum of the molar amounts of hydrogen peroxide added to the first solution and the second solution is 'a', and the sum of the molar amounts of divalent manganese ions and divalent iron ions in the metal ion solution is 'b', where a / b is 0.6-1.5. This facilitates the complete oxidation of manganese and iron ions.

[0077] As an example, a / b can be 0.6, 0.8, 1, 1.1, 1.2, 1.3, 1.4, or 1.5.

[0078] In some embodiments, the pH of the alkaline conditions is 7.5, 8, 8.5, 9, or 9.5. Alkaline conditions facilitate the oxidative change of manganese ions.

[0079] In some embodiments, the temperature of the first heat treatment is 40°C-70°C.

[0080] As an example, the temperature of the first heat treatment can be 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, or 70°C.

[0081] In some embodiments, an alkaline solution can be added to the first solution to create an alkaline environment in the reaction system.

[0082] As an example, the solute in an alkaline solution may include at least one of sodium hydroxide and potassium hydroxide. The solvent for an alkaline solution may be deionized water.

[0083] As an example, the concentration of the alkaline solution can be 1.0 mol / L, 2.0 mol / L, 3.0 mol / L, 4.0 mol / L, 5.0 mol / L, 6 mol / L, 7.0 mol / L, or 8.0 mol / L.

[0084] In some embodiments, stirring can be performed simultaneously with the first heating treatment to facilitate a complete reaction. For example, the stirring speed can be 500 rpm to 1000 rpm.

[0085] S3: Add supplementary oxidant to the second solution.

[0086] In some embodiments, a supplementary oxidant is added to the second solution to obtain a third solution. The supplementary oxidant has a stronger oxidizing power than hydrogen peroxide. Hydrogen peroxide oxidizes manganese ions relatively thoroughly, but the oxidation rate is relatively slow. By adding a strong oxidant, the oxidation of manganese ions can be effectively accelerated, preventing a significant decrease in production efficiency due to excessively long reaction times.

[0087] As an example, the temperature of the reaction system can be either room temperature or heated when a supplementary oxidant is added.

[0088] In some embodiments, the supplementary oxidant includes one or more of ammonium persulfate, sodium persulfate, potassium permanganate, and potassium perchlorate.

[0089] As an example, the molar amount of oxidant added to the third solution is c, and the sum of the molar amounts of divalent manganese ions and divalent iron ions in the metal ion solution is b, where c / b ≤ 0.1. Therefore, the addition of a small amount of oxidant helps to rapidly and fully oxidize manganese ions.

[0090] S4: Add phosphoric acid to the third solution and perform a second heat treatment.

[0091] In some embodiments, phosphoric acid is added in this step, and a second heat treatment is performed to generate ferromanganese phosphate by crystallization. After the crystallization is completed, ferromanganese phosphate is obtained by filtration, washing, drying and other steps.

[0092] Since the crystal transformation is completed during the gradual heating process, the initial crystal transformation temperature is low, the decomposition rate of excess hydrogen peroxide in the reaction system is slow, and less gas is produced, resulting in fewer pore structures. As the crystal transformation temperature increases, the decomposition rate of excess hydrogen peroxide in the system gradually accelerates, the gas produced gradually increases, and more pore structures are generated, thus forming a structural feature where the shell porosity is higher than the core porosity.

[0093] In some embodiments, the molar amount of phosphoric acid added is d, and the sum of the molar amounts of divalent manganese ions and divalent iron ions in the metal ion solution is b, where d / b is 1-1.2. This facilitates the complete crystallization of manganese-iron precipitate to form manganese-iron phosphate.

[0094] In some embodiments, the temperature of the second heat treatment is 60°C-90°C. This helps to increase the yield of ferromanganese phosphate.

[0095] As an example, the temperature of the second heat treatment can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, or 90°C.

[0096] In some embodiments, stirring can be performed simultaneously with the second heating treatment to facilitate a complete reaction. For example, the stirring speed can be 300 rpm to 800 rpm.

[0097] In a third aspect, this application proposes a method for preparing lithium manganese iron phosphate, comprising: mixing and sintering ferromanganese phosphate, a lithium source, and a carbon source at a high temperature to obtain the lithium manganese iron phosphate, wherein the ferromanganese phosphate includes ferromanganese phosphate prepared by the aforementioned method, or the aforementioned ferromanganese phosphate. Thus, lithium manganese iron phosphate with excellent electrochemical performance can be prepared by a simple process.

[0098] As an example, manganese iron phosphate and a lithium source (such as lithium carbonate) can be weighed according to a ratio of 1.02-1.06 between the molar amounts of Li and the sum of the molar amounts of manganese and iron. Based on the mass of the mixture, 6wt%-20wt% of a carbon source (such as glucose, polyethylene glycol, or one or more carbon nanotubes) is added, and the mixture is then milled. After milling, spray drying is performed for preliminary granulation, with the outlet air temperature at 80℃-105℃. Finally, the spray-dried powder is sintered at 600℃-750℃ for 8-16 hours to obtain lithium manganese iron phosphate.

