Preparation method of lithium iron phosphate positive electrode material

Abstract

The application relates to the technical field of batteries, in particular to a preparation method of a lithium iron phosphate positive electrode material. The preparation method of the lithium iron phosphate positive electrode material provided by the application comprises the following steps: sequentially subjecting a mixed solution containing chelates of ferrous salt, phosphate and soluble lithium salt to first drying and first sintering to obtain first sintered materials; and sequentially subjecting the first sintered materials and a carbon source to grinding, second drying, second sintering and crushing to obtain the lithium iron phosphate positive electrode material. The preparation method of the lithium iron phosphate positive electrode material is simple in steps, low in sewage yield and low in production cost, and improves the comprehensive performance such as the electrochemical performance and the compaction density of the lithium iron phosphate positive electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a method for preparing a lithium iron phosphate positive electrode material. Background Art

[0002] As human civilization continues to develop, society is gradually entering a period of rapid growth, and the demand for energy is exploding. However, the depletion of traditional fossil fuels and environmental pollution have led to the urgent need for an energy revolution and transformation. The rapid development of automotive powertrains, energy storage, and consumer electronics has awakened significant interest in resource-saving, environmentally friendly, efficient, clean, and affordable new energy sources.

[0003] As a high-performance, rechargeable, green power source, lithium-ion batteries have recently gained widespread application in various portable electronic products and communication tools, and are gradually being developed into power sources for electric vehicles. The research and development of new electrode materials, particularly cathode materials, is crucial in lithium-ion battery research. Among these, lithium iron phosphate cathode materials have attracted significant interest due to their stable phosphorus-oxygen bonds, abundant raw materials, excellent stability, service life, safety, low cost, and suitable energy density.

[0004] Traditional preparation methods for lithium iron phosphate cathode materials use pre-synthesized iron phosphate, a lithium source, a carbon source, and additives as raw materials, and employ wet grinding, drying, and a high-temperature solid-phase method. However, this method presents the following challenges: the entire process is lengthy and cumbersome, with high energy consumption; iron phosphate uses a large amount of water, creating environmental pressures for wastewater treatment; and uneven ion distribution leads to low product performance, impacting both intra-batch and inter-batch uniformity, exposing downstream battery manufacturers to the potential risk of inconsistent battery capacity.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a lithium iron phosphate positive electrode material, which fully or partially solves the problems of the prior art in the preparation of lithium iron phosphate positive electrode materials, such as complex steps, high wastewater output, high production cost, poor comprehensive performance such as electrochemical performance and compaction density of the prepared products, poor uniformity within batches, and poor consistency between batches.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0008] The present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0009] subjecting a mixed solution containing a chelate of ferrous salt, a phosphate and a soluble lithium salt to a first drying and a first sintering in sequence to obtain the first sintered material;

[0010] The lithium iron phosphate positive electrode material is obtained by grinding, drying, sintering and crushing the first sintered material and the carbon source in sequence.

[0011] Furthermore, the preparation method of the ferrous salt chelate comprises the following steps:

[0012] A soluble ferrous salt solution, a chelating agent and ammonia gas undergo a first reaction to obtain a chelate of the ferrous salt;

[0013] Wherein, the chelating agent includes ethylenediaminetetraacetic acid and / or N-hydroxyethylethylenediaminetriacetic acid.

[0014] Furthermore, the molar ratio of the ferrous salt in the soluble ferrous salt solution, the chelating agent and the ammonia gas is 1:(1-1.3):(1-3).

[0015] Furthermore, the phosphate includes at least one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate and ammonium phosphate.

[0016] Further, the first drying includes spray drying;

[0017] And / or, the first sintering includes: in an inert atmosphere, heating to 150-220°C and keeping warm for 1.5-2.5 hours; heating to 280-340°C and keeping warm for 1.5-2.5 hours; heating to 400-500°C and keeping warm for 1.5-2.5 hours; heating to 700-800°C and keeping warm for 3-6 hours.

[0018] Furthermore, the carbon source includes at least one of glycogen, glucose, white sugar, polyethylene glycol, citric acid and soluble starch;

[0019] And / or, the mass ratio of the first sintering material to the carbon source is 1000:(90-150).

[0020] Further, the second drying comprises spray drying;

[0021] And / or, the second sintering includes: heating to 240-330° C. and keeping the temperature for 1.5-2.5 hours under an inert atmosphere; and then heating to 500-600° C. and keeping the temperature for 3-5 hours.

