Process for preparing spherical iron phosphate and lithium manganese iron phosphate

Abstract

This invention proposes a method for preparing spherical iron phosphate, comprising the following steps: S1, mixing a trivalent iron salt with a complexing agent to obtain an iron salt complexing solution; S2, mixing the iron salt complexing solution and phosphorus source I, and then adding a surfactant premix to obtain a mixed solution; S3, reacting the above mixed solution under water bath conditions, separating the solid and liquid after the reaction, and sintering the obtained solid to obtain the spherical iron phosphate; wherein the surfactant premix includes an ionic surfactant and an acidulant. This invention, by introducing a surfactant premix, can effectively control the particle size and morphology, avoid or reduce particle agglomeration, thereby obtaining a powder with good morphology and uniform particle size. The premix contains an ionic surfactant and an acidulant, which acts as an acidifier, buffer, and chelate, and also improves the performance of the surfactant.

Description

Technical Field

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

[0002] Similar to lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP) has widely available and environmentally friendly raw materials, a high theoretical specific capacity (170 mAh / g), and an ordered olivine-like crystal structure that gives it advantages such as stable discharge voltage, good thermal stability, and good cycle stability. It is a promising cathode material for lithium-ion batteries. Compared with the 3.4V (vs. Li / Li+) voltage plateau of lithium iron phosphate, the prepared lithium manganese iron phosphate material has a 4.1V (vs. Li / Li+) voltage plateau. Under the same capacity conditions, lithium manganese iron phosphate batteries have a 20% higher energy density than lithium iron phosphate batteries.

[0003] During the charge-discharge process, the product of lithium iron phosphate after complete delithiation is the iron phosphate phase. Due to its advantages such as low cost and high chemical stability, FePO4, as a trivalent iron source, is considered an ideal material for synthesizing lithium iron phosphate and lithium manganese iron phosphate. The structure, morphology, and particle size distribution of iron phosphate all affect the performance of lithium manganese iron phosphate. By controlling the crystal size and morphology of the iron phosphate precursor, it is possible to achieve large-scale production of lithium manganese iron phosphate with high tap density, high discharge rate, and high capacity.

[0004] Studies have shown that particle spheroidization is key to improving tap density. Using iron phosphate with uniform particle size and regular morphology as a precursor, lithium iron phosphate and lithium manganese iron phosphate with uniform and regular morphology can also be produced. This can increase the tap density of lithium ions and improve the electrochemical performance of the cathode material. Preparing iron phosphate precursors with small and narrowly distributed spherical particle size is an important basis for obtaining lithium iron phosphate with small and narrowly distributed spherical particle size. Currently, iron phosphate production processes include sol-gel method, hydrothermal method, liquid phase precipitation, microwave method, etc. However, the above preparation methods have many defects, such as: complex operation process, reaction conditions that do not meet the requirements of industrial production, low yield, unstable product morphology and quality, etc. Furthermore, the iron phosphate prepared cannot be used as an iron source to prepare high-performance lithium manganese iron phosphate. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for synthesizing spherical iron phosphate, aiming to solve the problem that existing preparation methods are difficult to use to synthesize high-performance lithium manganese iron phosphate cathode materials. Using this spherical iron phosphate as an iron source can effectively improve the compaction density and electrochemical performance of lithium manganese iron phosphate.

[0006] In a first aspect, the present invention provides a method for preparing spherical iron phosphate, the method comprising the following steps:

[0007] S1. Mix the ferric salt with the complexing agent to obtain the ferric salt complex solution;

[0008] S2. After mixing the iron salt complex solution and phosphorus source I, add the surfactant premix solution to obtain a mixed solution;

[0009] S3. The above mixed solution is reacted under water bath conditions. After the reaction is completed, the solid and liquid are separated, and the obtained solid is sintered to obtain the spherical iron phosphate.

[0010] The surfactant premix includes ionic surfactants and acidulants.

[0011] In the above preparation method, on the one hand, by introducing a surfactant premix, the particle size and morphology can be effectively controlled by dispersing the particles, avoiding or reducing particle agglomeration, thereby obtaining a powder with good morphology and uniform particle size. This powder contains ionic surfactants and acidulants. The acidulants play the roles of acidity, buffering and chelation, and can also improve the performance of the surfactants. On the other hand, the introduction of a complexing agent can form a complex with iron ions, enhancing the stability of iron ions, making the iron phosphate crystals obtained from the reaction more stable and with controllable morphology.

[0012] According to embodiments of the present invention, the ionic surfactant is one or both of anionic and cationic surfactants; the acidulant is one or both of malic acid and citric acid.