[0099] The aforementioned ferromanganese phosphate has a small particle size and a porous structure, which can greatly improve the grinding efficiency. Furthermore, the high uniformity of manganese and iron element distribution in the aforementioned ferromanganese phosphate results in a relatively uniform distribution of manganese and iron elements in the prepared lithium manganese iron phosphate, which helps to improve the synergistic effect of manganese and iron and enhance electrochemical performance.

[0100] In a fourth aspect, this application proposes a lithium manganese iron phosphate, prepared using the aforementioned method. Therefore, this lithium manganese iron phosphate possesses all the features and advantages of the aforementioned method for preparing lithium manganese iron phosphate, which will not be repeated here.

[0101] As an example, the molecular formula of the aforementioned prepared lithium manganese iron phosphate satisfies LiFe y Mn 1-y PO4, 0 < y ≤ 0.8. The aforementioned lithium manganese iron phosphate material has a discharge capacity of 148.0 mAh / g - 153.0 mAh / g at a 0.1C rate.

[0102] The following specific embodiments illustrate the solution of this application. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0103] Example 1

[0104] (1) Add manganese sulfate and ferrous sulfate together in a molar ratio of 6:4 to deionized water, and add dilute sulfuric acid to prepare a metal ion solution with a pH of 3. The sum of the molar concentrations of manganese ions and ferrous ions in the metal ion solution is 1.0 mol / L.

[0105] (2) Prepare an alkaline solution with a molar concentration of 5.0 mol / L using sodium hydroxide.

[0106] (3) Using 5.0 L of mixed metal solution as the base solution, the stirring speed was adjusted to 800 rpm. Then, 198.0 g (1.75 mol) of 30% hydrogen peroxide was added for the first time. After the addition was completed, the temperature was raised to 60 °C, and the aforementioned alkaline solution was added to adjust the pH of the solution to 8.5. Then, 396.9 g (3.50 mol) of 30% hydrogen peroxide was added. After reacting for half an hour, 100 g (0.42 mol) of sodium persulfate and 580 g (5.92 mol) of phosphoric acid were added, the temperature was raised to 90 °C, and the stirring speed was adjusted to 500 rpm for 3 hours. Finally, the reaction slurry was filtered, washed, and dried to obtain ferromanganese phosphate.

[0107] (4) Manganese iron phosphate and lithium carbonate are mixed according to the ratio of the number of moles of lithium to the sum of the number of moles of manganese and iron elements of 1.03. After adding 8 wt% glucose and 6 wt% polyethylene glycol, the mixture is sand-milled and sprayed, and then sintered at 670℃ for 10 h to obtain lithium manganese iron phosphate.

[0108] The differences between the remaining embodiments and comparative examples and Example 1 are shown in Table 1. In Comparative Example 1, no hydrogen peroxide was added in the first instance, and 594.9g of 30% hydrogen peroxide was added only in the second instance. In Comparative Example 2, 594.9g of 30% hydrogen peroxide was added only in the first instance, and no hydrogen peroxide was added in the second instance.

[0109] Table 1

[0110]

[0111]

[0112] The specific surface area and porosity of the aforementioned manganese ferric phosphate were tested using the following methods, and the test results are shown in Table 2.

[0113] Porosity of ferromanganese phosphate: Porosity α = a1 / a2 × 100%; where a1 is the projected area of ​​the pores of the secondary ferromanganese phosphate particles obtained by CP (ion milling) test, and a2 is the projected area of ​​the secondary ferromanganese phosphate particles obtained by CP test.

[0114] Specific surface area of ​​manganese iron phosphate: (1) Sample pretreatment: Vacuum degassing of the sample under vacuum high temperature conditions to remove impurities and water vapor adsorbed on the sample surface; (2) Place the sample in a low temperature environment (liquid nitrogen temperature 77K), introduce nitrogen gas to gradually increase the pressure, and record the adsorption amount; (3) Calculate the adsorption amount according to the BET equation, obtain the corresponding volume through linear fitting, and then calculate the specific surface area.

[0115] Table 2

[0116]

[0117] Test results show that the SEM image and cross-sectional view of the manganese iron phosphate prepared in Example 1 are shown in the figure. Figure 1 and Figure 2 The results of the cross-sectional EDS test are as follows: Figure 3 As shown, the manganese and iron elements in the manganese iron phosphate in Example 1 are evenly distributed, which helps to improve the electrochemical performance of the prepared lithium manganese iron phosphate. Figure 4 The X-ray diffraction pattern of lithium iron phosphate prepared in Example 1 of this application shows that it corresponds to the characteristic peaks of the standard lithium iron phosphate card.

[0118] The compaction density and discharge capacity of the aforementioned lithium manganese iron phosphate were tested using the following methods, and the test results are shown in Table 3:

[0119] Preparation of positive electrode sheet: The aforementioned positive electrode active material lithium manganese iron phosphate, conductive agent Super P and polyvinylidene fluoride (PVDF) are thoroughly mixed with an appropriate amount of N-methylpyrrolidone (NMP) in a mass ratio of 90:5:5 to form a uniform slurry. The slurry is coated on aluminum foil and dried at 120°C for 12 hours. Then, it is pressed into shape using a pressure of 100 MPa to produce a positive electrode sheet with a diameter of 12 mm and a thickness of 120 μm.