[0022] Furthermore, the preparation method of the chelating agent comprises the following steps:

[0023] A mixed solution containing compound A, sodium cyanide and sodium hydroxide is subjected to a second reaction with a formaldehyde solution to obtain ammonia gas and compound B; the compound B is acidified to obtain the chelating agent;

[0024] The compound A includes ethylenediamine or hydroxyethylethylenediamine; the compound B includes sodium ethylenediaminetetraacetate or sodium N-hydroxyethylethylenediaminetriacetate.

[0025] Furthermore, the molar ratio of the ethylenediamine, the sodium cyanide and the formaldehyde is 1:(3.5-5):(3-5).

[0026] Furthermore, the ammonia gas obtained from the second reaction is used to carry out the first reaction in the preparation of the chelate of the ferrous salt;

[0027] And / or, the ammonia obtained from the second reaction is used to prepare the ammonium dihydrogen phosphate.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The preparation method of the lithium iron phosphate positive electrode material of the present invention omits the step of synthesizing the precursor iron phosphate, thereby reducing production costs and reducing the generation of wastewater.

[0030] 2. The present invention improves the stability and dispersibility of ferrous salts by adopting chelates of ferrous salts and utilizing the characteristics of chelates; the three-dimensional structure of the chelates ensures the morphology of particles in subsequent steps and shortens the mass transfer distance of ions, thereby improving the rate performance and conductivity of lithium iron phosphate positive electrode materials.

[0031] 3. The present invention mixes chelates of ferrous salts, phosphates and soluble lithium salts to form a solution, thereby achieving a gradation of the atomic solutions of iron, phosphorus and lithium, thereby improving the distribution uniformity of each atom, which is beneficial to improving product performance, uniformity within batches, and consistency between batches.

[0032] 4. The present invention improves the comprehensive performance indicators such as the electrochemical performance and compaction density of the lithium iron phosphate positive electrode material through secondary sintering and optimization of the sintering process parameters.

[0033] 5. The present invention implements resource-integrated comprehensive recycling of ammonia gas, a by-product generated during the preparation process, thereby reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 This is a schematic flow chart of a method for preparing a lithium iron phosphate positive electrode material according to Example 1 of the present invention. DETAILED DESCRIPTION

[0036] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. Those who do not specify specific conditions in the embodiments are carried out according to normal conditions or the conditions recommended by the manufacturer. Those whose reagents or instruments are not specified by the manufacturer are conventional products that can be purchased commercially.

[0037] The following is a detailed description of a method for preparing a lithium iron phosphate positive electrode material according to an embodiment of the present invention.

[0038] The present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0039] A mixed solution containing a chelate of ferrous salt, a phosphate and a soluble lithium salt is sequentially subjected to a first drying and a first sintering to obtain a first sintered material;

[0040] The first sintered material and the carbon source are sequentially ground, dried, sintered and crushed to obtain a lithium iron phosphate positive electrode material.

[0041] The present invention synthesizes lithium iron phosphate positive electrode material through a liquid phase method and two-step sintering process steps; the preparation method has simple process steps, low energy consumption, omits the synthesis process of the precursor iron phosphate, reduces the preparation cost of iron phosphate, and reduces the generation of sewage caused by filter cake washing.

[0042] By adopting ferrous salt chelates and utilizing the characteristics of the chelates, the stability and dispersibility of ferrous salts are improved; at the same time, the three-dimensional structure of the chelates ensures the morphology of the particles in the subsequent preparation process and shortens the mass transfer distance of ions, which is beneficial to improving the rate characteristics and conductivity of lithium iron phosphate positive electrode materials.

[0043] Water-soluble ferrous salt chelates, phosphates and soluble lithium salts are mixed to form a mixed solution containing Li, Fe and P, thereby achieving the gradation of atomic solution of iron, phosphorus and lithium, omitting the grinding process. With the help of the characteristics of the solution, the uniform distribution of each atom is improved, ensuring subsequent preparation, and is conducive to improving product performance, uniformity within batches, and consistency between batches.

[0044] Through two sintering processes, nano-sized particles can be prepared; by optimizing the process parameters of the two sintering processes, it is beneficial to improve the comprehensive performance of lithium iron phosphate positive electrode materials, such as electrochemical properties and compaction density.