[0013] Preferably, the ionic surfactant is a combination of anionic and cationic surfactants.

[0014] According to an embodiment of the present invention, the anionic surfactant is at least one of sodium dodecylbenzenesulfonate (SDBS), sodium dodecyl sulfate (SDS), sodium octylsulfonate (SOS), and potassium octylsulfonate (KOS).

[0015] According to an embodiment of the present invention, the cationic surfactant is at least one of hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), and octadecyldimethylhydroxyethylammonium nitrate.

[0016] When the ionic surfactant is a combination of anionic and cationic surfactants, the ratio of the anionic surfactant to the cationic surfactant can be chosen arbitrarily. According to an embodiment of the present invention, the mass ratio of the anionic surfactant to the cationic surfactant is 1:(0.5-1), preferably 1:(0.8-1). For example, the mass ratio of the anionic surfactant to the cationic surfactant is 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, etc.

[0017] According to an embodiment of the present invention, in the surfactant premix, the mass ratio of the ionic surfactant to the acidulant is (1.5-2):(0.1-0.5), more preferably (1.8-2):(0.1-0.3). For example, the mass ratio of the ionic surfactant to the acidulant is 1.8:0.1, 1.8:0.2, 1.8:0.3, 1.9:0.1, 1.9:0.2, 1.9:0.3, 2:0.1, 2:0.2, 2:0.3, etc.

[0018] When the ionic surfactant is a combination of anionic and cationic surfactants, the mass ratio of the anionic surfactant, cationic surfactant, and acidulant is 1:(0.5-1):(0.1-0.5), preferably 1:(0.8-1):(0.1-0.3).

[0019] According to an embodiment of the present invention, step S1 of the above method may further include at least one of the following additional technical features:

[0020] According to an embodiment of the present invention, the trivalent ferric salt is at least one of ferric nitrate, ferric sulfate, and ferric chloride.

[0021] According to embodiments of the present invention, the complexing agent is one or both of ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA).

[0022] According to embodiments of the present invention, the molar ratio of the ferric salt to the complexing agent is 1:(0.2-0.5), preferably 1:(0.3-0.4), for example, the molar ratio of the ferric salt to the complexing agent is 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, 1:0.5, etc.

[0023] According to embodiments of the present invention, the concentration of the iron salt complex solution is 0.1-0.3 mol / L, preferably 0.1-0.15 mol / L. For example, the concentration of the iron salt complex solution is 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, etc.

[0024] According to an embodiment of the present invention, step S2 of the above method may further include at least one of the following additional technical features:

[0025] According to an embodiment of the present invention, phosphorus source I is at least one of phosphoric acid and ammonium dihydrogen phosphate.

[0026] According to an embodiment of the present invention, the mixing ratio of the iron salt complex solution and phosphorus source I is such that the molar ratio of iron to phosphorus is 1:(1-1.2), preferably 1:(1.01-1.1). For example, the molar ratio of iron to phosphorus is 1:1, 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.1, 1:1.12, 1:1.14, 1:1.16, 1.18, 1.2, etc.

[0027] According to an embodiment of the present invention, the amount of the surfactant premix added is such that the mass of the ionic surfactant in the surfactant premix is ​​1-10% of the mass of the ferric salt, more preferably 1-3%, for example, the mass of the ionic surfactant is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% of the mass of the ferric salt.

[0028] According to an embodiment of the present invention, in step S2, the mixed solution further includes a soluble rare earth metal salt, preferably, the rare earth metal is one or both of lanthanum (La) or yttrium (Y).

[0029] According to an embodiment of the present invention, step S2 specifically involves: mixing an iron salt complex solution and phosphorus source I, then adding a surfactant premix and a soluble rare earth metal salt to obtain a mixed solution.

[0030] Preferably, the soluble rare earth metal salt is a lanthanum or yttrium nitrate.

[0031] In this invention, rare earth elements have larger ionic radii, which, according to the Bragg equation, increase the interplanar spacing. This increased lattice provides a larger migration path for lithium ions. Due to the doping of rare earth elements, on the one hand, lattice defects in iron phosphate are created, affecting the nucleation and growth process of iron phosphate, and consequently, the average particle size. On the other hand, the complex variable valence effect of rare earth elements can be utilized to improve the mobility of lithium ions in the battery. Simultaneously, the combined action of rare earth metal ions and surfactants promotes the formation of spherical iron phosphate particles with uniform size and regular morphology.

[0032] When the mixed solution includes a soluble rare earth metal salt, the amount of the soluble rare earth metal salt added is such that the molar ratio of iron to rare earth metal in the mixed solution of step S2 is 1:(0.01-0.1), preferably 1:(0.01-0.04). For example, the molar ratio of iron to rare earth metal is 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1, etc.