[0120] Battery Assembly: In an argon-filled glove box with both water and oxygen content less than 5 ppm, the positive electrode, separator, negative electrode, and electrolyte were assembled into an R2025 coin cell and left to stand for 6 hours. The negative electrode used a 15.6 mm diameter, 0.45 mm thick lithium metal sheet; the separator used a 25 μm thick polypropylene microporous membrane (Celgard 2325); and the electrolyte used was 1 mol / L LiPF6, with the electrolyte solvent being an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0121] Table 3

[0122] Example 1 152.8 2.16 Example 2 151.3 2.07 Example 3 149.7 2.03 Example 4 148.7 2.03 Example 5 148.3 2.11 Example 6 148.0 2.06 Comparative Example 1 141.0 1.96 Comparative Example 2 135.6 1.99

[0123] As shown in Table 3, the core porosity of the manganese iron phosphate is lower than that of the shell, forming a porosity gradient distribution. The thickness ratio of the core and the shell is relatively moderate, resulting in high elemental uniformity and structural stability of the prepared lithium manganese iron phosphate, as well as high specific capacity and compaction density.

[0124] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A type of manganese iron phosphate, characterized in that, The manganese ferrophosphate is a secondary particle formed by the agglomeration of primary particles. The manganese ferrophosphate includes a core and a shell layer located on the surface of the core. The porosity of the core is less than that of the shell layer. The radius of the manganese ferrophosphate is R, the radius of the core is r1, the thickness of the shell layer is r2, r1 / R=0.3-0.6, r2 / R=0.4-0.7, and (r1+r2) / R=1. The manganese and iron elements in the manganese ferric phosphate are uniformly distributed in the core and the shell.

2. The manganese iron phosphate according to claim 1, characterized in that, The porosity of the core is 8%-20%, and / or the pore size of the core is 10nm-50nm.

3. The manganese iron phosphate according to claim 1, characterized in that, The porosity of the shell is 25%-50%, and / or the pore size of the shell is 10nm-60nm.

4. The manganese iron phosphate according to claim 2 or 3, characterized in that, The porosity of the ferromanganese phosphate is 25%-40%, and / or the specific surface area of ​​the ferromanganese phosphate is 40 m². 2 / g-100m 2 / g.

5. The manganese iron phosphate according to claim 1, characterized in that, The Dv50 particle size of the ferromanganese phosphate is 1.0 μm-2.5 μm, and / or the primary particle size of the ferromanganese phosphate is 20 nm-80 nm.

6. A method for preparing ferric manganese phosphate according to any one of claims 1-5, characterized in that, include: Hydrogen peroxide is added to a metal ion solution to obtain a first solution, wherein the metal ion solution includes divalent manganese ions and divalent iron ions, the pH of the metal ion solution is ≤3, and the molar ratio of manganese ions to iron ions in the metal ion solution is (5-8):(2-5). Under alkaline conditions, hydrogen peroxide is added to the first solution, and a first heating treatment is performed to obtain a second solution; A supplementary oxidant is added to the second solution to obtain a third solution; the supplementary oxidant includes one or more of ammonium persulfate, sodium persulfate, potassium permanganate, and potassium perchlorate. Phosphoric acid is added to the third solution, and a second heating treatment is performed to obtain the manganese iron phosphate.

7. The method according to claim 6, characterized in that, The ratio of the molar amount of hydrogen peroxide added in the first solution to the molar amount of hydrogen peroxide added in the second solution is 1-3.

8. The method according to claim 6 or 7, characterized in that, The sum of the molar amounts of hydrogen peroxide added in the first solution and the second solution is a, and the sum of the molar amounts of divalent manganese ions and divalent iron ions in the metal ion solution is b, where a / b is 0.6-1.

5.

9. The method according to claim 8, characterized in that, The molar amount of the supplementary oxidant is c, where c / b ≤ 0.

1.

10. The method according to claim 8, characterized in that, The molar amount of phosphoric acid added is d, and the d / b ratio is 1-1.

2.

11. The method according to claim 6, characterized in that, The alkaline conditions have a pH of 7.5-9.5, and / or the temperature of the first heat treatment is 40℃-70℃, and / or the temperature of the second heat treatment is 60℃-90℃.

12. A method for preparing lithium manganese iron phosphate, characterized in that, include: The lithium manganese iron phosphate, lithium source, and carbon source are mixed and sintered at high temperature to obtain the lithium manganese iron phosphate, wherein the lithium manganese iron phosphate includes the lithium manganese iron phosphate according to any one of claims 1-5, or the lithium manganese iron phosphate prepared by the method according to any one of claims 6-11.

13. A lithium manganese iron phosphate, characterized in that, It is prepared by the method described in claim 12.

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