[0045] In some embodiments of the present invention, the method for preparing the chelate of ferrous salt comprises the following steps:

[0046] A soluble ferrous salt solution, a chelating agent and ammonia gas undergo a first reaction to obtain a chelate of the ferrous salt;

[0047] Wherein, the chelating agent includes ethylenediaminetetraacetic acid (EDTA) and / or N-hydroxyethylethylenediaminetriacetic acid (HEDTA).

[0048] In some specific embodiments of the present invention, a soluble ferrous salt solution, ethylenediaminetetraacetic acid and ammonia are reacted in a first reaction to obtain ethylenediaminetetraacetic acid ferric ammonium (EDTA-Fe 2+ / NH4 + ).

[0049] The present invention utilizes the three-dimensional network structure characteristics of the chelate solution to improve the dispersibility and uniformity of Fe(II) ions and achieve atomic grading. At the same time, the formed three-dimensional network structure of ammonium ethylenediaminetetraacetate iron can improve the specific surface area and particle morphology, thereby improving the ion mass transfer distance, electrical conductivity, electrical properties, etc. of the prepared lithium iron phosphate positive electrode material. The ammonium ions present in the ammonium ethylenediaminetetraacetate iron can improve the porosity during the sintering process, thereby improving the specific surface area, and no impurities are introduced. If other chelating agents, such as disodium ethylenediaminetetraacetate, are used, there is a risk of introducing impurity sodium ions.

[0050] In some specific embodiments of the present invention, a soluble ferrous salt solution, N-hydroxyethylethylenediaminetriacetic acid and ammonia are reacted in a first reaction to obtain N-hydroxyethylethylenediaminetriacetic acid ferric ammonium (HEDTA-Fe 2+ / NH4 + ).

[0051] Three-dimensional structure of HEDTA-Fe 2+ / NH4 + Chelate, increases Fe 2+ The spatial network structure of LiFePO4 is improved, thereby improving the morphology, resistivity and cycle characteristics.

[0052] In some embodiments of the present invention, the ferrous salt in the soluble ferrous salt solution includes at least one of ferrous chloride, ferrous nitrate, ferrous oxalate, ferrous sulfate and ferrous acetate.

[0053] In some embodiments of the present invention, the molar ratio of ferrous salt, chelating agent and ammonia in the soluble ferrous salt solution is 1:(1-1.3):(1-3); preferably, the molar ratio of ferrous salt, ethylenediaminetetraacetic acid and ammonia in the soluble ferrous salt solution is 1:(1-1.3):(2-3); the molar ratio of ferrous salt, N-hydroxyethylethylenediaminetriacetic acid and ammonia in the soluble ferrous salt solution is 1:(1-1.3):(1-2).

[0054] In some embodiments of the present invention, the temperature of the first reaction is 90-100° C., and the time of the first reaction is 2-4 hours.

[0055] In some embodiments of the present invention, the phosphate includes at least one of diammonium phosphate, monoammonium phosphate, and ammonium phosphate.

[0056] In some embodiments of the present invention, the soluble lithium salt includes at least one of lithium carbonate, lithium nitrate, lithium oxalate, lithium chloride, and lithium acetate.

[0057] In some embodiments of the present invention, in the mixed solution, the molar ratio of Li element, Fe element and P element is (1.0-1.2):(0.97-0.99):1.

[0058] In some embodiments of the present invention, the first drying comprises spray drying; preferably, the spray drying temperature is 220-280°C.

[0059] In some embodiments of the present invention, the first sintering process includes: heating to 150-220°C and holding for 1.5-2.5 hours; heating to 280-340°C and holding for 1.5-2.5 hours; heating to 400-500°C and holding for 1.5-2.5 hours; and heating to 700-800°C and holding for 3-6 hours under an inert atmosphere. Preferably, the inert atmosphere during the first sintering process includes argon and / or nitrogen.

[0060] In some embodiments of the present invention, the carbon source includes at least one of glycogen, glucose, white sugar, polyethylene glycol, citric acid and soluble starch; preferably, the carbon source includes glycogen.

[0061] In some embodiments of the present invention, the first sintering material and the aqueous solution of glycogen are mixed and then ground.

[0062] In some embodiments of the present invention, the mass ratio of the first sintering material to the carbon source is 1000:(90-150); typically but not limitatively, for example, the mass ratio of the first sintering material to the carbon source can be 1000:90, 1000:100, 1000:110, 1000:120, 1000:130, 1000:140, 1000:150 or a range value consisting of any two thereof.