[0033] According to an embodiment of the present invention, step S3 of the above method may further include at least one of the following additional technical features:

[0034] According to an embodiment of the present invention, the reaction conditions under the water bath include at least one of the following conditions:

[0035] (1) The reaction temperature is 20-100℃, preferably 80-100℃, for example, the reaction temperature is 20℃, 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 85℃, 90℃, 95℃, 100℃, etc.

[0036] (2) The reaction time is 4-10h, preferably 5-7h, for example, the reaction time is 4h, 5h, 6h, 7h, 8h, 9h, 10h, etc.;

[0037] (3) The stirring rate is 200-400 rpm / min, preferably 150-300 rpm / min, for example, stirring rates of 200 rpm / min, 250 rpm / min, 300 rpm / min, 350 rpm / min, 400 rpm / min, etc.

[0038] (4) The pH value of the reaction solution is 2-7, preferably 2-4. For example, the pH value of the reaction solution is 2, 2.5, 3, 3.5, 4, 5, 6, 7, etc.

[0039] According to an embodiment of the present invention, before the mixed solution reacts under water bath conditions, the following operation is further included: adding an alkaline solution to the mixed solution to adjust the pH value of the solution to 2-7, preferably 2-4, wherein the alkaline solution is at least one of ammonium monohydrate solution or ammonia water.

[0040] According to an embodiment of the present invention, the sintering conditions include at least one of the following:

[0041] (1) The heating rate is 2-5℃ / min, for example, the heating rates are 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, etc.;

[0042] (2) The sintering temperature is 400-500℃, for example, sintering temperatures of 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, etc.

[0043] (3) The sintering cycle is 2-10h, for example, the sintering cycle is 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, etc.

[0044] According to an embodiment of the present invention, before sintering, the solid after solid-liquid separation further includes the following operations: first washing with water at least three times, and then drying at 70-110°C.

[0045] According to embodiments of the present invention, the sintering atmosphere includes, but is not limited to, an inert atmosphere, such as a nitrogen atmosphere.

[0046] In a second aspect of the invention, a spherical iron phosphate is provided, which is prepared by the preparation method described in the first aspect.

[0047] In a third aspect, the present invention provides a method for preparing lithium manganese iron phosphate, wherein the lithium manganese iron phosphate is prepared using the above-mentioned spherical iron phosphate as an iron source.

[0048] Furthermore, the preparation method of the lithium manganese iron phosphate includes: mixing the above-mentioned spherical iron phosphate, manganese source, lithium source and phosphorus source II and then sintering them to obtain the lithium manganese iron phosphate.

[0049] According to an embodiment of the present invention, the lithium, iron, manganese, and phosphorus elements in the spherical iron phosphate, manganese source, lithium source, and phosphorus source II satisfy at least one of the following conditions:

[0050] (1) The molar ratio of iron to the sum of iron and manganese is (0.4-0.9):1, that is, Fe / (Fe+Mn)=0.4-0.9. For example, Fe:Fe+Mn is 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, etc.

[0051] (2) The molar ratio of lithium to the sum of iron and manganese is (0.95-1.1):1, that is, Li / (Fe+Mn)=0.95-1.1. For example, Li:Fe+Mn is 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, 1.09:1, 1.1:1, etc.

[0052] (3) The molar ratio of the sum of iron and manganese to phosphorus is (0.95-1.06):1, that is, (Fe+Mn) / P=0.95-1.06. For example, Fe+Mn:P is 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, etc.

[0053] According to an embodiment of the present invention, the preparation method of lithium manganese iron phosphate specifically includes the following steps:

[0054] S4. Spherical iron phosphate, manganese source, lithium source, phosphorus source II, carbon source I, dopant and water are mixed, and after coarse grinding, sand grinding and spray drying, powder material I is obtained;

[0055] S5. The above-mentioned powder material I is subjected to solid-state sintering for the first time. After cooling, a lithium manganese iron phosphate precursor is obtained. The precursor is mixed with carbon source II and water, and then subjected to coarse grinding, sand grinding and spray drying to obtain powder material II.

[0056] S6. The above-mentioned powder material II is subjected to a second solid-state sintering, and after cooling, the lithium manganese iron phosphate is obtained.

[0057] The method for preparing lithium manganese iron phosphate cathode material provided by this invention results in a more uniform distribution of lithium manganese iron phosphate particles. Some of these particles inherit the properties of spherical iron phosphate, and the combination of large and small particles in the lithium manganese iron phosphate results in a more uniform and regular morphology and size, which is beneficial for compaction performance. At the same time, the uniform presence of small particles ensures the excellent electrical performance of lithium manganese iron phosphate.

[0058] According to an embodiment of the present invention, step S4 of the above method may further include at least one of the following additional technical features:

[0059] According to an embodiment of the present invention, the manganese source is at least one of manganese tetroxide, manganese carbonate, and manganese pyrophosphate.

[0060] According to an embodiment of the present invention, the lithium source is at least one selected from lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate.

[0061] According to an embodiment of the present invention, the phosphorus source II is at least one selected from phosphoric acid, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, iron phosphate, and manganese pyrophosphate.

[0062] According to embodiments of the present invention, carbon source I and carbon source II are independently at least one of toluene, xylene, graphene, citric acid, polyethylene glycol, sucrose, and carbon nanotubes.

[0063] According to an embodiment of the present invention, the doping element in the dopant is at least one selected from fluorine, vanadium, magnesium, niobium, titanium, and zirconium.

[0064] According to an embodiment of the present invention, the dopant is at least one selected from ammonium fluoride, ammonium metavanadate, magnesium oxide, niobium pentoxide, titanium dioxide, and zirconium oxide.

[0065] According to an embodiment of the present invention, the mass of carbon source I accounts for 1-4% of the total mass of solid raw materials in S4. For example, the mass of carbon source I accounts for 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 3.95%, 4%, etc. of the total mass of solid raw materials in S4. The solid raw materials refer to: iron source, manganese source, lithium source, phosphorus source II, carbon source I, and dopant.

[0066] According to an embodiment of the present invention, the mass of the dopant element in the dopant accounts for 700-1400 ppm of the total mass of the solid raw material, for example, the mass of the dopant element accounts for 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, etc. of the total mass of the solid raw material.

[0067] According to an embodiment of the present invention, after mixing spherical iron phosphate, manganese source, lithium source, phosphorus source II, carbon source I, dopant and water, the solid content of the resulting mixed slurry is 20%-30%.

[0068] According to an embodiment of the present invention, step S5 of the above method may further include at least one of the following additional technical features:

[0069] According to an embodiment of the present invention, the conditions for the first solid-state sintering include at least one of the following:

[0070] (1) The sintering temperature is 400-600℃, preferably 400-500℃, for example, sintering temperatures of 400℃, 450℃, 500℃, 550℃, 600℃, etc.;

[0071] (2) The heating rate is 2-5℃ / min, for example, the heating rates are 2℃ / min, 2.3℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, etc.;

[0072] (3) The sintering cycle is 2-6 hours, for example, the sintering cycle is 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, etc.

[0073] According to an embodiment of the present invention, the mass of carbon source II accounts for 2-6% of the total mass of solid raw materials in S5, for example, the mass of carbon source II accounts for 2%, 3%, 4%, 5%, 6%, etc. of the total mass of solid raw materials in S5. The solid raw materials refer to: lithium manganese iron phosphate precursor and carbon source II.

[0074] According to an embodiment of the present invention, after mixing the lithium manganese iron phosphate precursor with carbon source II and water, the solid content of the resulting mixed slurry is 20%-30%.

[0075] According to an embodiment of the present invention, step S6 of the above method may further include at least one of the following additional technical features:

[0076] According to an embodiment of the present invention, the conditions for the second solid-state sintering include at least one of the following:

[0077] (1) The sintering temperature is 500-800℃, preferably 700-780℃, for example, sintering temperatures of 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 780℃, 800℃, etc.

[0078] (2) The heating rate is 2-5℃ / min, for example, the heating rates are 2℃ / min, 2.3℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, etc.;

[0079] (3) The sintering cycle is 6-20h, preferably 6-10h, for example, the sintering cycle is 6h, 7h, 8h, 9h, 10h, 12h, 14h, 15h, 16h, 18h, 20h, etc.

[0080] In a fourth aspect of the invention, the present invention provides a lithium manganese iron phosphate cathode material, wherein the lithium manganese iron phosphate is prepared by the preparation method of the third aspect.

[0081] The lithium manganese iron phosphate particles provided by this invention are uniformly distributed, with some particles inheriting the properties of spherical iron phosphate. The combination of large block particles and small spherical particles results in a more uniform and regular morphology and size, which is beneficial for compaction performance. At the same time, the uniform presence of small spherical particles ensures the excellent electrical performance of lithium manganese iron phosphate.

[0082] According to an embodiment of the present invention, the primary particle size of the lithium manganese iron phosphate is 0.35-0.4 μm, and the secondary particle size is 0.4-0.45 μm, wherein the particle size is the average particle size.