[0063] In some embodiments of the present invention, the second drying comprises spray drying; preferably, the spray drying temperature is 220-280°C.

[0064] In some embodiments of the present invention, the second sintering step includes: heating the material to 240-330°C and holding the temperature for 1.5-2.5 hours under an inert atmosphere; then heating the material to 500-600°C and holding the temperature for 3-5 hours. Preferably, the inert atmosphere during the second sintering step includes argon and / or nitrogen.

[0065] The second sintering of the present invention can fully crack the organic glycogen through different temperature gradients and holding times, and prevent untimely cracking and entering the high-temperature stage, thereby causing unnecessary side reactions; and can effectively and accurately regulate the coated carbon.

[0066] In some specific embodiments of the present invention, the method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0067] S1, a soluble ferrous salt solution, a chelating agent and ammonia (NH3) undergo a first reaction to obtain a chelate; wherein the chelating agent comprises ethylenediaminetetraacetic acid or N-hydroxyethylethylenediaminetriacetic acid;

[0068] S2. Spray-drying a mixed solution containing a chelate, ammonium dihydrogen phosphate, and a soluble lithium salt to obtain a first dry powder; heating the first dry powder to 150-220° C. and maintaining it for 1.5-2.5 hours; heating it to 280-340° C. and maintaining it for 1.5-2.5 hours; heating it to 400-500° C. and maintaining it for 1.5-2.5 hours; heating it to 700-800° C. and maintaining it for 3-6 hours under an inert atmosphere; and obtaining a first sintered material (LiFePO4 material);

[0069] S3. The first sintered material and glycogen are wet-ground and spray-dried in sequence to obtain a second dry powder; the second dry powder is heated to 240-330° C. under an inert atmosphere and kept warm for 1.5-2.5 hours; then heated to 500-600° C. and kept warm for 3-5 hours, and then crushed to obtain a lithium iron phosphate positive electrode material.

[0070] When the chelating agent is ethylenediaminetetraacetic acid, in step S1, the following reaction occurs:

[0071]

[0072] In step S2, the following reaction occurs:

[0073] EDTA-Fe 2+ / NH4 + +Li + +NH4H2PO4→LiFePO4.

[0074] When the chelating agent is N-hydroxyethylethylenediaminetriacetic acid, in step S1, the following reaction occurs:

[0075]

[0076] In some embodiments of the present invention, the preparation method of the chelating agent comprises the following steps:

[0077] A mixture containing compound A, sodium cyanide (NaCN) and sodium hydroxide (NaOH) and a formaldehyde (HCHO) solution undergo a second reaction to produce ammonia gas and compound B; compound B is acidified to produce a chelating agent;

[0078] Compound A includes ethylenediamine (H2N-CH2-CH2-NH2) or hydroxyethylethylenediamine (H2N-CH2-CH2-NH-CH2-CH2-OH);

[0079] Compound B includes sodium edetate or sodium N-hydroxyethylethylenediaminetriacetate.

[0080] In some embodiments of the present invention, the molar ratio of compound A, sodium cyanide and formaldehyde is 1:(3.5-5):(3-5).

[0081] In some embodiments of the present invention, the method for preparing EDTA comprises the following steps:

[0082] mixing an aqueous solution of ethylenediamine, an aqueous solution of sodium cyanide, and an aqueous solution of sodium hydroxide to obtain a mixed solution;

[0083] The aqueous solution of formaldehyde and the above mixed solution undergo a second reaction, the pH of the second reaction system is 8 to 12, and ammonia gas and a reaction solution are obtained after the reaction is completed;

[0084] A hydrochloric acid solution is added to the reaction solution to adjust the pH of the system to 2-5. After washing and filtering, EDTA crystals and a solution containing sodium chloride are obtained.

[0085] In some embodiments of the present invention, the molar ratio of ethylenediamine, sodium cyanide and formaldehyde is 1:(4-5):(4-5).

[0086] During the preparation of EDTA, the following chemical reactions occur:

[0087]

[0088] In some embodiments of the present invention, the preparation method of N-hydroxyethylethylenediaminetriacetic acid comprises the following steps:

[0089] mixing an aqueous solution of hydroxyethylethylenediamine, an aqueous solution of sodium cyanide, and an aqueous solution of sodium hydroxide to obtain a mixed solution;

[0090] The aqueous solution of formaldehyde and the above mixed solution undergo a second reaction, the pH of the second reaction system is 8 to 12, and ammonia gas and a reaction solution are obtained after the reaction is completed;

[0091] A hydrochloric acid solution is added to the reaction solution to adjust the pH of the system to 2-5. After washing and filtering, N-hydroxyethylethylenediaminetriacetic acid crystals and a solution containing sodium chloride are obtained.

[0092] In some embodiments of the present invention, the molar ratio of hydroxyethylethylenediamine, sodium cyanide and formaldehyde is 1:(3.5-5):(3-4).

[0093] During the preparation of N-hydroxyethylethylenediaminetriacetic acid, the following chemical reactions occur:

[0094]

[0095] During the preparation of HEDTA-trisodium, a NaOH solution is added to prevent the hydrolysis of NaCN, and the generated ammonia can be recycled to prepare a HEDTA-ammonium ferric chelate with a three-dimensional structure.

[0096] In some embodiments of the present invention, the mass percentages of the aqueous solution of ethylenediamine and the aqueous solution of hydroxyethylethylenediamine are each independently 50% to 80%; the mass percentage of the aqueous solution of sodium cyanide is 20% to 50%; the mass percentage of the aqueous solution of sodium hydroxide is 32% to 50%; the aqueous solution of sodium hydroxide is added dropwise to adjust the pH of the second reaction system to 8 to 12; and the mass percentage of the aqueous solution of formaldehyde is 30% to 40%.

[0097] In some embodiments of the present invention, the mixing temperature is 20° C. to 40° C., and the mixing time is 0.5 h to 2 h.

[0098] In some embodiments of the present invention, the temperature of the second reaction is 70° C. to 110° C., and the time of the second reaction is 2 h to 6 h.

[0099] In the preparation method of the chelating agent of the present invention, the by-product ammonia gas can be reused; and the solution containing sodium chloride, as a by-product, can be utilized as a resource.

[0100] In some embodiments of the present invention, the ammonia obtained in the second reaction is used in the first reaction to prepare the chelate of ferrous salt. The by-product ammonia obtained in the preparation of the chelating agent can be used in the preparation of the chelate of ferrous salt.

[0101] In some embodiments of the present invention, the ammonia produced in the second reaction is used to prepare phosphate. Preferably, the ammonia produced in the second reaction reacts with phosphoric acid to produce ammonium dihydrogen phosphate; the chemical reaction equation is: NH3 + H3PO4 → NH4H2PO4; preferably, the molar ratio of phosphoric acid to ammonia is 1:(1-1.3). The ammonia produced as a byproduct during the preparation of the chelating agent can also be used in the preparation of ammonium dihydrogen phosphate, thereby providing phosphate for the preparation of the lithium iron phosphate cathode material.

[0102] The present invention realizes the integrated and comprehensive recycling of the by-product ammonia in the preparation method of the chelating agent, thereby reducing the production cost of the lithium iron phosphate positive electrode material.

[0103] Example 1

[0104] The method for preparing the lithium iron phosphate positive electrode material provided in this embodiment includes the following steps:

[0105] S1, 76.16 g of a 60 wt % aqueous solution of ethylenediamine and 440 g of a 35 wt % aqueous solution of sodium cyanide were mixed, and a 35 wt % aqueous solution of sodium hydroxide was added, the pH of the system was adjusted to 8.3, and then stirred at 35 ° C for 1.0 h to obtain solution A;

[0106] 267 g of a 36 wt % formaldehyde aqueous solution was slowly added to solution A, reacted at 85° C. for 2.0 h, and water and ammonia were evaporated under reduced pressure to obtain solution B;

[0107] A hydrochloric acid solution with a concentration of 1+1 was added to solution B to adjust the pH of the system to 2.8, and ethylenediaminetetraacetic acid and solution C were obtained after washing and filtering.

[0108] S2. Add 268 g of ethylenediaminetetraacetic acid to 1.45 L of an 80 g / L aqueous solution of ferrous sulfate, introduce ammonia gas from step S1 into the solution, and react at 90° C. for 3 h to obtain solution D.