[0083] The beneficial effects of this invention are as follows:

[0084] 1. This invention uses a complexing agent in the preparation of spherical iron phosphate. The complexing agent can complex with iron ions, enhancing the stability of iron ions and making the resulting iron phosphate crystals more stable and with controllable morphology. The use of a surfactant premix allows for particle dispersion, effectively controlling particle size and morphology, avoiding or reducing particle agglomeration, thereby obtaining powder with better morphology and uniform particle size to improve tap density. The combined use of anionic / cationic surfactants and acidulants further enhances the surfactant effect.

[0085] 2. Doping with rare earth elements has two effects. On the one hand, rare earth elements can be doped into iron phosphate, causing lattice defects in iron phosphate, which affects the nucleation and growth process of iron phosphate, and thus affects the average particle size of iron phosphate. On the other hand, by using rare earth element doping, the complex variable valence effect of rare earth elements can be utilized to improve the mobility of lithium ions in the battery.

[0086] 3. Due to the vigorous reaction between iron ions and phosphate ions, the rapid co-reaction leads to poor crystallization. The iron phosphate material prepared in this invention slows down the reaction rate by controlling the pH of the solution. At the same time, the addition of rare earth ions can inhibit the growth of grains, allowing phosphate ions to fully react with iron ions. This is beneficial for controlling the morphology of iron phosphate and improving the electrochemical performance of lithium manganese iron phosphate.

[0087] 4. The lithium manganese iron phosphate material prepared by the preparation method of the present invention has a more uniform particle distribution. Some of the particles inherit the properties of spherical iron phosphate. The combination of large block particles and small spherical particles results in a more uniform and regular morphology and size, which is beneficial to the compaction performance. At the same time, the uniform existence of small particles ensures the excellent electrical performance of lithium manganese iron phosphate.

[0088] 5. The method for preparing lithium manganese iron phosphate cathode material provided by the present invention uses spherical iron phosphate as the iron source and is prepared by a secondary grinding process, which effectively improves the compaction density and electrochemical performance of lithium manganese iron phosphate cathode material.

[0089] 6. The preparation process of this invention is simple, the process parameters are easy to control, and the final product has excellent properties, making it suitable for industrial mass production.

[0090] Terminology Definition

[0091] Unless explicitly stated otherwise, all scopes referenced in this invention include end values.

[0092] The term "at least one" is used in this invention to describe the elements and components described herein. This is done merely for convenience and to provide a general meaning regarding the scope of the invention. This description should be understood to include one or at least one, and the singular includes the plural, unless clearly otherwise indicated.

[0093] All figures in this invention are approximate values, regardless of whether words such as "approximately" or "about" are used. The numerical values ​​may vary by 1%, 2%, 5%, 7%, 8%, 10%, etc. Whenever a number with a value of N is disclosed, any number having a value of N+ / -1%, N+ / -2%, N+ / -3%, N+ / -5%, N+ / -7%, N+ / -8%, or N+ / -10% will be explicitly disclosed, where "+ / -" indicates addition or subtraction, and the range between N-10% and N+10% is also disclosed.

[0094] Unless otherwise stated, the following definitions as used in this invention shall apply. For the purposes of this invention, chemical elements are consistent with the CAS version of the periodic table and the 75th edition of the *Handbook of Chemistry and Physics*, 1994. Furthermore, general principles of organic chemistry can be found in *Organic Chemistry*, Thomas Sorrell, University Science Books, Sausalito: 1999, and *March's Advanced Organic Chemistry* by Michael B. Smith and Jerry March, John Wiley & Sons, New York: 2007, the entire contents of which are incorporated herein by reference.

[0095] Unless otherwise defined, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although similar or equivalent methods and materials may be used in the implementation or testing of embodiments of this invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned in this invention are incorporated herein by reference in their entirety, except where specific paragraphs are cited. In case of any conflict, this specification and its included definitions shall prevail. Furthermore, materials, methods, and examples are illustrative only and are not intended to be limiting. Attached Figure Description

[0096] Figure 1 This is a SEM image of the iron phosphate material prepared in Example 1 of the present invention;

[0097] Figure 2 This is a SEM image of the lithium manganese iron phosphate material prepared in Example 1 of the present invention;

[0098] Figure 3 This is a 0.2C charge-discharge curve of the lithium manganese iron phosphate material prepared in Example 1 of the present invention. Detailed Implementation

[0099] The following description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0100] Example 1

[0101] This embodiment provides a method for preparing spherical iron phosphate:

[0102] Prepare a surfactant premix solution with a mass ratio of SDBS:CTAB:citric acid of 1:1:0.3 and set aside for later use;

[0103] Prepare a 0.1 mol / L EDTA-Fe complex solution with a molar ratio of Fe(NO3)3·9H2O:EDTA of 1:0.3 for later use;

[0104] The EDTA-Fe complex solution was mixed with phosphoric acid solution according to a Fe:P molar ratio of 1:1.03. Three times the volume of pure water was added, and the mixture was placed in a heated and stirred container. After thorough mixing, a prepared surfactant premix and lanthanum nitrate were added, ensuring that the total mass of SDBS and CTAB accounted for 1% of the mass of Fe(NO3)3, and the molar ratio of Fe(NO3)3 to lanthanum nitrate was 1:0.01. The resulting mixed solution was heated to 82°C in a constant-temperature reaction bath while stirring at 300 rpm / min. Ammonia was added dropwise to adjust the pH to 3, at which point a co-precipitation of rare earth elements and iron phosphate occurred. The precipitate was filtered and washed, then transferred to a forced-air drying oven for drying at 80°C for 4 hours. After drying, the powder was transferred to a muffle furnace for sintering under a nitrogen atmosphere at a heating rate of 3°C / min, a sintering temperature of 400°C, and a sintering time of 9 hours. The mixture was then naturally cooled to room temperature to obtain the iron phosphate material.

[0105] The obtained iron phosphate material was subjected to SEM testing, and the results are shown in the attached figure. Figure 1 As shown.

[0106] The SEM results show that the iron phosphate particles are spherical with regular external shapes and no significant bumps or depressions.

[0107] The obtained iron phosphate material was subjected to particle size testing, and the particle sizes were D10 = 0.882 μm, D50 = 2.827 μm, and D90 = 5.954 μm.

[0108] This embodiment also provides a method for preparing lithium manganese iron phosphate cathode material:

[0109] According to the molar ratios Mn:Fe = 0.6:0.4, (Mn+Fe):P = 0.96:1, and Li:(Mn+Fe) = 1.05:1, the iron phosphate, lithium dihydrogen phosphate, lithium carbonate, manganese tetroxide, glucose, polyethylene glycol, and magnesium oxide prepared above were mixed and dissolved in water. The mass of glucose and polyethylene glycol was controlled to account for 3.95% of the total mass of solid raw materials, and the mass of magnesium was controlled to account for 700 ppm of the total mass of solid raw materials. After being mixed evenly, a mixed slurry with a solid content of 30% was obtained. After coarse grinding, sand grinding, and spray drying, powder material I was obtained.

[0110] The above-mentioned powder material I was subjected to a first solid-state sintering treatment under a nitrogen atmosphere. The sintering temperature was 500℃, the heating rate was 2.3℃ / min, and the sintering cycle was 4h. After cooling to room temperature, a lithium manganese iron phosphate precursor was obtained. The precursor was mixed with glucose and polyethylene glycol and dissolved in water to obtain a mixed solution. After coarse grinding, sand grinding, and spray drying, a gray powder material II was obtained.

[0111] The above-mentioned powder material II was subjected to a second solid-state sintering treatment under a nitrogen atmosphere. The sintering temperature was 780℃, the heating rate was 2.3℃ / min, and the sintering cycle was 9h. After automatic cooling, it was crushed, sieved, and demagnetized to obtain lithium iron phosphate cathode material for lithium batteries.

[0112] The obtained lithium manganese iron phosphate material was subjected to SEM testing, and the results are shown in the attached figure. Figure 2 As shown in the figure, the lithium manganese iron phosphate material has a uniform particle distribution. Some of the particles inherit the properties of spherical iron phosphate and grow into large block particles. The combination of large block particles and small spherical particles results in a uniform and regular morphology and size.

[0113] Example 2

[0114] This embodiment provides a method for preparing spherical iron phosphate, referring to Example 1, except that the molar ratio of Fe(NO3)3 to lanthanum nitrate is 1:0.03, and the rest of the operation is the same as in Example 1.

[0115] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material. Using the above-mentioned spherical iron phosphate as the iron source, the preparation method is the same as in Example 1, and the lithium manganese iron phosphate cathode material is obtained.

[0116] Example 3

[0117] This embodiment provides a method for preparing spherical iron phosphate, referring to Example 1, except that in the surfactant premix, the mass ratio of SDBS:CTAB:citric acid is 1:1:0.1, and the rest of the operation is the same as in Example 1.

[0118] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material. Using the above-mentioned spherical iron phosphate as the iron source, the preparation method is the same as in Example 1, and the lithium manganese iron phosphate cathode material is obtained.