[0109] S3, solution D, ammonium dihydrogen phosphate solution and lithium carbonate were mixed to obtain solution E, wherein the molar ratio of Fe, P and Li in solution E was 0.972:1:1.03; solution E was spray-dried to obtain a first dry powder with a particle size of 20 μm; the first dry powder was heated to 160° C. at a rate of 3° C. / min and kept warm for 2.0 h, then heated to 280° C. and kept warm for 2.5 h, then heated to 450° C. and kept warm for 1.8 h, and finally heated to 720° C. and kept warm for 3.5 h under a nitrogen atmosphere to obtain a first sintered material;

[0110] The method for preparing the ammonium dihydrogen phosphate solution comprises: introducing the ammonia gas obtained in step S1 into an aqueous solution of phosphoric acid to react and obtain the ammonium dihydrogen phosphate solution; the molar ratio of ammonia gas to phosphoric acid is 1.02:1.

[0111] S4. Slowly add 100 g of the first sintered material to 24 g of a 50 wt% aqueous solution of glycogen, mix evenly, and grind for 2.0 h to obtain a slurry; the slurry is spray-dried to obtain a second dry powder with a particle size of 15.58 μm; the second dry powder is heated to 280°C and kept warm for 2.5 h under a nitrogen atmosphere; then heated to 580°C and kept warm for 4.0 h; and then crushed to obtain a lithium iron phosphate positive electrode material with a particle size of 1.2 μm.

[0112] The process diagram of the preparation method of the lithium iron phosphate positive electrode material of this embodiment is as follows Figure 1 shown.

[0113] Example 2

[0114] The method for preparing the lithium iron phosphate positive electrode material provided in this embodiment includes the following steps:

[0115] S1, 70.1g of a 65wt% aqueous solution of ethylenediamine and 450g of a 35wt% aqueous solution of sodium cyanide were mixed, and a 35wt% aqueous solution of sodium hydroxide was added, the pH of the system was adjusted to 9.0, and the mixture was stirred at 30°C for 1.5h to obtain solution A;

[0116] 280 g of a 36 wt % formaldehyde aqueous solution was slowly added to solution A, reacted at 92° C. for 2.5 h, and water and ammonia were evaporated under reduced pressure to obtain solution B;

[0117] A hydrochloric acid solution with a concentration of 1+1 was added to solution B to adjust the pH of the system to 2.4, and ethylenediaminetetraacetic acid and solution C were obtained after washing and filtering.

[0118] S2. Add 275 g of ethylenediaminetetraacetic acid to 1.50 L of an 80 g / L aqueous solution of ferrous sulfate, introduce ammonia gas from step S1 into the solution, and react at 92° C. for 3.5 h to obtain solution D.

[0119] S3, solution D, ammonium dihydrogen phosphate solution and lithium carbonate were mixed to obtain solution E, wherein the molar ratio of Fe, P and Li in solution E was 0.974:1:1.015; solution E was spray-dried to obtain a first dry powder with a particle size of 25 μm; the first dry powder was heated to 180°C at a rate of 3°C / min under a nitrogen atmosphere and kept warm for 1.5 h, then heated to 290°C and kept warm for 2.0 h; then heated to 430°C and kept warm for 2.2 h; and finally heated to 750°C and kept warm for 4.0 h to obtain a first sintered material;

[0120] The method for preparing the ammonium dihydrogen phosphate solution comprises: introducing the ammonia gas obtained in step S1 into an aqueous solution of phosphoric acid to react and obtain the ammonium dihydrogen phosphate solution; the molar ratio of ammonia gas to phosphoric acid is 1.08:1.

[0121] S4. Slowly add 100g of the first sintered material to 20g of a 50wt% aqueous solution of glycogen, mix evenly, and grind for 1.5h to obtain a slurry; the slurry is spray-dried to obtain a second dry powder with a particle size of 22.03μm; the second dry powder is heated to 300℃ under a nitrogen atmosphere and kept warm for 2.0h; then heated to 550℃ and kept warm for 5.0h; and then crushed to obtain a lithium iron phosphate positive electrode material with a particle size of 1.15μm.

[0122] Example 3

[0123] The method for preparing the lithium iron phosphate positive electrode material provided in this embodiment includes the following steps:

[0124] S1, 61g of a 75wt% aqueous solution of ethylenediamine, 340g of a 45wt% aqueous solution of sodium cyanide, and a 35wt% aqueous solution of sodium hydroxide were added, the pH of the system was adjusted to 9.5, and the mixture was stirred at 35°C for 2.5h to obtain solution A;

[0125] 300 g of a 36 wt % formaldehyde aqueous solution was slowly added to solution A, reacted at 92° C. for 3.0 h, and water and ammonia were evaporated under reduced pressure to obtain solution B;

[0126] A hydrochloric acid solution with a concentration of 1+1 was added to solution B to adjust the pH of the system to 2.1, and ethylenediaminetetraacetic acid and solution C were obtained after washing and filtering.

[0127] S2. Add 260 g of ethylenediaminetetraacetic acid to 1.3 L of an 80 g / L aqueous solution of ferrous sulfate, introduce ammonia gas from step S1 into the solution, and react at 96° C. for 2.0 h to obtain solution D.

[0128] S3, solution D, ammonium dihydrogen phosphate solution and lithium carbonate were mixed to obtain solution E, wherein the molar ratio of Fe, P and Li in solution E was 0.981:1:1.08; solution E was spray dried to obtain a first dry powder with a particle size of 13.2 μm; the first dry powder was heated to 210° C. at a rate of 2.5° C. / min and kept warm for 1.6 h, then heated to 330° C. and kept warm for 2.5 h, then heated to 480° C. and kept warm for 2.3 h, and finally heated to 750° C. and kept warm for 4.5 h under a nitrogen atmosphere to obtain a first sintered material;

[0129] The method for preparing the ammonium dihydrogen phosphate solution comprises: introducing the ammonia gas obtained in step S1 into an aqueous solution of phosphoric acid to react and obtain the ammonium dihydrogen phosphate solution; the molar ratio of ammonia gas to phosphoric acid is 1.21:1.

[0130] S4. Slowly add 100g of the first sintered material to 19g of a 50wt% aqueous solution of glycogen, mix evenly, and grind for 1.3h to obtain a slurry; the slurry is spray-dried to obtain a second dry powder with a particle size of 26μm; the second dry powder is heated to 320℃ under a nitrogen atmosphere and kept warm for 1.8h; then heated to 570℃ and kept warm for 5.0h; and then crushed to obtain a lithium iron phosphate positive electrode material with a particle size of 1.32μm.

[0131] Example 4

[0132] The preparation method of the lithium iron phosphate positive electrode material provided in this embodiment refers to Example 1, except that, in step S3, the first dry powder is heated to 240°C at a rate of 6°C / min under a nitrogen atmosphere and kept warm for 2.0 h, then heated to 360°C and kept warm for 3.0 h; heated to 520°C and kept warm for 3.0 h; and heated to 820°C and kept warm for 6.0 h to obtain a first sintered material.

[0133] Example 5

[0134] The preparation method of the lithium iron phosphate positive electrode material provided in this embodiment refers to that in embodiment 1, except that, in step S4, the second dry powder is heated to 350° C. and kept warm for 3.0 h under a nitrogen atmosphere; and then heated to 630° C. and kept warm for 4.0 h.

[0135] Example 6

[0136] The preparation method of the lithium iron phosphate positive electrode material provided in this embodiment refers to Example 1, with the only difference being that in step S3, the first dry powder is heated to 670°C at a rate of 3.0°C / min under a nitrogen atmosphere and kept warm for 4.0 hours; in step S4, the second dry powder is heated to 670°C under a nitrogen atmosphere and kept warm for 4.0 hours.

[0137] Example 7

[0138] The preparation method of the lithium iron phosphate positive electrode material provided in this embodiment refers to Example 1, except that, in S1, 200 g of a 50 wt % aqueous solution of hydroxyethylethylenediamine and 450 g of a 35 wt % aqueous solution of sodium cyanide were mixed, and a 35 wt % aqueous solution of sodium hydroxide was added, the pH of the system was adjusted to 10.2, and the mixture was stirred at 30° C. for 1.5 h to obtain solution A;

[0139] 260 g of a 36 wt % formaldehyde aqueous solution was slowly added to solution A, reacted at 92° C. for 2.5 h, and water and ammonia were evaporated under reduced pressure to obtain solution B;

[0140] A hydrochloric acid solution with a concentration of 1+1 was added to solution B to adjust the pH of the system to 2.4, and N-hydroxyethylethylenediaminetriacetic acid and solution C were obtained after washing and filtering.

[0141] S2. Add 275 g of N-hydroxyethylethylenediaminetriacetic acid to 1.50 L of an 80 g / L aqueous solution of ferrous sulfate, introduce ammonia gas from step S1 into the solution, and react at 92° C. for 3.5 h to obtain solution D.