[0119] Example 4

[0120] This embodiment provides a method for preparing spherical iron phosphate, referring to Example 1, except that the molar ratio of Fe(NO3)3·9H2O:EDTA is 1:0.2, and the rest of the operation is the same as in Example 1.

[0121] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material. Using the above-mentioned spherical iron phosphate as the iron source, the preparation method is the same as in Example 1, and the lithium manganese iron phosphate cathode material is obtained.

[0122] Example 5

[0123] This embodiment provides a method for preparing spherical iron phosphate, referring to Example 1, except that the surfactant premix is ​​a mixture of SDBS and citric acid in a mass ratio of 2:0.3, and the rest of the operation is the same as in Example 1.

[0124] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material. Using the above-mentioned spherical iron phosphate as the iron source, the preparation method is the same as in Example 1, and the lithium manganese iron phosphate cathode material is obtained.

[0125] Example 6

[0126] This embodiment provides a method for preparing spherical iron phosphate, referring to Example 1, except that the surfactant premix is ​​a mixture of CTAB and citric acid in a mass ratio of 2:0.3, and the rest of the operation is the same as in Example 1.

[0127] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material. Using the above-mentioned spherical iron phosphate as the iron source, the preparation method is the same as in Example 1, and the lithium manganese iron phosphate cathode material is obtained.

[0128] Comparative Example 1

[0129] The difference between Comparative Example 1 and Example 1 is that lanthanum nitrate is not added, but the rest of the operations are the same.

[0130] Comparative Example 2

[0131] The difference between Comparative Example 2 and Example 1 is that no surfactant premix was added; the rest of the operations are the same.

[0132] Comparative Example 3

[0133] The difference between Comparative Example 3 and Example 1 is that the surfactant premix is ​​a mixture of PVA (polyvinyl alcohol, a nonionic surfactant) and citric acid, with a mass ratio of PVA to citric acid of 2:0.3. The rest of the operation is the same.

[0134] Comparative Example 4

[0135] The difference between Comparative Example 4 and Example 1 is that EDTA is not used to complex Fe(NO3)3, but the rest of the operations are the same.

[0136] Comparative Example 5

[0137] The difference between Comparative Example 5 and Example 1 is that the surfactant premix is ​​a mixture of SDBS and CTAB, citric acid is not used, the mass ratio of SDBS to CTAB is 1:1, and the rest of the operations are the same.

[0138] Performance Evaluation

[0139] The lithium manganese iron phosphate positive electrode material, binder PVDF, NMP-prepared mixture, and conductive agent Super-P prepared in the above examples and comparative examples were weighed in a ratio of 92:4:4. After being ball-milled and dispersed evenly, the mixture was coated onto aluminum foil, vacuum-dried at 110°C, and then rolled and punched to obtain the positive electrode sheet. The electrolyte was 1 mol / L LiPF6, with a solvent volume ratio of EC:DMC:EMC = 1:1:1. The separator was a Celgard polypropylene membrane, and the negative electrode was a lithium metal sheet. All components were assembled into a coin cell in a vacuum glove box. Electrochemical tests were performed on the coin cell using a Blue Dot battery testing system, with a test voltage range of 2V-4.5V.

[0140] The test results are shown in Table 1.

[0141] Table 1. Test data of lithium manganese iron phosphate in each embodiment and comparative example.

[0142]

[0143]

[0144] As can be seen from the table, the lithium manganese iron phosphate cathode material obtained by the method in this embodiment has significantly better performance than the comparative example.

[0145] Comparative Example 1, which did not contain lanthanum nitrate, had poor electrical performance. This may be because: La ions affect the nucleation and growth process of iron phosphate, thus affecting the average particle size of iron phosphate; on the other hand, using rare earth element doping can utilize the complex variable valence effect of rare earth elements to increase the migration rate of lithium ions in the battery, thereby improving electrical performance.

[0146] As can be seen from Comparative Example 2, surfactants can disperse particles to effectively control particle size and morphology, avoid or reduce particle agglomeration, and thus obtain powder with better morphology and uniform particle size. Without the addition of surfactants, iron phosphate particles will agglomerate, resulting in irregular shapes and the appearance of spherical and rod-shaped particles.

[0147] As can be seen from Comparative Example 3, compared with nonionic surfactants, ionic surfactants have stronger emulsifying and dispersing abilities, and higher stability in weak acids. The combined use of anionic and cationic surfactants can enhance surfactants, improve emulsifying and dispersing abilities, enhance chemical stability, and expand the scope of application.