[0142] Comparative Example 1

[0143] The preparation method of the lithium iron phosphate positive electrode material provided in this comparative example comprises the following steps:

[0144] S1, preparing ferric phosphate by sodium method;

[0145] S2. 100 g of iron phosphate, 249.6 g of lithium carbonate and 14 g of glucose were mixed and ground for 2.5 h, and then spray-dried at 240 ° C.; sintered under nitrogen conditions for 5 h, heated to 760 ° C, and then kept warm for 10 h; and air flow pulverized to obtain lithium iron phosphate positive electrode material.

[0146] Test Example 1

[0147] The electrochemical properties of the lithium iron phosphate cathode materials prepared in Examples 1 to 6 and Comparative Example 1 were tested. The test methods refer to the national standards for lithium iron phosphate cathode materials. The results are shown in Table 1.

[0148] Table 1

[0149]

[0150]

[0151] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps include: subjecting a mixed solution containing a chelate of ferrous salt, a phosphate and a soluble lithium salt to a first drying and a first sintering in sequence to obtain the first sintered material; The first sintered material and the carbon source are sequentially ground, dried, sintered and crushed to obtain the lithium iron phosphate positive electrode material; The preparation method of the ferrous salt chelate comprises the following steps: A soluble ferrous salt solution, a chelating agent and ammonia gas undergo a first reaction to obtain a chelate of the ferrous salt; Wherein, the chelating agent is ethylenediaminetetraacetic acid and / or N-hydroxyethylethylenediaminetriacetic acid; Soluble ferrous salt solution, ethylenediaminetetraacetic acid and ammonia react to obtain ethylenediaminetetraacetic acid ferric ammonium EDTA-Fe 2+ / NH4 + ; Soluble ferrous salt solution, N-hydroxyethylethylenediaminetriacetic acid and ammonia react to obtain N-hydroxyethylethylenediaminetriacetic acid ferric ammonium HEDTA-Fe 2+ / NH4 + .

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The molar ratio of the ferrous salt, the chelating agent and the ammonia gas in the soluble ferrous salt solution is 1:(1-1.3):(1-3).

3. The method for preparing the lithium iron phosphate cathode material according to claim 2, wherein: The phosphate includes at least one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate and ammonium phosphate.

4. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The first drying comprises spray drying; And / or, the first sintering includes: in an inert atmosphere, heating to 150-220°C and keeping warm for 1.5-2.5 hours; heating to 280-340°C and keeping warm for 1.5-2.5 hours; heating to 400-500°C and keeping warm for 1.5-2.5 hours; heating to 700-800°C and keeping warm for 3-6 hours.

5. The method for preparing the lithium iron phosphate cathode material according to claim 1, wherein: The carbon source comprises at least one of glycogen, glucose, white sugar, polyethylene glycol, citric acid and soluble starch; And / or, the mass ratio of the first sintering material to the carbon source is 1000:(90~150).

6. The method for preparing a lithium iron phosphate cathode material according to claim 1, wherein: The second drying comprises spray drying; And / or, the second sintering includes: heating to 240-330° C. and keeping the temperature for 1.5-2.5 hours under an inert atmosphere; and then heating to 500-600° C. and keeping the temperature for 3-5 hours.

7. The method for preparing a lithium iron phosphate cathode material according to claim 3, wherein: The preparation method of the chelating agent comprises the following steps: A mixed solution containing compound A, sodium cyanide and sodium hydroxide is subjected to a second reaction with a formaldehyde solution to obtain ammonia gas and compound B; the compound B is acidified to obtain the chelating agent; The compound A includes ethylenediamine or hydroxyethylethylenediamine; the compound B includes sodium ethylenediaminetetraacetate or sodium N-hydroxyethylethylenediaminetriacetate.

8. The method for preparing a lithium iron phosphate cathode material according to claim 7, wherein: The molar ratio of the compound A, the sodium cyanide and the formaldehyde is 1:(3.5-5):(3-5).

9. The method for preparing a lithium iron phosphate cathode material according to claim 7, wherein: The ammonia obtained by the second reaction is used to carry out the first reaction in the preparation of the chelate of the ferrous salt; And / or, the ammonia obtained from the second reaction is used to prepare the ammonium dihydrogen phosphate.

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