[0148] Figure 3 The graph shows the 0.2C charge-discharge curve of the lithium manganese iron phosphate material prepared in Example 1 of this invention. It can be seen from the graph that the lithium manganese iron phosphate obtained by the preparation method provided by this invention has good electrochemical performance.

[0149] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing spherical iron phosphate, characterized in that, The preparation method includes: S1. Mix the ferric salt with the complexing agent to obtain the ferric salt complex solution; S2. After mixing the iron salt complex solution and phosphorus source I, add the surfactant premix solution to obtain a mixed solution; S3. The above mixed solution is reacted under water bath conditions. After the reaction is completed, the solid and liquid are separated, and the obtained solid is sintered to obtain the spherical iron phosphate. The surfactant premix includes ionic surfactants and acidulants; The complexing agent is one or both of ethylenediaminetetraacetic acid or diethylenetriaminepentaacetic acid; The ionic surfactant is one or both of anionic and cationic surfactants; the acidulant is one or both of malic acid and citric acid.

2. The preparation method according to claim 1, characterized in that, The anionic surfactant is at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, sodium octylsulfonate, and potassium octylsulfonate; the cationic surfactant is at least one of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and octadecyldimethylhydroxyethylammonium nitrate.

3. The preparation method according to claim 1, characterized in that, The mass ratio of the ionic surfactant to the acidulant is (1.5-2):(0.1-0.5).

4. The preparation method according to claim 1, characterized in that, The amount of the ionic surfactant is 1-10% of the mass of the ferric salt.

5. The preparation method according to claim 2, characterized in that, The ionic surfactant is a combination of anionic and cationic surfactants.

6. The preparation method according to claim 5, characterized in that, The mass ratio of the anionic surfactant to the cationic surfactant is 1:(0.5-1).

7. The preparation method according to claim 1, characterized in that, In step S1, the ferric salt is at least one of ferric nitrate, ferric sulfate, and ferric chloride.

8. The preparation method according to claim 1, characterized in that, The molar ratio of the trivalent iron salt to the complexing agent is 1:(0.2-0.5).

9. The preparation method according to claim 1, characterized in that, The concentration of the iron salt complex solution is 0.1-0.3 mol / L.

10. The preparation method according to claim 1, characterized in that, In step S2, the phosphorus source I is at least one of phosphoric acid and ammonium dihydrogen phosphate.

11. The preparation method according to claim 1, characterized in that, In step S2, the iron salt complex solution and the phosphorus source are mixed according to the Fe:P elemental molar ratio of 1:(1-1.2).

12. The preparation method according to claim 1, characterized in that, In step S2, the mixed solution also includes soluble rare earth metal salts.

13. The preparation method according to claim 12, characterized in that, The rare earth metal is one or both of lanthanum and yttrium.

14. The preparation method according to claim 12, characterized in that, In the mixed solution of step S2, the molar ratio of iron to rare earth metal is 1:(0.01-0.1).

15. The preparation method according to claim 1, characterized in that, The reaction conditions in step S3 include: a reaction temperature of 20-100℃, a reaction time of 4-10 h, a stirring rate of 200-400 rpm / min, and a pH value of 2-7 for the reaction solution.

16. The preparation method according to claim 1, characterized in that, The sintering conditions in step S3 include: a heating rate of 2-5℃ / min, a sintering temperature of 400-500℃, and a sintering cycle of 2-10 h.

17. A method for preparing lithium manganese iron phosphate, characterized in that, include: The iron source, manganese source, lithium source and phosphorus source II are mixed and sintered to obtain lithium manganese iron phosphate, wherein the iron source is spherical iron phosphate obtained by the preparation method according to any one of claims 1 to 16.

18. The preparation method according to claim 17, characterized in that, The molar ratio of iron to the sum of iron and manganese is (0.4-0.9):1; and / or The molar ratio of lithium to the sum of iron and manganese is (0.95-1.1):1; and / or The molar ratio of iron and manganese to phosphorus is (0.95-1.06):

1.

19. The preparation method according to claim 17, characterized in that, The preparation method specifically includes: S4. Spherical iron phosphate, manganese source, lithium source, phosphorus source II, carbon source I, dopant and water are mixed, and after coarse grinding, sand grinding and spray drying, powder material I is obtained; S5. The above-mentioned powder material I is subjected to solid-state sintering for the first time. After cooling, a lithium manganese iron phosphate precursor is obtained. The precursor is mixed with carbon source II and water, and then subjected to coarse grinding, sand grinding and spray drying to obtain powder material II. S6. The above-mentioned powder material II is subjected to a second solid-state sintering, and after cooling, the lithium manganese iron phosphate is obtained.

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